v>EPA
United States
Environmental Protection
Agency
Industrial Environmental Research EPA-600/7-79-110
Laboratory May 1979
Cincinnati OH 45268
Research and Development
Evaluation of the
Environmental
Effects of Western
Surface Coal Mining
Volume I
Interagency
Energy/Environment
R&D Program
Report
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series. Reports in this series result from the
effort funded under the 17-agency Federal Energy/Environment Research and
Development Program. These studies relate to EPA's mission to protectthe public
health and welfare from adverse effects of pollutants associated with energy sys-
tems. The goal of the Program is to assure the rapid development of domestic
energy supplies in.an environmentally-compatible manner by providing the nec-
essary environmental data and control technology. Investigations include analy-
ses of the transport of energy-related pollutants and their health and ecological
effects; assessments of, and development of, control technologies for energy
systems; and integrated assessments of a wide range of energy-related environ-
mental issues.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/7-79-110
May 1979
EVALUATION OF THE ENVIRONMENTAL EFFECTS
OF WESTERN SURFACE COAL MINING
Volume I
by
Frank Cook
Mathematica, Inc.
Princeton, New Jersey 08540
Contract No. 68-03-2226
Project Officer
S. Jackson Hubbard
Extraction Technology Branch
Industrial Environmental Research Laboratory
Cincinnati, Ohio 45268
INDUSTRIAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Industrial Environmental
Research Laboratory-Cincinnati, U.S. Environmental Protection Agency,
and approved for publication. Approval does not signify that the contents
necessarily reflect the views and policies of the U.S. Environmental
Protection Agency, nor does mention of trade names or commercial
products constitute endorsement or recommendation for use.
ii
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FOREWORD
When energy and material resources are extracted, processed,
converted, and used, the pollutional impact on our environment and even
on our health often requires that new and increasingly more efficient
pollution control methods be used. The Industrial Environmental Research
Laboratory-Cincinnati (IERL-CI) assists in developing and demonstrating
new and improved methodologies that will meet these needs both efficiently
and economically.
An evaluation of the surface coal mining methods presently used in
arid and semi-arid regions of the western United States and a description
of the effects that those methods have on the environment are presented in
this volume. In addition, recommendations on how those methods might
be altered to reduce both long-term and short-term environmental effects
are presented. For further information, contact the Resource Extraction
and Handling Division.
David G. Stephan
Director
Industrial Environmental Research Laboratory
Cincinnati
iii
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ABSTRACT
The objectives of this project were to describe and evaluate the
methods presently used for surface mining of coal in the western United
States, to identify and evaluate the effects that use of those methods
have on the environment, and to recommend ways in which the methods
might be altered to reduce both long-term and short-term environmental
damage.
This was accomplished by statistical analysis of comprehensive pro-
duction and reclamation data for all 44 western surface mines active or
under development in 1975, and through qualitative evaluations based on
personal interviews of state and Federal reclamation specialists and
field surveys of nine mines during three seasons of the year.
On a regional basis, the short-term environmental damages caused by
western surface coal mining in 1975 and 1976 were deemed to be neither
severe nor extensive. There are, however, areas in which current or
projected concentration of large-scale surface coal mining activities
could produce serious local effects in both the short- and long-terms.
Additionally, there are regional uncertainties regarding the long-term
severity and extent of certain potential environmental damages.
There appears to be one significant way in which current mining
methods may need to be altered to reduce environmental damage. It is
modification of operating practices to enable more selective removal of
overburden and placement of spoil, particularly where multiple seams are
mined. This would include not placing high-sodium clays and shales on
spoil surfaces and preventing the placement on pit floors of materials
high in soluble minerals or trace elements.
This report was submitted in fulfillment of Contract No. 68-03-2226
by Mathtech, Inc. under the sponsorship of the U.S. Environmental Protection
Agency. This report covers a period from June 25, 1975, to April 30, 1977,
and work was completed on June 3, 1977.
iv
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CONTENTS
Foreword iii
Abstract iv
Figures vii
Tables xi
Acknowledgments xiii
1. Introduction 1
2. Conclusions 2
3. Recommendations 4
4. Background, Objectives, Scope 6
Background 6
Study Objectives 7
Scope of Study 7
Study Methodology 7
Synopsis of Subsequent Sections 9
Summary 9
5. Issues and Impacts 11
Introduction 11
The General Setting for Development 11
Factors Affecting Public Attitudes Toward
Development of Strippable Coal Reserves 13
Preview of Mining and Reclamation Technology ... 22
Physical Characteristics of the Region 24
Further Perspectives 31
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6. Current Mining and Reclamation Systems 37
Introduction 37
Classification of Mining Situations 37
Mine Planning 44
Mine Operating Procedures 49
Summary: Integration of Mining and
Reclamation Practices 102
Reclamation Practices 106
Other Reclamation-Related Practices
and Problems 115
Summary 120
7. Discussion of Recommendations . 124
Selective Overburden Removal and Spoil Placement . . 124
Means for Minimizing Groundwater Impacts 125
Bucket Wheel Excavators 125
References 129
Appendix A
Comparison of Dragline and Bucket Wheel
Excavator Production Rates and Costs 131
VI
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FIGURES
Number Page
1. Trends in Surface Coal Mine Production in the
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Trends in Production Share for Surface Coal Mining
Strippable Coal Reserves of the Western United States . .
Current and Forecasted 1980 Surface Coal Mine
Production Percentages for the Western United States . . .
Geographic Distribution of Coal Surface -Mined in
Existing and Proposed Railroad Lines and Coal Slurry
Locations of Planned Coal Gasification Plants and
Coal-Fired Power Plants
Section View of Results of Single Seam Dragline
Distribution of Average Annual Precipitation at
Histogram of Average Altitudes at Western Mines ....
Regional Trends in Average Surface Coal Mine Size . . .
Regional Trends in the Number of Active Surface
Histogram of Premining Stocking Rates at
XT
14
15
16
18
19
21
22
25
26
32
32
35
38
39
Vll
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16. Photograph Showing Truck and Shovel Stripping
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
Classification of Western U.S. Surface Coal
Photograph Showing a Typical Drainage Diversion Ditch . .
Photograph Showing a Revegetated Diversion Ditch ....
Photograph Showing Headcut at Outlet of Drainage
Photograph Showing Rock-Lined Outlet of Drainage
Diversion Ditch, Used to Prevent Headcutting
Histogram of Average Topsoil Salvaging Depths
Photograph Showing Topsoil Removal by Dozer
Photograph Showing Topsoil Replacement by Scraper . . .
Photograph Showing An Area That Has Been Topsoiled . .
Photograph Showing Dragline Excavating Box Cut
Photograph Showing Dragline Placing Box Cut Spoils . . .
Methods for Opening a Box Cut in Deep Overburden ....
Plan and Section Views Showing Dragline Keycutting
Comparison of Spoil Piles with 27 Meter (90 Foot)
and 37 Meter (120 Foot) Crest-to-Crest Spacing
Section View Showing Coal Wedge
Photograph Showing Removal of Coal Wedge and
Section View Showing Dragging of Filled Bucket
j. j.
43
50
50
51
51
53
54
54
55
57
57
60
60
61
63
64
65
66
67
68
Vlll
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37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
SAR Analysis of Core Drill Sample From
Illustration of Selective Spoil Placement Procedure
in Single Seam Dragline Stripping Situation
Plan and Section Views Showing Side -Benching Procedure .
Comparison of Spoil Profiles in Two Side -Benching
Photograph Showing a Typical Dragline Bucket
Procedure to Bury Surface Overburden Strata in
Middle of Spoil Profile
Maximum No-Rehandle Depth vs. Dragline Boom Length
An Extended Bench Procedure with Selective Spoil
Comparison of Spoil Rehandle Percentages for Two
Multiple Seam Situation With No Spoil Rehandle
Multiple Seam Method With Rehandle to Enable
A Two-Pass Extended Bench Method for Stripping
Single Pass Extended Bench Method Stripping for
Two Coal Seams
Comparison of Final Surface Contours for Dragline
Photograph Showing Fugitive Dust Caused by
Blasting of Coal
Photograph Showing Typical Pit Incline
Photograph Showing Reclaimed and Adjacent
69
70
75
76
77
78
80
82
83
85
88
90
92
93
95
97
100
101
102
109
IX
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57. Photograph Showing Graded Box Cut Spoil Pile 110
58. Photograph Showing a Reclaimed Final Cut 110
59. Photograph Showing Sediment Carried Into
Roadside Ditch 114
60. Photograph Showing Contour Ditches Used to Reduce
Erosion 114
61. Photograph Showing Small Sink Hole in Orphan
Sodic Spoil 118
62. Photograph Showing Erosion on Steep Glacial Spoils ... 119
63. Photograph Showing Disposal of Fly Ash in a Strip Pit . . 119
64. Operating Plan for Bucket Wheel Excavator 126
x
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TABLES
Number Page
1. Frequency of Occurrence of Premining Land Uses
at Mines Active or Under Development in 1975 12
2. Net Exports of Coal Surface-Mined in the
Western United States During 1975 (Actual) and
1980 (Forecasted) 17
3. Capacity of Coal-Fired Steam Electric Power Plants
Dedicated to Out-of-State Consumers 20
4. Frequency of Occurrence of Reclamation Research
at Western Mines in 1976 36
5. Frequency of Occurrence of Parameter Values in 1975 . . 42
6. Production Percentages for the Five Most Frequently
Occurring Western Surface Coal Mining Situations .... 42
7. Ranking Factors for the Five Most Frequently
Occurring Western Surface Coal Mining Situations .... 44
8. Frequency of Use and Evaluation of Drainage Control
and Treatment Practices Used During 1975 53
9. Frequency of Use and Evaluation of Topsoil
Salvaging Practices 58
10. Production Summary for Selective Spoil Placement
Illustration 71
11. Cost Estimate for Selective Spoil Placement Example ... 73
12. Production Summary for Non-Selective Spoil
Placement Alternative 74
13. Production Estimate for Extended Bench Procedure
with Selective Spoil Placement 86
14. Effect of Bench Height and Swing Angle on Dragline Output . 89
XI
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15. Ways to Integrate Mining and Reclamation in
16.
17.
18.
19.
A-l.
A-2.
A-3.
A-4.
A-5.
Dragline Stripping Situations
Frequency of Use and Evaluation of Grading Practices . .
Frequency of Use and Evaluation of Revegetation and
Frequency of Occurrence of Potential Reclamation
Problems as Identified by Mining Companies
Comparison of Mining Plans and Costs for Dragline
and Bucket Wheel Excavator Systems
Production Estimate for Walking Dragline (Marion 7620)
Cost Estimate for Walking Dragline (Marion 7620) ....
Cost Estimate for Walking Dragline (BE 1370)
Production Estimate for Bucket Wheel Excavator ....
112
116
117
127
131
133
134
135
136
Xll
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ACKNOWLEDGMENTS
The guidance and assistance of Mr. S. Jackson Hubbard, Project
Officer, are gratefully acknowledged, particularly in allowing the latitude
necessary to place the problem in its proper perspective.
Field surveys and assessments of the environmental impacts of
mining activities at those mines were conducted by Hittman Associates of
Columbia, Maryland under subcontract to Mathematica. Peter Briggs and
Thomas Mills were the principal project participants for Hittman.
Mr. James E. Biller, Mining Engineering Consultant, helped
greatly during conduct of the field survey and analysis of modified mining
methods. Dragline and bucket wheel excavator operating alternatives,
production analyses, and cost analyses were prepared by Mr. Wilbur A.
Weimer, retired Vice President of Engineering for Peabody Coal Company,
and now an independent mining consultant.
The cooperation of mining company personnel in allowing unlimited
and repeated access to their mining and reclamation operations, and in
providing extensive information, was invaluable. Listed below are the
names and affiliations of some of the people who helped:
Consolidation Coal Company (Glenharold Mine):
Buddy Beach, David Kirtz, Richard Huscka.
Decker Coal Company (Decker Mine):
Leonard Skretteberg, William Rosewarne.
Energy Fuels Company (Energy No. 1 and No. 2 Mines):
Dwane Johnson.
Kaiser Steel Corporation (York Canyon Mine):
David Bacca, William Sykes.
Kemmerer Coal Company (Elkol and Sorenson Mines):
Gus Tuason, Louis Engstrom, James Brophy.
Knife River Mining Company (Savage Mine):
Dean Dishon.
Pacific Power and Light Company (Dave Johnston Mine):
Glen Goss, Ed Paisley, Ken Beech.
xiii
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Western Coal Company (San Juan Mine):
R. W. Allen, L. Lumpkins, Dwayne Walker.
Wyodak Resource Development Corporation (Wyodak Mine):
A. J. Westre, Syd Garrans, Conrad Ballwegs.
Utah International, Inc. (San Juan Mine):
R. J. Ballmer, A. G. House.
The authors are also indebted to representatives of state and
Federal reclamation agencies, who provided unlimited and repeated access
to permit files, were generous with their time and information, and
reviewed mine inventory data presented in this report. Listed below are
the names and affiliations of some of the people who helped:
Colorado Division of Mines:
Robert Campbell.
Montana Department of State Lands, Reclamation Division:
C. C. McCall, Richard Juntunen, Art Olsen, Jack Schmidt.
New Mexico Bureau of Mines and Mineral Resources:
Frank Kottlowski.
North Dakota Public Service Commission:
Edward Englerth.
Wyoming Land Quality Division:
Homer Derrer, Terrence Larson.
U. S. Geological Survey:
Al Czarnowsky (Billings), Archie Carver, Joseph Poole,
Tim Steele (Denver), Wilbur Ballance, Warren Hodson
(Cheyenne).
The following additional specialists also gave freely of their time
and information:
Montana Bureau of Mines and Geology:
Robert Hedges.
Montana State University:
Richard Hodder, Bernard Jensen.
U. S. Bureau of Land Management:
Ron Kuhlman (Denver).
U.S. Department of Agriculture, Forest Service, SEAM
Program (Billings):
Grant Davis.
Finally, while much of this report is based on information provided
by the foregoing people, all conclusions are those of the authors.
xiv
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SECTION 1
INTRODUCTION
Rapid growth since 1969 of production of western coal by the
surface mining method has aroused public interest in the possible
environmental, social, economic, and political effects not only of mining,
but also of coal transportation, conversion, and utilization. This report
addresses the environmental effects of mining particularly the physical
effects.
In recent years, doubts over the feasibility of adequate reclama-
tion of mined lands in arid and semi-arid western climates have been
voiced. Some have questioned the feasibility of revegetating mined lands;
others believe that groundwater systems may be disrupted or that ground-
water will be contaminated by mining. Additional uncertainties, such as
those concerning the effects of mining on air quality, persist as well.
Historically in other parts of the country, particularly in
Appalachia, it has been concluded that reclamation as an add-on-tech-
nology, appended to existing mining methods, would not be adequate to
prevent severe and extensive environmental damage. Instead, basic
modification of the mining methods themselves, to incorporate
reclamation-related practices, has been shown, to be necessary. The
purpose of this study was to detail the environmental damage which results
from the mining methods currently being utilized.
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SECTION 2
CONCLUSIONS
On a regional basis, the short-term environmental damages
caused by western surface coal mining in 1975 and 1976 were deemed to
be neither severe nor extensive. This is due not only to unique features
of the western environment, such as gradual topography and virtual
absence of acid overburden strata, but also to stringent reclamation laws,
organizational changes at the mines, and commitment by mining companies
of the resources needed for adequate reclamation. The relatively low rate
of land disturbance by present western surface coal mining activities is
also a factor.
These general conditions notwithstanding, however, there are
areas in which current or projected concentration of large-scale surface
coal mining activities could produce serious local effects in both the
short- and long-terms. Additionally, because there has not yet been a
"long-term" in western surface coal mining as currently practiced, there
are uncertainties regarding the long-term severity and extent of certain
potential environmental damages. For example, for the region as a whole,
there is uncertainty regarding the feasibility of permanently restoring
mined lands to pre-mining productivity levels.
Based on available information, though, it now appears in general
that mined lands can be adequately revegetated with species suitable for
erosion control and planned post-mining land uses, so long as the best
available topsoiling material is salvaged (as it is at 96 percent of the
active mines), and proper techniques for seedbed preparation, seeding,
and amendment are used. Time will tell whether additional procedures,
such as irrigation (infrequently used at present), will be needed to
guarantee long-term vegetation success. There are notable exception to
this such as the very arid areas located in the southwestern portions of
the United States.
The potential effects of mining on groundwater quantity and quality
appear to be local ones, since indications are that the aquifers disturbed
by mining are shallow and generally discontinuous. Mining activities have
in some cases caused dewatering of wells located close to the mines, but
an apparently effective remedy has been for the mining companies to drill
deeper wells. Additionally, there are uncertainties regarding the long-
term effects of mining on local groundwater quality. If current research
indicates that there are problems, it may prove to be necessary on site-
specific bases to remove and place overburden materials selectively as a
means of minimizing degradation of the quality of local groundwater supplies.
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The effects of mining activities on air quality have not yet been
widely assessed, so that no conclusions have been drawn about the need
for, or lack of need for, control measures to reduce fugitive dust
emissions.
Other kinds of environmental damages, such as creation of closed
surface depressions, erosion, and mineralization of surface waters,
could possibly be fairly serious local problems if adequate preventive
technology was not brought to bear. But in general, the technology is
available; it is being used; and it costs relatively little on a per-ton basis
where the thickness of the coal exceeds about six meters (20 feet).
The predominant mining methods presently used in the west are
dragline stripping of single and multiple coal seams. There appears to be
one significant way in which those methods may need be altered to reduce
environmental damage. It is modification of operating practices to enable
more selective removal of overburden and placement of spoil, particularly
where multiple seams are mined, to prevent placement of high-sodium
clays and shales on spoil surfaces, or to prevent placement on pit floors
of materials high in soluble minerals or trace elements.
It is desirable to devise mining equipment or methods that would
enable placement of topsoiling materials directly on graded spoils by the
main stripping machine.
It is also desirable to identify and evaluate alternative means for
minimizing the effects of mining on groundwater quality and quantity,
particularly where alluvial valley floors are to be mined. This would be
a logical supplement to research now being conducted to assess the effects
of mining on aquifer systems.
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SECTION 3
RECOMMENDATIONS
Extensive research is currently being conducted to assess the
magnitudes of the environmental effects of western surface coal mining,
and to identify and evaluate means for mitigating those effects. The
research recommendations presented in this report are intended to
supplement the current research program.
It is recommended that the Environmental Protection Agency
sponsor a symposium on selective overburden and spoil placement
techniques used or proposed for use in dragline stripping situations. In
this regard, it is desirable that symposium material be prepared by and
for mine engineering and operating personnel rather than by and for
contract researchers, and that participation of midwestern mine personnel
as well as those in the wes.t be solicited. Emphasis should be placed on
practical aspects, including information requirements, methods used in
advance of mining to identify desirable and undesirable strata, operating
practices used to enable dragline operators to identify the various strata,
management techniques used to ensure that dragline operators follow plans,
overburden removal and spoil placement techniques, production and cost
characteristics, alternatives considered, and operational performance
and problems.
It is further recommended that consideration be given to conduct of
laboratory or field studies to determine the effects of selective removal
and placement practices on required topsoil replacement depths, revegeta-
tion success, and water quality.
Consideration should also be given to conduct of a study to identify
and evaluate alternative methods for minimizing the effects of mining on
groundwater quantity and quality, both during and after mining. This
would be a logical supplement to the research projects now being conducted
to assess groundwater effects.
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Finally, with industry cooperation, an analysis of the feasibility,
costs, and effectiveness of bucket wheel excavators for overburden
removal at large mines in North Dakota should be considered. Use of
such a system may enable placement of topsoil by the main stripping
machine directly on graded spoil surfaces.*
e
A bucket wheel excavator was at one time used at a North Dakota mine
for removal of overburden and interburden and mining of three coal
seams from one machine position. This application was considered
unsuccessful primarily because mining of the coal by the bucket wheel
excavator resulted in unacceptably high ash content in the coal.
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SECTION 4
BACKGROUND, OBJECTIVES, SCOPE
BACKGROUND
Recent rapid development of the thick, shallow, low-sulfur coal
reserves of the western United States -- initially stimulated by increased
demand for low-cost, low-sulfur coal -- has aroused public interest in
the possible social, economic, and environmental effects of western coal
mining, transportation, conversion, and utilization. The feasibility of
reclamation of surface-mined lands in arid and semi-arid western
regions, for example, has repeatedly been challenged, most notably by
the Sierra Club in a suit brought against the U. S. Department of Interior
to slow or stop surface mining of Federally owned coal in the eastern
Powder River Basin. Although the injunction won by the Sierra Club in
that suit has since been lifted by the U.S. Supreme Court, thereby
permitting the opening of several large mines in eastern Wyoming,
skepticism over reclamation feasibility persists. This is manifested not
only by the numerous surface coal mining and reclamation research
projects now being conducted in the region, but also by the determination
of tiie people in states like Montana to control the rate of growth of coal
production.
But, environmental concerns notwithstanding, the social and
economic effects of mining, transportation, conversion, and utilization
of coal are also important. For example, it is probably fair to state that
opposition to construction and operation of the Four Corners (New Mexico)
and Colstrip Units 3 and 4 (Montana) power plants was far greater than
the opposition to the mines from which those power plants are supplied.
Such opposition, motivated in part by concern over air pollution, was
intensified by the fact that, although the coal is mined and burned in-state,
the electricity is transmitted to out-of-state users.
The transportation, conversion, and utilization of coal may entail
the use of large quantities of water, whether for coal slurry pipelines,
water-cooled power plants, or coal gasification plants. To many people
in the arid west, this prospect is alarming at best. Additionally, in the
case of coal gasification, the spectre of huge gasification plants looming
on quiet, rural western horizons, is sufficient to unnerve the local
populace.
There are many more examples; the effects of rapid population
growth are the most notable of those. They include overcrowded schools,
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increased taxes and, overall, a kind of shattering of the rural tranquility.
Grace Lichtenstein of the New York Times put it this way:
"In recent years, Gillette [in northeastern Wyoming]
has exhibited many symptoms of the trailer-lined
energy boom town that is scarring the Western
landscape -- inflated rents, mobile homes on dirt
strips, overcrowded schools, crime and mental
health problems. The one local car rental agency
charges premium rates for cars with 75, 000 miles
on them. Liquor stores outnumber groceries." [1]
The focus of the present study was the environmental impacts of
western surface coal mining as practiced during 1975 and 1976, but it is
clear that a balanced perspective on this subject can be achieved only in
the context of other types of impacts, other activities associated with coal
development, and other uses of land.
STUDY OBJECTIVES
The purposes of this study were threefold:
To evaluate, qualitatively, the surface coal mining
methods presently used in arid and semi-arid
regions of the western United States.
To describe and evaluate the effects that those
methods have on the environment.
To recommend ways in which those methods might
be altered to reduce both short-term and long-term
environmental damage.
SCOPE OF STUDY
The study included current surface coal mining and reclamation
methods and equipment used in the states of North Dakota, Montana,
Wyoming, Colorado, New Mexico, and Arizona. Social and economic
aspects, and other phases of coal development and utilization were
considered to be within the scope where necessary to provide needed
perspective.
STUDY METHODOLOGY
The methodology used to achieve study objectives was basically a
qualitative one involving the following tasks:
Compilation of a comprehensive inventory of information
on climate, topography, geology, mining methods, and
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reclamation practices for all western surface coal
mines active during 1976 or planned for opening prior
to 1980. This inventory is documented in an
accompanying volume.
Statistical classification and analysis of inventory data
to determine, quantitatively, the frequency of occur-
rence of various physical and technological conditions.
A survey of literature on mining methods, reclamation
practices, and environmental impacts, with emphasis
on research studies and environmental impact state-
ments recently completed or currently in progress.
Personal interviews of state and Federal government
personnel responsible for regulation of reclamation
of surface coal mines.
A field survey of nine surface coal mines. Each mine
was visited during three different seasons of the year.
Regarding the personal interview phase of the project, an attempt
was made to identify consensuses of opinion regarding the magnitudes of
the environmental impacts caused by surface coal mining in the region.
This was generally possible, although individual differences of opinion
were encountered. In one state, for example, a reclamation specialist,
an ecologist by training, indicated his opinion that surface mining destroys
soils that may have required hundreds of years for development. He felt
that current methods for salvaging and replacement of topsoil were not
Affective, for two reasons. First, he indicated, the various soil horizons
are mixed during removal of soil by scraper prior to main stripping.
Secondly, he felt that the fertility of the soil was destroyed during the
period in which the soil remained stockpiled prior to replacement on
graded spoil surfaces.
In the same state, another reclamation specialist, a soils scientist,
stated that mining and reclamation as practiced in that state improved the
overall plant-growing medium. He felt that topsoil salvaging and replace-
ment as currently practiced was an effective reclamation practice, that
stockpiling of soil for relatively long periods of time had little effect on
soil fertility, and that breaking up of the clay hardpan normally near the
surface in many western mining areas was a beneficial effect.
The foregoing opinions were weighed by members of the study
team in light of reclamation performance observed at the nine field
survey mines. Judgments regarding the effectiveness of the reclamation
practices were then made.
The nine field survey mines each were visited during October 1975,
January 1976, and April 1976. In each instance, mining and reclamation
personnel at the mines were interviewed. Additionally, mining operations,
reclamation operations, and areas in various stages of reclamation were
8
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observed and photographed. In all instances, members of the field
survey team were allowed unlimited access to all parts of the mines.
The principal field survey investigators were a hydrologist and a
mining engineer. On various occasions, these individuals were also
accompanied by a biologist, a geological engineer, and a mechanical
engineer.
Average annual production for the field survey mines was 1, 250, 000
tons per year. The mines included the following ones:
Two area lignite mines located in semi-arid areas.
Three area, mines located in semi-arid areas. One
of those was in Colorado, where the coal seams
pitched moderately.
Two open pit coal mines, one located in an arid
area, the other in a semi-arid area.
Two area coal mines located in an arid area.
SYNOPSIS OF SUBSEQUENT SECTIONS
Major environmental issues and impacts associated with western
surface coal mining are identified in the next section. This is accom-
plished after describing the growth and structure of the mining industry,
the environmental setting, and the impacts of other phases of coal
development and other land uses.
Section 6 contains a classification and description of current
mining and reclamation practices. Relationships between mining methods
and reclamation performance are emphasized. Statistics describing the
frequency of use of various mining and reclamation practices are also
presented in this section, as are assessments of the effectiveness of
current reclamation practices.
Possible means for improving reclamation performance or
reducing the costs of achieving current reclamation results through
modification of mining equipment or methods are discussed in Section 7.
SUMMARY
The recent growth of surface coal mining activity in the western
United States has aroused interest not only in the environmental effects of
the mining operations themselves, but also in the social, economic, and
environmental effects of coal transportation, conversion, and utilization.
Thus, although the focus of the present study was the environmental
impact of mining, related issues are discussed in this report to provide
needed perspective.
-------
The basic purposes of this study were to evaluate the surface coal
mining methods currently used in the western United States, to describe
and evaluate the effects that those methods have on the environment, and
to recommend ways in which the methods might be altered to reduce both
short-term and long-term environmental damage.
The study included current surface coal mining operations in
North Dakota, Montana, Wyoming, Colorado, New Mexico, and Arizona.
Evaluation of mining and reclamation equipment and methods was within
the scope of the study.
The approach taken to achieve study objectives was basically a
qualitative one, involving judgments made by study team personnel on the
basis of information gathered from literature surveys, personnel inter-
views of government and industry mining and reclamation specialists, and
field surveys of nine western surface coal mines in three different
seasons of the year.
10
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SECTION 5
ISSUES AND IMPACTS
INTRODUCTION
In evaluating the environmental effects of western surface coal
mining and identifying research needs in that area, it is important first to
identify the major issues associated with development of strippable western
coal reserves, to differentiate between apparent issues and real ones, and
to identify the factors underlying the real issues. Additionally, prior to
any discussion or evaluation of mining methods and reclamation practices,
it is desirable to define the types, potential extent and potential severity of
the environmental effects of surface coal mining in the region. These
objectives are accomplished in the present section.
First, the general setting for coal development is described. Next,
the factors that have caused concern over development of strippable western
coal reserves are identified. The physical, institutional, and technological
factors that affect the potential extent and severity of environmental
damages are then discussed, and types of environmental effects are
classified as insignificant or potentially significant.
THE GENERAL SETTING FOR DEVELOPMENT
Development of the vast strippable coal reserves of the western
United States is taking place in sparsely populated agricultural and ranching
areas located in arid and semi-arid climates. These three regional
factors -- population density, land use, and climate -- more than any
others, have greatly influenced public attitudes toward development of
strippable coal reserves in the region.
Although, because productivity in western surface coal mining is
very high, the effects of mine employment on population density in areas
other than Campbell County, Wyoming, Colorado's Yampa Basin, and the
Beulah/Stanton area of North Dakota (where very rapid growth in mining
is now occurring) are generally small, the effects on population of associ-
ated activities, such as construction of mine-mouth power plants, may be
large. *
Average direct employment in 1975 was 87 employees per mine. Total
direct employment in the six-state area was approximately 4,000
employees for 45 surface coal mines.
11
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The climate in western surface coal mining areas is arid or semi-
arid, with average annual precipitation ranging from 18 centimeters
(seven inches) in northwestern New Mexico to 51 centimeters (20 inches)
in western North Dakota. In such a setting, any land use that affects
surface or groundwater quantity or quality in any potentially adverse way
will be viewed critically by many people. This is particularly true of
groundwater (aquifer) systems.
The predominant land use in western surface coal mining areas is
grazing of cattle and sheep. In fact, as shown in Table 1, 97 percent of
the area, disturbed by mining in 1975 was used for grazing of cattle or
sheep prior to mining. Coal mining is a relatively new land use, at least
on present and projected scales, and there is naturally some skepticism
over the compatibility of the existing land uses with this new use. *
Additionally, the new land use brings with it new kinds of people, with
economic, educational, political, and social backgrounds different from
those of the native ranchers and farmers. The result is a kind of cultural
shock for natives and newcomers alike.
Although there has been surface coal mining activity in the region
for the past fifty years, it was not until the early 1970's, when the boom
in western surface coal production began, that the foregoing factors came
into play in an important way.
TABLE 1. FREQUENCY OF OCCURRENCE OF PREMINING
LAND USES AT MINES ACTIVE OR UNDER
DEVELOPMENT IN 1975
Predominant Premining
Land Use
Cropland or Hayland
Grazing of Cattle or Sheep
Wildlife Habitat
Frequency of Occurrence at
Mines in 1975
Percent of Acres
3
97
Negligible
Percent of Mines
11
89
Negligible
The importance of this kind of skepticism is illustrated in another part of
the country by the public relations campaign sponsored by a major coal
company to popularize the slogan, "Cows, Coal, and Corn are
Compatible. "
12
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FACTORS AFFECTING PUBLIC ATTITUDES TOWARD DEVELOPMENT
OF STRIPPABLE COAL RESERVES
As a further means of presenting the issues surrounding western
surface coal mining, an elaboration of the foregoing discussion is presented
next.
The Growth of Mining
For the three decades preceding 19&9ป total western surface coal
mine output remained fairly constant at 1. 8 million metric tonnes (two
million tons) annually. Since 1969, as shown in Figure 1, production
growth has been truly startling, increasing at an average annual compound
rate of 30 percent, from 14. 5 million metric tonnes (16 million tons) in
1969 to 68 million metric tonnes (75 million tons) in 1975. At this rate,
annual production doubles roughly every three years. By 1985, it is
conservatively forecast that regional production will have reached
318 million metric tonnes (350 million tons) annually, resulting in an
annual mining disturbance of approximately 6, 962 hectares (17, 203 acres).
At the same time, as illustrated in Figure 2, the relative importance of
underground coal mining, at least in the next decade, will continue to
decline. For 1985, for instance, it is estimated that surface mining will
account for nearly 95 percent of total western coal production.
Geographically, production comes from most of the strippable coal
reserves areas shown in Figure 3, but, as shown in Figure 4, the
majority (approximately two-thirds) of actual 1975 and forecasted 1980
regional production is accounted for by two states -- Wyoming and
Montana. The latter Figure also shows that production growth between
the years 1975 and 1980 will occur in all states individually.
Historically, in the eastern and midwestern surface coal mining
regions of the United States, rapid growth in surface coal mining coupled
with a lack of stringent reclamation laws underlay the extensive environ-
mental damage that resulted. The damage was widely and graphically
publicized. Thus, in 1969, when growth in western mining began in
earnest, it is understandable that the average citizen in the west envisioned
thousands of miles of eroding, unstable, unsightly spoil banks; exposed
highwalls; and heavily sedimented and acidified streams.
Markets for Western Coal
As though the perceived environmental costs of mining were not
bad enough, there appeared to be few in-state benefits to counterbalance
those costs. On a regional scale, the employment effects of mining were
not seen to be large. State severance taxes on coal were small. Worse
still, much of the coal was not to be used to provide energy for in-state
*v
This estimate is based on the actual 1975 average for the region of
20, 345 tons per acre.
13
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350 -\
300-
250-
200-
SURFACF MINED
TONNAGE
(In millions)
150
100-j
100
90
75-
60-
SURFACE MINED
TONNAGE
AS A PERCENT OF
TOTAL REGIONAL 45
TONNAGE
30-
15-
40
50 60
YEAR
70
80
Figure 1. Trends in Surface Coal Mine
Production in the Western
United States
Figure 2. Trends in Production Share for
Surface Coal Mining in the
Western United States
-------
MONTANA
ARIZONA
; SUBBIT'JMINOUS
BITUMINOUS
LIGNITE
STRIPP1BLE DEPOSITS
Figure 3. Strippable Coal Reserves of the
Western United States*
Small amounts of strippable coal are present in Utah, but are not shown
in this Figure.
15
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MONTANA
NORTH DAKOTA
COLORADO
ARIZONA
NฃW MEXICO
PERCENT OF TOTAL
REGIONAL TONNAGE
Figure 4. Current and Forecasted 1980 Surface
Coal Mine Production Percentages for
the Western United States
16
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residents. In fact, as shown in Table 2, more than half of the coal that
was surface-rained in the region during 1975 was exported from the region.
The export percentages for Wyoming and Montana, the two largest pro-
ducing states, were 74 and 95 percent, respectively.
Figure 5 shows the states of final destination for coal that was
surface-mined in Wyoming during 1975. Most of the market areas were
in the midwestern United States. The export picture for Montana was
similar.
The transportation of coal both within and out of the region has
associated social, economic, and environmental effects. Existing railroad
track is insufficient to meet projected transportation demand, thus new
track must be constructed. Some ranches will be split in the process.
Additionally, existing transportation patterns may be disrupted. Coal
slurry pipelines are proposed; water will be required to operate them.
Although, as shown in Figure 6, new railroad track requirements are
relatively small on a regional scale, several new coal slurry pipelines, to
Oregon, Arkansas, and Texas, are proposed. The associated impacts are
perceived by some as potentially severe.
Coal Conversion and Utilization
The foregoing export patterns do not tell the whole story because
much of the coal that is not exported is (or will be) converted to electricity
or gas which, in turn, will be exported from the states. For example, as
TABLE 2. NET EXPORTS OF COAL SURFACE-MINED IN
THE WESTERN UNITED STATES DURING
1975 (ACTUAL) AND 1980 (FORECASTED)
State
Arizona
Colorado
Montana
New Mexico
North Dakota
Wyoming
REGIONAL
Net Exports as a Percent of Total
Surface Coal Mine Production
1975
50
(42)
95
8
27
73.5
53
1980
(10)*
7
91
32
17
66
52
Parentheses indicate net imports.
17
-------
00
I Available Data Does Nol SpecHy Tonnages
Figure 5. Geographic Distribution of Coal Surface-Mined
in Wyoming During 1975
-------
Seattle
Portland
MONTANA
To
Soardman,
Oregon
10 mty
capcity
Minneapolis
SI Piul
Chicago
Salt Lake
City
.X* '
^ *
'-*
W ""
ฃ*..
^ '*%
ป V
Kansas Cily
MILLION TONS
PER YEAR:
PROPOSED COAL
SLURRY PIPELINES
EXISTING COAL
SLURRY PIPELINE
MEW RAILROAD
LINES
MAJOR EXISTING
RAILROAD ROUTES
Figure 6. Existing and Proposed Railroad Lines and
Coal Slurry Pipelines in the Western Region
-------
shown in Table 3, 95 percent of the electricity generated in new coal-fired
power plant units planned for construction in Wyoming and North Dakota is
dedicated to out-of-state consumers. In such cases, although construction
and operation of the power plants will yield in-state economic benefits, it
will also cause adverse social and environmental effects. Additionally, in
many areas, especially those that are particularly scenic, the construc-
tion of large transmission towers and long lines may fuel local opposition.
Similarly, the construction and operation of coal gasification plants,
and the transmission of gas, although economically beneficial in many
respects, will cause social and environmental disruptions. The use of
water, mentioned earlier, is perceived by some as a major disadvantage
of the plants, although aesthetic and air quality considerations also come
into play.
In total at the present time, ten coal gasification plants are planned
for construction in the six-state region. The locations of those proposed
plants, along with the locations of planned new coal-fired plants and addi-
tions to existing power plants, are shown in Figure 7.
TABLE 3. CAPACITY OF COAL-FIRED STEAM
ELECTRIC POWER PLANTS DEDICATED
TO OUT-OF-STATE CONSUMERS
State
Arizona
Colorado
Montana
New Mexico
North Dakota
Wyoming
REGIONAL
(Approximate )
Percent of Installed
Capacity Dedicated to
Out-of -State Consumers
1975
Not
determined
8
30
93
55
80
71
1980
Not
determined
10
37
Not
determined
79
85
60
Percent of Planned
New Capacity
Dedicated to
Out-of -State
Consumers
Not
determined
10
50
Not
determined
95
95
Not
determined
20
-------
MONTANA
ARIZONA
NEW COAL FIRED
POWER PLANT
ADDITIONS TO
EXISTING COAL
FIRED POWER PLANTS
COAL C
UNITS
IASSIFICAT1ON
Figure 7. Locations of Planned Coal Gasification
Plants and Coal-Fired Power Plants
21
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PREVIEW OF MINING AND RECLAMATION TECHNOLOGY
The types, potential extent, and potential severity of the environ-
mental effects caused by surface coal mining are dependent primarily upon
three factors: the physical characteristics of the mine site, the mining
technology, and the reclamation technology. In order to provide a back-
drop for discussion of physical factors and their relationships to
environmental effects, a brief preview of frequently used mining and
reclamation technologies is presented next.
Mining Technology
The predominant mining technology used in the west during 1975 was
stripping of single and multiple coal seams using a single walking dragline
as the prime stripping machine. * A simplified representation of the
operating procedures used in the single seam case is shown in Figure 8.
First, the initial or box cut is opened, generally parallel to the coal seam
cropline or burn line. Prior to actual overburden removal, in all western
states but North Dakota, the overburden is drilled and blasted. Blasting
may cause ground vibration, air shock, noise, and dust. Additionally,
both drilling and blasting cause emission of fugitive dust.
Subsequently, working from a position on or near the original
ground surface, box cut overburden is removed and spoiled by the dragline
onto the natural ground adjacent to the cut. The height of the box cut spoil
pile, which depends on the width and depth of the box cut and the natural
repose angle of the spoil, usually ranges between 30 and 50 feet. The
permanent topographic change that results may degrade the appearance of
the area., and may also affect water table levels after mining.
BOX CUT SPOILS
VCOAL SEAM
' WATER tN FINAL PIT
Figure 8. Section View of Results of Single
Seam Dragline Stripping
Henceforth, the term "mining" will mean surface coal mining and the
term "west" will mean the surface coal-producing areas of the western
United States.
22
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Additionally, the outslope of the spoil pile, which is initially 35 to
40 degrees from the horizontal, drains externally; that is, surface runoff
from the outslope enters natural drainageways external to the actual
mining operation. Since, immediately after spoil placement at least, the
outslope is long, steep, and unvegetated, erosion and sedimentation may
result.
If the overburden or coal seam carried groundwater prior to
mining, groundwater will be intercepted by the box cut and may collect in
the pit. Possible effects include lowering of the water table and sedimenta-
tion or chemical degradation of the water that enters the pit. Since, for
production reasons, pit water is usually pumped out to external drainage-
ways or storage basins, surface water quality might also be affected.
After the box cut has been completed and coal has been loaded out,
a second cut is made roughly parallel to the first one. Excavated over-
burden is placed by sidecasting it into the adjacent open cut to rest at its
natural angle of repose. This topographic change, from rolling terrain to
steep-sided piles, might degrade land use if nothing was done about it.
Moreover, since overburden strata that were originally nearest the coal
are usually placed on or near the surfaces of the spoil piles, the chemical
quality of surface water that comes in contact with the spoil may be affected.
Groundwater, if initially present in the overburden or coal, will eventually
flow through this spoil. * Minerals or toxic chemicals in the spoil may then
dissolve in the groundwater. Further, since the topography and perme-
ability of the spoil is or may be different than that of the overburden, rates
of percolation and runoff of surface water may be altered, thereby possibly
changing the physical and chemical characteristics of the water. It is
notable, however, that sedimentation of external surface waters is not a
problem even potentially at this stage, since the spoils from the second
and subsequent cuts drain internally.
After removal of overburden from the cut, coal is loaded into off-
highway trucks, which haul the coal on dirt- or rock-surfaced roads to a
loading facility. Coal loading, haulage, and unloading generates dust.
Stripping and loading procedures in subsequent cuts are similar to
those used in the second cut. When the overburden becomes too deep for
profitable stripping, or a property boundary is reached, stripping ceases.
Overburden excavated from the last cut is placed in the open second-to-
last cut; thus the last cut would remain open permanently if special efforts
were not made to fill it.
*It has been assumed here that the water table will return to approximately
the original level after mining has been completed.
23
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Reclamation Technology
Clearly, in the absence of use of special preventive procedures,
western mining, both during and after actual conduct of mining operations,
would adversely affect land use, water quality, and air quality both on and
off the mine site. Special procedures are generally used, however.
Prior to mining, for example, drainage diversion ditches or
impoundments are constructed to reduce the amount of surface water that
will enter the active pit. Although used primarily for production reasons,
this practice is usually environmentally desirable. Additionally, sediment
control basins are constructed "below" mining operations.
In order to reduce noise and ground vibration, millisecond delays
are used in blasting of overburden. The total weight of explosive detonated
at a given instant is determined by using a formula developed by the U. S.
Bureau of Mines.
Bulldozers are used to reduce the steepness of the box cut spoil
outslopes so that surface runoff and erosion will be reduced. Terraces
may be constructed on the outslope as an additional erosion-control
measure. Salvaged top soil may or may not be spread on the slopes of the
box cut spoil piles and, whether or not topsoil is used, the slopes will be
seeded, generally with a mixture of native and introduced grasses.
Fertilizer and mulch may'also be used.
In the second and subsequent cuts, special procedures may be used
to avoid placement of undesirable materials either below the water table or
near spoil surfaces. Angle-of-repose spoil piles are graded by dozer to a
topography suitable for the planned post-mining land use. Salvaged topsoil
is generally spread on graded spoils, and topsoiled areas are seeded,
fertilized, and mulched. They may also be irrigated, although this is not
a common practice. Restoration of suitable topography and revegetation
both have dual purposes; they reduce erosion and sedimentation and
improve post-mining land use potential. The final cut, in some states,
will be partially filled after mining. Production- or safety-related
practices, such as watering of haul roads, also are environmentally
beneficial. They may also be needed to reduce fugitive dust emissions.
The purpose of use of the foregoing practices is to maintain
satisfactory air and water quality on and off the mine site during and after
mining, and to restore the land to productive use after mining. The
effectiveness of those practices is dependent not only on the quality of
reclamation efforts but also on the physical characteristics of the mine
site, the topic of the next section.
PHYSICAL CHARACTERISTICS OF THE REGION
Climate, topography, geology, hydrology, and land uses differ in
many important respects among the three major surface coal mining
regions of the United States -- east, central, and west. Some character-
istics of the western region are environmentally desirable; others are not.
24
-------
Climate
The climate in western mining areas is arid to semi-arid with
average annual precipitation ranging between 18 centimeters (seven inches)
and 51 centimeters (20 inches). * The distribution of average annual pre-
cipitation at surface coal mines active in the region during 1975 is shown
in Figure 9. Much of the precipitation is snow. When it does rain, the
rain often falls during infrequent, intense thunderstorms. Surface runoff
is high. Because the precipitation is infrequent, soil moisture content is
generally low. Even if the soil moisture was high, it still might not be
available at the proper time for seed germination and plant growth.
High summer temperatures, wind, and low average humidity cause
rapid evaporation of water from the ground. Water is also lost by plant
transpiration. Wind, another climatic factor, carries water vapor from
the site of evaporation or transpiration, thereby increasing evapotranspira-
tion potential. Wind also may pick up fine soil particles, resulting in loss
of soil and increases in particulates suspended in the air.
As shown in Figure 10, the altitude at western mines ranges between
305 meters (1,000 feet) and 2,440 meters (8,000 feet), and averages
1, 545 meters (5, 070 feet). The growing season at these altitudes is
relatively short; 60 to 120 days is a typical range. Additionally, the
numbers of vegetative species available for reclamation at such altitudes
may be very limited.
25
Percent of
Mines at
Which
Average
Annual
Precipitation
Equals x
20
15
10 . .
LO
15 20
x = Average Annual
Precipitation (Inches)
Figure 9. Distribution of Average Annual
Precipitation at Western Mines
fa
Arid regions are those in which average annual rainfall is 30 centimeters
(12 inches) or less. From a reclamation standpoint, extremes of annual
precipitation may be more relevant than averages.
25
-------
Ts'o. of Mines
at Which
Average Altitude
Altitude is
i. x
10 ..
s .,
Average = 5, 070
ZOOO 4000 6000 3000
x = Mine Altitude (Ft. i
Figure 10. Histogram of Average Altitudes
at Western Mines
The relevance of the foregoing is in part that revegetation of mined
areas may be difficult. Fugitive dust may be troublesome. Preservation
of shallow aquifer systems may assume special importance. On the other
hand, pit flooding is relatively infrequent, undesirable spoil materials
need not be buried too deeply (e. g., by more than several feet) to protect
against leaching, and erosion may be potentially less troublesome than in
areas of heavy precipitation. *
Topography
Approximately 70 percent of the acreage disturbed by western
surface coal mining during 1975 was characterized by gently rolling topo-
graphy. The remainder was characterized by either flat or hilly topography.
There were few if any mountainous areas, such as those typical of central
Appalachian surface mining areas.
Western topography is generally desirable from an environmental
standpoint. First, restoration of approximate original contours, although
possibly costly, is not too difficult. Additionally, because the west is
naturally characterized by sharply contrasting landforms -- with buttes
and mesas rising out of vast plains areas, and landforms often changing at
fence lines -- it is relatively easy to blend reclaimed topography with
adjacent undisturbed topography.
The combination of low rainfall and gradual topography is also
desirable because, in some cases, erosion problems are potentially less
serious than those in the eastern United States, where rainfall is heavy
and topography is steep.
The potential for erosion is also greatly dependent upon soil
characteristics, however.
26
-------
Soils
Most of the soils in western mining areas are residual soils,
derived over long periods of time from the underlying rock strata. A
common rule of thumb in such cases is that 100 years are required for
formation of one inch of soil. The other two types of soils sometimes
present in western mining areas are alluvial soils -- those carried to
their present location by water -- and glacial soils -- those developed
from glacial drift. The occurrence of these latter two types of soils,
although possibly frequent in some locales, is infrequent on a regional
scale.
Western residual soils are typically sandy or clayey, relatively
infertile, often highly credible, and sometimes high in salts. The sandy
soils have low moisture-holding capability, whereas the clay soils tend to
be very hard when dry. Additionally, clays tend to grip the water mole-
cules so tightly that plants cannot use the water in the soil.
Nonetheless, although western soils are often inferior to those,
for example, in the midwestern United States, they are frequently the
best plant-growing media available in strip mine reclamation; that is,
they are usually superior to underlying overburden materials. This fact
underscores the importance of salvaging and replacement of topsoiling
materials, usually surface soils.
Overburden
Western overburden usually consists of clays and interbedded
shales and sandstones. The shales are often weakly consolidated.
Massive sandstone, such as that common in the eastern United States, is
uncommon in the west. Limestone and slate in the overburden are also
uncommon. Glacial drift is found in parts of the lignite fields of North
Dakota and northeastern Montana, but it occurs very infrequently on a
regional scale.
When placed as spoil, the shales tend to weather fairly rapidly to
clay-sized particles. Where the shale is high in exchangeable sodium
indicated by sodium adsorption ratios (SAR) greater than about ten -- the
weathered material will become poorly permeable, thereby impeding
revegetation attempts. In fact, until fairly recently, revegetation failures
due to permeability problems were not uncommon in North Dakota and
parts of Montana and Wyoming, where high-SAR shales and clays frequently
occur in the overburden or in the interburden separating coal seams.
Sandstones, if placed on spoil surfaces, also tend to weather fairly
rapidly, although less rapidly than shales, producing a droughty condition.
Most often, the color of surface spoil materials is light gray or
light brown. This is desirable because dark-colored spoil materials, such
as those often found in other parts of the country -- western Kentucky is a
good example -- cause excessively high surface temperatures during the
growing season. This may inhibit seed germination and plant growth.
27
-------
Overburden and spoil materials in the west are almost always
basic. The acid-producing pyritic and marcasitic overburden frequently
associated with high sulfur coal in the midwestern United States is rare in
the west. This factor, coupled with low precipitation, means that surface
coal mining in the west will rarely if ever cause acid mine drainage. On
the other hand, the overburden may contain high proportions of soluble
salts that might dissolve in surface water or groundwater that passes over
or through spoiled overburden. Dissolution of trace elements in surface
water and groundwater is another potential problem, although trace
element pollution has not been widely reported.
In sum, the chemical problems related to overburden in other parts
of the country, especially acid mine drainage, may not be major problems
in the west, although there is little hard data to refute or support this
assertion. Physical problems related to failure of spoil materials to
weather quickly -- such as is the case in Illinois, for example, if lime-
stone is placed on spoil surfaces -- are not particularly troublesome in
the west. In contrast, however, a physical problem -- impermeability of
spoil surfaces -- is a characteristic of many western mining operations.
Thus a need for selective placement of undesirable overburden materials,
namely high-SAR shales and clays, exists in the west, just as it does in
other parts of the country, although for different reasons.
Vegetation
The predominant types of native vegetation in western mining areas
are long grasses, short grasses, and shrubs. The long grasses provide
fairly dense cover and protection from erosion, but in many cases, long
grass areas have been converted to production of wheat. Vegetative
density in short grass and shrub areas is generally less than that in long
grass areas. Additionally, extensive grazing of such areas has frequently
resulted in a change of vegetative species mixes, with less palatable
species such as sagebrush predominating. This has resulted in reduction
in land productivity and increases in runoff and erosion. In fact, in some
cases, environmental impact statements are required where Federally
owned lands are to be grazed.
Surface Water
The minor streams of the region are predominantly ephemeral;
that is, they flow only during snowmelt and rainfall. These streams
contribute little to perennial water bodies. Indeed, the quality of surface
waters flowing in perennial water bodies in the region is relatively poor,
a condition resulting from heavy natural sediment loads and high concen-
trations of sulfates, bicarbonates, and dissolved solids. [2-15]
The implication here is that the potential effects of mining on
surface -water quality, whether physical or chemical, do not appear to be
great. At least, they seem to be of lesser concern to state regulatory
personnel and mining company reclamation personnel than are other
potential problem areas. Possible adverse effects on water quality, due
28
-------
to evaporation of surface water impounded on mine sites are also
considered to be of lesser importance. *
Groundwater
At the present time, in the authors' opinion, one of the major
remaining uncertainties regarding the environmental effects of western
surface coal mining concerns the effects on groundwater quantity and
quality. This is an emotional and fairly complex area of concern, and one
in which it is fairly easy to fuel the flames of protest.
There are, in the west, three types of aquifers of interest: glacial,
alluvial, and rock aquifers. Glacial aquifers are composed of unconsoli-
dated sand and gravel deposits associated with the glacial drift overlying
bedrock. These kinds of aquifers occur only in parts of North Dakota and
Montana, but even there, their extent is limited because proportionately
very little of strippable western coal reserves are overlain by glacial
drift. Alluvial aquifers, which consist of unconsolidated sand and gravel
deposited on floodplains in modern geologic times and deposits that filled
preglacial river valleys, are important sources of water in the region.
They generally lie along major rivers and act as storage for those rivers.
Any significant disruption of the alluvial aquifers might affect the quantity
and quality of the surface waters that flow in the rivers. Some western
mines are located in floodplains but unless the scale and location of mining
changes drastically in relation to forecasts, the potential effects of mining
on alluvial aquifers may not be large in a regional sense, although local
effects may be significant.
Rock aquifers consist primarily of sandstones, limestones, and
coal seams. Unlike many sandstones, the coal has relatively low porosity.
Fractures serve as the primary conduits for groundwater in coal seams.
The result is that yields from coal seam aquifers are usually small. In
fact, in mining areas during 1975, the average yield from wells which
derived their water from overburden or coal was less than 3. 7 liters per
minute (seven gallons per minute). In all cases but one, the well water
was used for domestic purposes and stock watering. In general, the
important non-coal rock aquifers are located below current stripping
depths, with water tables typically found at depths of 50 to 150 meters
below the ground surface. Those kinds of aquifers will not normally be
affected by mining.
Nonetheless, shallow aquifers, which can be important water
sources for stock watering and domestic purposes, are affected by mining.
In 1975, for example, this was true at 65 percent of the mines in the
It should be noted that high levels of total dissolved solids in western
surface waters and groundwater continue to be of concern to some
environmental specialists. Although agriculture is currently a major
cause of this problem, they believe that large-scale surface coal mining
may also ultimately be a problem-source.
29
-------
region. Since many people in rural areas draw well water from these
shallow aquifers, the effects of mining on the quantity and quality of well
water are of legitimate concern.
It is known, for example, that water levels in wells located within
a few kilometers of active mining operations have been lowered by three
to six meters (10 to 20 feet). Although the water table may rise to approxi-
mately its original level after mining and reclamation have been completed,
the drawdown of the water table during mining is an effect that should be
mitigated in some way. * Means for doing so are described and evaluated
later in this report. For perspective, however, the following is noted
here. A review of mining and reclamation plans in the region revealed
that no more than 10 or 20 wells or springs are usually affected by any
single mining operation. At the present time, there are about 28 mines
at which shallow aquifers are disrupted in some way. Therefore, on a
regional scale, possibly 400 wells or springs may be affected during the
production periods of mines that are currently active. This number will
of course increase as the scale of mining increases.
Lowering of the water table is not the only consequence of concern;
replacement of coal or rock aquifers by less permeable or more permeable
spoil may permanently alter rates of groundwater flow and storage.
Scientific research projects currently in progress should shed light on this
soon. One such project, recently completed at strip mines in the Powder
River Basin, indicates that spoils placed by dragline are generally as
permeable as the shallow sandstone aquifers in the area, but that spoils
placed by scraper or truck are not as permeable. [16]
Degradation of the quality of groundwater that passes through spoils
is another possible problem caused by mining. In some areas, this
potential problem is not viewed with much concern, because the natural
quality of the groundwater in shallow aquifers is poor before any mining
disturbance takes place. In others, it is known that mining has changed
groundwater quality. Considerable amounts of data, now becoming
available as a result of groundwater monitoring and modeling by coal
companies, state agencies, and Federal contractors, should help in
determining whether or not problems exist.
Summarizing, mining may affect groundwater in shallow aquifers
by lowering the water table, reducing the rate of flow, or changing the
chemical quality of the water. Because, prior to mining, the groundwater
in those aquifers is often low in yield and poor in quality, and because the
aquifers are frequently discontinuous, the regional effects of current
mining activities are not viewed as highly significant. In certain locales,
of course, the effects could be significant.
If the aquifer that was disturbed was a confined aquifer, the original
water table may not "recover" after mining.
30
-------
Land Use
Although there are many different uses of western land, the pre-
mining land use for almost all of the land that has been or is being mined
in the west is grazing of cattle, sheep, and horses. This is a desirable
feature from mining and reclamation standpoints. One reason is the
relative ease with which a suitable topography can be restored after mining.
Restoration of positive drainage from the mine site is not too difficult
where the original rolling topography is restored. This is not always the
case when a relatively flat topography suitable for farming is restored.
Differential settling of spoils presents far fewer problems on land
reclaimed to grazing than on that reclaimed to row-crop or feed grain
production. Vegetative species used for revegetation can be native range-
land grasses which, once established, should require relatively little
maintenance for grazing purposes.
Summary; Potential Environmental Problems
Summing up, in a relative sense, it is felt that lowering of water
tables, reduction of aquifer yields, chemical changes in groundwater,
inability to establish adequate long-term vegetation, and emission of fugitive
dust are the potential problems associated with western mining at present.
This is not to suggest that they are actual problems on a regional scale --
that issue is discussed later in this report -- but rather to suggest that
other types of physical and chemical effects are less significant. Those
latter effects include aesthetic degradation, landslides, and acid mine drain-
age. Sedimentation of surface waters does not appear to be a problem on the
present scale of mining, although some believe that the projected mining growth
could change this. Biological and socioeconomic effects were not evaluated.
FURTHER PERSPECTIVES
It was stated earlier in this section that the extent and magnitude of
the environmental damages caused by mining are dependent on three
things: the physical characteristics of the mining area, the mining method,
and the quality of reclamation efforts. This latter factor, the quality of
reclamation efforts, cannot be discussed fully merely by describing
reclamation practices. There are important institutional and technological
factors that come into play. The major ones are discussed below. Since
public attitudes toward western mining -- which surely have had a great
influence on the kinds of research planned or in progress -- were in part
formed on the basis of knowledge of past environmental damages in the
eastern coalfields of the United States, the format for the subsequent
discussion is a contrast of factors in each geographic region.
Industry Structure
In the east, both the mines and mining companies are small, on the
average. As shown in Figure 11, average annual production for eastern
mines has remained fairly constant at 50,000 metric tonnes (55, 000 tons)
for over thirty years. As a result, mine lives are short; six to twelve
31
-------
Co
ro
1,200 --
1,000
800 --
AVERAGE
MINE
SIZE
(Thousands
WEST
1932 1940 1948 '.95o 1104 197J
3000
2500
2000
NCMBER
OF 1500 J
ACTIVE
MINES
1000
!00
Q
, CAST
1
/'
1
\
]
1
1
I
/
,J
/""X /
1 \ ,' * *
1 \
' s /
I V
/
/
/
/
/ ^
f . _; - ~* ~^- *^v^_^ .CENTRA
/ ' ~"~ --/^"y
.^^ '.VEST
1932 1940
1948 115o
YEAR
Figure 11. Regional Trends in Average
Surface Coal Mine Size
Figure 12. Regional Trends in the
Number of Active
Surface Coal Mines
-------
months is common. Capital requirements for an average mine are
relatively low; approximately $1.5 million capital is required to open a
mine that will produce 91, 000 metric tonnes (100, 000 tons) per year. The
equipment used is mobile and multipurpose. It can be used for highway
construction work as readily as for strip mining.
These characteristics have several important effects. The first
of these is that there are many mines, the number of active mines having
increased, as shown in Figure 12, from 1, 500 in I960 to over 3, 000 in
1973. This is an average of about 450 mines in each of the eastern states.
The sheer volume of mining operations taxes the ability of state reclama-
tion agencies to effectively administer reclamation laws. Additionally,
because capital requirements are fairly low, many new mine operators
enter the market when coal prices rise, as has been the case over the
past several years. Some of these operators lack expertise in reclama-
tion technology. Many of the companies have neither engineers nor
reclamation specialists. Bankruptcies are not uncommon. When this
happens, the environment may suffer.
Things are vastly different in the west. There are and have been
relatively few active mines. The number has remained fairly steady at
about 50 for 30 years. This is an average of about eight mines per state,
a figure that does not tax the administrative capability of state reclama-
tion agencies.
Although western production has increased dramatically in the past
decade, this increase has resulted not from an increase in the number of
mines, but rather from an expansion of the capacities of existing ones.
In 1976, the average annual production for western mines was 1. 45 million
metric tonnes (1. 6 million tons). Operation of such mines requires not
only huge capital investment, but also considerable engineering and
reclamation expertise. The mines are long-lived; 27 years is the average
remaining life of mines now active. The coal companies are large and
well-established. Many are wholly owned subsidiaries of public relations-
conscious power companies.
Under such circumstances, it is not surprising that the coal
companies have made a commitment to reclaim the land, if only to avoid
jeopardizing a $20 to $50 million capital investment. This commitment,
coupled with resident reclamation expertise, often means good reclama-
tion.
Reclamation Laws
The reclamation laws in the west are fairly new. In drafting those
laws, the experience of reclamation agencies in midwestern and eastern
states weighed heavily. As a result, western reclamation laws are
generally comprehensive and stringent. Since the coal industry was until
recently not a major economic factor in the west, the lobbying that often
proved effective in other regions was not effective in the west. Good
laws, coupled with the quality of enforcement possible when there are only
ten or so mines in a given state, translated into effective reclamation, at
least insofar as current knowledge and technology is concerned.
33
-------
An additional factor is the extensive ownership of western coal by
the Federal government. This means, under the National Environmental
Policy Act, that comprehensive, detailed environmental impact statements
must be prepared before the coal can be mined. Such statements may
require several man-years to prepare.
Disturbed Acreage and Land Use
At present, the acreage disturbed annually by western mining is
less than that disturbed annually by eastern mining. There are two
reasons for this. First, western coal is much thicker than eastern coal.
In 1975, for example, the average total thickness of the coal seams mined
in the west was 11. 9 meters (39 feet), whereas for the east the average
was 1.5 meters (five feet).* Other things being equal, this means that
approximately 25, 000 metric tonnes of coal are produced per hectare
disturbed (67, 860 tons per acre) in the west, but only 3, 300 metric tonnes
of coal are produced per hectare disturbed (9, 000 tons per acre) in the
east. **
Other things are not equal, however, since for a given coal seam
thickness, the acreage disturbed by the typical one-cut contour methods
used in the east is approximately 60 percent greater than the corresponding
acreage when area mining methods are used. This is offset by the fact
that, on the average, the heating value of western coal is only about
65 percent as great as that of eastern coal. Overall, though, even on the
basis of heating value, roughly seven and one-half times fewer hectares
are disturbed per metric tonne of western coal than in the east.
Another factor is the premining productivity of western lands.
This is depicted in Figure 13 which shows that the premining stocking
rates at mines active in 1975 ranged between 4 and 48 hectares (10 and
120 acres) per animal unit year, and averaged 25 hectares (62 acres) per
animal unit year. In 1975, about 1,458 hectares (3, 605 acres) were
disturbed by mining. This caused a temporary productivity loss of only
58 animal unit years of grazing for the six-state region. Even if coal
production tripled or quadrupled and several years were required to
return the land to full or near-full productivity, the loss of grazing
capacity in the west would still be small. Of course, since large-scale
surface coal mining and reclamation are relatively new in the west,
there are still doubts that productivity can be restored in the long run.
*Total coal seam thickness is defined as the total thickness of all seams
mined in a given pit.
**Western coal: 1,740 tons per acre-foot. Eastern coal: 1,800 tons per
acre-foot. Only the acreage directly disturbed by overburden removal
is included in this example.
34
-------
No. of Mines
at Which
Average Stocking
Rate is
4 x
10 ..
3
6
4 4-
Average = 62
20
40
60 SO
100
120
x = Stocking Rate 'Acres per
Animal Unit Year)
Figure 13. Histogram of Premining Stocking Rates
at Western Mines
and Reclamation Costs
In 1976, the average stripping ratio at western mines was 4. 5 cubic
yards of overburden per ton of coal mined. The corresponding average
for the east and midwest was approximately 16:1 cubic yards per ton. [17]
If overburden removal costs per cubic yard of overburden were the same
in all regions, this would imply that western costs on a tonnage basis
would be only 28 percent as great as those in the east and midwest. Of
course, this cost difference is reflected in the coal prices which, for
steam coal in 1976, averaged about $5 per ton f. o. b. mine in the west
versus $17 per ton f. o.b. mine in the east.
An additional effect of the difference in coal seam thicknesses is
the disproportionate effect of reclamation cost increases among the
regions. For example, a $1,000 per acre reclamation cost increase
would on the average be equivalent to 1. 4 cents per ton in the west and
11 cents per ton in the east. Stated another way, the average cost
increase on a per-ton basis would be 0. 28 percent of the selling price in
the west, but 0. 64 percent of the selling price in the east.
R e clamation Res ear ch
Reclamation research was being conducted at 71 percent of the 45
western mines active in 1976. The types and frequencies of research
activities are summarized in Table 4. Revegetation research is common.
Experiments are being conducted to evaluate alternative vegetative
species, irrigation procedures, various topsoiling depths, and water
harvesting techniques. In many cases, the mining companies themselves
are conducting or paying for the conduct of this research.
35
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TABLE 4. FREQUENCY OF OCCURRENCE OF
RECLAMATION RESEARCH AT
WESTERN MINES IN 1976
Subject of
Research
Revegetation
Hydrology
Overburden Analyses
Spoil Grading
Mining Method
Percent of Mines at Which
Research is Being Conducted*
69
31
18
7
4
Percentages total to more than 100 percent because
two or more types of research are being conducted
at some mines.
Exposed Highwalls
In typical eastern contour mining situations, the predominant
situation in the east, only one cut is made; thus every cut is a final cut,
resulting in a final highwall, whether it is eventually buried or not. * At
the typical western mine, many cuts are made, but only one (per pit area)
is a final cut. Thus the length of exposed highwalls in the west is far
smaller than that in the east. This is aesthetically desirable.
The exception is mountaintop removal mining, a method in which no final
highwalls are left.
36
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SECTION 6
CURRENT MINING AND RECLAMATION SYSTEMS
INTRODUCTION
The purposes of this chapter are to describe and evaluate the mining
and reclamation practices most frequently used in the west during 1975 and
1976. Ways in which environmental considerations have influenced or
should influence mining practices are emphasized. Mining and environ-
mental problems are identified.
Many, different types of mining situations occur in the west; it is
neither necessary nor desirable to discuss all of them. Rather, the
different situations have been classified in accordance with a series of
physical and technological parameters, and the frequency of occurrence of
each class of situations has been determined. Only those which occurred
frequently are discussed.
CLASSIFICATION OF MINING SITUATIONS
The choice of mining methods and equipment is dependent on many
economic, physical, and technological factors. In classifying the various
mining situations into groups with common production or environmental
characteristics, however, the following four factors (parameters) are felt
to be sufficient:
Mining method
Number of coal seams mined per pit
Main stripping equipment
Spoil placement method
Mining Method
The area method of mining, used at 82 percent of western mines,
accounted for 97 percent of the acreage disturbed in 1975. In this method,
overburden is removed in fairly long and roughly parallel strips, each
averaging about 43 meters (140 feet) in width, with overburden from a
37
-------
given cut being hauled, pushed, or cast into the adjacent open cut. A
typical cut and spoil pile are shown in Figure 14. *
The open pit method of coal mining is an infrequently used method
in which spoil is hauled from the pit by trucks and placed in a permanent
storage area outside the pit. Although the maximum pit depth in area
mining is currently about 45 meters (150 feet), an open pit may be as much
as 305 meters (1,000 feet) deep. Because of the great depths, the high-
walls in open pits are benched, giving the pits the terraced appearance
shown in Figure 15. A large permanent spoil "mesa" shows in the fore-
ground of that Figure. Unlike area mining, when open pit mining is
completed a large hole remains; there is no backfilling.
Modified open pit mining, although infrequently used at present, is
a method that will be widely used in the eastern Powder River Basin of
Wyoming where the coal seams are very thick. It is similar to area
mining in that spoil is placed back in the open cut, but more similar to
open pit mining in that spoil is hauled and the cuts are not long and
narrow. Additionally, although spoil is placed back in the open cut, a
Figure 14. Photograph Showing Area Mining Site
Spoils to the right of the angle-of-repose spoil pile in the photograph have
been graded to approximate original contour.
38
-------
Figure 15. Photograph Showing an Open Pit Coal Mine
permanent depression in the land surface usually remains after mining.
This is because the coal is often much thicker than the overburden where
the method is used.
Number of Coal Seams Mined Per Pit
Multiple seam mining, defined as the mining of two or more seams
in one pit (or highwall), accounted for about half of the acreage disturbed
by western mining in 1975. Single seam mining accounted for the
remaining acreage. A unique environmental feature of multiple seam
mining is that the interburden separating two coal seams is often
chemically or physically undesirable and must be buried in the spoil to
prevent subsequent spoil revegetation failures. * From a production
standpoint, multiple seam mining is more complex than single seam
mining.
Stripping Equipment
Three kinds of stripping equipment are used in the west. These
are sidecast equipment, construction equipment, and open pit (truck and
shovel) equipment.
In some states, the interburden material must be placed above the water
table in addition to below the spoil surface.
39
-------
Sidecast equipment, which includes draglines and stripping shovels,
is equipment that is used to remove overburden and place it in the adjacent
open cut by sidecasting. Stripping shovels are rarely used in the west at
present nor will they be widely used in the future. Draglines are widely
used, having accounted for 88 percent of the acreage disturbed by western
mining in 1975.
Two characteristics of dragline stripping are notable from an
environmental standpoint. One is that spoils are cast to rest at their
natural angle of repose; thus the spoil piles require grading by other
equipment to restore approximate original contours. The second is that
it may be difficult to place spoil materials selectively, particularly where
multiple seams are mined. Draglines are used only in the area method of
mining, never in open pit or modified open pit mining.
Construction equipment consists of dozers, end loaders, scrapers,
and relatively small spoil haulage trucks. These kinds of equipment are
rarely used in the west. When they are used, it is usually to supplement
dragline production. The equipment has production drawbacks compared
to draglines, in that operating costs are fairly high and production
capacity is fairly low. On the other hand, construction equipment has
desirable environmental characteristics. Approximate original contours
can be restored as part of the spoil placement process and spoil materials
can be placed selectively.
Open pit equipment consists of loading shovels and spoil haulage
trucks. The loading shovels are used to excavate overburden and load it
into trucks. * The trucks haul and place the excavated material. Although
the operating costs for truck and shovel stripping are higher than those
for dragline stripping, they are lower than the corresponding costs for
construction equipment. Additionally, fairly large rates of production can
be achieved with truck and shovel systems. Moreover, they have the same
environmental advantages as construction equipment. Truck and shovel
equipment, shown in Figure 16, is presently used primarily in conjunction
with the open pit and modified open pit mining methods.
Use of Spoil Segregation Techniques
Spoil segregation refers to selective placement of spoil to place
undesirable materials below spoil surfaces or above the water table or
both. As indicated earlier, adequate spoil segregation can be difficult to
achieve where draglines are used for overburden removal and spoil place-
ment, but should not be difficult where construction or open pit equipment
is used. This practice, which accounted for 39 percent of 1975 western
production, is the principal way in which western mining and reclamation
operations can be integrated.
C
Loading shovels differ greatly from stripping shovels.
40
-------
Figure 16.
Photograph Showing Truck and Shovel
Stripping Combination
Classification of Mining Situations
A summary of the frequencies of occurrence in 1975 of the values
of the foregoing parameters is presented in Table 5. The Table shows
that area mining accounted for 97 percent of the acreage disturbed by
western mining in 1975. Single and multiple seam mining occurred with
roughly equal frequency. Draglines were by far the most widely used type
of stripping equipment. According to coal company mining and reclama-
tion plans, spoil segregation was practiced at 40 percent of the mines
active in 1975.
A classification of western mining situations based on the fore-
going parameters is shown in Figure 17. Alternative measures of the
frequency of occurrence of each situation are shown on the right side of
that Figure. Although 13 different mining situations are depicted, only
five occurred with sufficient frequency to warrant attention in this report.
Those five situations and their associated production percentages are
summarized in Table 6. Dragline area mining of single coal seams
without spoil segregation, for example, accounted for roughly one-third
of 1975 strip pits and disturbed acreage. In total, dragline stripping
situations, including single and multiple seams, both with and without
spoil segregation, accounted for 69 percent of the strip pits, 83 percent
of the tonnage, and 88 percent of the acreage disturbed by western mining
in 1975.
41
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TABLE 5. FREQUENCY OF OCCURRENCE OF
PARAMETER VALUES IN 1975
Parameter
Mining
Method
No. of Coal
Seams Mined
Per Pit
Stripping
Equipment
Use of Spoil
Segregation
Techniques
Parameter
Value
Area
Modified Open Pit
Open Pit
Single
Multiple
Dragline
Truck and Shovel
Construction
No
Yes
Percent of
1975 Acres
97
1
Z
51
49
88
5
7
61
39
Percent of
1975 Mines
82
15
3
37
63
70
15
15
60
40
TABLE 6. PRODUCTION PERCENTAGES FOR THE FIVE MOST
FREQUENTLY OCCURRING WESTERN SURFACE
COAL MINING SITUATIONS
Rank
1
Z
3
4
5
TOTAL
Mining
Method
Area
Area
Area
Area
Modified
Open Pit
--
Number
of Seams
Single
Single
Multiple
Multiple
Multiple
Main
Stripping
Equipment
Dragline
Dragline
Dragline
Dragline
Truck b
Shovel
Spoil
Segregation?
No
Yes
Yes
No
Yes
Percent of ...
Pits
34
11
11
13
3
72
1975
Tonnage
23
25
20
15
4
87
1975
Acres
30
11
24
23
1
89
Maximum
Tonnage
16
18
11
11
12
68
42
-------
1 ALL MINES 1
(.MINING
METHOD
1 AREA
H MODIFIED 1
OPEN PIT
H OPEN PIT 1
MMMMMM
H
H
H
rl
i |
rl
H
II. NO. OF
COAL
SEAMS
SINGLE 1
III. MAIN
STRIPPING IV. SPOIL
EQUIPMENT SEGREGATION?
H
H DRAGLINE 1 1
r-T
sKf H -
m
1 jrnMSTnnn!j 1
i K
i J DRAGLINE 1 1
.
ซซ~H : : H
' f
SINGLE 1
NO
ซ
YES
NO
YES
NO
NO
YES
J
I
I
I
I
|
I
L 1 ALL OTHER 1 NEGLIGIBLE
i HI
Pi SHOVEL M r
H.
HCQNSTRUC-1 i
TION | I
-T
MULTIPLE | 1 TRUCK* |_| ^
<-L
???
YES
YES
???
M^H^^^^
YES
I
I
I
1
SINGLE 1 NEGLIGIBLE
MULTIPLE | |COIi}fJ|3UC'| 1
NO
1
NO. OF
PITS
21
7
1
1
3
8
7
3
1
1
2
1
1
1
1
PERCENT
OF 1975
TONNAGE
23.2
25.3
Neg.
5.3
1.3
15.3
19.7
None
None
4.5
None
None
None
2.4
PERCENT
OF 1975
ACRES 1
29.9
10.8
Neg.
4.1
22.8
24.3
1.5
None
None
0.8
None
None
None
2.0
PERCE
OF MA
FONNA
16.0
18.2
Neg.
5.7
1.0
11.2
11.4
1.8
6.0
4.2
12.4
7.0
4.2
Neg.
1.4
Figure 17. Classification of Western U.S. Surface Coal Mining Situations
-------
TABLE 7. RANKING FACTORS FOR THE FIVE MOST
FREQUENTLY OCCURRING WESTERN
SURFACE COAL MINING SITUATIONS
Mining
Method
Area
Area
Area
Area
Modified
Open Pit
Number
of Seam a
Single
Single
Multiple
Multiple
Multiple
Main
Stripping
Equipment
Dragline
Dragline
Dragline
Dragline
Truck &
Shovel
Spoil
Segregation?
No
Yes
Yes
No
Yes
Rank, Based on Percent of ...
Pits
1
4
3
2
6
1975
Tonnage
2
1
3
4
6
1975
Acres
1
4
2
3
8
Maximum
Tonnage
2
1
4
5
3
Overall
Rank
1
2
3
4
5
A ranking of the top five situations, based on a composite of the
various frequency measures, is presented in Table 7. Area mining of a
single coal seam using a single dragline without and with spoil segregation,
respectively, ranked first and second. Area mining of multiple coal seams
using a single dragline with and without spoil segregation, respectively,
ranked third and fourth. Finally, modified open pit mining of multiple
seams with spoil segregation ranked fifth. This is not because the situa-
tion occurred frequently in 1975, but rather because it will occur fairly
frequently by the early 1980's.
The foregoing five mining situations are discussed in this chapter.
Dragline systems receive special emphasis, particularly with regard to
ways in which production and reclamation practices can be integrated.
Briefly, this refers to the practices of selectively placing spoil and
choosing pit geometries that minimize the time lag between spoil place-
ment and spoil grading.
MINE PLANNING
Planning of new western mines requires several years. Frequently
the first step is to obtain a prospecting lease on a property. Exploration
drilling is then conducted to determine the general characteristics of the
coal seams, including depth, thickness, dip, heating content, sulfur
content, ash content, moisture content, and, in some parts of the west,
sodium content. In this phase the spacing of core drill holes is generally
about one mile. Ordinarily only the coal is cored at this stage. If pre-
liminary analysis indicates that mining is feasible, markets for the coal
will be sought and a mining lease will be obtained. If surface ownership
differs from coal ownership, attempts will be made to lease or purchase
surface rights.
44
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Subsequently, a detailed mapping program will be undertaken. The
property will be drilled on 1/4 to 1/2 mile centers and, for some holes,
the overburden as well as the coal will be cored. Overburden stratigraphy
will be analyzed to estimate overburden drilling, blasting, removal, and
placement costs. Additionally, some overburden samples will be
artificially weathered in a laboratory and tests will be conducted to
determine the plant-growing potential of the various overburden strata.
The overburden will also be analyzed chemically and physically to
determine those materials which must be placed selectively as spoil to
comply with state and Federal reclamation laws.
Various maps will be developed. One type of map will show lines
of equal overburden depth (isopachs). Others will show lines of equal
stripping ratio, coal seam thickness, heating value, and sulfur content.
The latter types of maps, depicting the characteristics of the coal on an
areal basis, are particularly important where multiple coal seams are to
be mined. This is because the characteristics of the coal seams may
vary considerably, and blending of coal from two or more seams may be
required to meet sales contract requirements. Blending may also be
required where only one seam is mined as well, because the character-
istics of a given seam may vary significantly over the mining property.
In North Dakota, for example, the sodium content of the coal is some-
times troublesome. Near the coal seam outcrop, the sodium content is
frequently low because much of the sodium has been leached out. In
deeper overburden, the sodium content is typically higher. In such cases,
it may be necessary to work two or more pits simultaneously, so that
coal from the various pits can be blended to keep the average sodium
content within contract limits.
The total strippable coal reserves will then be reestimated with
increased accuracy made possible by the detailed drilling program. For
a given area, the strippable reserves are dependent primarily on the
selling price and thickness of the coal, the overburden depth, and the coal
recovery percentage. Based on estimated coal selling prices and mining
costs, a recovery line is defined. This line shows the estimated location
of the final highwall, that is, the line of maximum strippable overburden
depth. Of course, during the life of the mine, economics may change,
usually resulting in redefinition of the recovery line and thus the strippable
coal reserves and the estimated mine life.
Once the reserves, overburden depths, and stripping ratios have
been estimated, alternative mining equipments will be evaluated. Limits
on the total allowable capital investment for equipment are usually set in
accordance with a rule of thumb which states that the total capitalization
should not exceed $1. 11 per metric tonne (one dollar per ton) of strippable
reserves. Thus, for example, the total capitalization for an area under-
lain by 45 million metric tonnes (50 million tons) of strippable coal should
not exceed 50 million dollars if the foregoing rule is used. Limits on
total capitalization imply limits on equipment capacity and thus on rates
of coal production. Those limits affect markets for the coal and the coal
selling price.
45
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Selection of main stripping equipment, the so-called prime
movers, receives major emphasis. Draglines are widely favored as
prime movers in the west today because, for a given rate of production,
the unit ownership and operating costs for a dragline are lower than those
for alternative types of equipment. Required overburden removal capa-
cities for the dragline are determined by multiplying the required rate of
coal production by the stripping ratio. For example, if the required rate
of production is 1. 8 million metric tonnes (two million tons) of coal per
year and the average stripping ratio is 2. 72 bank cubic meters of over-
burden per metric tonne of coal (three bank cubic yards per ton), then the
required stripping capacity would be 4. 9 million cubic meters (six million
cubic yards) per year. * The dragline bucket size necessary to provide
this capacity is dependent on the operating schedule, overburden type, and
machine availability. The overburden in part determines cycle times,
spoil swell factors, and bucket fill factors, which influence production
capacity. Typical planning factors are shown below:
Scheduled operations: 720 hours per month
Machine availability: 75 percent
Cycle time: 60 seconds
Spoil swell factor: 30 percent
Bucket fill factor: 85 percent
Application of those factors results in an estimated production of 200, 000
cubic meters (260, 000 cubic yards) of overburden per year per cubic
meter of dragline bucket capacity. Thus, in the foregoing illustration, a
dragline with a bucket capacity of 17. 6 cubic meters (23 cubic yards)
would be required.
In practice, determination of required equipment capacity is
complicated by the fact that, because mining usually begins along the coal
seam cropline where the overburden is shallow, the average stripping
ratio increases steadily as mining progresses. This means that required
equipment capacity will be greater in the later years of mining than in
early ones. Accordingly, a stripping machine sized to meet early year
production requirements will be too small to meet those in later years.
Conversely, a machine sized to meet later year requirements will have
excess capacity in early years.
Although there is no set rule for resolving this problem, common
practice appears to be to size the machine to meet production require-
ments in early years, and then to purchase additional equipment in later
years. The primary alternative is to purchase a machine capable of
In this simplified illustration, it has been assumed that there is no
appreciable spoil rehandle.
46
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meeting maximum production requirements and then to use it only one or
two shifts per day in the early years of mining.
The problem of selection of equipment capacity is compounded if
the rate of coal production itself will increase over the life of the mine.
Then the required rates of overburden removal increase as a result of
both increasing stripping ratio and increasing coal production. This
situation occurs most notably in Wyoming's eastern Powder River Basin
where, because the coal is very thick, one company may have mining
rights to billions of metric tonnes of strippable coal -- too much for
which to find a market all at one time. As a result, production rates at
a given mine will increase as new markets for the coal are found. This
necessitates the use of a flexible stripping system whose capacity can be
increased in the required increments.
This can be difficult where the prime movers are draglines,
particularly since the present lead time for purchase of new machines is
seven years; thus an alternative choice has frequently been made in the
eastern Powder River Basin. The choice is truck and shovel stripping
equipment. The equipment is proven, readily available, and can be
augmented in small increments by adding trucks and shovels. The
disadvantage of this alternative is that it is more costly to operate than
draglines, a factor that may not be too significant when the coal is
30 meters (100 feet) thick, but is very significant otherwise. It is notable
in this regard that the average thickness of individual seams mined in
western areas other than the eastern Powder River Basin was 6. 7 meters
(22 feet) in 1975.
The implication here is that draglines will continue to dominate
the western mining scene for the foreseeable future. This is but another
reason for the strong emphasis on dragline mining situations in this
report.
Additional equipment that must be selected includes that for top-
soil removal, coal loading, coal haulage, and spoil grading, amendment,
and seeding. Specifications for materials handling, maintenance, and
office facilities are also defined at this stage. These decisions, although
important, are not too relevant from an environmental standpoint.
Initial mining and reclamation plans will also be developed during
the planning phase. The mining plan will typically show the sequence and
geometry of stripping cuts, the methods used for overburden removal and
spoil placement, and the approximate locations of drainage control
facilities, coal haulage roads, and coal loading facilities.
In preparation of reclamation plans and environmental impact
statements, several kinds of environmental surveys and analyses are
generally conducted. These include, but are not limited to, the following:
Overburden analyses.
Soil surveys to determine topsoiling materials that are
suitable for placement on the surfaces of graded spoils.
47
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Hydrologic surveys to determine the premining
characteristics of surface water and groundwater in
the mining area.
Vegetation surveys to determine the types and
densities of native vegetative species prior to mining.
Archeological surveys to identify historically
significant archeological features that should be
preserved.
Wildlife surveys to determine the types and numbers
of wildlife that inhabit the mining area.
Reclamation plans include specification of at least the following
kinds of procedures:
Drainage control procedures, which may include
stream diversions, above-highwall diversion ditches,
impoundments, culverts under haul roads, and
ditches alongside haul roads.
Topsoil salvaging procedures, including depths of
topsoil removed and replaced, locations of topsoil
stockpiles, and ages of stockpiles at the time of
placement of topsoil on graded spoils.
Spoil segregation procedures to selectively place
undesirable spoil materials during the mining
process.
Erosion and sediment control procedures including
terracing, contour ditching, and construction and
maintenance of sediment basins.
Procedures for grading of spoil piles to restore
approximate original contours. The plans usually
include maps showing the topography after grading.
Seeding and spoil amendment procedures including
types and densities of vegetative species to be used,
methods and times of year for seeding, and amounts
of fertilizer, lime, gypsum or other spoil amend-
ments to be used.
Procedures for reducing the height and slope of the
final highwall.
Procedures to minimize noise, ground vibration, and
air shock resulting from overburden and coal blasting.
Procedures to control fugitive dust emissions.
48
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After all necessary permits and licenses have been obtained,
planning has been completed, a market for the coal has been defined, and
financing has been obtained, mine development can begin. It includes
purchase and assembly of equipment and construction of facilities and
haul roads.
MINE OPERATING PROCEDURES
Operating procedures for frequently occurring mining situations
are described below in more-or-less chronological order.
Drainage Control Practices
At 31 percent of the mines active or under development in 1975,
intermittent stream courses that originally crossed planned pit areas were
diverted to prevent surface water from entering strip pits. An apparent
tendency in such cases is to straighten the stream beds thereby increasing
the hydraulic gradient, possibly accelerating erosion of stream banks.
This phenomenon was observed at one of the nine field survey mines.
Above-highwall diversion ditches were constructed at 71 percent of
the mines active or under development in 1975 as a means of reducing the
amount of surface runoff that entered strip pits. Generally those ditches
were placed at or near the perimeters of the mining areas to eliminate
the need to reconstruct the ditches as mining progresses. Although
erosion of the ditches is a potential problem, it did not appear to be an
actual one. This is because the gradients of the ditches are controlled to
some extent, the ditches are often naturally vegetated, and rainfall,
although sometimes intense, is infrequent. Figure 18 shows a typical
diversion ditch. After mining has been completed, the ditches are
revegetated as part of the reclamation effort. A reclaimed ditch is shown
in Figure 19.
The ditches usually empty onto undisturbed ground. Headcutting
at the ditch outfalls, shown in Figure 20, had occurred at two of the field
survey mines where no attempt had been made to dissipate the energy of
the outflow. At other mines, effective means had been devised to prevent
headcutting. One of those, shown in Figure 21, was construction of a
shallow rock-lined catch basin at the outfall of the ditch.
At some mines, small earth dams were constructed across
natural drainageways to impound surface runoff and prevent it from
entering strip pits or crossing haul roads. Those impoundments were
generally deemed to be effective in controlling runoff. Potentially,
however, they could have adverse effects on the water budget, due to
increased evaporation of impounded water, and on surface water quality.
But losses to the water budget due to the impoundments were deemed to be
insignificant. Some grab samples taken from the impoundments were
salty, however.
49
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Figure 18. Photograph Showing a Typical Drainage Diversion Ditch
Figure 19. Photograph Showing a Revegetated Diversion Ditch
50
-------
.
Figure ZO. Photograph Showing Headcut at Outlet of
Drainage Diversion Ditch
Figure 21. Photograph Showing Rock-Lined Outlet of Drainage
Diversion Ditch, Used to Prevent Headcutting
51
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Sediment basins were used at 87 percent of the mines active or
under development in 1975. Although many of the basins at the field
survey mines were shallow, they appeared to be effective. The conclusion
is that sediment is being controlled adequately at the field survey mines.
Some on-site sedimentation was observed, however.
Drainage treatment facilities, commonly used in other parts of the
country, are not used in the west. This is taken to imply that chemical
treatment of mine area discharge is not needed.
Of course, despite efforts to prevent water from entering strip pits,
some surface water or groundwater usually does enter the pits. In such
cases, water is pumped from the pits and discharged onto natural ground,
or into drainage diversion ditches, or is routed through plastic tubing to
sediment basins. Pit water was pumped at 82 percent of the mines active
in 1975. In other parts of the country, particularly in midwestern acid
areas, water that is pumped out of the pit must be treated to settle iron
and reduce acidity. This is not required in the west because there are
few acid areas. Of course, as pumped, the water may be heavily sedi-
mented or high in soluble salts or trace elements. Sediment appears to
be settled out in basins. At some mines there is no indication that pit
discharge is chemically undesirable; in fact the water is used at some
mines to irrigate reseeded spoil areas. At others, it appears that water
quality has been changed.
Drainage at haul roads is controlled in the following three ways at
most western mines:
By installing culverts under the roads wherever the
roads cross natural drainageways.
By constructing and maintaining roadside ditches
that carry runoff from the roads.
By crowning the roads to divert surface runoff to
the roadside ditches.
At the field survey mines, culverts, ditches, and crowning were used and
appeared to be adequate.
A summary of the frequencies of use of the foregoing drainage
control practices is presented in Table 8. An overall evaluation of the
effectiveness of those practices is also presented.
Tops oiling Practices
Salvaging of topsoiling material was used at 96 percent of the
western mines active in 1975. Typically, tops oil is removed by scrapers
about one to six months ahead of overburden removal. In some northern
areas, topsoil is not removed during winter months but in other areas it is
removed year-round. The average depths of topsoil that were removed
were determined from the premining soil surveys, and ranged from 10 to
52
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TABLE 8. FREQUENCY OF USE AND EVALUATION OF
DRAINAGE CONTROL AND TREATMENT
PRACTICES USED DURING 1975
Drainage Control
Practice
Stream Diversion
Drainage Diversion
Ditches
Impoundments
Sediment Basins
Drainage Treatment
Pump Water From*
Pits
Frequency of Use of Practice in 1975
Percent of Acres
12
55
No Data
72
1
89
Percent of Mines
31
71
No Data
87
2
82
Overall Evaluation
Some stream bank erosion due
to increased hydraulic gradient
Effective, but headcutting at out-
fall of ditch should be prevented
No data
Appear to contain most or all
sediment on -site
Apparently not needed, but current
research will provide resolution
No apparent significant adverse
effects
122 centimeters (four to 48 inches) during 1975, as depicted in Figure 22.
For the region as a -whole, the average depth was 33 centimeters
(13 inches). Figure 23 shows two scrapers removing topsoil. Figure 24
shows topsoil being piled by a dozer for subsequent loading into trucks. In
North Dakota, topsoil and subsoil are removed separately. At most mines
in other states, either only topsoil is saved, or topsoil and subsoil are
removed and stockpiled together.
No. of Mines
at Which
Average Depth
of Topsoil
Salvaged is
Equal to x
25
20
15
10
Average = 13 Inches
0 10 ZO 30 40 50
x = Topsoil Depth (Inches)
Figure 22. Histogram of Average Topsoil Salvaging Depths
53
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Figure 23. Photograph Showing Scrapers Removing Topsoil
Figure 24. Photograph Showing Topsoil Removal by Dozer
54
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At 96 percent of the mines active in 1975, some topsoil was
temporarily stockpiled prior to placement on graded spoils. There are
several reasons for this. One is that in the early years of mining there
are only small acreages of graded spoil upon which to spread the topsoil.
This is particularly true where topsoil is removed well in advance of over-
burden removal -- a practice that may expose large areas to wind erosion.
Another is that it is not always desirable to place topsoil during winter
months, when seeding is not done and wind erosion may occur. In
general, however, it is desirable to place topsoil without stockpiling
because a transplant effect may occur. This could be accomplished if
topsoil was transferred across the active pit by the prime mover, a
practice which is not feasible under current technology, but in some cases
could be done if the prime mover was a bucket wheel excavator.
Transport of topsoil around the active strip pits to graded spoil
areas can be costly, particularly where the pits are long. Typical costs
range from $0. 33 to $0. 66 per cubic meter of topsoil ($0. 25 to $0. 50 per
cubic yard). In some cases, to reduce costs, spoil bridges are constructed
across the pits.
Wind and water erosion of stockpiled topsoil is a potential problem,
and in some instances an actual one. Seeding of stockpiles to reduce water
erosion is an effective but infrequently used practice -- during 1975, it was
used at only 22 percent of the active mines. Where not used, erosion -was
observed to occur, as shown in Figure 25. The vegetation on the stockpile
in that Figure is volunteer.
Figure 25. Photograph Showing Erosion on Topsoil Stockpile
55
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The percentage of topsoil that is stockpiled varies from mine to
mine and year to year. Thirty to fifty percent appears to be a representa-
tive figure.
Some time after stockpiling, usually six to 24 months, the topsoil
is spread by scraper on graded spoils. Figure 26 shows topsoil replace-
ment by scraper. Figure 27 shows an area that has been topsoiled,
adjacent to an area that has not. The topsoiled area is at the right of the
Figure.
In recent years, some ecologists have expressed concern that
presently used topsoil salvaging practices may not be adequate. Their
concerns are twofold. The first is that mixing of the various horizons may
"contaminate" the A-Horizon material. The second is that stockpiling of
topsoil for long periods of time may reduce or destroy the fertility of the
soil. Field observations suggest that those concerns are unfounded in the
short-term at least. At all of the field survey mines, vegetative density
on topsoiled and seeded areas was good to excellent. Of course, through
refinement of topsoiling salvaging techniques, it might be possible to
reduce the time required to establish a given vegetative density on
reclaimed areas, but, allowing for the apparent success of current
practices, those refinements do not appear to be critical.
The frequencies of use and evaluations of topsoil salvaging practices
are summarized in Table 9.
Overburden Drilling and Blasting
In North Dakota where the overburden consists of clays, soft
shales and, in some areas, glacial drift, blasting of overburden is not
required. At most mines in the other states, the overburden is drilled
and blasted prior to excavation. Blasthole drill diameters at the field
survey mines ranged between 19 and 27 centimeters (7-1/2 and 10-5/8
inches). Holes are generally drilled on 6 x 6 or 7. 6 x 7. 6 meter (20 x 20
or 25 x 25 foot) centers. Holes are typically loaded with bulk ANFO.
Ordinarily, the overburden does not require hard blasting; typical powder
factors range between 0. 12 and 0. 33 kilograms per cubic meter of over-
burden (0. 2 and 0. 55 pounds per cubic yard). This is much less than the
0. 6 kilogram per cubic meter (one pound per cubic yard) average in the
eastern United States. Frequently, where draglines are the prime movers,
overburden is blasted only in the keycut. At some mines where loading
shovels are used for overburden removal, the breakout force of the shovels
is sufficient to remove overburden without blasting.
At the field survey mines, overburden is drilled three to six
months in advance of stripping. Holes are usually loaded and shot within
ten days after drilling. These practices appear to be unique to the west.
In the midwestern and eastern United States where rainfall is fairly
heavy, overburden blastholes are usually drilled only a few shifts ahead
of stripping and the holes are loaded and shot within a few hours after
drilling. This is because surface runoff or shallow groundwater may
cause clogging of the blastholes.
56
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Figure 26. Photograph Showing Topsoil Replacement by Scraper
Figure 27. Photograph Showing An Area That Has Been Topsoiled
-------
TABLE 9. FREQUENCY OF USE AND EVALUATION
OF TOPSOIL SALVAGING PRACTICES
Activity
Tops oil Removal
and Replacement
Topsoil Stockpiling
Prior to Placement
on Graded Spoils
Use of Special
Measures to Control
Erosion on Topsoil
Stockpiles
Frequency of Use
Percent of
1975 Acres
94
94
15
Percent of
1975 Mines
96
96
ZZ
Remarks
Scrapers used
extensively.
Includes 65 percent of
mines at which stock-
piling will be done "if
necessary". Usually
it is necessary in the
early years of mining.
Excludes 36 percent of
mines at which erosion
control measures will
be used "if necessary".
Blasting of overburden can have adverse environmental conse-
quences, primarily noise, ground vibration, and air shock. These effects
are major problems in some parts of the midwestern and eastern United
States, but they are not too troublesome in the west. There are two
reasons for this. The first is that there are relatively few people living
close to western mines. The second is that overburden is not blasted too
hard in the west.
There are a few mines close to towns, however, and two proce-
dures are used at those mines to minimize the environmental effects of
blasting. The first is to delay the shots by row so that the amount of
explosive detonated at a given instant is less than or equal to an amount
determined by the U.S. Bureau of Mines to be acceptable. The second is
to replace primercord detonators with electrically wired blasting caps.
Primer cord is a cord made of explosive material which is lain on the
surface of the overburden connecting the blastholes. Ordinarily several
hundred meters of primercord will be exposed on the surface of the
ground for any given shot. The cord is detonated by a blasting cap and
then detonates caps buried in each hole. Since the primercord is itself an
explosive and is on the surface, its detonation usually makes a lot of
noise. The alternative is to use electrical wire running to the caps in the
blastholes. Detonation of the buried caps makes relatively little noise.
Thus the substitution of electric caps for primercord is an effective noise-
reducing technique.
Some researchers have made the claim that overly hard blasting
of overburden may result in excessive amounts of fines in the shot
58
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material, thereby causing permeability problems when the material is
placed as spoil. This potential problem does not appear to occur in
practice. Blasting costs can be a fairly large component of total over-
burden removal costs and western mine operators are careful not to blast
the overburden any harder than necessary.
There have been some complaints on the part of ranchers living
near mines that blasting has caused wells or springs to dry up. In some
cases, it has been determined that blasting could not have been the cause
of the problems. In others -- the Absolaka mine is an example -- it has
been determined that blasting has disrupted groundwater systems.
Drilling and blasting may also cause emission of fugitive dust.
Neither the magnitude nor the extent of this potential problem was evaluated
as part of this study.
Box-Cutting Procedures
The first step in the actual stripping process is to open the box cut.
This cut is usually made along or near and roughly parallel to the coal
seam cropline, where the overburden is shallow. In dragline stripping
situations, the dragline rests on or near the natural ground surface, as
shown in Figure 28, to excavate the box cut overburden. Excavated
material is sidecast onto the natural ground adjacent to the box cut as
shown in Figure 29. Where shovels and trucks are used to open the box
cut, the spoil need not be stacked next to the cut and in practice it usually
isn't. Generally, regardless of the equipment used, the box cut spoils
are placed on areas that are not underlain by strippable coal; in dragline
stripping this would be immediately outside of the coal seam cropline.
On occasion, for production reasons, the box cut may be opened
fairly far back from the cropline. If draglines are used for box-cutting,
this usually means that the box cut spoils are placed on land that is under-
lain by strippable coal. Consequently, the box cut spoils must eventually
be moved to enable mining of the underlying coal. In such cases, mine
operators may be reluctant to grade and seed the box cut spoil piles
because they will be redisturbed. This is a minor environmental dis-
advantage of opening the box cut with a dragline back from the coal seam
cropline.
Where the box cut overburden is fairly shallow, as is usually the
case, box-cutting with shovels and trucks costs more than box-cutting
with draglines. As a result, where shovels and trucks are the major
overburden removal and spoil placement equipments, small draglines are
often used to open the box cuts. They are also used for auxiliary work
such as ditch-digging. On the other hand, where draglines are the prime
movers, it is very unlikely that shovels and trucks would be used to open
the box cuts. Rather, the prime mover or a small auxiliary dragline
would be used.
59
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Figure 28. Photograph Showing Dragline Excavating
Box Cut Overburden
Figure 29.
Photograph Showing Dragline Placing
Box Cut Spoils
60
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There are three situations in which box-cutting costs for draglines
could be fairly high. One is where the box cut overburden is deep and spoil
rehandle is required, either because a borrow pit is required or because
box cut spoil must be placed on both sides of the cut as shown in Figure 30.
The second is the case where the coal seam pitches steeply down from the
cropline. In this case, standard practice is to open the box cut as wide as
possible to allow for progressive narrowing of subsequent cuts to reduce
overall spoil rehandle and maximize the amount of coal that can be
recovered by stripping.
The third case is that in which two or more coal seams are to be
mined in a given pit. In such cases, if a dragline is used to open the box
cut and the overburden is fairly deep, it may be difficult or costly to open
up to both seams in the box cut. This is environmentally relevant in states
like Montana where the law requires that all seams which are economically
recoverable be mined. One purpose of this law is to prevent redisturbance
of reclaimed areas to recover lower lying seams, the mining of which may
become economically feasible in later years.
Single Seam Dragline Stripping Procedures
The most common mining situation in the west in 1975 was stripping
of a single seam using a single dragline as the prime mover. Several
alternative operating procedures were used, depending on the overburden
depth and the chemical and physical characteristics of the overburden.
Each procedure is discussed in succeeding paragraphs.
BORROW SPOIL
BORROW SPOIL
REHANDLE MATERIAL
Figure 30. Methods for Opening a Box Cut in Deep Overburden
61
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One-Lift, One-Pass Stripping Without Spoil Rehandle --
The least complex and most frequently occurring single seam
dragline mining situation is one in which the dragline rests on or near the
natural ground surface and overburden is removed in one lift and one pass
in each pit.
In each cut the dragline is centered over the keycut, a narrow
trench made to establish the new highwall. This is shown in position #1
in Figure 31. From that position, the keycut material is cast into the
adjacent open cut. Eventually, if the dragline was not moved from the
keycut position, the resulting spoil pile would become so large that the toe
of the spoil would ride up the existing highwall above the top of the coal
seam, thus necessitating subsequent spoil rehandle to enable recovery of
all the coal. In order to prevent this, the dragline is moved sideways out
toward the open cut to enable casting of the remaining spoil at an increased
distance from the existing highwall. The resulting working position for
the dragline is shown as position 2 in Figure 31.
After digging out a complete block, called a digout or move, the
dragline is moved to the new keycut position to begin the next digout.
Overburden handling procedures on this digout are similar to those on the
previous one. When the end of the pit is reached, the machine is usually
deadheaded (moved without excavating any overburden) back to the opposite
end of the pit to begin the next cut.
The primary operating decision variables in this case are the width
of the pit and the length of the digout. The choices of pit width and digout
length affect both production costs and spoil grading costs. As the pit is
narrowed, for example, the swing time and thus the overall cycle time for
the dragline will be reduced. In theory, of course, the pit could be
narrowed so far that swing angles would become so small that the drag-
line bucket could not be hoisted the required distance during the time
required for the loaded swing. If this were to happen, narrowing the pit
might increase cycle times and reduce productivity. In practice, however,
this situation probably would rarely occur because the minimum pit width
is dictated by safety considerations or the operating room required for
coal loading and haulage equipment. At western mines, -where electric
shovels are used for coal loading, the minimum pit width in practice is
30 meters (100 feet). Where front end loaders are used for coal loading,
this can be reduced to 27 meters (90 feet). Under such circumstances,
where the dragline operates on or near the natural ground surface, overall
cycle times are usually determined by swing rather than hoist times.
Intangible factors also affect the minimum practical pit width. For
example, if the pit is deep and narrow, people may not want to work in it.
In fact, at some surface coal mines in Australia, union contracts require
a minimum pit width of 43 meters (140 feet). In the United States, a good
rule of thumb is that the pit should be at least as wide as it is deep.
62
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Figure 31. Plan and Section Views Showing
Dragline Keycutting Procedures
63
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The possible effect of this rule on pit widths can be gauged by
considering the average overburden depths at western mines. As shown
in Figure 32, those depths ranged between six and 55 meters (20 and
180 feet), and for the region as a whole averaged 24 meters (80 feet).
Since 30 meters (100 feet) is the minimum width dictated by working room
requirements for coal loading and haulage equipment, it would appear that
the foregoing rule affects minimum pit widths only where overburden
depths exceed 30 meters (100 feet). But for overburden that deep,
production factors usually dictate that the pits be fairly wide. In 1975,
the average width of dragline pits at western mines was 43 meters
(140 feet).
The reason for emphasis on pit widths in this situation is that time
lags between spoil placement and spoil grading can be reduced, and spoil
grading costs as well as overburden handling costs can also be reduced by
narrowing the pit.* Grading costs decrease as the crest-to-crest spacing
of the spoil piles decreases. That spacing, in turn, is identical to the pit
width. It has been estimated, for example, that per-acre grading costs
for spoils with 27 meter (90 foot) crest-to-crest spacing are roughly half
of the costs for grading of spoils with 37 meter (120 foot) crest-to-crest
spacing. [19] The reason for this cost difference is illustrated in
Figure 33, which shows that the width and depth of the vee between adjacent
piles increases as the crest-to-crest spacing of the piles increases.
Since spoils are graded by pushing spoil from the crests into the vee,
decreasing the width and'depth of the vee decreases the amounts and
distances that spoil must be moved to achieve a desired final contour.
No. of Mines
at Which
Average
Overburden
Depth Equals x
16
1Z
Average = 80 Ft.
0 40 80 120 160
x = Overburden Depth (Ft.)
Figure 32. Histogram of Average Overburden Depths
This applies only to the no-rehandle situation under discussion.
64
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Crest-to-Crest Spacing: 90 feet
Crest-to-Crest Spacing: 120 feet
Figure 33.
Comparison of Spoil Piles with 27 Meter (90 Foot)
and 37 Meter (120 Foot) Crest-to-Crest Spacing
At one western mine in 1975, for instance, pit width was 120 feet
and spoil grading costs were estimated to be $3, 954 per hectare ($1, 600
per acre). The average coal seam thickness was 4. 6 meters (15 feet),
yielding a grading cost of 5-1/2 cents per metric tonne of coal (six cents
per ton). It is estimated that narrowing the pit to 27 meters (90 feet)
would result in a grading cost reduction of 2-3/4 cents per metric tonne
(three cents per ton) of coal produced.
This modest decrease in grading costs must be balanced against the
increased non-productive dragline walking time and possible decrease in
spoil stability that may result when the pit is narrowed. The former effect
results from the fact that, for a given area to be mined, the number of pits
and dragline digouts required increases as the pit width is decreased.
This in turn increases the non-productive movement time for the dragline.
Additionally, as the pit is narrowed, the base of the spoil pile is also
narrowed and, although spoil instability is not a major problem at western
mines, the stability of the spoil pile may be decreased.
Coal recovery percentages are also affected by the choice of pit
width in that, in each pit, a wedge or "fender" of coal is usually left in
place at the toe of the spoil pile, as shown in Figure 34, either to buttress
the spoil toe or because clean coal cannot be loaded near the spoil toe. The
height and width of the wedge depend on the coal seam thickness and the
position of the spoil toe. If the pit is narrow, it may be possible to keep
the spoil toe low on the coal seam in the highwall as shown in the top section
view in Figure 34. The height of the coal wedge would then be corres-
pondingly low. For a given overburden depth, as the pit is widened, the
spoil toe will generally ride higher on the coal in the highwall, necessitating
65
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COAL: '
WEDGE
L NARROW PIT
COAL
WEDGE
2. WIDE PIT
Figure 34. Section View Showing Coal Wedge
a higher coal wedge. Of course, although in a given pit the size of the
wedge may increase as the pit width is increased, the number of pits and
thus the number of wedges decreases.
Operators of western mines generally estimate losses of coal in
wedges as a function of the pit widths. An example is a mine in Montana
at which the average coal seam thickness is 4. 6 meters. The average coal
loss for a pit width of 33. 5 meters (110 feet) is 1. 7 percent; for a pit
width of 45. 7 meters (150 feet) it is 2. 3 percent. The higher within-pit
coal losses for the wider pits are counterbalanced to some extent by the
reduction in total number of pits required.
Most mine operators are interested in finding ways to reduce the
losses in coal wedges. In fact, in Montana, this is required by law. One
solution, observed in use at a field survey mine, is to use a backhoe to
recover the coal wedge after the main part of the seam has been loaded out
by shovel. The left-hand side of Figure 35 shows the appearance of an
area from which the coal wedge has been removed. The wedge is still in
place at the right-hand side of that Figure.
The length of the digout also affects production and spoil grading
costs. If the digout is long, the spoil is placed in conical piles and the
resulting spoil ridge line as viewed from the highwall is undulating. In
such cases, before spoils can be graded by pushing material into the vee's
between adjacent spoil piles, dozers must be used to create a fairly level
spoil crestline from which they will subsequently work to push material
into the vee's. Nonetheless, some mine operators prefer long digouts
66
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Figure 35.
Photograph Showing Removal of Coal
Wedge and Subsequent Collapse of Spoil
because they reduce the total number of digouts required and the non-
productive sideways and diagonal movement of the dragline.
The alternative is to shorten the digout so that, rather than being
placed in conical piles, the spoil is placed so that the crestline is fairly
even, similar to the crestline for spoils placed by a stripping shovel. In
this case, it is fairly easy to construct a road on the crestline from which
spoil will be pushed into the vee's. Although use of the shorter digout does
therefore reduce spoil grading costs, total non-productive sideways and
diagonal movement time for the dragline is increased.
This latter disadvantage is frequently offset to some degree by a
decrease in digging time for the dragline. This is because, although the
bucket can usually be filled in two or three bucket lengths, conventional
practice is to continue pulling the bucket in to the machine fairleads to tip
the bucket upward before hoisting, thus preventing material from spilling
from the bucket when it is hoisted. This practice is depicted in Figure 36.
By shortening the digout, the average distance that the filled bucket is
dragged before hoisting is reduced; thus overall cycle times are also
reduced. In one instance known to the authors, the digout was shortened
to reduce spoil grading costs and dragline productivity increased by two
percent. *
Use of a patented device known as the Miracle Hitch enables hoisting of the
loaded bucket at any point without substantial spillage, thereby eliminating
the practice of dragging the filled bucket into the fairleads before hoisting.
67
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Roll
Position
required
to hold load
Bucket filled by this point
Figure 36. Section View Showing Dragging of Filled
Bucket Before Hoisting to Prevent Spillage
The foregoing discussion has illustrated some interrelationships
between pit width and digout length on one hand and spoil grading costs and
timeliness on the other. It may be possible through consideration of
those relationships in choosing pit widths and digout lengths to "integrate"
mining and reclamation decisions. Overall, however, the effects of
those choices on reclamation performance are seen to be modest,
particularly where the coal seams being mined are thick.
There is another,'potentially more significant way to integrate
mining and reclamation practices, however. It involves selective
removal of overburden and placement of spoil materials to ensure that the
"best" spoil materials are placed on or near spoil surfaces, or below the
water table, if any exists above the pit floor. In conventional practice,
where special spoil placement methods are not used, overburden materials
are inverted on the spoil piles. Thus surface overburden materials are
placed near the pit floor and overburden materials nearest the coal seam
are placed on the surfaces of the spoil piles. This is frequently undesirable
from an environmental standpoint.
One reason is illustrated in Figure 37 which shows the sodium
adsorption ratio (SAR) vs. overburden depth for a core drill sample from
a surface coal mine in North Dakota. In this case, the SAR value increases
with overburden depth and reaches the critical value of ten at a depth of
about six meters (20 feet). The SAR value for the overburden stratum
immediately above the coal seam is 45, indicating that significant perme-
ability, and, therefore reclamation problems would result if the material
was placed on the surfaces of the spoil piles, as would be the case if
selective placement procedures were not used -- and often in practice
they are not. In fact, old unvegetated orphan spoils in several of the
western states are testimony to the spoil permeability and vegetative
cover problems that have resulted from placement of high-SAR materials
on spoil surfaces. *
There are also old orphan spoils which have a good cover of volunteer
vegetation but in some cases the spoils resulted from stripping of shallow
overburden which may not have had high SAR values.
68
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SODIUM ADSORPTION RATIO (SAR)
0
10
20
30
OVERBURDEN 40
50
60
70
80
90
100
110
DEPTH
(FT.)
10
20
30
40
50
LIGNITE
TZJ
LIGNITE
Figure 37.
SAR Analysis of Core Drill Sample
From North Dakota Mine
Of course, to a great degree, potential revegetation problems due
to impermeable surface spoil materials are reduced through spreading of
topsoil on spoil surfaces, but the topsoil depths required to prevent contact
of plant roots with the underlying impermeable spoil layer may be fairly
large. In North Dakota, for example, the law requires replacement of up
to 1-1/2 meters (five feet) of topsoil to prevent occurrence of this problem.
In Montana, undesirable spoil materials must also be buried under a
minimum of 1-1/2 meters of acceptable material. In such instances,
selective placement of spoil materials could result in a reduction of
necessary topsoil replacement depths. Although selective spoil placement
practices in dragline stripping situations are not widely used in the west,
they are widely used in the midwestern United States, indicating that such
practices are both technologically and economically feasible.
One method for ensuring that the overburden strata immediately
above the coal seam are buried in the spoil pile is illustrated in Figure 38
which depicts stripping of 27 meters (90 feet) of overburden by a dragline
with a 72 meter (235 foot) boom at 30-1/2 degrees and a 23 cubic meter
(30 cubic yard) bucket. It has been assumed that the lowest six meters
(20 feet) of the overburden consist of undesirable material which must be
buried in the spoil.
An operational procedure that can be used to achieve this objective
is as follows. First, after removal of 1-1/2 meters (five feet) of topsoil
by scraper, the top 20 meters (65 feet) of the overburden are excavated
and cast into the open pit at an average swing angle of 82 degrees from the
digging position. Next, the lowest six meters of the overburden are
excavated, and the boom is swung through an average angle of 120 degrees
to place the material ahead of the main spoil pile near the pit floor. This
procedure is known as leading the spoil.
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<,CLfiY \ 4
2O' TOXIC 3TRflTf) ,
ELEVATION
Figure 38. Illustration of Selective Spoil Placement
Procedure in Single Seam Dragline
Stripping Situation
70
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Selective digging of the undesirable overburden material results
from a digging procedure known as layer loading, a practice in which
overburden is excavated by dragging the bucket parallel to the overburden
strata. This practice is not only feasible, it is also desirable from a
production standpoint because it minimizes digging time. An exception is
digging of sandstone strata, in which case layer loading may be difficult.
The foregoing selective spoil placement procedure, although both
feasible and desirable, has an adverse effect on dragline productivity.
This is because the swing angle required to lead the undesirable spoil
material is greater than that which would be required if the selective place-
ment procedure was not used. A production estimate for the selective
placement case is shown in Table 10. Assuming a three meter (10 foot)
coal seam, the estimated average annual production in 18 to 27 meters of
overburden (60 to 90 feet) cover is one million metric tonnes (1, 125,000
tons) of coal. *
TABLE 10. PRODUCTION SUMMARY FOR SELECTIVE
SPOIL PLACEMENT ILLUSTRATION
Walking Dragline (Marion 7620) 235' boom with a 30 cubic yard bucket,
cut 125' wide in 90' overburden cover and a 100' digout.
_ . Swing Swings Bank Cu. Yds.
Component Angle per Hr Bucket Carry
Topsoil (Move by scrapers)
Sand, Clay ^ ^ &
fe Gravel
Toxic Strata 120 57 21
Total
Moved by dragline
Operating time for a 100' digout 80%
Delays 20<7o
Scheduled time
Cubic yards per digging hour
Cubic yards per month (576 hours)
Cubic yards per year (bank)
Mining ratio (cubic yards per ton)
Tons per year
Tons per year at 60' cover overburden (ratio = 6.67)
Annual average tons in 60' to 90' cover
Bank Time
Cubic Yards Hours
2,315
30,093 22.47
9,260 7.73
41,668
39,353
30.20
7.55
37.75
1,303
751,000
9,007,000
10
900, 000
1,350,000
1, 125,000
*In this example, 27 meters (90 feet) has been assumed to be the maximum
overburden depth.
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Estimated costs for this case are presented in Table 11. Highlights
from that Table are summarized below:
Capitalization: $10 million.
Stripping and grading cost: $1. 09 per metric tonne
($1. 20 per ton).
Total mine operating costs: $2.23 per metric tonne
($2.46 per ton).
After-tax income: $0. 32 per metric tonne ($0. 35 per
ton).
After-tax net cash flow: $0. 72 per metric tonne
($0. 80 per ton).
The effect on production of use of the selective placement proce-
dure is shown in Table 12. If all spoil was placed non-selectively, the
average swing angle for the dragline would be 85 degrees, where, in
contrast, the average for the selective placement case was 91 degrees.
This reduction of swing angle would result in a two percent increase in
the amount of overburden moved and coal exposed each year. Overall
stripping costs per unit of coal produced would also decrease, in this
example from $1.09 per metric tonne ($1. 20 per ton) to about $1.07 per
metric tonne ($1. 18 per ton). Thus the estimated incremental cost to
selectively place overburden in this case is roughly two cents per metric
tonne of coal produced. Based on an average stripping ratio of 6. 7 cubic
meters per metric tonne (eight cubic yards per ton), this is equivalent
to an incremental overburden removal cost of much less than one cent per
meter (one cent per cubic yard).
The cost of moving topsoil has been reported by operators of
western mines to be $0. 37 to $1.11 per cubic meter ($0. 25 to $0. 85 per
cubic yard) of topsoil replaced. * Thus, in the present example, if
selective spoil placement would enable reduction of required topsoil
replacement depths, reclamation costs might also be reduced.
Of course the foregoing example is just one of many possible
situations. For example, at one mine at which the average overburden
depth is 24 meters (80 feet), soil and overburden analyses indicated that
the top six meters (20 feet) of the overburden excluding topsoil were the
least desirable of all the materials in the bank. Burial of such material
in the spoil presents no problem unless, to prevent possible contamina-
tion of groundwater, the materials cannot be placed on or near the pit
floor. Even in this case, however, the required placement of high bank
(or low bank) overburden materials in the middle of the spoil profile is
The upper end of the cost range generally applies where topsoil salvaging
is contracted.
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TABLE 11. COST ESTIMATE FOR SELECTIVE
SPOIL PLACEMENT EXAMPLE
Walking Dragline (M 76ZO)
Cubic yards moved per year in 60' to 90' overburden 9,000,000
Tons of coal produced annually, 10' seam @ 90% recovery 1, 125,000
Average Mining Ratio 8. 00
CAPITAL COSTS
1- Walking dragline, 235' boom, 30 cu. yd. $ 5, 300,000
1- Coal loader (M 151) 900,000
1- Cat. 992B, F. E. L. 200,000
2- Cat. D8 dozers + 1 Cat. 633C scraper 450,000
1- Cat. 16 patrol grader 125,000
3- Coal haul trucks (120 ton) 450,000
1- Coal dump hopper, crusher and car mover 400,000
1 - Railway lead and hold yard 500, 000
1- Building and miscellaneous equipment I, 675, OOP
Total $10,000,000
MINE OPERATING COSTS PER TON
Stripping
Labor $0.52
Repairs and Supplies 0. 32
Power 0, 16
Reclamation 0.20
Total 1.20
Coal loading and shooting 0. 10
Coal hauling (2 miles) 0. 12
Crushing and loading RR cars 0. 10
Supervision and office 0. 16
Administration and general 0. 06
Western Miners Union royalty 0.40
Interest on one-half of the capital costs 0. 18
Miscellaneous 0. 14
TOTAL $2.46
SALES REVENUE ($0. 35 per mm BTU)
Royalty
REALIZATION NET
Mine Costs
Depreciation
Depletion
Net for income taxes
North Dakota income tax 4%
Federal income tax
INCOME PER TON
Add back
NET CASH FLOW $0. 80
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TABLE 12. PRODUCTION SUMMARY FOR NON-SELECTIVE
SPOIL PLACEMENT ALTERNATIVE
Walking Dragline (Marion 7620) 235" boom with a 30 cubic yard bucket,
cut 125' wide in 90' overburden cover and a 100' digout
_ Swing Swings Bank Cu. Yds. Bank Time
Component Angle Per Hr. Bucket Carry Cubic Yards Hours
Topsoil (Move by scrapers) 2, 315
85ฐ 61'5 21'5 39.353 29.54
Total 41,668
Moved by dragline 39, 353
Operating time for a 100' digout 80% 29.54
Delays 20% 7.38
Scheduled time 36. 92
Cubic yards per digging hour 1, 332
Cubic yards per month (576 hours) . 767,000
Cubic yards per year (bank) 9, 205, 000
Mining ratio (cubic yards per ton) 10
Tons per year 920, 000
Tons per year at 60' cover overburden (ratio = 6. 67) 1, 380, 000
Annual average tons in 60" to 90' cover 1, 150, 000
feasible and can often be achieved without appreciable cost increase. It
may be costly, however, where the dragline operates from a position on
or near the natural ground surface. In such cases, it would be advisable
to revise operation procedures so that the dragline operates from a bench,
as described next.
Two-Lift, One Pass Stripping Without Rehandle
In some cases it is impossible or undesirable to work the dragline
from the top of the ground, so instead the machine works from a bench
that is cut some distance below the ground surface. There are several
possible reasons for operating from a bench. One is to keep the dragline
at a constant elevation above the coal even though the terrain may be
rolling. Another, particularly applicable in glaciated areas, is to provide
a working surface for the dragline on bedrock. In wet weather, this
reduces problems that might otherwise result from walking a large drag-
line through mud. A third reason is to prevent casting of unconsolidated
surface overburden materials on the pit floor. This is sometimes
necessary to minimize spoil instability problems.
74
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At large mines, the dragline is usually used to cut its own bench.
The operating procedure involves making two overburden lifts in one pass
of each pit. First, working from the machine-supporting bench, the over-
burden beneath the bench is excavated and spoiled in conventional fashion.
Then the bench for the next pass -- not the next digout -- is excavated to
the side of the bench from which the dragline operates. This is accomplished
by turning the boom in the direction of the next pit, as shown in the left-hand
portion of Figure 39, and excavating overburden above the dragline bench.
Then, as shown in the right-hand portion of Figure 39, the boom is turned
through 180 degrees and the spoil is cast on top of the existing spoil pile.
An artist's conception of this procedure is shown in Figure 40.
PLAN VIEW
1 DRAGLINE DIGGING UMCONSOLIDAT E D OVERBURDEN
PLAN VIEW
2 DRAGLINE DUMPING UNCONSOLIDATED OVf BBURDEN
wstm ^-:-:^----
SECTION
SECTION
Figure 39. Plan and Section Views Showing
Side-Benching Procedure
*In practice the average swing angle in this operation is usually about
135 degrees.
75
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Figure 40. Artist's Conception of Side-Benching Procedure
The bench from which the dragline operates is called the
established bench; the bench which is cut to the side for the next pass is
called the side bench. The side bench is used extensively for production
reasons at mines in the midwestern United States but is infrequently used
at western mines. This is because in the west precipitation is low and
soils are shallow. Thus problems with spoil instability and movement of
draglines through deep mud are infrequent. But although, historically,
the side bench method has evolved principally as a solution to production
problems, it also has environmental advantages that may make it desirable
for more widespread use in the west.
Where the surface overburden materials are better plant-growing
media than lower bank materials, the advantage is that the former
materials can readily be placed on the surfaces of the spoil piles. In
fact, in the midwestern United States this is one of several reasons for
the successful revegetation efforts that characterize parts of the region.
There is a potential drawback, however, in that the underlying spoil
materials, which may be undesirable, are sometimes exposed during
grading of the spoil piles. A case where this might happen is illustrated
in the top half of Figure 41 which shows the appearance of side bench
spoils for a situation in which the overburden depth is 30 meters (100 feet)
and the bench depth is three meters (10 feet). In this instance, it is
likely that spoil materials immediately underlying the side bench spoil
76
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Side Bench Material
1. Overburden Depth = 100 feet
Bench Depth = 10 feet
Side Bench Material
2. Overburden Depth = 100 feet
Bench Depth = 30 feet
Figure 41. Comparison of Spoil Profiles in
Two Side-Benching Situations
will be exposed during spoil grading. In practice, however, this potential
problem can be remedied by rehandling of spoil materials by the dozers
during grading to ensure that the side bench materials remain on the
surface after grading.
Another possible solution, shown in the lower half of Figure 41, is
to make the dragline bench deeper to increase the depth of the side bench
materials on the spoil piles. In this illustration, it is virtually impossible
to expose the underlying spoil materials during grading.
The potential reclamation advantages of the deeper dragline bench
notwithstanding, however, many mine superintendents are opposed to its
use. There are several reasons for this. One is that the swing angles
required for placement of side bench materials are very large. Thus, for
a given overburden depth, the deeper the bench, the greater the average
swing angle on each digout. Additionally, although all draglines were once
used in this manner, many dragline operators dislike digging overburden
high above the bench. Further, if the bench is very deep, it may be
difficult to fully load the bucket during the side-bench digging cycle. One
reason for this is the fact that, when the bucket is dropped from above onto
the surface of the overburden, a procedure known as "chopping, " the
77
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bucket jaw plates hit the surface before the teeth; thus bucket penetration
may be poor. A typical bucket is shown in Figure 42. It is seen in that
Figure that the jaw plates protrude far beyond the teeth.
A second reason for difficulty in filling the bucket in overhead
digging is that some of the material to be excavated moves out ahead of
the bucket and falls down onto the dragline bench. This phenomenon,
called "chasing the dirt, " in combination with reduced bucket penetration
in chopping, often means that bucket fill factors are lower in side-
benching than in conventional digging.
Where the overburden is not extremely deep or spoil rehandle is
not required, or both, there are few production advantages to offset the
foregoing disadvantages. As a result, if the side bench is not required
for production reasons, and in the west it often is not, then mine operators
will be very reluctant to use the method, or, if they do use it, they'll
carry the bench high in the bank, thus minimizing potential reclamation
advantages. For reasons presented later in this section, deep overburden
or the necessity to rehandle spoil may soften this reluctance.
In the situations discussed thus far, it has been assumed that
surface overburden materials beneath the topsoil are best for placement
on spoil surfaces and that the strata immediately above the coal seam are
the worst. This is not always the case. At one mine, for example, the
top six meters (20 feet) of a total of 24 meters (80 feet) of overburden have
been determined to be the least desirable materials for placement either
Figure 42. Photograph Showing a Typical Dragline Bucket
78
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on spoil surfaces or on the pit floor. Thus it is necessary to place the
surface overburden materials in approximately the middle of the spoil
profile. This is an unusual situation and, at first glance, one for which
there may not be a simple solution.
But there is one. It involves working the dragline from a bench six
meters (20 feet) below the ground surface. Then, on a given digout, a
depth of about 12 meters (40 feet) of overburden is excavated below the
bench by layer loading and is cast onto the floor of the adjacent cut. This
is the material shown as #1 in Figure 43. Next the side bench is cut from
the top six meters (20 feet) of overburden and the material is cast onto the
existing spoil. Finally, the remaining six meters (20 feet) of overburden
below the bench, that is, immediately above the coal, are excavated by
layer loading and cast on top of the side bench spoils. In this manner, the
side bench material is buried in the middle of the spoil pile.
If in addition it was necessary to keep the three or so meters of
overburden immediately above the coal from being placed on the top of the
spoil piles, this too could be accomplished by leading that material ahead
of the main spoil and placing it low in the spoil profile.
Summarizing, in single seam dragline stripping, it is generally
possible to achieve the required degree of spoil placement control if the
side bench procedure is used. Use of those procedures is more costly
than use of conventional sidecasting procedures. To the best of the
authors' knowledge, although it would be a fairly straightforward task, no
one has tried to determine those costs for a range of operating situations.
Single Seam Stripping With Spoil Rehandle --
It is desirable in surface mining to handle overburden materials
only once. In practice, where draglines are used to strip deep overburden,
rehandling of some material, typically 20 to 30 percent of the bank
material, is sometimes required. The need to rehandle spoil affects the
choice of pit width and digout length, and often affects the capability to
selectively place spoil material, usually for the better.
The maximum depth of overburden that can be excavated and side-
cast without the necessity for subsequent spoil rehandle is a function of
the operating procedure, the dragline dumping radius, the spoil repose
and highwall angles, and the amount of swell of the spoil. A mathematical
expression of that function is given below for the case in which the drag-
line operates from the surface of the ground:
d = e - ฐ-25 w (1)
a cot0+ (1+f) cot 9 { '
where
= maximum depth of overburden that can be excavated and
spoiled without rehandle (meters).
79
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PLAN
1ST
ELEVATION
Figure 43. Procedure to Bury Surface Overburden
Strata in Middle of Spoil Profile
80
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e = effective spoil radius of the dragline, defined below
(meters).
w = pit width (meters).
<ฃ = angle of the highwall from the horizontal (degrees from
horizontal).
0 = natural repose angle of the spoil from the horizontal
(degrees from horizontal).
f = swell fraction for the spoil (decimal).
The effective spoil radius of the dragline is defined as the horizontal
distance from the top edge of the highwall to the crest of the final spoil pile,
when the dragline is positioned as close to the highwall edge as possible.
Generally this is a position in which the rim of the dragline tub is about
three meters (10 feet) from the edge of the highwall. Mathematically, the
following expression applies:
e = r - a - 3 meters (10 feet) (Z)
where
r = dragline dumping radius; the horizontal distance from the
centerline of the tub to the centerline of the point sheave
(meters).
a = the radius of the tub (meters).
In the west in 1975, the following averages applied:
Overburden depth: 24 meters (80 feet).
Pit width: 43 meters (140 feet).
Effective spoil radius: 61 meters (200 feet). This
corresponds in practice to a boom length of about
79 meters (260 feet).
Spoil swell factor: 0. 30.
Highwall angle: 75 degrees from horizontal.
Spoil repose angle: 37 degrees from horizontal.
The maximum no-rehandle depth for these conditions is approximately
25 meters (83 feet). Figure 44 is a graph showing the general relation-
ship between maximum no-rehandle depth and dragline boom length. *
The graph shows theoretical values for which pit curvature and variations
in overburden depth within a given pit have not been considered.
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Maximum
Overburden
Depth for a
No-Re handle
Operation
(Feet)
120
100
Notes
Highwall angle: 75ฐ
Spoil repose angle: 37
Spoil swell: 28%
Pit Width = 100 ft.
Pit Width = 140 ft.
260
100 200 ~" 300
Dragline Boom .Length (Feet)
Figure 44. Maximum No-Rehandle Depth
vs. Dragline Boom Length
In practice, because mining usually begins along the coal seam
cropline in shallow overburden, average overburden depth increases
steadily from cut to cut. When the maximum no-rehandle depth is
reached, the pit is generally narrowed to increase that depth. In the
preceding example, for instance, narrowing the pit from 43 meters to
27 meters (140 feet to 90 feet) would increase the maximum no-rehandle
depth from 25 meters (83 feet) to 27 meters (89 feet). When the pit has
been narrowed to its practical minimum, the toe of spoil is allowed to
ride up the highwall to the top of the coal seam. This procedure further
increases the maximum no-rehandle depth.
Eventually, however, it becomes necessary at most mines to
extend the effective spoil radius of the dragline to enable stripping of still
deeper overburden. The method used to accomplish this, known as the
extended bench method, was patented by Weimer and Mullins in 1941. A
typical extended bench procedure is depicted in Figure 45. Working from
an established bench, the first activity on the dig out is to dig the keycut.
But instead of casting the keycut spoils at relatively small swing angles
as in conventional practice, the keycut spoils are swung at an angle of
about 130 degrees from the digging position, and cast against the existing
highwall in the open cut. This spoil, which is then leveled by dozers,
extends the dragline bench out into the open cut. Eventually, on each
digout, the dragline will be moved out sideways onto this extended (spoil)
bench to complete the digout. This includes excavation or rehandle of the
extended bench material from the previous digout. The rehandle material,
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UNCONSOL1DATED
MATERIAL
PLAN VIEW
^-UNCONSOLIDATED
f MATERIAL
SECTION
Figure 45. Method Used to Extend the Dragline Bench
83
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shown cross-hatched in Figure 45, is the last material handled on each
digout, so it always becomes the surface spoil material.
The environmental relevance of the extended bench method, which
is a one-pass, two-lift method, is that overburden can be excavated and
spoil placed very selectively through proper choice of the bench depth and
the material used to extend the bench. In practice, for reasons presented
previously, many mine superintendents prefer shallow (high) benches.
This preference may not always be optimal, either from production or
environmental standpoints. Additionally, although standard practice in
the west is to use keycut material to extend the bench, use of other
material would sometimes be better as a means of achieving reclamation
objectives. Briefly, the recommended practice would be to determine the
best material for placement on the surfaces of the spoil piles and then to
use that material to extend the bench.
An example of a procedure proposed for use at one mine involves
mining of a single coal seam by removal of 38 meters (125 feet) of over-
burden using a large walking dragline. In this case, maximum overburden
depth for no rehandle is 27 meters (88 feet) with a 46 meter (150 foot) pit
width. The overburden consists of clay and hard shale. Spoil must be
placed selectively in two ways. First, the spoil placed on the pit floor as
the first activity in the digout must be the hard shale, which is lower bank
material. This is because placement of incompetent clay materials on the
pit floor as the spoil pile base would probably result in instability of the
pile after additional spoil had been placed on top of the clay. The second
requirement is that the top 7. 6 meters (25 feet) of the overburden be placed
on the tops of the spoil piles.
Plan and section views of the pit are shown in Figure 46. The
operating procedure is as follows. The dragline works from an established
bench which is 15 meters (50 feet) below the ground surface, roughly at
the interface of the clay and shale strata. On each digout, material below
the bench is excavated conventionally. Side bench material, above the
elevation of the established bench, is dug by chopping, as previously
described.
The first activity on each digout is to make the keycut below the
bench. Since all of the keycut material is competent hard shale, it is
suitable for placement on the pit floor. This is accomplished by swinging
the keycut material through an angle of about 120 degrees from the digging
position and placing it on the pit floor, ahead of the main spoil pile. The
keycut spoil thus forms the spoil-stabilizing "buckwall11. It is not used to
extend the bench, rather the toe of the keycut spoil pile intersects the
bottom of the coal seam at the highwall. Next, the remaining overburden
below the elevation of the bench is excavated and cast on top of the buckwall.
The third activity on each digout is to dig ahead and to the side and
excavate surface overburden materials by digging above the bench. Those
materials, shown as #3 in Figure 46, are used to extend the dragline bench.
The swing angles required in this phase are fairly small because the boom
is swung counterclockwise from the bank to the extended bench.
84
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PLAN
SECTION
Figure 46. An Extended Bench Procedure with
Selective Spoil Placement
85
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Subsequently the remaining side bench material, #4 in Figure 46, is
excavated and cast on the main spoil pile.
As the last activity on each digout, the dragline is moved sideways
onto the extended bench for that digout, and the extended bench material
from the previous digout is excavated and cast onto the top of the spoil pile.
This material was originally the surface overburden material. The
rehandle percentage in this example is 18 percent.
A production estimate for this example is presented in Table 13.
The effect on production of the swing angles required for various compon-
ents of the digout can be guaged from the swings per hour for each
component. In this case, for the bench depth shown, 18 percent rehandle
would be required regardless of the spoil placement method used. Rehandle
in this case is thus necessitated by overburden depth, not by reclamation
requirements.
TABLE 13.
PRODUCTION ESTIMATE FOR EXTENDED BENCH
PROCEDURE WITH SELECTIVE SPOIL PLACEMENT
Extended Bench Method Using Walking Dragline (B-E 2570),
335' boomฎ 35ฐ with 110 cu. yd. bucket, pit 150' wide by a
maximum of 125' overburden cover and 100' digout.
_ . Swing Swings
Component Anglป pgr ^
0
Q
Q
Q
Keycutto o 5Q
Buckwall
Lower lift 8Q 58
to spoil
Surface to 6Q ?0
Bench
Upper lift
to spoil
Bank Cu. Yds.
Bucket Carry
56
59
81
81
Bank Time
Cubic Yards Hours
10,416 3.72
31,250 9.13
12,543 2.21
15,235 3.55
Rehandle
to Spoil
70
70
Operating time for a 100' digout
Delays - Variable due to many conditions
Scheduled time
Cubic yards per digging hour
Cubic yards moved per month (540 hours)
Bank cubic yards moved per month
Bank cubic yards moved per year
Ratio at overburden of 125'
Tons per year
99
(75%)
(25%)
(100%)
69,444 - 82% 18.61
15,296 - 18 2.21
84,740 - 100%
20.82
6.94
27.76
4,070
2,198,000
1,802,000
21, 163,000
4.6
4,600,000
86
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The foregoing examples were intended to illustrate that, contrary
to folklore, selective removal of overburden and placement of spoil can be
accomplished in very deep overburden where a dragline is the prime
mover. In some cases, however, the ability to place surface overburden
materials on the surfaces of the spoil piles may be contingent upon the use
of a fairly deep bench. This may be resisted by mine operators because
of the large swing angles required for casting of side bench materials and
the need to chop out overburden above the bench.
In deep overburden, however, where spoil rehandle is required,
the deep bench has certain notable advantages. The major one is that, for
a given operating situation, the rehandle percentage decreases as the
bench is deepened. This is illustrated in Figure 47 for a situation in
which average overburden depth is 30 meters (100 feet). As the Figure
shows, if the bench depth is six meters (20 feet), the rehandle percentage
is 30 percent. Deepening the bench to 50 feet reduces the rehandle to
14 percent.
An additional advantage of the deep bench in deep overburden is the
improved position of the dragline for digging strata below the bench. For
a given dragline and type of overburden strata, there is an optimal digging
depth. Below that depth, productivity declines somewhat because of
difficulties in filling the bucket when digging far below the bench. This is
illustrated in Table 14, which shows the effect of bench height on producti-
vity for several swing angles. [20] The reason for difficulties in filling
the bucket is that the vertical component of the drag force increases as
the digging depth increases. Beyond some optimal depth this is undesirable
because the vertical force becomes so large that it tends to pull the bucket
out of the bank before the bucket has been fully loaded.
In a specific situation, the advantages and disadvantages of different
bench depths must be weighed. Often, the disadvantages of the deep bench
will outweigh its advantages. On the other hand, there are some situations
in which a fairly deep bench, about one-third to one-half of the distance
from the surface of the bank to the coal, appears to be the best choice.
An example is the case where the upper half of the bank consists of
unconsolidated material and the lower half consists of material that is
hard to dig, such as sandstone or limestone.
In light of the apparent reclamation advantages of the deeper bench,
if selective overburden removal and spoil placement capability is needed
on a large scale in deep western stripping, then a systematic evaluation of
the effects of bench depth on productivity and reclamation performance
might be warranted.
Multiple Seam Dragline Stripping Procedures
Mining of two or more coal seams in a given pit accounted for
roughly half of the acreage disturbed by western mining in 1975. Most
often these situations involved uncovering of two coal seams by a single
dragline. From a production standpoint, multiple seam mining is more
complex than single seam mining and much could be said here about
87
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HIGH BENCH (30% P-EHANDLE)
Extended bench
(Rehandle)
1. 100 foot overburden depth, 20 foot bench depth.
Rehandle = 30%
DEEP BENCH (14% F.EHANDLE)
Extended bench
(Rehandle)
2. 100 foot overburden, 50 foot bench depth.
Rehandle = 14%
Figure 47. Comparison of Spoil Rehandle Percentages
for Two Bench Depths
88
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TABLE 14. EFFECT OF BENCH HEIGHT AND SWING ANGLE
ON DRAGLINE OUTPUT [20]
Percent of
Most Favorable
Cut Depth
20
40
60
80
100
120
140
160
180
200
Swing Angle
30ฐ
1.06
1.17
1.24
1.29
1.32
1.29
1.25
1.20
1.15
1.10
45ฐ
0.99
1.08
1.13
1.17
1.19
1. 17
1. 14
1. 10
1.05
1.00
60ฐ
0.94
1.02
1.06
1.09
1.11
1.09
1.06
1.02
0.98
0.94
75ฐ
0.90
0.97
1.01
1.04
1.05
1.03
1.00
0.97
0.94
0.90
90ฐ
0.87
0.93
0.97
0,99
1.00
0.985
0.96
0.93
0.90
0.87
120ฐ
0.81
0. 85
0.88
0.90
0.91
0.90
0.88
0.85
0.82
0.79
150ฐ
0.75
0.78
0.80
0.82
0.83
0.82
0.81
0.79
0.76
0.73
180ฐ
0.70
0.72
0.74
0.76
0.77
0.76
0.75
0.73
0.71
0.69
current operational procedures and ways in which they can be improved.
But from the standpoint of the effect of western mining on the environment,
only two differences between single and multiple seam mining are relevant
and significant. They are the following:
Frequently the interburden material separating two
coal seams is physically or chemically undesirable
and must be buried in the spoil to ensure successful
reclamation. This is the case, for example, where
the interburden has a high sodium adsorption ratio.
In cases in which spoil rehandle is not required for
production reasons, it may be difficult or costly to
bury the interburden material.
Three alternative multiple seam mining methods are used at
western mines. The choice of method is dictated by overburden, inter-
burden, and dragline characteristics. Each involves making two passes
in a given pit, one to uncover the top seam and the second to uncover the
bottom seam. The methods differ primarily in the position of the dragline
on the second pass of each pit. That position in turn is determined by the
dumping radius and dumping height of the dragline in relation to require-
ments imposed by the overburden and interburden depths.
89
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Multiple Seam Stripping Without Rehandle --
The most desirable situation from a production standpoint, although
often least desirable from an environmental standpoint, is one in which
two seams are stripped without appreciable spoil rehandle. The proce-
dure, as used at a particular western mine several years ago, is illustrated
in Figure 48. On the first pass, the dragline works from the natural
ground surface and overburden is excavated and cast conventionally,
exposing the upper coal seam. The coal is loaded out fairly closely behind
the stripping operation.
After this first pass of the pit, the dragline is deadheaded back to
the opposite end of the pit the end from which the upper coal seam has
been loaded out -- and is moved down a ramp to the top of the interburden.
Now the second pass begins, with interburden materials excavated conven-
tionally and cast on top of the spoil pile that resulted from first-pass
stripping. This exposes the lower coal seam, which is loaded out close
behind the interburden stripping operation. At the end of the pit, the drag-
line is ramped up to the ground surface and deadheaded back to the opposite
end of the pit to begin the first pass in the next cut.
FIRST PASS
SECOND PASS
Figure 48. Multiple Seam Situation With No Spoil Rehandle
90
-------
The feasibility of this method is dependent on the following two
conditions:
The dumping radius of the dragline must be sufficient
to enable placement of all overburden and interburden
materials without the use of an extended bench.
The dumping height of the dragline must be sufficient
so that, when the dragline is positioned on the inter-
burden, it is possible to cast all interburden material
on top of the spoil pile that resulted from the first
pass.
In practice, it is rare for both of these conditions to hold true and,
as a result, the operating method just described is rarely used. When it
is, the interburden material always ends up on the top of the spoil piles.
To the best of the authors' knowledge, there is no way to prevent this
without incurring some spoil rehandle.
In the actual case under discussion, during the early 1970's, the
interburden was placed on top of the spoil piles and, because of imperme-
ability of the resulting spoil surface, revegetation failures were common.
Subsequently, operating procedures were revised to enable burial of the
interburden material in the spoil pile. The procedure, which is depicted
in Figure 49, is as follows. On the first pass, on overburden, the
material is deliberately cast in close to the lower highwall. Otherwise,
the first pass operating procedure is identical to the method just described.
After deadheading and ramping down to a position on the interburden,
the interburden is excavated and cast on top of the spoil pile which resulted
from first-pass stripping. The last activity on each digout, however, is
to rehandle a portion of the overburden material that had been cast in
close to the lower highwall during the first pass, and to cast that material
on the top of the spoil pile. If the rehandle material is deep enough on the
angle-of-repose spoil piles so that the buried interburden material is not
exposed during grading, then reclamation objectives will have been
achieved.
Burial of the interburden material in this kind of situation can be
costly since rehandle which would not have been required otherwise is
required to meet reclamation requirements. If, however, spoil rehandle
is required for production reasons, as in the situations discussed next,
the effects of reclamation requirements on production costs may not be as
great.
Extended Bench Stripping of Two Coal Seams --
When the dumping height of the dragline is sufficient to enable
casting of all interburden materials on top of the spoiled overburden, from
a dragline working position on top of the interburden, but the dumping
radius is too small, then a two-pass extended bench method is used. The
first pass on overburden is similar to that described in the preceding
91
-------
FIRST PASS
SECOND PASS
Figure 49.
Multiple Seam Method With Rehandle to
Enable Burial of Interburden Material
section. On that pass, some of the overburden material is cast in close to
the lower highwall and, as shown in Figure 50, will subsequently be used
to extend the bench for the second pass. Ultimately, the extended bench
material will be rehandled and placed on top of the spoil piles.
After completion of the first pass, the dragline is deadheaded and
ramped down onto the interburden as in the previous example. Interburden
material is excavated and cast on top of the spoiled overburden.
Eventually, on each digout, the machine will be moved out onto the extended
bench to complete the digout. The last activity on each digout is to
excavate the material that formed the extended bench for the previous dig-
out, and to cast that material on the spoil pile. If this rehandle material
is suitable for reclamation and it is deep enough on the angle-of-repose
spoil piles so that the buried interburden material will not be exposed
during grading, then reclamation objectives will have been achieved.
Spoil rehandle is always required in use of this extended bench
method whether or not burial of the interburden material is required.
But the amount of rehandle incurred might be increased substantially by a
requirement for interburden burial. No specific data were determined for
92
-------
NO 1
SOIL REMOVAL AND KEYCUT
MADE IN OVERBURDEN
. ' .' f ',-. '.. .1 r\ it e . ',--.
' ฐ - ' 'c''''""'.' ' ' c '-'' '\ I'I f-' - '' "ซ ' "
&ป-:&yi:'-ฃ-^
::-..-. r--V :.'.-;. r'.y. :;;. '.-.: :' . ..-....\/i
NO. 2
REMOVAL OF REMAINING
OVERBURDEN
NO. 3
KEYCUIMADE IN INTERBURDEN
i^mmzm
..
>J
NO.
REMOVAL OF INTERBURDEN
AND EXTENDED BENCH
Figure 50. A Two-Pass Extended Bench Method
for Stripping of Two Coal Seams
93
-------
the west but for one midwestern mine at which acid interburden had to be
buried and topsoil was not replaced, 15 percent rehandle was required for
production reasons. In order to adequately bury the interburden and
ensure that it stayed buried after spoil grading, the rehandle had to be
increased to 40 percent.
The Horseshoe Method for Two-Seam Stripping --
Most frequently in practice, both the effective spoil radius and the
effective dumping height of the dragline must somehow be extended to
enable stripping of the lower seam. Extension of the effective dumping
height is required when the dragline does not have sufficient height, when
working from a second-pass position on the interburden, to cast all of the
interburden material on top of the spoiled overburden pile. The opera-
tional solution to this problem is to increase the effective dumping height
by raising the elevation of the bench from which the dragline will work
when stripping the interburden.
Although there are two different ways to do this, only one is widely
used. It is a two-pass method in which, on the first pass, the dragline
operates from the ground surface or a shallow bench. Overburden is
excavated conventionally and cast into the open pit at maximum effective
range. This usually means that the toe of the spoil pile thus made will
intersect the face of the lower coal seam at the highwall.
At the end of the pit, the dragline is moved over into the spoil pile
that resulted from the first pass and is deadheaded in the spoil to the
opposite end of the pit. During the deadheading operation the dragline,
assisted by a dozer, is used to knock the tops off of the first-pass spoil
pile and create a flat bench in the spoil pile. When the opposite end of the
pit has been reached, the dragline remains on the spoil bench that has just
been constructed and digs the interburden. (The upper coal seam has by
this time been loaded out at this end of the pit.) The elevation of the
machine on the spoil bench is roughly the same as the elevation of the
surface of the natural ground, thus working from the bench in the spoil
pile increases the effective dumping height of the dragline considerably as
contrasted with a machine position on the interburden.
The interburden is excavated by underbench chopping, a procedure
illustrated in Figure 51. Facing the highwall from a position across the
pit on the spoil bench, the dragline bucket is dropped onto the surface of
the interburden. Because the jaw plates on the bucket protrude beyond the
teeth, bucket penetration in this chopping operation is generally poor. As
the bucket is dragged toward the spoil pile for loading, some material rolls
out ahead of the bucket and falls into the vee between the lower highwall
and the spoil pile, thus bucket fill factors in this operation are fairly low.
When the digging cycle has been completed, the bucket is hoisted and
swung through an angle of about 135 degrees to cast the interburden
materials on top of the spoil bench from the previous cut. The lower coal
seam is loaded out behind the stripping operation.
94
-------
I LOWER COAL SEAM
Figure 51. Removal of Interburden by Underbench Chopping
When the end of the pit is reached, the machine is moved to the
overburden and is again deadheaded back to the opposite end of the cut to
begin the first pass on the next cut. Generally excavation of the over-
burden cannot begin until all of the lower coal has been loaded out of the
previous cut.
Dragline productivity in removal of the interburden depends on
many site-specific factors, but is always lower than productivity in
removal of the overburden. According to mine operators queried by the
authors in a recent nationwide survey of surface coal mining operations,
productivity in removal of interburden from a spoil bench ranges between
15 and 60 percent of the productivity for the same machine when used in
conventional overburden removal and spoil placement, depending on on-site
conditions. [17] A typical range for mine planning is 35 to 50 percent.
There are several reasons and partial remedies for the low
productivity. One is the need to construct a bench in the spoil pile for the
second pass in each pit. Another is the large average swing angle required
for placement of interburden material. A third is difficulty in crossing
inclines (coal haulage ramps) if the inclines enter the pit at other than the
ends of the pit. This problem can be avoided by having only one incline
and placing it at the end of the pit, but then the entire pit of lower coal
must be loaded out before removal of overburden in a subsequent cut can
begin.
A fourth reason is the low bucket fill factor in underbench chopping.
Frequently the interburden is blasted very hard to raise the fill factor
during chopping. Another partial solution is to use dozers and end loaders
to keycut the interburden so that the dragline bucket can be dropped into
the keycut to excavate the interburden. A further partial remedy, only
conceptual at present, is to design a special chopping bucket for use during
the interburden removal pass in each pit. This bucket might have longer
teeth, hinged jaw plates, or a modified center of gravity, to improve
penetration of the bank when chopping down from the spoil bench.
95
-------
Environmentally, the method may not be good if the interburden is
undesirable as a spoil surface material. This is because the interburden
material is usually placed on the surfaces of the spoil piles.
At many mines, each of the three foregoing methods is ordinarily
used at some time during the life of the mine. Initially, when the over-
burden is shallow, the two-pass method without appreciable spoil rehandle
can be used. Later, as overburden depth increases, the two-pass
extended bench method must be used to extend the effective spoil radius
of the dragline on the second pass. Finally, when the overburden gets
deep enough, the two-pass horseshoe method just described must be used
to extend both the effective spoil radius and the effective dumping height of
the dragline. For all three methods, particularly the last one, burial of
the interburden material in the spoil pile may be difficult and costly. The
magnitude of the problem has not been evaluated quantitatively, but it
would be fairly easy to do so for a wide range of operating conditions.
One-Pass Stripping of Two Coal Seams --
It is in some cases possible to improve both productivity and
selective overburden removal and spoil placement capability in two-seam
mining through use of a novel one-pass method. As a rule of thumb, from
a production standpoint, the method is superior to two-pass methods only
for situations in which the coal seams are thin and the interburden depth
is similar to or greater than the overburden depth. Such situations rarely
occur in the west; thus the one-pass method is rarely, if ever, used there.
Nonetheless, selective removal of overburden and placement of
spoil can be achieved fairly readily through use of one-pass methods, and
they are illustrated here for that reason. The first illustration is a
situation in which the effective spoil radius but not the effective dumping
height of the dragline must be increased. It is the counterpart of the two-
pass extended bench method, except that only one pass is made in each pit.
A characteristic of the one-pass extended bench method is that the
dragline always works from a position on the interburden, never on the
overburden. This eliminates the need to ramp the machine up and down at
the end of each pit. The method is illustrated below for a situation in
which the following three selective placement requirements must be
satisfied:
Competent interburden materials must be used to build
the spoil-stabilizing buckwall.
Interburden materials must be buried in the spoil pile.
Surface overburden materials must be placed on the
tops of the spoil piles.
It would be difficult or impossible to satisfy all three of these
requirements if a two-pass method was used. The one-pass method
shown in Figure 52 will work, however. As shown in that Figure, the
96
-------
I /
SECTION
Figure 52. Single Pass Extended Bench Method
for Stripping of Two Coal Seams
97
-------
dragline is positioned on the interburden. The first activity on each
digout is to dig a keycut in the interburden, subsequently swinging the
keycut spoil through an average angle of 130 degrees to place the spoil on
the pit floor, thereby forming the buckwall. Next, the top nine meters
(30 feet) of the overburden are dug to the side and above the bench. These
materials are cast against the lower highwall on top of the buckwall to
form the extended bench for the next digout.
The third component of the digout involves above-bench digging of
the remaining overburden, which is cast onto the main spoil pile. The
remaining interburden is then dug conventionally, below the bench, and
also is cast onto the main spoil pile.
Finally, on each digout, the dragline is moved out onto the extended
bench and the material used to extend the bench for the previous digout is
excavated and placed on top of the main spoil pile. This rehandle material
came from the surface of the overburden.
The method is seen to be environmentally desirable for situations
in which interburden materials are to be buried in the spoil and surface
overburden materials are to be placed on the surfaces of the spoil piles.
It also has certain production advantages over the two-pass method.
These include the following:
Elimination of underbench chopping.
Elimination of ramping of the dragline up or down at
the end of each pit.
A fifty percent reduction in dragline deadheading.
A spoil rehandle percentage identical to the two-pass
method.
Unfortunately, the method also has a significant disadvantage, the
one which would probably prevent its use in most western mining situations.
It is the need to chop all overburden to the side and above the dragline
bench. Most mine superintendents and dragline operators will resist
this, particularly where the overburden is hard and deep. It is equivalent
to working from a deep bench in one-pass stripping of a single seam.
Still, all things considered, the one-pass extended bench method
for uncovering of two seams might be the best choice in the west where
the following conditions hold:
The average overburden depth is 12 meters (40 feet)
or less.
The overburden strata are not too hard.
The thickness of the upper coal seam is three meters
(10 feet) or less.
98
-------
It is also possible to use a one-pass method in situations in which
both the effective spoil radius and the effective dumping height must be
increased to enable stripping of the interburden. The method is not
discussed here. Suffice it to say that it has advantages and disadvantages
similar to the one-pass extended bench method.
Truck and Shovel Stripping Systems
For reasons presented earlier in this section, loading shovels and
spoil haulage trucks will be used for overburden removal and spoil place-
ment at several large mines in the eastern Powder River Basin of
Wyoming, where the coal is very thick. In general, although the operating
costs of such equipment are higher than those for draglines, use of shovels
and trucks enables very selective placement of spoil, better control of
reclaimed topography, and grading of spoils concurrent with placement.
Overburden is excavated by loading shovels, generally in benches
because of the limited digging height of the shovels and to make safe high-
walls in deep overburden, and is loaded into haul trucks. The resulting
spoil is then hauled to designated placement areas, usually to a mined-
out portion of the active pit, and is dumped. As the spoil is placed,
dozers are used to maintain a fairly level working place for the trucks,
thus spoil grading is accomplished nearly concurrently with placement.
One possible drawback of this method of spoil placement is that the
spoil materials are compacted fairly tightly. Where there is a water table
above the pit floor, researchers have found that the resulting spoils may be
less permeable than the original coal or soft sandstone aquifers. [16]
Thus rates of groundwater flow through the spoil will probably be less than
those in the original aquifers.
Because the coal is thick -- sometimes thicker than the over-
burden -- where shovels and trucks are used for stripping, the ground
surface will usually be lowered appreciably by mining. (This is not a
function of the stripping equipment, however, but rather depends on the
depth of the overburden and the thickness of the coal.) Keefer and Hadley
have speculated that lowering of the ground surface may result in creation
of extensive closed depressions. [21] They further maintain that
restoration and maintenance of through-flowing drainageways will be
difficult and that water flowing in stream channels that are intersected by
mining may become impounded unless measures are taken to ensure
proper outflow. They also point out, however, that the technology for
stabilizing stream-gradient breaks by using engineering structures is
available.
Use of shovels and trucks for stripping in the foregoing kinds of
situations is actually beneficial, because the reclaimed topography can be
shaped more precisely than in cases where spoil is placed by dragline.
An example of this is depicted in Figure 53, which shows projected
reclaimed contours for both dragline and truck and shovel stripping
systems.
99
-------
41N
DRAGLINE FINAL SURFACE
Figure 53. Comparison of Final Surface Contours for Dragline
and Truck and Shovel Stripping Systems
A final environmental advantage of some truck and shovel systems
is that it may be fairly simple to remove topsoil and subsoil separately.
This may aid in revegetation efforts.
Coal Loading and Haulage
Although coal loading and haulage activities are of lesser environ-
mental relevance than are overburden removal and spoil placement
activities, they do cause some environmental impacts. The main ones
are generation of fugitive dust, potential spoil grading delays, disruption
of surface drainage patterns, and erosion of haul road surfaces.
The standard coal loading and haulage system is based on the use
of electric shovels for coal loading and bottom-dump trucks for coal
haulage. Prior to loading, because western coal is fairly thick, it is
usually drilled and blasted. Similar to overburden blasting, this may
cause generation of fugitive dust, such as that shown in Figure 54. It is
not generally known if that dust is carried off of the mine sites.
After blasting, the coal is excavated by shovel and loaded into haul
trucks. The trucks are then driven in the pit to the closest incline, up
the incline to the main haul road network, and then to the loading facility.
Truck traffic causes generation of fugitive dust. At most mines, the
roads are periodically watered to maintain some safe level of visibility.
This is the main dust suppression procedure used.
The design, location, and spacing of inclines into the pits are of
interest from an environmental standpoint. This is because grading of
spoils may be delayed near incline areas and because it may be difficult
to backfill the inclines after completion of mining without leaving topo-
graphic depressions that adversely affect surface drainage patterns on
reclaimed lands.
100
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Figure 54. Photograph Showing Fugitive Dust
Caused by Blasting of Coal
A typical incline is shown in Figure 55. Standard practice at many
mines is to carry the inclines low in the spoil profile, maintaining a
maximum grade of about eight percent into the pits. In single seam
mining, the incline usually enters the pit at the level of the top of the coal
seam. Where two seams are mined in a given pit, the incline enters at
the elevation of the upper seam and an additional ramp, sometimes steeply
graded, is constructed from the incline entrance down to the lower seam.
In addition to the fact that the incline is often carried low in the
spoil profile, the spoil piles on either side of the incline are usually
higher than the remaining spoil piles in a given pit. This is because the
overburden materials that otherwise would have been placed in the opening
left for the incline entrance must be stacked high on either side of the
incline. The crest-to-crest spacing across the resulting spoil piles
probably averages about 114 meters (375 feet). Grading cannot usually
take place within this "band" so long as the incline is used for coal haulage
out of the pit.
The average spacing of inclines at western mines in 1975 was
609 meters (2,000 feet). Delays in grading of 114 meter-wide strips along
each incline can be significant. Additionally, where the inclines are very
deep, it may be difficult to completely backfill them when they no longer
are needed.
101
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Figure 55. Photograph Showing Typical Pit Incline
As a remedy for this, many mining companies have made the
following changes:
The spacing of the inclines has been increased,
thereby reducing the number of inclines for a given
pit.
The inclines are carried high in the spoil profile,
and then brought down fairly sharply into the pit.
Where the inclines are carried high in the spoil profile, it is
possible to backfill and grade them as mining progresses. Where there
are two inclines in a given pit area, for example, one incline might be
used while backfilling and grading are taking place on a section of the other.
These kinds of procedures are required by law in some states, and
are sometimes used voluntarily in others. It would appear that their use
increases coal haulage costs, but improves reclamation.
SUMMARY: INTEGRATION OF MINING AND RECLAMATION PRACTICES
Certain reclamation activities, such as drainage diversion and
topsoil removal, take place in advance of stripping. Others are an integral
part of the mining process, and yet others, still to be discussed, take
102
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place after stripping, coal loading, and coal haulage have been completed
in a given pit area.
The phrase, "integration of raining and reclamation" has been
widely publicized in recent years in debates over the need for, or lack of
need for, new and more stringent reclamation laws. Indeed, in steep
slope mining in central Appalachia, where reclamation is in large part a
materials handling problem, the phrase has real meaning. Similarly, in
acid areas of the midwestern United States, where exposure of toxic
materials for even short periods of time may cause significant environ-
mental damage, mining and reclamation should be integrated to prevent
or minimize such exposure.
But things are different in the west. Although many words have
been used in this report to describe ways in which mining and reclamation
practices can be integrated, selective removal of overburden and place-
ment of spoil is the principal way in which this need be accomplished.
Nonetheless, various other ways have been proposed in the literature, so
before moving on to a discussion of reclamation practices per se, a recap
of actual and proposed methods for integration of mining and reclamation
is presented.
Selective Overburden Removal and Spoil Placement
Several features of the western environment are relevant in this
regard.
Since there are few acid a,reas, reduction of the time
lapse between spoil placement and spoil grading, say
by narrowing or shortening the pits, may not yield
significant environmental benefits. (In contrast, in
acid areas such as those in southern Indiana or
western Kentucky, these practices might yield very
significant benefits.)
Topsoil is salvaged and replaced at virtually all
western mines. In some cases, this may reduce the
need to selectively place spoil materials that will
eventually be covered by the topsoil.
Potential chemical contamination of groundwater is
of greater concern in the west than in other parts of
the country. This may impose unique selective
placement requirements on western mining companies.
Unique requirements or not, however, observation of existing
practices and analysis of proposed ones suggests that selective overburden
removal and spoil placement can be accomplished at a cost. Preliminary
analysis of several cases suggests that the cost may be fairly low. It
might be worthwhile, therefore, to determine and publicize those costs
for a wide range of operating conditions.
103
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Moreover, in a given situation, there may be many ways to achieve
a specified selective placement objective. Some will be less costly or
more effective than others. The authors' field experience suggests that
superintendents of mines in the midwestern United States have had more
experience in developing cost-effective procedures than those in the west.
If so, some "technology transfer" may prove beneficial. One way in
which such a transfer might be accomplished is discussed in Section 7 of
this report.
Narrow Pits and Short Digouts
It has previously been asserted in this section that, within limits,
spoil grading costs and the time lags between spoil placement and grading
can be reduced by narrowing dragline pits and shortening the digouts. For
example, it has been shown in a previous report for one specific case that
reduction of pit width from 43 meters (140 feet) to 27 meters (90 feet)
would reduce the time lag between spoil placement and grading from
20 weeks to 13 weeks. At most western mines, this change of pit widths
would also reduce spoil grading costs by several cents per ton. But these
effects are not felt to be significant in the west, where the coal is thick
and there are relatively few toxic overburden strata; thus research to
determine environmentally best pit widths and digout lengths appears to be
unwarranted.
Modified Incline Spacing and Design
Reduction of the number of inclines per pit and raising of the
inclines in the spoil profile appear to be desirable practices from a
reclamation management standpoint in that reclamation of the inclines can
be made more-or-less concurrent with mining.
Dipline Mining
With the exception of a few mines in Colorado, the strip pits at
western mines are oriented roughly parallel to the strike of the coal
(perpendicular to the dip). Since groundwater in rock aquifers generally
flows down-dip, opening of a long pit along the strike may result in inter-
ception of relatively large amounts of groundwater. Possibly, by orienting
the pits parallel to the dip of the coal seam, and thus to the direction of
groundwater flow, the amounts of groundwater intercepted by the active
pits could be reduced. * After mining and reclamation had been completed,
however, the net effects would be the same, regardless of the pit orienta-
tion.
The dipline mining concept does not appear to warrant serious
consideration as a means of reducing the environmental impacts of mining,
for the following reasons:
jf
The effect of pit orientation on groundwater levels and flow rates is a
matter of some speculation at present.
104
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Interception and drawdown of groundwater during
mining may not be significantly affected by pit
orientation.
In most area mining situations, the dipline method
has enormous economic disadvantages, related to
the fact that deep overburden is encountered in
every cut, including the first one. These dis-
advantages do not "average out" over the life of the
mine.
Piling Spoil on the Highwall in the Final Cut
Frequently, during removal of overburden from the final cut, some
spoil is piled on the highwall for subsequent use in burying the final high-
wall. This is a relatively minor way in which mining and reclamation
practices can be integrated. It may result in achievement of reclamation
requirements at least cost, but has the minor disadvantage that some
land beyond the coal recovery line (final highwall) will be disturbed.
Retreat Mining
Retreat mining, a concept occasionally proposed for use in steep
slope contour mining situations, is a method in which mining would begin
at the permit boundary or coal recovery line and proceed back out to the
coal seam cropline. It would involve first constructing coal haulage roads
all the way to the expected location of the final highwall, and then mining
or retreating back to the cropline. The purported advantage of the method
is that it would enable concurrent reclamation of haul roads and other
spoil areas which would never again be disturbed, by truck traffic or in
any other way.
In the authors' opinion, such a method is unwarranted and
impractical in most area mining situations for the following reasons:
Adequate reclamation of inclines and haul roads can be
achieved through use of practices already discussed.
Opening of the box cut in very deep overburden along
the recovery line would in most cases be technologically
or economically infeasible, or both. Even it it was
feasible, the resulting box cut spoil pile would be
enormous in size.
The recovery line, defined by the maximum depth of
overburden that can be stripped profitably, changes
greatly as the economics of mining change. For a
mine which has a 30 year life, it is virtually
impossible at the outset of mining to predict the
locations of final highwalls.
105
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The economics of retreat mining would be disastrous.
The equipment capacities required to meet production
schedules in the early years of mining would be
enormous. Not only that, the required equipment
capacity would actually decrease over time. Additionally,
the net cash outflows in the early years of mining would
probably be so large as to make mining economically
infeasible.
These enormous economic disadvantages coupled with questionable
environmental advantages should be sufficient to lay this concept to rest.
Side Bench Stripping
As indicated earlier in this section, in single seam dragline
stripping situations, cutting of a side bench by the dragline as the last
activity on each digout enables placement of surface overburden materials
on the surfaces of the spoil piles. This is a result that is often environ-
mentally desirable and may cost relatively little. Thus, use of the method
may sometimes be warranted even though possibly unnecessary from a
production standpoint.
Blasting Delays
Use of blasting delays and electric caps in place of primercord are
methods which are generally adequate, in the west, to reduce noise, air
shock, and ground vibration to acceptable levels.
Tabular Summary
A summary and evaluation of ways in which mining and reclamation
practices can be integrated in dragline stripping situations is presented in
Table 15.
RECLAMATION PRACTICES
The final reclamation category in this discussion consists of those
practices used in a given area after placement of spoils. The main
purposes of these "post-mining" reclamation practices are to restore
approximate original contours and re vegetate the restored areas. The
technology for doing so is fairly standard, much of it having been adapted
from western agricultural practice.
The Organization for Reclamation
In past years, reclamation was the responsibility of production
personnel at the mines. As a result, dual purpose equipment, that used
for both production and reclamation, was used for production purposes as
the need arose, often to the detriment of reclamation. The prime
examples of resulting problems were grading delays caused by diversion
of all operational dozers to production activities. Violations of grading
regulations were not uncommon in past years.
106
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TABLE 15.
WAYS TO INTEGRATE MINING AND RECLAMATION
IN DRAGLINE STRIPPING SITUATIONS
Mining Practice
Selective Overburden
Removal and Spoil
Placement
Narrow Pit and
Short Digout
Reduced Number of
Inclines
Carry Inclines High in
Spoil Profile and Drop
Sharply Into Pit
Dipline Pit Orientation
Piling Spoil on Highwall
in Final Cut
Retreat Mining
Placement of Side Bench
Material on Spoil
Surfaces
Blasting Delays for
Overburden and Coal
Effect on Reclamation
May improve revegetation
capability and reduce
potential adverse effects
on groundwater quality.
Reduces time lag between
spoil placement and grading.
Reduces spoil grading costs.
Enables timelier spoil
grading.
Enables timelier backfilling
and grading of inclines.
May reduce groundwater
drawdown during mining.
Reduces costs of backfilling
final cut or reducing final
highwall.
Enables complete back-
filling of inclines
concurrent with mining.
May reduce depth of topsoil
required.
Reduces noise, air shock,
and ground vibration.
Effect on Mining Productivity
or Cost
Productivity usually reduced
because average dragline
swing angle is increased.
Productivity may be increased
or decreased depending on
overburden and dragline
characteristics.
Increases coal haulage costs.
Not known but may increase
coal haulage costs.
Increases capital investment,
reduces profitability, degrades
cash flow pattern.
Probably minimal
Felt to be economically
infeasible.
Productivity usually reduced
because average dragline
swing angle is increased.
Not known, but may Increase
overburden removal costs.
Remarks
Used at 40 percent of
western mines in
1975.
Safety and other problems
may result if pit is too
narrow. Coal recovery
decreases slightly as pit
is narrowed.
May also cause scheduling
problems.
Required by law in some
states.
Rarely if ever used in
practice except in Colorado
where topography is unique.
If used there, erosion
problems may result.
Common practice in other
parts of the country.
Never used. Costs greatly
outweigh benefits.
Widely used in midwest,
but infrequently used in
west.
Widely used
-------
The first step taken at many mines to remedy this situation was to
dedicate certain equipment to grading activities. Today, at about 60 per-
cent of the western mines, some dozers are dedicated to grading and other
reclamation activities. This has resulted in significant improvement in
the timeliness of grading. Some minor problems persist, among them the
fact that maintenance and repair of reclamation dozers is often given
lower priority than maintenance and repair of production dozers; but,
overall, dedication of equipment to reclamation has eliminated some past
problems.
Another step taken by some companies was to create more-or-less
autonomous reclamation organizations at the mines. Centralization of all
reclamation activities under a reclamation superintendent has improved
reclamation by reducing or eliminating possible conflicts between produc-
tion and reclamation which frequently arose when both activities were
administered by a single person.
Spoil Grading
Grading technology is largely standardized, involving the use of
bulldozers to grade spoils and restore approximate original contours
after placement of spoil. Grading costs per ton of coal are relatively low
where the coal is thick.
In dragline stripping situations, grading is usually kept current to
within two or three spoil ridges of the active pits, depending primarily on
state or Federal reclamation regulations. It is not unusual, however, for
grading to be kept current within one spoil ridge of the active pit. In
fact, at some of the field survey mines, the spoil piles had been graded
right up to the active pits.
The appearance of graded areas is generally excellent. Fre-
quently, from a topographic standpoint, it is impossible to distinguish
between areas that have been mined and those that have not. An example
of such a case is shown in Figure 56. The area to the right of the road in
that Figure has been mined and reclaimed. The area to the left was not
disturbed by mining.
In general, surface drainage patterns at active mining sites are
felt to be adequately restored by grading. * Often, new drainageways
constructed on graded spoils are connected with natural drainageways at
the boundaries of the mining operations. Of course, positive drainage
from mining sites cannot be restored where the open pit mining method is
used. Additionally, special care must be taken to restore adequate
drainage in modified open pit mining situations, where the ground surface
is lowered appreciably by mining.
It should be noted that there are knowledgeable specialists who feel that
restoration of surface drainage patterns may be one of the most critical
problems facing western surface coal miners -- particularly with regard
to stabilization of drainage channels over the final highwall.
108
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Figure 56. Photograph Showing Reclaimed and
Adjacent Undisturbed Areas
There may be occasional minor problems in grading during the
spring, when the spoils are sometimes muddy, but the capacity of grading
equipment appears to be large enough so that required grading timeliness
can be accomplished even if no grading is done during wet periods.
Dozers are also used to grade the box cut spoils, but approximate
original contours in box cut areas are restored at only 35 percent of the
active mines. At the other mines, the outslopes of the box cut spoils are
reduced to an average angle of about 15 degrees from the horizontal.
This angle might be as high as 20 degrees. In the latter two cases, a
spoil pile, such as that shown in Figure 57, remains and as such is a
permanent topographic change. Some erosion generally occurs on the
graded box cut piles.
Dozers are also used to reduce the height and slope of the final
highwall and to partially backfill the final cut. Final cuts have been
reached to date at very few western mines, but where they have been
reached, one of two procedures has been used to reduce the final highwall.
The first, mentioned previously, is to pile spoil on the highwall side of the
last cut during the stripping process. After the coal has been loaded out,
that spoil is pushed into the cut, thereby reducing the height and slope of
the final highwall. The second is to use explosives to blast the final high-
wall down into the open cut. After blasting, dozers are used to do finish
grading in the highwall area. The appearance of a reclaimed final cut at
a western mine is shown in Figure 58. The final highwall in that cut was
24 meters (80 feet) high before reclamation.
109
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-
Figure 57. Photograph Showing Graded Box Cut Spoil Pile
Figure 58. Photograph Showing a Reclaimed Final Cut
110
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A minor disadvantage of the foregoing methods of reducing final
highwalls is the disturbance of a narrow strip of land along the final high-
wall. Intuitively, however, it would seem that the acreage involved is but
a small fraction of the total acreage disturbed by mining.
Even after reduction of the final highwall, a depression of swale
will usually remain. If there was a water table above the elevation of the
pit floor prior to mining, a shallow lake might form in that depression.
A tabular summary of grading practices is presented in Table 16.
The Table shows that grading practices are widely used, generally with
excellent results.
Revegetation and Erosion Control
Revegetation of mined areas has two purposes, control of erosion
and return of the land to productive use. The vegetative species used for
erosion control might not be productive in terms of the planned post-mining
land use, which is usually grazing. Similarly, vegetative species suitable
for grazing might not provide adequate erosion control, particularly during
the initial years after seeding. A frequent solution to this potential problem
is to seed both fast-growing annual grasses, suitable for early erosion
control, and native species, suitable for the post-mining land use.
Several things are done before seeding. Common practice is to
scarify the spoil surfaces prior to replacement of topsoil to ensure a good
bond between the spoil and the soil. This is particularly important where
the spoil surface is impermeable. An'alternative sometimes used is to
spread a few inches of topsoil on the spoil surface, scarify, then replace
the remaining topsoil. Scarification is accomplished using a variety of
farm implements and homemade devices.
Seed is usually sown by drill seeding. The rangeland type of drill
is used at some mines and, in those cases, it apparently has worked very
well. Drill seeding is usually done on the contour, although seeding up and
down slopes was observed in occasional use at one field survey mine. In
many cases, broadcast seeding is used in areas that are inaccessible to the
drill; these are often areas that are fairly steep. Broadcasting is usually
less successful than drilling, and many attribute this to the seeding method
itself. One specialist, however, believes that broadcasting (at a double
rate) should be more widely used because it enables seeding to be done at
the proper times of year. In contrast, drill seeding during spring or fall
may be delayed when the ground is muddy, resulting in seeding at other
than optimal times. Hydros ceding has been tried at a few mines but,
according to mine operating personnel, it failed because the water often
made the surface of the spoil seal over. This occurred several years ago
before topsoiling was required in some states. Aerial seeding, widely
used at mines in the midwestern United States, is rarely used in the west.
Native species, required by regulation in some states, are widely
used for revegetation in all states. Specialists generally agree that this is
a good thing because the native species may persist without maintenance.
Ill
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TABLE 16. FREQUENCY OF USE AND EVALUATION OF GRADING PRACTICES
Grading Activity
Frequency of Use
Percent of
1975 Acres
Percent of
1975 Mines
Remarks
Overall Evaluation
Restoration of Approximate
Original Contour
97
96
Spoil grading required
in all states.
Generally excellent from aesthetic, drainage,
and land use standpoints. Occasional erosion
problems on long unbroken slopes, but not a
major problem overall.
Reduction of Outslope of
Box Cut Spoil Piles
100
100
Approximate original
contour restored at
35 percent of mines.
Average maximum
final grade at remain-
ing mines is 27 percent
(1 5 degrees).
Final grades are sometimes too steep for
effective revegetation and erosion control,
even if terracing is used.
Reduction of Highwall in
Final Cut
74
82
Few impoundments
permitted in final cuts.
Approximate original
contour required at
25 percent of mines.
Average final grade of
highwall at remaining
mines is 33 percent
(18-1/Z degrees).
Direct evaluation difficult because final cuts
have been reached at very few mines.
Observations at a few mines indicated
excellent results.
-------
Stocking rates on lands returned to grazing can be increased by as much as
15 times through use of introduced species, but annual fertilization and
reseeding each decade would be required to maintain the land productivity.
This is currently the range management procedure for "tame pasture"
which is used for about one month each year during the spring calving
period. The stocking rate for tame pasture might average about one acre
per animal unit month as contrasted with an average of about 4-1/2 acres
per animal unit month for rangeland. This evidences the well-known fact
that grazing productivity can be increased through seeding of introduced
grasses and legumes, but the range management requirements in such
cases would probably be excessive for large acreages.
Several years ago, there was a widely publicized shortage of native
seed supplies, brought on by rapidly increased demand for seed for strip
mine reclamation. Although this shortage persists for many species, it
has not resulted in widespread revegetation delays. Today, several
companies are planning to grow their own seed. This is being encouraged
by state reclamation personnel.
In almost all cases, requirements for amendments such as
fertilizer or gypsum are determined by specialists whose services are
retained by mining companies for that purpose. Amendments, mainly
fertilizer, were used at 42 percent of the mines active in 1975. In the
remaining cases, according to information provided by mining companies,
tests had indicated that amendments were not needed.
Mulching, a practice employed at 24 percent of the mines active in
1975, is an effective means of conserving soil moisture and reducing
erosion, if the mulch is used in sufficient quantity and is tacked or
crimped into the soil so that it is not blown off by the wind. During the
period of the field survey, for instance, operators of a mine located in a
glaciated area started using mulch to reduce erosion on reclaimed areas.
It appeared to be effective. Figure 59 shows sediment carried from
reclaimed areas into roadside ditches prior to the use of mulch at that
mine.
Terraces and ditches constructed on the contour are additional
measures used to control erosion at 16 percent of the mines active in 1975.
In general, however, these measures are not felt to be needed where
mulching is used, except on long slopes. Terraces had been constructed
on the outslopes of box cut spoils at a few mines, for example, but did not
appear to have had any effect on erosion rates. On the other hand, contour
ditches such as those shown in Figure 60 were effective in reducing erosion
on long slopes in areas where the original topography was fairly steep.
Seeded areas were irrigated at only 16 percent of the mines active
in 1975, and most of those were located in arid regions. The irrigation is
used only for one or two growing seasons. Some problems with sealing of
saline soils after wetting have occurred.
113
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Figure 59. Photograph Showing Sediment Carried
Into Roadside Ditch
Figure 60. Photograph Showing Contour Ditches
Used to Reduce Erosion
114
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The overall evaluation of revegetation and erosion control practices
is good. Field observations at nine mines during three seasons of the
year, coupled with information gathered from state and Federal regulatory
personnel, indicate that mine spoils in the west can be revegetated to
produce as good or better vegetative cover than the pre-mining cover.
This is particularly true where topsoil is replaced and proper seedbed
preparation, seeding, and amendment procedures are used. There is,
however, still uncertainty regarding the long-term survival capability of
vegetation on reclaimed lands.
Extensive revegetation research is now being conducted at western
mines, as it should be; but the objectives of current research are mainly
to reduce revegetation costs, decrease the time required to establish
climax features, and increase post-mining land productivity.
A summary and evaluation of the revegetation and erosion control
practices used at western mines during 1975 is presented in Table 17.
OTHER RECLAMATION-RELATED PRACTICES AND PROBLEMS
Insofar as the environmental effects of western surface coal mining
are concerned, the trend seems clear. Initially, there was considerable
skepticism over the feasibility of reclamation in general; many things were
unknown, many questions were unanswered. But as information became
available, and questions were answered, the answers usually indicated two
things:
The potential impacts were neither as severe nor
extensive as had originally been anticipated.
Reclamation is technologically and economically
feasible.
There are still unanswered questions, however. What depths of
topsoil are needed to ensure successful long-term vegetative growth? Will
complex and interconnected aquifer systems be forever disrupted by
mining? Is fugitive dust from mining operations a real problem? Will
large sink holes appear in graded spoils years after grading? Some of
these questions are legitimate ones, and research now in progress should
provide answers.
An indication of the general areas of concern is given in Table 18,
which shows potential environmental problems as identified by mining
companies in their environmental impact statements and reclamation
plans. The problems are termed potential ones because use of suitable
reclamation techniques should often prevent them from becoming actual
ones.
The potential problem condition most frequently cited was over-
burden or interburden materials that were clayey in texture and high in
exchangeable sodium, as indicated by SAR values greater than ten. This
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TABLE 17.
FREQUENCY OF USE AND EVALUATION OF REVEGETATION
AND OTHER EROSION CONTROL PRACTICES
Activity
Terracing of Graded
Spoil Surfaces
Spoil Amendment,
Primarily Fertilizer
Seeding
Drill
Broadcast
Drill & Broadcast
Mulching
Irrigation
Frequency of Use
Percent of
1975 Acres
34
47
44
5
51
28
24
Percent of
1975 Mines
16
42
71
7
22
24
16
Remarks
Terracing and contour
ditching should be
more widely used.
Not needed in all
cases.
Broadcast used on
areas inaccessible for
drilling.
Frequency of use is
Increasing.
Overall Evaluation
Moderately effective in controlling erosion
where used.
Broadcasting not felt to be as effective as
drilling, although broadcasting at the right
time of year may be more effective than
drilling at the wrong time. Revegetation
success generally good if proper seedbed
has been prepared.
An effective measure if crimping or tacking
of mulch are used to prevent wind loss.
Appears to be very effective where used,
although may cause sealing of high SAR
clays.
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TABLE 18.
FREQUENCY OF OCCURRENCE OF POTENTIAL
RECLAMATION PROBLEMS AS IDENTIFIED BY
MINING COMPANIES
Problem Type
Clayey, high SAR material
in overburden or interburden
Fugitive dust
Sink holes
Sedimentation
High trace element
concentrations
%
Chemical water pollution
Frequency of Occurrence in 1975
Percent of Acres
64
33
U
10
1
0
Percent of Mines
49
22
9
9
4
0
condition occurred at half of the mines active in 1975. Means for
preventing or minimizing the impermeability and attendant revegetation
problems that could be caused by this condition have been discussed
extensively in this report.
The second most frequently cited potential problem was wind
erosion and fugitive dust. There are several mines at which fugitive dust
appears to be a real problem. One is a large mine which is located right
next to a town. During winter months in particular, coal dust is blown
from large uncovered coal stockpiles toward the town. The state
regulatory agency intends to require that the company monitor fugitive
dust using high-volume air samplers.
In general, it is not known if fugitive dust is carried beyond the
boundaries of mining properties. Research now in progress should shed
some light on this question.
Some additional potential problems were cited by mining companies,
but only infrequently. One is the possibility that sink holes will occur in
graded spoils. Sink holes are surface depressions ranging from a few
centimeters in diameter and depth, up to 10 meters in diameter and
15 meters in depth. To date, the problem, illustrated in Figure 61,
appears to have occurred principally in orphan areas (ungraded spoils)
where the spoil materials were high in sodium, primarily in North Dakota.
These are typically areas that had been stripped by dragline without
blasting of the overburden. As a result, large shale or clay "boulders"
were placed in the spoil. It is possible that those boulders in combination
with differential settling, weathering, and piping caused the subsidence
(sink holes).
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Figure 61. Photograph Showing Small Sink Hole
in Orphan Sodic Spoil
To the best of the authors' knowledge, the sink hole problem has
not yet occurred extensively in graded spoils. This doesn't necessarily
mean that it won't. In fact, as a means of preventing occurrence of the
problem, one company segregates large boulders during the stripping
process.
Other infrequently cited potential problems included erosion,
sedimentation, and high trace element concentrations in overburden strata.
These do not appear to be significant problems on a regional scale,
although severe erosion can occur on steep, glacial spoils, as shown in
Figure 62. That Figure shows an isolated incident rather than a general
condition, however.
There are also a handful of potential local problems. One is burial
of power plant fly ash in the strip pits at captive mines. An example is
shown in Figure 63. The fly ash is sometimes high in toxic trace elements
such as boron, and some fear that burial of the fly ash in strip pits may
result in chemical pollution of the groundwater. This potential problem
was not studied as part of the current project.
A final comment applies to dewatering of wells during mining.
Thus far, this has not occurred too frequently on lands not owned or
leased by mining companies. Where it has occurred, mining companies
have drilled deeper wells on affected properties. In every such case
known to the authors, it has been possible to find lower-lying aquifers of
118
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Figure 62. Photograph Showing Erosion on Steep Glacial Spoils
Figure 63. Photograph Showing Disposal of Fly Ash in a Strip Pit
119
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yield and quality at least comparable to the ground-water in the aquifer that
was dewatered.
SUMMARY
Mining of single and multiple coal seams by single draglines
accounted for 68 percent of the strip pits, 83 percent of the tonnage pro-
duced, and 88 percent of the acres disturbed by western surface coal
mining in 1975. Because of the actual frequency of occurrence of dragline
stripping situations in 1975 and the forecasted frequency for J.980, those
situations were emphasized in this study. Other situations, such as
modified open pit mining of thick coal seams using loading shovels and
trucks for overburden removal and spoil placement, occurred infrequently
in 1975 and, although such situations will occur more frequently by 1980,
they have received little emphasis in this report.
Procedures used to plan production and reclamation at western
mines are sophisticated and thorough. The information gathered and
analyses conducted to satisfy state and Federal reclamation regulations
are extensive.
Prior to and during mining, adequate drainage control procedures
are generally used. They include diversion of ephemeral streams that
cross mining areas, construction of above-highwall ditches to divert
surface runoff so that it does not enter strip pits, construction of earth
dams across natural drainageways for the same purpose, construction of
culverts under haulage roads and drainage ditches alongside the roads,
crowning of haul roads to divert surface runoff to the ditches, and construc-
tion and maintenance of sediment basins. Although there are some
occasional problems such as stream bank erosion in stream diversions,
and creation of headcuts at outfalls of drainage diversion ditches, drainage
control procedures were deemed effective overall. Diversion ditches are
in general adequate in capacity, have suitable grades, and are adequately
vegetated. Impoundment of surface water to prevent it from entering strip
pits or crossing haul roads does not appear to have had a significant
impact on water budgets or water quality. Sediment basins, although
sometimes fairly shallow, appear to contain most or all sediment on site.
Topsoil was salvaged at 96 percent of the mines active in 1975.
The depths of topsoil salvaged averaged 33 centimeters (13 inches). The
soil is removed one to six months ahead of stripping, and some of it is
stockpiled prior to replacement on graded spoils. Techniques for control
of erosion on topsoil stockpiles were used at only 22 percent of the mines
active in 1975, and wider use of such techniques appears warranted to
reduce soil losses due to water erosion. Although direct replacement of
topsoil without stockpiling appears to be desirable, at least during growing
seasons, it is not always possible. Additionally, judging from the success
of revegetation, claims that stockpiling destroys the fertility of the soil
appear to be unfounded.
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Drilling and blasting of overburden and coal do not appear to have
caused significant environmental problems in the west, with the possible
exception of emission of fugitive dust. There are several reasons for this,
among them the low population densities in mining areas and the use of low
powder factors in the relatively soft consolidated overburden that overlies
strippable western coal. Where mines are located close to population
centers, two techniques have been used to reduce noise, air shock, and
ground vibration. One is to delay the shots so that the amount of explosive
detonated at any given instant is less than or equal to an amount determined
by the U.S. Bureau of Mines to be acceptable. The second is to replace
primercord detonator with electric blasting caps to reduce blasting noise.
Both of these techniques appear to have been effective and, overall, blasting
is not felt to cause significant environmental impacts in the west.
There are many ways to integrate mining and reclamation practices
in dragline stripping situations, but the only significant one in the west is
selective removal of overburden and placement of spoil. This technique,
planned for use at one-third of the dragline operations active in 1975, is
needed for the following reasons:
If special handling techniques are not used, overburden
materials will be inverted on the spoil piles. Thus
surface overburden strata will be placed on the pit
floor, and the overburden strata immediately above
the coal seam being mined will be placed on the surfaces
of the spoil piles.
Overburden or interburden at about half of the mines
active in 1975 contained strata that were clayey in
texture and high in exchangeable sodium, the latter
factor indicated by a sodium adsorption ratio (SAR)
greater than about ten. If placed on the surfaces of
the spoil piles, as was often the case where selective
placement procedures were not used, those materials
soon become impermeable, increasing surface runoff
and causing subsequent revegetation failures.
Where there is a water table above the pit floor,
placement on the pit floor of spoil materials high in
soluble salts and trace elements may lead to
mineralization or pollution of ground-water.
The objective in dragline stripping situations, therefore, should be
to place the most desirable materials on the pit floors and on the surfaces
of the spoil piles. This is generally possible at relatively low cost in
single seam dragline stripping situations by using lead spoil principles
and the side bench technique, neither of which appeared to be widely used
in 1975. The problem is more difficult in multiple seam mining because
two of the three prevailing multiple seam mining techniques result in
placement of interburden materials, which are very often undesirable, on
the surfaces of the spoil piles. Mine operators in the midwestern United
States have had considerable experience in selective placement, however,
and it is suggested that some means for "technology transfer" be devised.
121
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The foregoing remarks are not meant to imply that better selective
placement techniques are always needed to satisfy reclamation require-
ments, because, with topsoil replacement, they often are not. On the
other hand, improvement in spoil placement control might enable reduction
of the depths of topsoil required to achieve specified reclamation goals.
The design and spacing of inclines used to haul coal out of the strip
pits have environmental relevance. Typical practice until recently was to
space inclines about 457 meters (1, 500 feet) apart and to have them at the
level of the pit floor for most of their length. This frequently resulted in
spoil grading delays and made it difficult, from a management standpoint,
to ensure that the inclines would eventually be completely backfilled. An
apparently effective remedy, required by law in one state and sometimes
used voluntarily in others, is to reduce the number of inclines used in a
given pit area, and to keep them on top of graded spoil for as great a
length as possible before dropping down to the level of the pit floor.
Economic considerations usually limit truck and shovel stripping
systems to areas in which the coal is very thick. Such areas occur most
notably in the eastern Powder River Basin of Wyoming. It is not
reasonable to assume that trucks and shovels will come into widespread
use in areas other than those in which the coal is very thick. Where used,
however, such systems are environmentally superior to draglines in
several respects in that they enable very selective placement of spoil,
better control of reclaimed topography, and grading of spoils concurrent
with placement. By the same token, though, the spoils are compacted
more than spoils placed by dragline.
In dragline stripping situations, there are several additional ways
to integrate mining and reclamation practices, but none of these is felt to
warrant serious consideration. These include narrow pits, dipline mining,
and retreat mining.
The organization for reclamation has been greatly improved at
many mines through dedication of equipment to reclamation and centrali-
zation of reclamation activities under a reclamation superintendent. This
has generally improved reclamation performance significantly.
Spoil grading was deemed to be good to excellent in all respects,
including restoration of approximate original contours and adequate surface
drainage patterns, appearance, control of erosion, and return of the land
to productive use. Many mine superintendents would still like a cheaper
means of grading, however. Additionally, there are specialists who feel
that restoration of drainage patterns is one of the most serious problems
facing western surface coal miners.
Revegetation success was generally felt to be good. In fact, the
consensus of study team members and specialists interviewed during the
course of the study is that, if proper spoil placement, topsoiling, seedbed
preparation, seeding, amendment, and mulching techniques are used, it
would appear that the vegetation on mined areas will be equal or superior
in productivity to pre-mining vegetation. However, there are still some
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uncertainties regarding long-term revegetation success. Extensive
revegetation research is now being conducted, but the primary objectives
of those research activities are to reduce revegetation costs, reduce the
time required to achieve climax conditions, and further increase land
productivity.
Some questions remain unanswered, among them the following:
What depths of topsoil are really needed to achieve
specified revegetation objectives?
Is fugitive dust from mining operations a significant
problem on local or regional scales?
What techniques can be used to minimize groundwater
impacts in cases where they are potentially severe?
Will sink holes (subsidence) occur extensively in
graded sodic spoils?
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SECTION 7
DISCUSSION OF RECOMMENDATIONS
SELECTIVE OVERBURDEN REMOVAL AND SPOIL PLACEMENT
A Symposium
It is recommended that the Environmental Protection Agency
sponsor a symposium on the managerial, technical, and operational
aspects of selective overburden removal and spoil placement in dragline
stripping situations. The call for papers should be issued nationwide to
enable some technology transfer from east to west, and possibly vice
versa. The papers and presentations should deal with all aspects of
selective removal and placement, including the following ones:
Means used-in advance of mining to identify desirable
and undesirable spoil materials.
Descriptions of planned operational procedures
devised to enable selective removal and placement
of undesirable materials.
Actual operating procedures, including means for
identification of undesirable materials by dragline
operators, and management techniques employed to
ensure that operators follow the plans.
Operating problems.
Adequacy of the procedures.
Effects on production rates and costs.
Alternative procedures considered.
Participation of both mine engineering and operating personnel
would be essential to the success of such a symposium. Unfortunately,
many of those people have neither the time nor the inclination to write or
present technical papers. This could be remedied to some extent by
appointing a select group of qualified individuals to survey mines at which
selective removal and placement procedures are used, and to determine
and document the needed information. The resulting document would then
be reviewed by mining company personnel.
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It would be essential that the symposium be a medium for exchange
of practical information among practitioners. It would not be good if it
turned out to be a progress report on environmental research.
Effectiveness of Selective Removal and Placement
It would be desirable to determine the effectiveness of selective
overburden removal and spoil placement practices in terms of answers to
the following questions:
How does selective placement affect the amounts of
topsoil that need to be replaced?
How is the quality of groundwater and surface water
affected?
How is revegetation success affected?
In order to be useful, it would be necessary to obtain quick results
for a wide range of conditions. This may not be possible; if it isn't, the
project isn't worth doing.
MEANS FOR MINIMIZING GROUNDWATER IMPACTS
It may be worthwhile to conduct a study to identify and evaluate
means for minimizing groundwater impacts. These might include selective
overburden removal and placement, dipline mining, advance dewatering,
deepening of wells on neighboring properties, and impoundments in final
cuts, to name a few. The objectives of the study would be to describe
alternatives and to present estimates of the costs and effectiveness of
each alternative.
BUCKET WHEEL EXCAVATORS
Bucket wheel excavators can be used to excavate any overburden
materials that do not require blasting. [22] Overburden does not generally
require blasting at mines in North Dakota, thus bucket wheel excavators
("wheels") can in some cases be used there for overburden removal and
spoil placement. Wheels cannot be used for overburden removal in most
other western mining areas.
The interest here in these machines was motivated by attempts to
devise overburden removal and spoil placement concepts that would have
cost characteristics comparable to draglines, but also enable placement
of topsoil by the main stripping machine directly onto graded spoils. This
would appear to be a desirable mining and reclamation system.
In fact, it appears that use of a wheel would accomplish those
objectives at some North Dakota mines. The operating procedure for a
single seam case is shown in Figure 64. Working from a position on coal,
125
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PLAN
ELEVATION
Figure 64. Operating Plan for Bucket Wheel Excavator
126
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overburden would be excavated in lifts by the wheel and conveyed to the
spoil pile via an articulated stacker. A dozer would work in the spoil
leveling it as placed. After completing a given digout, the wheel would be
advanced and topsoil for the next digout would be excavated. Now, how-
ever, the stacker would be swung in a counterclockwise direction, and the
topsoil would be placed directly on leveled spoils from the previous pass
(not the previous digout). This might be an excellent way to integrate
raining and reclamation practices.
Additionally, if the rate of production was high enough, the wheel
would actually be cheaper than a dragline of comparable capacity. This
is shown in Table 19, which is a comparison of production rates and costs
for dragline and bucket wheel excavator used under identical operating
conditions where the coal seam is three meters (ten feet) thick and the
overburden is 27 meters (90 feet) deep. For a given rate of production,
1. 8 million metric tonnes of coal per year, total costs for the wheel are
estimated to be 20 percent less than those for a large dragline. *
The wheel would have several disadvantages as well, among them
the following:
A large-capacity wheel would be required to operate
in the typical overburden depths at western mines.
This is because the long spoil dumping radius needed
in, say, 25 meters of overburden can only be obtained
TABLE 19.
COMPARISON OF MINING PLANS AND
COSTS FOR DRAGLINE AND BUCKET
WHEEL EXCAVATOR SYSTEMS
Stripping Machines
Marion 7620 30 CY.
B-E 1370 60 CY.
BWE Krupp 20 CY.
Annual
Cubic Yards
9, 007, 000
16,000,000
16,848,000
Tons
1,125,000
2,000,000
2,105,000
Annual Per Ton
Mining Cost
$2.46
2. 30
1.85
Earnings
$0.35
0.42
0.66
The mining plans have been drawn and the costs estimated for the strip mining of
the Gascoyne lignite deposits, Bowman County, North Dakota.
The costs estimates represents mining in the maximum overburden cover,
approximately to a 90 foot recovery line.
Details of production and cost estimates are presented in Appendix A.
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on wheels with large capacity. Unlike draglines, it is
not possible to buy a wheel with long reach and small
capacity. This means that wheels are limited in use
to situations in which large production capacity is
needed.
The presence of occasional sandstone lenses in the
overburden, as is sometimes the case in North Dakota,
might greatly complicate overburden removal by wheel.
Unless special selective placement procedures were
used, overburden materials beneath the tops oil would
be inverted on the spoil piles.
During North Dakota -winters, the frost line at surface
mines has averaged about 1.2 meters (four feet) deep
in recent years. Frozen soil could not be excavated
by the wheel.
All things considered, the wheel still might be a good choice for
some large mines in North Dakota. Thus a study of the mining and
reclamation characteristics of such a system should be considered.
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REFERENCES
1. Lichtenstein, Grace. Town's Boom Halted by a Strip Mine Ban.
The New York Times, Thursday, November 20, 1975.
2. U. S. Geological Survey. Water Resources Data for North Dakota,
Part 1, Water Quality Records. 1971.
3. U.S. Geological Survey. Water Resources Data for North Dakota,
Part 2, Water Quality Records. 1971.
4. U.S. Geological Survey. Division of Water Resources. Water
Resources Data for Montana, Part 1, Surface Water Records. 1972.
5. State Engineers Office. Water and Related Land Resources of the
Green River Basin, Wyoming, Report No. 3. Wyoming Water
Planning Program, Cheyenne. 1970.
6. U.S. Geological Survey. Water Resources Data for Montana,
Part 2, Water Quality Records. 1971.
7. U.S. Geological Survey. Water Resources Data for Wyoming,
Part 2, Water Quality Records. 1971.
8. lorons, Hembree, Oakland. Water Resources of the Upper Colorado
River Basin. USGS Professional Paper 442.
9. Surface Water Records of U.S. USGS Water Supply Paper 1918.
10. Surface Water Records of U.S. USGS Water Supply Paper 1925.
11. Surface Water Records of U.S. 'USGS Water Supply Paper 1934.
12. Quality of Surface Waters of the United States, 1958, Parts 9-14,
Colorado River Basin to Pacific Slope Basins in Oregon and Lower
Columbia River Basin. USGS Water Supply Paper 1574. 1964.
13. U.S. Geological Survey. Water Resources Data for New Mexico,
Part 2, Water Quality Records. 1970.
14. U.S. Geological Survey. Water Resources Data for New Mexico,
Part 1, Surface Water Records. 1971.
15. U.S. Geological Survey. Water Resources Data for Arizona,
Part 1, Surface Water Records. Part 2, Water Quality Records.
1971.
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16. Rahn, P. H. Potential of Coal Strip Mine Spoils as Aquifers in
the Powder River Basin. Study conducted by the South Dakota
School of Mines and Technology for the Old West Regional
Commission. 1976.
17. Cook, Frank. Evaluation of Current Surface Coal Mine Overburden
Handling Techniques and Reclamation Practices. U.S. Bureau of
Mines Contract Report No. S0144081. Prepared by Mathtech, Inc.,
December 24, 1976.
18. State of Wyoming. Range Condition Report: National Resource
Lands for Wyoming. January 1975.
19. Personal communication from Jake Rowland of Pittsburgh &
Midway Coal Company. September 1975.
20. Boulter, George W. Open Pit and Strip Mining Systems and
Equipment: Excavation and Loading. In: SME Mining Engineering
Handbook, A. B. Cummins and I. A. Given, eds. American
Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.,
1973, Volume H. 17-43 pp.
21. Keefer, W. R., and R. J. Hadley. Land and Natural Resource
Information and Some Potential Environmental Effects of Surface
Mining of Coal in the Gillette Area, Wyoming. USGS Circular 743.
1976.
22. Personal communication from Wilbur A. Weimer, Belleville,
Illinois. February 1977.
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APPENDIX A
COMPARISON OF DRAGLINE AND BUCKET WHEEL
EXCAVATOR PRODUCTION RATES AND COSTS
This Appendix contains the details of production and cost analyses
of dragline and bucket wheel excavator stripping systems for a surface
mining situation in North Dakota. Table A-l shows the estimated produc-
tion rate for a stripping situation involving a single dragline with a
TABLE A-l. PRODUCTION ESTIMATE FOR WALKING
DRAGLINE (Marion 7620)
Swing Swings Bank Cu. Yds.
Component Angle Per Hr. Bucket Carry
Topsoil (Move by scrapers)
Sand, Clay 82o ^ &
t Gravel
Side Bench 120 57 21
Total
Moved by Dragline
Operating Time for a 100' Digout 80%
Delays 20%
Scheduled Time
Cubic Yards Per Digging Hour
Cubic Yards Per Month (576 Hours)
Bank Cubic Yards Per Year
Mining Ratio
Tons Per Year
Tons Per Year at 60' Cover Overburden (Ratio = 6.67)
Annual Average Tons in 60' to 90' Cover
Bank Time
Cubic Yards Hours
2,315
30,093 22.47
9,260 7.73
41.668
39,353
30.20
7.55
37.75
1,303
751,000
9,007,000
10
900, 000
1,350,000
1,125,000
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72 meter (235 foot) boom and a 23 cubic meter (30 cubic yard) bucket used
to uncover a three meter (ten foot) coal seam overlain by an average of
23 meters (75 feet) of overburden cover. Estimated annual production is
1, 020, 000 metric tonnes (1,125, 000 tons). Table A-2 shows a cost analysis
for the same situation. Estimated total operating costs are $2. 71 per metric
tonne ($2. 46 per ton).
Table A-3 shows the cost estimate for a larger dragline used under
the same operating conditions. This dragline has a 79 meter (260 foot)
boom and a 46 cubic meter (60 cubic yard) bucket. Estimated annual
production in an average of 23 meters (75 feet) of overburden cover is
1, 814, 000 metric tonnes (2, 000, 000 tons). Total estimated operating costs
are $2. 54 per metric tonne ($2. 30 per ton).
Estimated production rate for a Krupp Sch. Rs. 1500/5 30. 5 bucket
wheel excavator used in the same situation is 1, 909, 000 metric tonnes
(2,105, 000 tons) annually, as shown in Table A-4. The cost estimate,
Table A-5, shows total operating costs of $2. 04 per metric tonne ($1. 85 per
ton). Thus, for an annual production rate of 1, 909, 000 metric tonnes, the
bucket wheel is the lower cost alternative.
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TABLE A-2. COST ESTIMATE FOR WALKING DRAGLINE
(Marion 7620)
CAPITAL, COSTS
1- Walking dragline, 235' boom, 30 cu. yd.
1- Coal loader (M 151)
1- Cat. 992B, F.E. L.
2- Cat. D8 dozers -f 1- Cat. 633C scraper
1- Cat. 16 patrol grader
3- Coal haul trucks (120 ton)
1- Coal dump hopper, crusher and car mover
1- Rail-way lead and hold yard
1- Building and miscellaneous equipment
Total
MINE OPERATING COSTS
Stripping
Labor
Repairs and Supplies
Power
Reclamation
Total
Coal loading and shooting
Coal hauling {2 miles)
Crushing and loading RR cars
Supervision and office
Administration and general
Western Miners Union royalty
Interest on 1/2 of the capital costs
Mi s c ellane o us
TOTAL
SALES REVENUE ($0. 35 per mm BTU)
Royalty
REALIZATION NET
Mine costs
Depreciation
Depletion
Net for income taxes
North Dakota income tax (4%)
I.R.S.
EARNINGS PER TON
Add back
CASH FLOW
Strip mining with a walking dragline
Cubic yards moved per year in 60' to 90' overburden
Tons coal produced annually, 10' seam @ 90% recovery
Mining Ratio
$ 5,300,000
900,000
200,000
450,000
125,000
450,000
400,000
500,000
1,675,000
$10,000,000
Per Ton
$0.52
0.32
0.16
0.20
1.20
0.10
0.12
0. 10
0.16
0.06
0.40
0.18
0. 14
$2.46
$4.25
0.25
$4.00
2.46
1.54
-0.45
-0.40
0.70
0.03
0.32
$0.35
0.45
$0.80
(M 7620)
9,000,000
1,125,000
8.00
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TABLE A-3. COST ESTIMATE FOR WALKING DRAGLINE
(BE 1370)
CAPITAL COSTS
1- B-E 1370, 260' boom, 60 cu. yd. bucket $10,500,000
1- Coal loader B-E 195LR 1, 160, 000
1- Cat. 992B, F.E. L. 200,000
3- Cat. D8 dozers + 1- Cat. 16 patrol grader 575, 000
4- Coal haul trucks 600, 000
1- Coal dump hopper, crusher and car mover 400, 000
1- Railway lead and hold yards 500, 000
1- Building and miscellaneous equipment 2, 065, OOP
Total $16,000,000
MINE OPERATING COSTS Per Ton
Stripping
Labor $0.33
Repairs and Supplies 0. 32
Power 0.25
Reclamation 0.20
Total 1.10
Coal loading and shooting 0. 10
Coal hauling (2 miles) 0.12
Crushing and loading RR cars 0. 08
Supervision and office 0.10
Administration and general 0.05
Western Miners Union royalty 0.40
Interest on 1/2 of the capital costs 0. 17
Miscellaneous 0.18
TOTAL $2.30
SALES REVENUE ($0. 35 per mm BTU) $4. 25
Royalty 0.25
REALIZATION NET $4. 00
Mine costs 2. 30
1.70
Depreciation -0.40
Depletion -0.40
Net for income taxes 0. 90
North Dakota income tax (4%) 0. 04
I. R. S. 0. 44
EARNINGS PER TON $0.42
Add back 0. 40
CASH FLOW $0. 82
Strip mining with a walking dragline (BE 1370)
Cubic yards moved per year in 60' to 90' overburden 16, 000, 000
Tons coal produced annually, 10' seam @ 90% recovery 2,000,000
Mining ratio 8. 00
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TABLE A-4. PRODUCTION ESTIMATE FOR
BUCKET WHEEL EXCAVATOR
Scheduled hours per month 720
Operating time per month at 60% 432
Delay time per month at 40% 288
Cubic yards per digging hour 3,250
Bank cubic yards per month 1, 404, 000
Bank cubic yards per year 16, 848, 000
Mining ratio 10
Tons per year at 90' overburden 1,685,000
Tons per year at 60" overburden 2, 525, 000
(Ratio = 6. 67)
Annual average tons in 60' to 901 cover 2, 105,000
Krupp bucket wheel excavator sch. rs. 1500/5 30.5
Cut 1001 wide in 90' overburden
Direct topsoil excavation and placement by the bucket wheel
excavator
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TABLE A-5. COST ESTIMATE FOR BUCKET WHEEL EXCAVATOR
CAPITAL COSTS
1- B.W.E, Sch. Rs. 1500/5 30.5 complete
1- Coal loader (B-E 195LR)
1- Cat. 992B, F.E. L.
2- Cat. D8 + 1- Cat. 16 patrol grader
4- Coal hual trucks (120 ton)
1- Coal dump hopper, crusher and car mover
1- Railway lead and hold yards
1- Building and miscellaneous equipment
Total
MINE OPERATING COSTS
Stripping
Labor
Repairs and Supplies
Auxiliary equipment
Power
Total (including reclamation)
Coal loading and shooting
Coal hauling (2 miles)
Crushing and loading RR cars
Supervision and office
Administration and general
Western Miners Union royalty
Interest on 1/2 of the capital costs @ 8%
Miscellaneous
TOTAL
SALES REVENUE
Royalty
REALIZATION
Mine costs
Depreciation
Depletion
Net for income taxes
North Dakota income tax (4%)
I. R. S.
EARNINGS PER TON
Add back
CASH FLOW
$12,500.000
1,160,000
200,000
425,000
600,000
400,000
500,000
2,215.000
$18.000,000
Per Ton
$0.33
0.32
0.04
0. 15
0.84
0.10
0.12
0.08
0.10
0.05
0.40
0.18
0. 16
$1.85
$4.25
0.25
$4.00
1.85
2.15
-0.43
-0.40
1.32
0.05
0.61
0.66
0.43
$1.09
Strip mining with a bucket wheel excavator
Cubic yards moved per year in 60' to 90' overburden 16,848,000
Tons coal produced annually, 10' seamฎ 90% recovery 2,105,000
Mining ratio 8. 00
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse beforelfompletmg)
1. REPORT NO.
EPA-600/7-79-110
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
EVALUATION OF THE ENVIRONMENTAL EFFECTS OF WESTERN
SURFACE COAL MININGVOLUME I
5. REPORT DATE
May 1979 issuing date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Frank Cook
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Mathematica, Inc.
Mathtech Division
Princeton, New Jersey 085^0
10. PROGRAM ELEMENT NO.
EHE 623
11. CONTRACT/GRANT NO.
Contract No. 68-03-2226
12. SPONSORING AGENCY NAME AND ADDRESS
Industrial Environmental Research Laboratory-Ci
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati. Ohio U3268
13. TYPE OF REPORT AND PERIOD COVERED
Final - 6/75 - 6/17
14. SPONSORING AGENCY CODE
600/12
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This report is a companion volume to the report titled, "Evaluation of the
Environmental Effects of Western Surface Coal Mining - Volume II: Mine Inventory."
It describes and evaluates the methods presently used for surface mining of coal
in the western United States, identifies and evaluates the effects that use of
those methods have on the environment, and recommends ways in which the methods
might be altered to reduce both long-term and short-term environmental damage.
This was accomplished by statistical analysis of comprehensive production and
reclamation data for all UU western surface mines active or under development in
1975, and through qualitative evaluations based on personal interviews of state and
Federal reclamation specialists and field surveys of nine mines during three
seasons of the year.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COS AT I Field/Group
Coal Mining
Classification
Western Coal Mining
Alternative Reclamation
Practices
43F 68C
48A 68D
68A 91A
18. DISTRIBUTION STATEMENT
Release to the Public
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
151
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
137
U. S. GOVERNVENT PRINTING OFFICE: 1979 657-060/532-.
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