EPA 0107
THE DESIGN AND OPERATION
OF WASTE ROCK PILES
AT NONCOAL MINES
July 1995
U.S. Environmental Protection Agency
Office of Solid Waste
401 M Street, SW
Washington, DC 20460
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" DISCLAIMER
530R95083
This document was prepared with teclmical support from Science Applications International
Corporation in partial fulfillment of EPA Contract No. 68-W4-0030, Work Assignment 7. The
mention of company or product names is not to be considered an eooorsement by the U.S. '
Government or by the Environmental Protection Agen~.
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TABLE OF CONTENTS
1.0 IN1'RODUcnON ..............................................
1.1 Waste Rock Generation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Environmental Impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 Summary of Literature Review. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1
2
2
2.0 BASIC DESIGN AND OPERATION OF WASTE ROCK Pll..ES . . . . . . . . . . . . . . . .. 4
2.1 Preliminary Design Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4
2.1.1 Waste Rock Characterization. . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.2 Site Characterization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Stability Factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.1 Foundation Stability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2.2 Waste Rock Pile Stability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2.3 Assessing Waste Pile Stability. . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Types of Waste Rock Pile Configurations. . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4 Construction/Operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 10
2.4.1 Foundation Preparation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11
2.4.2 Methods of PiliDgIMaterial Placemem ...................... 12
2.4.3 Constructing aDd Operating Surface Watez Controls. . . . . . . . . . . . .. 14
2.5 Monitoring. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 17
2.5.1 Sttuctural Monitoring. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 17
2.5.2 Environmental Impact Monitoring. . . . . . . . . . . . . . . . . . . . . . . .. 20
3.0 DESIGN AND OPERATION TO ADDRESS ACID ROCK DRAINAGE. . . . . . . . . . .. 23
3.1 Acid Generation in Waste Rock Piles . . . . . . . . . . . . . . . . . . . . . . . . . . .. 23
3.1.1 Acid Generation Chemistry .. . . . . . . . . . . . . . . . . . . . . . . . . . .. 23
3.1.2 Acid Generation Pathways. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 25
3.1.3 Acid Generation Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 26
3.2 Design and Operating Factors Affecting ARD """"""""""" 28
3.2.1 ~ Pile ()peration "'" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 28
3.2.2 DiversioDS/runoff control to address ARD . . . . . . . . . . . . . . . . . . .. 29
3.2.3 Monitoring for ARD """""""""""""""'" 30
Predictive Testing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 30
ARD Modeling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 32
Engineered Designs for the Prevention of Acid Generation
in Waste Rock Piles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 33
3.6 Comparison of Engineered Designs. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 36
3.3
3.4
3.5
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4 0 CASE STUDY . . . . . . . . . . .. 39
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1 Me' ~1Jghlin Mine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 39
4.1.1 Mine Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 39
4.1.2 Waste Rock Piles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 42
4.1.3 Waste Rock Disposal Planning, Monitoring, Reevaluation and
'. Plan Modification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 43
4...1.4 What Went Wrong? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 44
4.1.5 Waste Rock Disposal Management and Cost .................. 45
5.0 BffiUOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 47
Figure 2-1.
Figure 2-2.
Figure 2-3.
Figure 2-4.
Figure 2-5.
Figure 2~.
Figure 3-1.
Figure 4-1.
Figure 4-2.
Figure 4-3.
LISI' OF FIGURES
Classification System for Waste Rock Pile CoDfiguratioDl .............. 9
Ascending and Descending Methods of Construction .................. 13
Stratification Within a Waste Rock Pile .......................... 14
Flow-1brougb Rock Drain .................................. 16
Wire1iDe and Buried ExteIIsometers ............................ 19
Typical Piezometer Types .................................. 21
Model for the Visualization of Long Term Am .................... 27
Mclagblin Mine Location Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 40
McUughlin Mine I..ayout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 42
Inter Barrier Construction at the McLaughlin Mine Waste Rock Piles ....... 46
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1.0 INTRODUcnON
The U.S. Environmental Protection Agency (EPA) is developing technical summary reports
on several issues related to the management of wastes produced by the non-coal mining and
beneficiation induStry. This document provides summary information on the design and operation of
waste rock piles.--The intent of this and similar reports is to provide state and federal regulators with
information on the newest technical designs and innovations used in the environment.aJ management of
mine waste.
Section 1.0 of this report introduces the subject of waste rock piles and provides information
on research being conducted on WIIt.e rock piles by goverrtJllP.Dt2l agencia in the United States and
intemationa11y. Section 2.0 summarizes the current understandings of finvt21N'!ftb1 waste rock pile
design and openDon. Section 3.0 summarizes information on the prevention of acid rock drainage
(ARD) through the design and operation of waste rock piles. Finally, sectioD 4.0 presaD a case
study of a waste rock pile.
1.1
Wute Rock GeDeratioa
The act of mini", iavolv. the excavation of rock conniniJ'll valuable miDerala. This rock .
known as ore. To accesa and euavIte ore, minen must move aDd store or dispose of rock that does
not contain ecoaomic miDenI vaIua. This rock is blown as waste rock. This report summarizes
recent and proposed future research efforts in the design and operation of waste rock piles, as they
influence the poteDtiaI for enviromDeIItal impactJ.
W Ute rock consists of non-minenlized and low-grade IDinualized rock removed from above
or within the ore body duriDg emaaioD activities. Waste rock includes graaular, brotea rock and
soils ranging in size from fine sand 10 large boulden, with fines conteDt largely depeadeat on the
nature of the formation and the methods employed during min.ing.
Waste rock is produced . nolKOal mines as a byproduct of excavating an identified economic
mineral ~ metal depoIit. MiDeI design their open pit and underground operaDonslO provide the
most cost-effective meaDS for recovering the ore. Since removed waste rock is usually transported to
some nearby locaDoa for disposal, mines generally attempt to limit the amount of waste rock removed
as much as economically feasible. Modern mines use computer models to determine the most
economical pit configurations, taking into account safety and reclamation requirements.
For open pit mines, the stripping ratio is the amount of overburden and waste rock that must
be removed for each unit of crude ore mined and varies with the mine site and the ore being mined.
Depending on the nature and depth to the ore deposit, mine waste rock may constitute the largest
volume waste stream generated by a mining project and can amount to thousands of tons per day.
The quantity of waste rock generated relative to ore extracted from a mine is typically larger for
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surface mines than underground mines, reflecting the greater costs of underground mining operation.
The ratio of waste rock to ore (i.e., the stripping ratio) at surface mines may range as high as 10:1
for some areas, with typical values ranging from 1: 1 to 3: 1 for most mineral types.
Because ore grades in mined material are generally continuous, waste rock with mineral
concentrations just-below the "cut-ofr grade (i.e., the grade at which the target mineral can be
recovered economically) may be stockpiled separately from other waste rock; this material is often
referred to as "subore" or "low-grade ore." In addition, the cut-off grade at a given mine may
change with the price of .the commodity, thus leading to more or less waste rock being disposed as the
stripping ratio changes. The ratio of waste rock to ore is much lower at underground mines than at
surface mines, refiectiDa the hiPer cost of UDderpouDd mini.. Because of the higher cosa,
underground mining is most suitable for relatively high-grade ores.
1.2
EamroameotaIlmpadl
Historic waste rock disposal practices provide evidence of the types of poteatiaI enviroJmu.-.dAi
impacts that may result from improper design, construction, or manaaemeat of waste rock piles. FO!
example, waste rock piles may experieDce slope or foundation failure. In addition, both abaDdooed
and active nolKOal miDiDg sites have ezperienced problems that result in i"V"dl to local surface and
ground water quality. Both physical aDd chemical surface water i~ may result from increased
sedimflJlt2tion in the stream due to rmraimnem of waste material in runoff from the piles.
The generation of acid rock drainage (ARD) at waste rock pilei is well ~Jmellted in both
U.S. and international scientific litenture. Acid drainage is generated during all steps of the mining
process via chemical oxidation of sulfide compounds, particularly iron sulfides, to sulfuric acid.
Future uses for surface and ground waters that receive acid drainage from waste rock piles or other
mining process wastes may be limited by the acidity of the waters.
.
w Ute rock also is used in the construction of roads, tailings starter dams, buttresses for heap
leach pads, and other em- aDd off-site COIISttUction. The formation of ARD from reactive rock used
in these applicatioDs aIJo baa led to significant environmental problems caused by le.al"hing of high
concentrations of heavy metals to surface and ground waters.
1.3
Slimnuary of Literature ReYiew
Through the research conducted for the preparation of this paper, Ca~I. in particular, was
found to be prominent in waste rock pile design research. In North America, most waste rock pile
research is currently being performed by several different Canadian government and industry-
sponsored groups. These groups include: the British Columbia Mine Waste Rock Pile Research
Committee, the Mine Environment Neutral Drainage Program (MEND), and the Canada Centre for
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Mineral and Energy Technology (CANMET).
Research c:oDducted by the British Columbia Mine W ute Rock Pile Research Committee is
focusing on the problems of stability experienced at mine waste piles constructed at coal mines in
British Columbia. Although this research focuses on continual stability problems associated with coal
mine waste piles, the results of the work sponsored to date by the committee can also be applied to
non-coal mine waste piles. The British Columbia Mine Waste Rock Pile Research Committee baa
funded the deve10pmem of five reports on design, operation, monitorin&, failure characteristics, and
review and analysis of failures of waste rock piles: Investigation and Design Manual (Pitem, 1991),
Operating and Monitorin& Manual (Klohn LeoDOff, 1991), Methods of Monitoring (HBT AGRA
Limited, 1992), Review and Evaluation of Failures (Broupton, 1992), and Failure RuDOut
Cbaracteristic:a (Goldec, 1992).
The Mille EnvironmaIt Neuual.Drainage Program (MEND) baa been vecy aetive in the
researd1 of acid rock dniDaae- 1bi8 coopfuti~e research proaram is spoDIOftId IIId fi----' by the
C2ML1ian mini", iDdusUy, the aovemmeat of C2n~!I aDd the aovemmentl of several ~i..
provincel. Its purpose is to aasiIt the mini,,& iDduauy aDd aovermnem ageaciea in deve10piDg and
imp)~1 tecbniqueI and tedmoIogi. for minimi~ IIId mitigatiD& Kid le..et.-on IIId ia
impadl . tailiDp and WIlle rock pilei (II wen as the auociated enviromneIItal ;~).
The C~n~!I Centre for Mineral and Energy Tedmology (CANMET) baa provided funding to
the MEND and British Columbia Mille W ute Pile Research Committee in their curreDt research
projects.
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2.0 BASIC DESIGN AND OPERATION OF WASTE ROCK PILFS
To design aDd operate stable waste rock piles, the operator mUSt eusure proper foUDdation
materials, allow fOr slope angles aDd construction processes that will ensure stability throughout the
life of the mine, and provide for proper water drainage to minimi7~ infiltration/seepage. The shear
strength and durabQity characteristics of both the fouDdation and waste materials must be considered,
as well as drainage patterDS and predicted pore water pressures. This section addresses the major
factors that affect the stability of waste rock piles and bow they should be considered during the
design and operation of the pile.
2.1
PreIimiDary Desip CoDsideratiODl
Geaen1ly, the first step in initiating the design of a waste rock pile is the as,-Mly of all
available iDformation aDd data ~IVY 10 characterize the waste rock aDd proposed site. Much of
the data is available from public or govemmeat orpni7mnaa (e.,., topographic mapa, clnn-e
information). This information is typically supplemented with field iDvestipOO.. that may iDdude
land surveying, sampliq from test pits, treDChes, or boreholes, grouDdwater 1IIOIIit.oriDg, aDd
piezometric and pereolmoa testing. Further detaiJs on field tatiD& requireIneIa aDd UJchniqu. are
provided in CANMET (1977), McCarter (1985), Piteau (1991), aud Brodie eLaI., (1992).
2.1.1 Waste Rock Characterization
Since mining sites vary in the types of materials enco11Dt«ed in the acavaDon of ore, a full
characterization of the anticipated waste materials should be completed concurrent with mine design
planning. However, Piteau Associates points out that the diversity of particle size aDd physical
properties associated with waste rock leads to a difficult and complex sampling and analysis process
relative to that required to characterize foundation soils and overburden materials. In addition,
material properties may chan&e over time due to stresses within the waste pile, weathering, chemical
changes, and other types of degradation. Although abrasion and durability tests attempt to measure
potential degradaDoa. the effect of combined factors over time is difficult to predict.
The waste rock material to be disposed in the pile should be analyzed for both physical and
chemical characteristic:. The strength of the proposed pile may be assessed by such parameters as
rock type (igneous, metamorphic or sedimentary), density, panicle size distribution, and pore water
pressures within the waste pile. The density and pore water pressures also are influenced by the pile
construction method and subsequent amounts of consolidation and settlement. Pore water pressures
decrease the stability of both the waste and foundation materials. With respect to shear strength, the
most favorable pile materials are hard, durable rock with little or no fines present. Failure can occur
when a pile containing material with excessive fines is constructed on a steep slope. In addition,
waste fines may become saturated from water runon and snow melt and trigger a failure. Ideal waste
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rock would be of sufticieat durability, hardness and coarseness to provide high shear strength and low
pore water pressure. A description of the ,mineralogy of the pile material is necessary to identify, for
example, the preIeDCe of sulfide materials such as pyrite, which indicate the potential for acid rock
drainage. Litewi8e, the preaeace aDd amount of basic minerals (e.g., calcite) must be determined in
order to evaluate the acid neutralization potential of the rock pile.
Once waste material characteristics are known, prop« design and construction methods can be
implemented. For example, as poor-quality waste materials are encountered during construction of a
waste pile, specific secbOOS of the pile can be prepared to receive the materials or additional
protection can be installed. Overburdea materiala (e.g., soils), due to their fine nature, would
contribute to instability in the waste rock pile aDd should, therefore, be placed in a separate location.
Likewise, aad-&eoentiD& rock may be segrepted 10 that immediate measures may be taken to
control acid geuenDon. Lastly, since the physicallDd, particularly, the chemical properties of mined
rock can chqe ov« time. then alIo should be a program of periodic or conrimlnlUl characterization
to eoaure that chaDpa caD be made to desip aDd operation II c:oaditioDI warnDt.
2.1.2 Site Characterizatio
A complete site characterization involves the collection aDd coosw.-rina of a divene set of
information that eacompasaeI site activities, layout, terrain, hydrololY, aDd cIimst-.. For example,
physiographic data address the proximity of the location to the source of the waste, nearby mining
activities such as blastiq that could affect pile stability, the site capacity, aDd topographic features
such as slopes aDd valleys that may detem1ine placrmem of the waste rock pile aDd surface wat«
flows. Hydrologic coDiidentioDa addreaa natural draiDa&e aDd climst-. concerDI iDdude storm evema,
tempenture, precipitation, aad wind paUelDI. The MWRPRC data indicate that more failures have
occurred duriD8 winter aDd SPriDa seasons, which typically briDa areater IIDOUIIII of precipitation,
than in summe:r aDd fall (Broupmn, 1992). The hydrogeology of the site, including the position of
the watec table, groundwatec flow systems, distribution of discluaqe aDd rechar&e areas, aDd
groUDdwatec US3p, assista in idatifyiD& pathways for poteDtial eavironmf'.llbl and human health
risks. In addition, pouDd aad surfxe watec quality, air quality, fish and wildlife habitat and
produ~, vea~ aDd ~ivi"l aDd future land use must all be determiDed in order to assess
potential eIfY~.tti impact (Piteau, 1991).
2.2
StabiUty F8dOn
A close look at the factors that affect waste dump failures provides important information
relative to the stability parameterS that should be considered during the design and operation of waste
rock piles. The Canadian Mine Waste Rock Pile Research Committee conducted an in-d~th study of
over 40 failures of waste rock dumps from coal mines aimed at improving the design and operation of
future dumps (Broughton, 1992). The research committee identified numerous factors that potentially
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contribute to waste dump failures. For example, the data indicated that most waste dump failures
occur on foundation slopes exceeding 20 degrees. Piteau and Associates (1991) identify seven major
factors that affect pile stability: dump configuration, foundation conditions, waste material properties,
method of constrUeUon, dumping rate, piezometric and climatic conditions, and seismic and blasting
activities .
2.2.1 Foundation Stability
An important aspect of site characterization includes an' accurate characterization of foundation
stability. Soil tests for shear strength, penneabilitylhydrau1ic coDductivity aDd consolidation, and
depth determinationa for any loose or incompetent soils, are important in assesam,the streDgth aDd
preparatory requiremeutl of the fouDdation. CompeteDt fouDdatioIIa refer to fomvl!8tinQ material with
higher shear strength 1han the waste materials; weak fouDdations have lower shear streagth than the
waste materials (CANMET, 1977). Level foundations are also more stable than sloping fouDdatioIIa.
The strength aDd durability of the UDder1ying bedrock should also be evaluated.
FouDdation soil conditions, including the type of soil and the amouDt of pore pressure, have &:
large effect on overall waste pile stability. In addition, excess pore pressures may result from hip
loading rates aDd steep (ooftdltion sIopeI. Where sloped fouDdatioIIa are preseIIt (Le., arear- tbaR 10
degrees), a stability aaalysil is Decellary to determine the m:nimmn poteatial dispI~ due to
base shearing.
Wbete a level fosJnd2tina (i.e., less than 10 degrees) is provided, the pile will geaeral1y DOt
be susceptible to mass sliding aloagthe base UDless it is CODStructed on very weak fouIVImnn
materials (e.g., organic soils). In general, sloped foundations present greater risks associated with
sliding than level foundations (found~tinns are less stable and material may move farther and more
quickly). Therefore, CANMET recommends higher safety factors for waste rock piles on sloped
foundations (CANMET, 1977). In addition, foundation stability also may be affected by temporal
conditions that are DOt considered duriD& a site characterization. For example, the Mine Waste Rock
Pile Research Committee fouDd that winter freezing of foUDdatioDl, before loading, may also
contribute to some failures (Broughton, 1992).
2.2.2 Waste Rock Pile Stability
The size and configuration of a waste pile directly affect its stability. The variables that need
to be considered in the configuration of a waste pile are height, volume, and slope angles. The height
of a pile is defined as the vertical distance from the ground at the toe of the pile to the pile crest.
Piles may range from 20 m up to 400 m (piteau, 1991). In the U.S., the size of waste rock in a pile
is usually defined as tonnage or acres covered; however, the Canadian protocol describes the size of a
waste rock pile as a volume unit. The slope angle of a pile is determined by the type of construction
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method used. Eod-piled materiab result in slopes at the angle of repose, appro][llm1t--ay 37 degrees,
the average male of repose for free-piled cohesionless rockfill. Steeper slopes may result if the piled
materials have some cohesive properties (such as significant fines) or consist of largely angular
boulden.
At sites Where waste rock consists of frictional, coarse materials, the ma][lmnm slope at the
pile perimeter is the angle of repose (as indicated previously, typically about 37 degrees). Where
competent foUDdation materials are foUDd and adequate drainage is provided, the height of the pile is
generally unlimited. Where fouDdation materials are weak, stability analyses must be performed to
determine the mnlmnm height and slope to ensure the desired level of stability. (CANMET, 1977,
provides guidance on such analyses, depending on the specific characteristics of the foundation
materials.) Compacting foundllion materiaIJ alooa the perimeter slopes (Where mnimum stresses are
found) caD increase stability and allow for greater slope angles aDdIor pile heigbta. Howevel',
compaction redueeI permeability, thereby iDaeasiDg the need for draiDap comrola.
In additioD to pile oven1l heiJbt, volume, and slope 1DIIe. the praeace of lifta or beDcha is
an importaDt pile configuration factor for aidiDg the stability of the pile. LiftI and beDcbes reduce the
overalllllgle of the pile &lope aDd coatroI nmoff from the pile. Be8cbeI are lliabdJ doped borimaaII
surfaces constructed iDID the slope of a waste rock pile. They are typically ~ ill pilei . :
part of reclamation, be it concurreat with construction or during final recl~. LiftI are die
working levels of a waste rock pile. A specific area of a waste met pile may be worked . a
particular lift level until the lift is completed. ADother lift may then be constructed on top of the
previous lift. Constructing a pile in lifts, or utilizing benches, duriD& the active operation of the pile
typicaUy resulll in lowei' slope mgla and, th«efore, increased stability. Other pile operation
med10da caD affect pile stability. well. Most importaDdy, a rapid rate of waste dumpina caD
contribute to the instability of a pile and baa been attributed to several pile failures. High dumping
rates can lead to increased pressure in the dump and DOt allow for adequate time for consolidation and
settling of the pile to eosure stability (Piteau, 1991). The direction of crest deve10pmem is aDOtber
operational factor that sboulcI be close1y monitored; deviatioDi from the design plan may direct the
waste pile to fonnd2tinDi of greater slope and lead to reduced stability.
2.2.3 Ass-.~ Waste Pile Stability
The ~ method of assessing the overall stability of a proposed waste rock pile is to
calcuiate a fador of safety (FS). The FS represents the ratio of the shear strength to the shear stress.
As noted above, the subility is directly related to the foundation and waste rock materials and
drainage, along with general dump features, including size, volume, slope angle, degree of
confinement, method of construction, and dumping rate. The accepuble FS for an individual pile will
depend upon site-specific conditions related to the potential impacts/risks of a slope failure.
CANMET (1971) recommends an FS (greater than unity) to account for differences between predicted
design parameters and actual conditions within the pile. Methods used to calculate safety factors
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generally focus on shear stress aDd pore water pressure along w critical surfaces" within the pile
(including the variability in these parameters along such surfaces). Selection of the methodology to
be used at a specific site depeDds on the operator's determination of the most likely failure mode.
Piteau (1991) includes a description of the types of failure modes associated with waste rock piles
(edge slumping rotational failure, liquefaction, etc.). .Mine dumps located in areas of high seismic
risk require specialized safety analyses.
For piles constrUcted on sloped foundations, the following equation can be used to determine
the maximum possible foundation slope angle (assuming DO hydrostatic pressures, i.e., proper
drainage exists) (CANMET, 1977):
tan i = tan {)IFS
i = fonnd2tino slope angle
() = friction angle betw- waste aDd fouDdation materials
, FS = factor of safety (stability factor)
When UDeVeD follnd~ slopeI are eooountered, the Wqe method described in CANMET (1977)
can be used to determiDe wbem. the calc::ul:ated factor of safety is acceptable to the opel".
Piteau (1991) indudel a dump stability rating system to provide a "semi-quaDtitative" metbod
for assessing waste rock pile failure poteDtial. Based on the resul1I of the ratiDa aualyais, dumpa can
then be placed in ODe of four dump stability classes. The classes are correlated to "negligible,"
"low," "moderate," and "high. failure potemial. The purpose of this s~ is to guide opeLators in
waste rock pile design (m conjuDCtion with information on the site-specific risb associated with a
failure).
2.3
Types of Waste Rock Pile Conragurations
Taylor and Greenwood (198S) presented a classification system for DOn-impounding waste
rock pile types based on the topOaraphic setting and the configuration (geometric shape) of a pile.
This classificatioD system waa developed ostensibly to provide waste rock pile vocabulary common to
industry and govenmeut repret--'ves. The pile types identified in Taylor and Greenwood (198S)
are illustrated in FIgU1'e 2-1 and discussed further below.
Valley-Fill
A valley-fill waste rock pile partially or completely fills a valley. It is typically constructed
by dumping waste rock at the head of the valley and extending the pile by continuous dumping of
waste rock on the downstteam slope. A valley-fill also can be constructed by building horizontal lifts
at the farthest downstream location of the pile toe and then proceeding upstream toward the head of
Final
8
July 19, 1995
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Waste RocJc PUe Repon
-.--'" /,//
--7/ff
,"((
"
I. VALLEY-FILL (COMPLETE)
",----)
'" ---"'"
",'" ",-- / /
" -"",,'- ",'- - ---.. ..~/
--~"" ~ 7"
......- -
---*"
---'" -:~~
---., 411'.
--- ///(
//1
,/, ,
II. CROSS- VALLEY
Dr. RIDGE
",.-.--:;;
ifffJ7 ~.
---J.7
/1 ((
DI. SIDE-HILL
Y. HEAPED
Figure 2-1. Classification System for Waste Rock Pile Configurations
(from: Taylor and Greenwood, 1985)
Final
July 19, 1995
9
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Waste RocJc PUe Repon
the valley. In this type of construction, surface water controls (i.e., diversions) are required to divert
upstream runoff from collecting behind the pile lifts. The surface of valley-fill waste rock piles is
graded or sloped to prevent poDding and to limit infiltration of surface water runon and precipitation.
Cross-Valley
"
A cross-valley waste rock pile is constructed from one side of a valley, across the drainage, to
the other side of the valley. It does DOt completely infill to the head of the valley as in a valley-fill
waste rock pile. The crest of this type of pile may be horizontal or sloped aDd the fill slopes can be
established in either the upstream or downstream direction. This type of pile requires surface wat«
controls (i.e., diversions, culvens or ftow-tbrougb rock drains) to avoid impouDding water behiDd the
pile. A valley-fill waste rock pile consuucted in an upstream progression can ess~2l1y be
considered a cross-valley type pile UDtil the head of the valley is infilled.
Side-Hill
A side-hill waste rock pile is constructed on the side of a valley but does DOt cross~er the
ftoor of the valley. The pile slopes are usually inclined in the same direction as the found2rioD (i.e..:
hill slope). These waste rock pile types do DOt block major drainages.
Rim
A ridge waste rock pile is constructed on the crest of a ridge, with the slopes of the pile
extendi." down both aides of the ridge. The crest of the pile may be sloped or horizootaI.
Heaped
A heaped waste rock pile is constructed by stacking or piling mounds of waste on a relatively
flat surface that is horizontal to moderately inclined.
Hybrid TY1)es
Waste lOCk piles, while lenerally classified as one of the above five types, may be hybrids.
Further, waste met may be backfilled into open pits or disposed in in-pit piles. In addition, waste
rock is often used . slope fill to construct roads or level ground surfaces for mine/mill buildings.
Waste rock is also used for start.u dams andJor buttresses in tailings impoundment construction.
2.4
Construction/Operation
Construction of waste rock piles begins concurrently with the excavation of waste rock from
the mine workings after all necessary siting and design work has been completed. This section
Final
10
July 19, 1995
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Waste RocJc Pile Report
addresses ---ures that may be required to prepare the foundation for construction of a waste pile,
COIDIDOD techniqa8 for CODItrUdin& the pile, and methods for controlling surface water, 1UlM)D, aDd
run-off.
2.4.1 Foundation-Preparation
Preparin& the land area that will serve as the pile foundation is the first step in constructing a
waste rock pile. If possible, the siting and design work should have resulted in the selection of a site
that requires mininW preparatioD. However, if the fonndltioD coDditiona are poor, aDd no other site
is available, ~ foUowm, ~ may be used to promote stability of the WIIte rock pile. All of
these ~es require various levels of equipmeut lCCeSIibility to the toe or 8footpriDt8 of the
planned waste rock pile.
ClearinJ Vesr..ntintt and Striwm, SaiJa
Unleaa topIOil baa 10 be saIvapd, the removal of tteeI IIId vecewioB from . site is DOt
normally p«fonned siBee it requireI 8dditioaaI ~iInrea IDd the veceatioD c:a provide *qdi-:
to the undsiyiDa lOila. If logiq of timber resourcea 0IHite is required prior to COIIICnIc:IioD (due .
U.S. Forest Service reaulJDo.. or odler reaIODI), it should be completed . dole to the iniri-v. of~
pile construction aa possible. In special situatio.., c1earin& vececation from . site may be necessary.
For example, if heavy vecetatioD aim OD . steep slope tb8 is to form the hnMf!ltina of the pile,
clearina may be advisable 10 prev- the covered veaetation from fDrmiq . weak zoae beneath the
pile. Also, if the site investiption indil-2t,.. weak soils that require removal prior to dumpm"
clearin& would be nee...,. FmaIly, areal of the pile tb8 will be CODStnICted to coavey war. (e.g.,
rock: drains) should be cleared of all vegetation that could poteDtially impair the hydnu1ic
performance of the structur..
As st3ted above, weak 1Oila, suc:b aa organic soila, may require excavaDon if it is determined
that the pile will not provide sufticieat colL1Olidation for an adequate fouDdItioIL After excavation of
soft soils, the pound surface sbouIcI be compacted to eusure that the DeW {"lIndhO is sufficiemly
dense to suppoIt .... pile. The acavated surface may also require gradina or draina&e if water is
present.
UndPrtlninaH ..... Prelifts
In areas of sballow groundwater or surface discharges/seepage (i.e., wetlands), sanuated soils
need to be drained to support waste materials. Finger drains (constructed of sand aDd gravel) and
gravel blankecs can be installed to collect water from the area beneath the pile footprint aDd convey it
to a central collection ditch. If the ground water seepage rate is significant, perforated pipes may be
placed in the gravel drains to increase the hydraulic capacity. Typically where underdrainage is
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11
July 19, 1995
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Waste Rock Pile Repon
necessary, rock drains are needed to CODVey upstream surface water flow aDd surface water
infiltration through the pile.
An alternative to the excavation of soft soils is the construction of prelifts. Access to the
foundation area is a limiting factor. A prelift of pile material from 5 to 15 m in thickness can be
used to consolida, span, or contain weak soils.
2.4.2 Methods of Piling/Material PlaamIeDt
Mine waste piles are built using either ascJO.lVli"l or
-------
Wane RocJc Pile Report
...
~ACe
~
.t A'tttn--. c............
..... Ten '.
~
881W. 'UIJNI8
~) 1).....6.. C...........
.... Wr"'"""f ...
Figure 2-2. Ascending and Descending Methods of Construction
(from: Piteau, 1991)
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13
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Waste Rock PUe Report
COARSE LAYER
Figure 2-3.
SLUMP
Stratification Within a Waste Rock Pile
(from: Call, 1985)
2.4.3
CoDSttuctmg and Operating Surface Wit« Controls
Waste rock piles often cover significant land areas, and surface water 1'UJH)n, 1'UJH)ff, mow,
and snow melt control mea~ are frequently necessary to preveat saturation of slopes or
developmem of phreatic surfaces in the pile, and to mitigate surface erosion, ARD, and other releases
of toxic pollutants to groUDd or surface waters (piteau, 1991). Environment21 regu1aDooa and
guidelines governing the design, CODStrUction, or operation of drainage control systems exist on the
Federal, State, and localleve1s. In &eoeral, these regulatioDa and pide1ines attempt to achieve two
primary objectives. First, the drainage system must be able to handle a calculated ftood-event and,
second, the system must result in a discharge of adequate quality (e.g., low toxicity, reasonable pH,
low suspended solids) (Claridge, 1986). Designing these structures to meet these needs requires
accurate precipitation data and the coDSideration of specific events that may result in extreme runoff
volumes. The control measures discussed below are often most effectively used in conjunction with
each other.
Direct precipitation onto a pile should be directed to a collection area to minimi7.e infiltration
into the pile surface. Slight grading of the pile crest and bench surfaces can be used to direct runoff
away from the outer face of the pile aDd into ditches or rock drains that quickly convey runoff/runon
to collection areasldiscll:lrges. In addition to rainwater, control of snow and ice IC-l"nmnlation also
must be considered for waste piles in northern regions. There is some evidence to support the theory
that failures in waste piles located in the Rocky Mountains have occurred as a result of weak zones
forming in areas where spaw and ice within the pile melted rapidly (piteau, 1991).
Final
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Waste Roclc PUe kpon
Thece are several rules of thumb utilized to minimi7.e the effect of snow aDd ice on waste pile
stability .
W ute rock should not be dumped on faces where significant amounts of snow or ice
are present (greater than 1 meter)
dump surfaces should be worked evenly so that depressions do not ac'.(;Ilnmlate large
amounts of snow
snow removed from the mine should not be dumped on an active pile - a separate
wSDOw-only. dump should be utilized
snow should not be dumped in draiDaae area or diversion mJlnnel. that will be
coveced by waste rock materialJ
dumpin& in the winter moDtbl should be planDed 10 that it occurs on exposed
windward faces with the least amount of mow 1("1'!lmulatioa (PitaII, 1991).
Surface \Vat« divenioDi sbouId be constructed to convey UParadi- 60wa (overiaDd . well
as ch:mnelized flow) arouud or throu8b the base of the pile in eu&ineend struc:tDrel. These ItrUdIIrII
prevellt surface water nDIOn and surface water flows from iDfiltratiD& the pile materials and, in .
extreme coDditioDl. ~-mppiDg the waste pile. Thece are three primary optioDI for divertiDI
surface waters in a waste pile. Water can be diverted 1) around the pile in a diversion channe12)
along the bottom of the pile in a rock cIrain, or 3) UDder the pile in a culvert (Claridge, 1986). The
method or methods used are depeodeut on site-specific coDditioDa aDd waste pile type. Diversion
('}umnels can be lined or unlined ditcbes and are often eagineered 10 convey the upgradieut flows
associated with a specific ca1cnl~ storm eveut. These cIuIIIMI IIID8t be regularly lIUIIinbined or
collected sedimeIIt and brush can impede the flow through the strUctureII.
Even in those waste pile typea whece it is feasible to divert surface water around a pile (side-
hill, ridge, and heaped designs), it is impossible to divert all surface water. AI a result, subsurface
rock drains are used in Dearly aU typeS of waste piles 10 enhaDce surface water flows tbrougb the pile
(see Figure 2-4). A rock draia it CODSttUcted by one of two medloda. Fint, waste rock material
(non-reactive 0lil)') of the appropriate size can be placed directly into the area that will constitute the
drain. Second,"" met material (igneous or metamorphic and DOlH'eactive) can be dumped from
trucks directly 018 the fbundatioD (eud dumping), which allows for natural segregation through the
dumping action (Li&bthaII, 1986). A minimum pile height of 20 10 30 meters is geuerally necessary
to ensure that natural ScpegaDon results in the formation of a coarse gradation rock drain (Das,
1990). If the pile is less than 20 to 30 meters in height or the waste rock material is DOt of adequate
quality, coarse, durable rock may need to be hauled to the site and used in constructing the drain.
Igneous and metamorphic rocks are the preferred rock types to be used to construct rock
drains due to their generally good resistance to mechanical degradation and compression (Das, 1990).
Final
15
July 19. 1995
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Waste Rock Pile Repon
MAX'" WATD 8UYAT108I
AT aGO-YUII PLOOD
-------
50rII
........
o
1C1G18
Figure~. FIow-TbI'ou&b Rock Drain (from: Ilgbtball and SeDan, 1985)
In additioD, the rock should be resiataDt to slaking and freeze/thaw activity.. DaI (1990) sugestl tbJt
ovec 80 pel'ceDt of the rock sbould grade larger than 100 mID. The requiremeat of coarse durable
rock for rock drains can significantly alter a facility's miniqg plm (i.e., alteriDa blaItiDa metbocIs to
ensure proper material size or avoidiD& the IDiniD& of certain areal c:onniniqg rock types that are DOt
suitable for rock drains) (Lighthall, 1986). In the evem tbJt acceptable rock draiDs canDOt be
constructed utilizing waste rock from the site, a manufactured rock drain can be used (Oaridge,
1986).
The rock drain should be capable of conveying the mean annual stream flow without buildup
of water upstream of the pBe. The inlet capacity and the outlet capacity are vital considerations in
designing and ua-Miaa a rock drain and ensuring the stability of a pBe. While it may be necessary
for the inlet to ~..._. the meaa umual stream flow, the outlet is typically desigJU'Jd to discharge up
to the 200-year 8Goci flow without a reduction in pile stability (Du, 1990). Commonly, the rock
drain is exteDded up the upstteam slope, or inlet, of the waste pile ~rovidina for additional capacity in
the event of blockages or increased flows. The outlet can be stabilized by constructiD& a flatter slope
and/or utilizing larger, more angular, material (Lighthall, 1986). A bypass channel should be
provided, in the event the inlet becomes blocked, to convey flood flows that cannot be passed through
the rock drain without undesirable buildup of head on the upstream side of the pile (piteau, 1991).
Final
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Waste Rock Pile Repon
The use of a rock drain to convey surface water through a pile in place of upstream diversions
creates a greater poteDtial for environmental impacts downstream of the pile. AB surface water flows
through the pile die water may become contaminated: these contaminants then migrate downstream.
In addition, ecoloaical habitats downstream may be compromised due to decreased flows resulting
from blockages in the pile (Das, 1990). However, it is not possible to divert surface wata' around all
types of waste pile -designs. AB economically and environmentally favorable dumpsites become more
and more scarce,-it may become necessary to use -cross-valley- or .valley-fill- designs utilizing rock
drains for stream conveyance and surface water control (Claridge, 1986).
1.5
MoaitoriD&
W ute roc:t piles sbould be moaitored on a replar basil to detect my eDVironm81t21 or
structural changes. Then are numerous reasons for (and benefits gained from) instituting a
monitorin& proJDID at a waste rock pile. These include, but are DOt limited to: 1) complyiD& with
regulatory requiremecu, 2) IoweriD& operatina coati (compreheosive IIIODitoriDa may allow a
reductio.n in the faaor of safety in the dump, and thia iI associated with lower dumpiq COIUI), 3)
improvin& worker aDd equ~meat safety, 4) reducin& the risk of dama&e to 1be eavironmeat (due to
dump failure, acid roc:t drainaae. etc.), S) improvin& dump behavior predictioaa, IDd 6) improvin&
future monitorin& metboda (HBT Agra Ltd., 1992). Two primary conditioaa are gmenD)'
monitored: structural stability (e.g., movement and internal pressures) and eavironllV!Jlt2l inw1CtS
(e.g., waste geochemistry, water quality) of the pile. Monitoring can be qualitative, quantitative, or
both.
2.5..1 Structural Monitorin&
Oualitative Strudllral Monitorin2 Methods
Qualitative monitoriq is the visual obsenration of a pile for structural changes that could lead
to stability probiema. Typical c:banges that should be noted during visual monitoring include
slumping. or JDOIVemeat within a section of a waste rock pile.
Quantitative Strummtl MonitDrin, Methods
Quantitative monitoring U$e5 instruments and/or sampling and analysis procedures. There are
numerous quantitative instruments available for monitoring water movement and pressure. Two
reports published by the British Columbia Mine Waste Rock Pile Research Committee (Operating and
Monitoring Manual prepared by Klohn Leonoff Ltd, 1991. and Mined Rock and Overburden Piles:
Final
17
. July 19, 1995
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Waste Rock PUe Repon
Methods of Monitoring prepared by HBT AGRA Ltd, 1992) provide detailed information on the
quantitative monitoring of waste rock pile stability.
The most common equipmem for quantitative monitoring of movement are wire1iDe
.extensometen (aI~ called tripod monitors or mechanical crest monitors) or buried exteDSOmeten (see
Figure 2-5). Extemometers measure the change in distance between two points through the use of a
wire attacl1ed betWeen a stand installed on stable ground, which bas low potential for movement, and
an anchor in the pile surface (usually on a slope). The stand is the reference point from which the
wire line may be read to determine the amount of movement occurring down the slope, parallel to the
wire. ExteIIsometen may be of the simple type with m2ft\l,! reading required at the stand, or a
recordel' UDit may be used to plot movemeat over time. ExteDsometen are limited, however, by the
fact that they are able to meuure only the degree of movement that occurs parallel to the wire. In
certain cases, this can significmdy under..mm2te the actua1 movemmlt that baI oc:eurred.
Th«e are a number of movemeut-monitoring tedmololies designed to collect vertical
settlement and total displacemeDt data from waste piles. These include electronic cfisn-
measuremeat (EDM), sealemeat poges, tiltmeten, and inclinometen. EDY is a tedmique based 0It
traditional surveyina tecbniquea, whel'eby an electronicaUy generated signal, controlled by a
microcomputec, is aimed at reflected tara-. After 10catiDI the positioD of the tarJet, the EDY
distance aDd new position of the tarpt can be measured. Unlike euemometen, EDM tecImoIolY is
able to accouDt for the full vector movement of the pile.
Settlement gauges are utilized to quantify the settling of the waste rock pile. Typical gauges
consist of a measuring device on the surface of the pile and a probe that is inserted into the pile. The
degree of settlement (movemem between the surface and the probe) can then be measured and
quantified. These data on vertical movements can supplement those gathered from ext.eDSometers.
Inclinometers and tiltmeters are used to measure the horizontal or angular displacement within an
embankment or across the foll1v1:at1nn boundary and are most applicable in monitoring for rotational
failure modes. Additional movemeat monitoring technologies currently are undel' development but
are DOt yet available. These tecbniquea include the Global Positioning System (GPS), acoustic
emission tecbniquea, electro~c monitoring (EM), videometry, and the use of lasei' camena
(HBT AGRA UII., 1992).
Pore pressure is one of the most critical measurements required to ensure pile aDd foundation
stability. Pore pressure-monitoring instruments can be divided into two categories: 1) devices that
are sealed into the pile and record pressures at that point (piezometers) and 2) open hole or well-type
standpipes that record an average pressure over the length of the borehole. Piezometers are the most
common pore pressure-monitoring devices employed in waste piles. Piezometers are available
utilizing many different types of technologies: pneumatic, electrical resistance, electrical vib~g
wire, standpipe, and multi-port or multi-level, as shown in Figure 2-6. Piezometers typically contain
Final
18
July 19, 1995
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Waste Rock PUe Repon
Light Weight Portable
Tripod Stands
Pin Driven into
Face of Waste Pile
Sus~nded
Weight
Reference line to
Monitor 'Total"
movement
a) Wire1ine Extens0R8eter (Tripod fotxIitor)
Anchor
t-teasuring
Statioo
Fill (2.5m thick)
Wire in flexible
casing (eg. PYC)
Enclosure
I
/
I
I
Original Surface
Monitors displacement parallel to
extensometer only.
Shear 7.one
b) Buried Extenscneter
F1gure 2-5. Wireline and Buried Extensometen
(from: HBT AGRA, Ltd., 1991)
a diaphragm that ia deflected UDdec pressure. The deflection in the diaphragm is measured utilizing
numerous typeS of tedmologies, and quantified as a pressure reading. All of these piezometer types
can be suitable and the decision on which type to use should be based on availability, prior
experience, requirements for data acquisition, and cost (HBT AGRA Ltd., 1992).
Final
19
July 19, 1995
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Waste Rock Pile Report
Current Monitorin, Methods EtnplQyed
In -Mined Rock aDd Overburden Piles: Methods of Monitoring, - published by the British
Columbia Mine Waste Rock Pile Research Committee, HBT AGRA Ltd. presents the results of a
survey of mining ':Ompanies, instrumentation companies, consultants, and other related companies
concerning current -monitoring methods employed. Over 170 companies were contacted. The results
of the survey indiCate that the most common monitoring practice is the use of simple mechanical
devices (extensometers) to monitor surface movement. Use of piezometers to monitor pore watec
pressure in foundation materials is less common, but is used by some facilities. In nearly all cases,
qualitative visual monitoring is performed (HBT AGRA Ltd., 1992).
Problems with Current Methods
There are a number of problema associated with curreat sttucturalmonitoring practices. For
example, disturbaDceI may occur in the location of surface monitoring stations. These refenace
points may be moved or disturbed due to rock falls/sloughs or activity on the pile. Data aI80 may be
incomplete due to iDfrequeut or irregular data collection, thereby resulting in an incomplete pidure of
the pile's state. In addition, wire1ine exteDSOmeten (the IDOIt COIDIDOB movemeat-monitorina
instruments) accurately measure only mov~ that occur in a direction parallel to the line made b,
the two tefeteo.:e poina. Movemeqts in other directions often are DOt recognized. Lastly, common
movemem-monitoring tedmiques (i.e., ~metcn and visual observation) provide a picture of the
performance of the surface of the pile, but do DOt provide data on the iDternal portion of the pile
where critical instabilities may initiSltp-. (HBT AGRA Ltd., 1992)
2.5.2 Enviromnentallmpaa Monitoring
Structural monitoring of the pile, while a useful tool for recording potential environmental
conditions that could lead to stability problems, is not effective in monitoring for environmemal
(water quality) chqes. Qm1ntit2tive ground-water and surface water monitoring techniques based on
chemical analYIII are required to assess water quality impacts.
Ouantitative r---'-Watf!r and Surface Water Monitorinr
Ground-water monitoring is generally not required by federal environmental programs, but is
often required by States. In addition to satisfying regulatory requirements and aiding in the protection
of the environment, a well- designed ground-water mOnitoring program is economically beneficial as
it can reduce expenditures and liabilities associated with broad-scale remediation (Maxfield and Mair,
1995).
Final
20
July 19, 1995
-------
--
~
--
-
p
.... ....
-
-
-...
...
-...
..-CiII
--.-
--
-
p
Wane Rock Pile Report
.... ....
-
p
Vibnting Wire
FIpre u. Typical "aometer 1'ypeI
(from: HBT AGRA, Ltd., 1991)
Pna8atic
£lectric:.a1 Resistance :-
Sandra Maxfield aDd AIm Nair, in an article entitled .Stratqic Desip of Groundwater
Monitoring Programs for IDorgania,. (1995) outline the requiremeDts for a well-designed grouDd-
water monitoring system. A well-designed detection monitoriDg program coataiDa several eJemems.
The first element is the developmeat of baseline ground-water quality data collected over a minimnm
of one year prior to foU1vl~n preparation aDd pile construction. A group of up-aradieut monitoring
wells should be used to establish ban"line grouudwater COncenttatiolll. These baseline data are then
used as the comparison for future data collected. These we1ls are theoretically unaffected by the
mining practices and are used to develop baseline data. On-site or down-gradieut monitoring we11a
are needed to determine if a release bas occurred. Data from these we11a are compared to baseline
data from up-gradjeat we1JJ. Comparisons are made based on a set of targeted parameter'S. The next
element of a mmai80riDa proaram is the sampling schedule and procedures. The sampling schedule
and procedures IIIoukI be consi9ent and well-documented, preferably in the form of a Sampling and
Analysis Plan (SAP). The final element crucial to a well-designed monitoring program is the
development of an action level or concentration limit that should not be exceeded in on-site or down-
gradient wells (Maxfield and Mair, 1995). The same procedure is applicable to the design of surface
water monitoring programs.
Quantitative monitoring for ground water and surface water quality changes downgradient of a
pile requires regular sampling and analysis of water samples (monthly, quarterly or another frequency
Final
July 19, 1995
21
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Waste Rode PUe Report
correlated to site-specific coDditions such as flow rates, that could affect pollutant release potential).
SufficieDt surface wit« sampling siteS and ground water wells must be established immediately
downgradieDt of the pile to address all potential water migration pathways. In addition, the
parameters selected for sample analysis should reflect the geochemistry of the waste rock and any
chemical agems ~ may be present as a result of the mining activities (e.g., acid drainage, heavy
metals, nitrates from blasting compounds).
Any changes in water chemistry may signal developing water quality problems. For example,
increasing concentrations of sulfate -and total dissolved solids may signal the initial generation of acid
rock drainage.
Final
22
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Waste Roclc PUe R£pon
3.0 DFmGN AND OPERATION TO ADDRESS ACID ROCK DRAINAGE
Que of the moat serious environmental problems resulting from waste rock disposal at DOn-
coal mines is the leaenbon of acid rock drainage. Acid rock drainage from waste rock piles is
responsible for the degradation of water quality in many streams in the western United States as well
, ,
as impacts to local ground waters. While much of the acid rock drainage is the result of historic mine
waste disposal prictices, discussions with federal and state mining regulaton indicate that the same
problems are seen at new waste rock piles where DO definitive long-term designs or CODttols are
implemented. Site-specific differences from one mine to another (e.g., rock type, climate variables,
etc.) require the application of amtroI teclmiques or designa that are customized to the site COnditioDl.
In auod1« report, EP A bas described in some detail the process of acid geaeration and
various predictive mecbocII iIa 1IIe 8Dd UDder deYeiopmeat (U.S. EPA. 1994). 1bia Iectioa
GlnnnmizeI some of tbIt iDfonDatioD . it applie8 to WIIt8 rock pil..
3.1
Add Generation in Waste Rock Pilei
Acid generation and JeJminl from waste rock piles can sevel'dy impact poteDtial usa for a :"
recemn, water. Acidic: wateI' fifth....... disaolution of 1DdalJ, raulti"l ill the dWJI2I'Ie of acidic
water connininl high coaceatntioDI of toxic metala from the wute rock pile. Low pH waters may
be corrosive to metal or concrete struaures, toxic heavy metaIJ may deplete 01' 11- :aqJ1!Iti~ life, and
higher oXYleD demand lowen the concemration of oXYleD that is available to sustain biotic
organisms.
In extreme cases of acid draiDage, receiving waters may exhibit a layer of brightly colored
yellow oraDge-red iron precipitale mown as .ye1low-boy.. The precipitate may alao exhibit colon
ranging from yellow-greea to purple or black, depmiin& on the preseace and oxidation stares of
minerals and meWs tbat are mobilized in the acid drainage. Other indicaaon of acid drainage include
mineral salt bloo., irregular melting of SIIOW over acid-geoentiD& materiaIa, aDd Kl"Iltnnlation of
whitish gypsum slimel Ilona draiDage pathways from the waste rock pile. (BLM, 1992)
3.1.1 Acid <*--- Chemistry
Acid drainage is generated from the chemical oxidation of sulfide compounds, particularly
iron sulfides found in pyrites, to sulfuric acid. These acid generating reactions require the preseace
of the pyrite or other sulfide compoUDds, oxygen, and water. The pyrite may be oxidized either by
oxygen or ferric iron. For the second reaction to proceed, ferrous iron must be oxidized to ferric
iron. The oxidation reaction rate is increased in the presence of iron bacteria such as 1hiobacillus
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Waste Rock Pile Repon
fe17'OtJXilJllM. Heat genented during reaction also accelerates the rate of oxidation (BLM, 1992,
Doyle, 1990). The chemical reactions are as follows:
1)
F~ + 7/2 ~ + H20 <==> Fe2+ + 2S0l + 2H+
2a)
Fe2+ + H+ + 1/40: < = = > Fe'+ + 1/2~0
2b)
F~ + 14Fe'+ + 8~ < = = > ISFe2+ + 2S0l" + 16H+
The net result of these reactioDa is the production of sulfuric acid, which migrates with the water
percolating through the pile.
Acid draiDage can be mitigated or suppressed by the presence of basic carboDIIe miDerals,
such as calcite (Cac~, magnesite MSO. + H20 + C~ [M = Ca, M&J
Some of the C~ leaent.ed by tbi8 reaction will remain dissolved in the aqueous solution and will ac&
as a buffer 10 1II2innin the solution pH near neutrality UDtiI the neutralization capacity of the
carboDate minerals is reached.
The add generating pountilll (AGP) and net nelllTalization pote1ItiIIl (NNP) of a rock sample
can be measured by static and kinetic tests (e.g., humidity cells) aDd by titration of a slurry of the
sample. Minerals connining hip levels of carbonates may exhibit higher NNPs, aDd thus be better
able to suppress ARD. Waste rock with a ratio of NNP to AGP of 3.0 or more is unlikely 10 pose a
threat of ARD. A NNP/AGP ratio of 1.0 or less will almost certainly result in ARD unless other
mitigation measures are tabID. NNP/AGP ratios between 1.0 aDd 3.0 mayor may DOt produce
significam amoUDtS of ARD; sud1 wate rock piles must be further evaluated individually.
The men preseace of ARD-neutralizing minerals in the waste rock or a high NNP is DOt
sufficient to emure that neutralization will occur. Carbonates may be concentrated in pockets
inaccessible to .. bulk of the ARD, or may be present beneath the rock surface. Even where such
minerals are preseut at the surfaces of the rocks and accessible for intimate contact with the ARD,
chemical reactioos can eveDtual.ly result in the deposition of slimes or scale deposits on the surface of
the carbonates, preventing contact of the ARD with the bulk of the carbonate minerals and leading to
depletion of the rock's neutralizing potential sooner than predicted.
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Waste Rock Pile Repon
3.1.2 Acid Geoention Pathways
,
As showa in the reactioDI above, acid genuation in waste rock piles requires the preseace of
both water aDd OXYIeD. Oxygen is transported within the waste rock pile by gaseous aDd aqueous
advection aDd ~ Gaseous diffusion, described by Fick's First Law, is the migration of
gaseous particles &om a region of high concentration to one of lowei' concentration. Fick's First Law
states that the gas-flux or rate of diffusion is proportional to the difference in gas coneeatratious; the
proportionality CODSWIt is refetred to 18 the diffusion coeffic:ieat. Upon coDStnu:tion of a waste pile,
the iDterna1 gal composition mimica the overall co~ition of the atmosphere without signific:aDt
gradieora between reaio- of the waste pile. As time pilla, howevel', chemical reactionI oc:aurinI
withia tile waite pile al- compocirioDa of localized pseoua space, aDd coDCeOta~ andiaIII
appeII'. (Northwest ~. 1991)
The rate of pllnDIpOrt aovened by these andiaIIIaeaenUy apIaiDa nadoaa pi
movemeat aDd is not likely to be II hip II movemem nteI c:auaed by advec:tioD, the predomiDaat pi
InDIpOrt procell. GaseoaIIdvec:tioD ia the bulk mov.- of ... duoup larp ponII or ~
in the waste roc:t pile drivea by pnuure andiaIII. Altboqb iDftueaced by the pc. .,~ility of the ~
waste rock pile, advec:tioa alIow8 for 0XJaeB InDIpOrt up to depdII of 60 meten (North.. !
'.
Geoch- 1991, citiDa WIdawonh, 1981). Pnuure andiaIII may be formed by pseoua tla.oapoct .;
resu1tiq from )~i,«t iDcreaseI in t.empalblre &om exoIbermic add-aeaeratiD& reactionI aDd
decreaMI in surface tanpel'1ture duriD& winter 1DOIdba. IDcrea8ed ~... Dear Kid-&enenDna
sites inaeaIe the local vapor prea8d(e, which pushes the psea up toward the cool« surfa:e of the
waste pile. 1bia pheDomeDon haI heeD obsKved by localized IDOW meIIa on WIIte rock pU.
(Northwllt G-.Mnt~ 1991). Oda.. facton in formiDa pnuure andiaIII include wind c:urr-. and
atmospheric pressure c:baDaea; bowever, MEND indicates that gaseous advection indue. by thermal
gradients is cousidered to be the primary pathway for traDSpOrtina the amoum of oxygen necessary to
create the large volume of acid produds that is observed in waste rock pUes.
Oxygen is also preaeat ia I diuolved state and transported through the waste rock via
movemeot of.... The rate of water movemem, whida is aoverned primarily by aqueoua
advectioa, is ~- upoD die hydraulic: conductivity and hydraulic: padieat. Where direct
precipitadoa, w8da .,.... the hydraulic: gradient, is the only source of oXJaeuated water to a
waste rock pile ... wben permeability is low, the transport of oxygen via aqueous advection is
expected to be IIIIICIa 1- than tnnsport via gaseous advection (Northwest Geochan, 1991). In fact,
research on tailings pUes iDdicate8 that, due to their smaller particle sizes and preseace II slurries,
tailings do DOt permit aqueous or gaseous advection to any significant degree.
War« tranSport through a waste rock pile is complicated by variatious in parameters such as
hydraulic: conductivity, which vary with the permeability of the rock and between layers of rock. As
a result, certain sectioDS of the waste pile may exhibit channeling or water flow across rock layers
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Wast~ Rock PUe Repon
while other sections exhibit stratification or water flow within rock layers. Cb~nneling is expected to
be the primary method of water tranSport (MEND, 1991, citing Whiting, 1981); however, MEND
indicates that monitoring of water movement through unsaturated rock is extremely difficult aDd that
field observations are sr.arce. Hydraulic gradients in waste rock piles are depeadeat on a number of
variables, including the rate of infiltration aDd nmotf, surface topography, height aDd width of the
pile, aDd distribution of hydraulic conductivity.
Solids tranSport within waste rock piles also affecta the migration of water' aDd oxygea needed
for acid generation. For example, suspeDded solids may plug water' channe1s, reducing the infiltration
capacity of the t".h~nne1a. Particularly problematic is the plugging of water chmnel. within large
particle- size bases that are iDteDded to drain excesa water' from the rock pile. Small partide-size
rocks, fines, weathered clays, and mineral precipitants may be flushed to the base, causing a decrease
in permeability over time and eventual geotechnical instability of the waste rock pile (MEND, 1991,
citing WhitiD&, 1981).
In addition to serviDg II a by factor in the geueration of acid draiDa&e, water movemellt in
and arouDd waste rock piles constitutel the primary pathway for such COnbminmln . acids, acid
byprodUdS, aDd meWs, 10 migrate from a pile 10 the eaviroDmeat. MEND (1991) paenlized the .:
hydrogeological conditio.. surroundinI the waste rock pile with three model.. 1be three modeIJ
describe waste rock pilea that are located in 1) a grouDdWlB' recharge area, 2) a p'OUDdwatcr
discharge area, aDd 3) between a recharge and disc:barge area on . sloping NCface. MEND reports
that the latter model is the most common scenario because mines often lie in slopinJ terrain. Many of
the same factors inf1ueacing WIt« movemeDt within the waste rock pile also influence wateI'
movement around the pile. These factors include infiltration aDd nmotf, evapondon, hydraulic
conductivities aDd gradients, cb~nMiDg and stratification, aDd mineral precipitation/dissolution.
3.1.3 Acid Generation Model
Robertson aDd BartoD-Bridges (1990) present a model for visualizing acid geueration in a
waste rock pile (see F1pI'e 3-1). Unfortunately, an understanding of the physical processes in a
waste rock pile"" 1- to the chemical environment necessary for the development of acid drainage
is not fully developed to support a waste rock pile design.
The model describes a waste rock pile as having -reactor sites" where optimum conditions for
the development of ARD (sufficient oxygen, water and microbes) are available in the presence of
sulfides. ARD develops at the "reactor sites" and is transported in water. seepage down through the
waste rock pile. As the acidic water flows contact basic materials along the flow path, the acidic
seepage is buffered and the neutralization capacity of the rock decreases. As more aDd more
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Waste Rod Pile Report
IIUCTOII PU&
b ""-.-
-.~c 1-1
-
I
I
I
t
. '.
U8C--1IOCIC
leD':
--- I8COIn'.--ATID-T8I
- /III:IG_IIR/WIJ'.
- - 8n11M81DA88
Figure 3-1. Model tor the Vasualizatioo of Lon& Term ARD
(from: Robertson and Bartoo-Bridas, 1990)
neutralizadoD ~ is ued, die pH of the acidic seepage is buffered less, resultin& in lower pH
solutioDS ..., ... the flow path.
Over ~ . the waste rock pile matures, the neutralization potential of the rock can
approach zero. ThUl, tile acid seepage in the flow can DO longer be buffered. This resu1ts in the acid
front reaching the base of the waste rock pile. (The buffering capacity, as well as the acid-generating
potential, for individual piles is highly site-specific.) The extent to which the entire pile becomes
acidic is dependent upon the distribution of acid generation and neutralization capacities in the pile.
The acid product of acid generation is stored in the pile not only in solution, but also as solid-
phase products. These are stored in zones where salts are precipitated as a result of evaporation
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Waste Rock PUe Repon
within the pile. These stored acid products are flushed from the pile during periods of increased
infiltration and flow (spring runoff and high precipitation events). Thus. high concentrations of acid
products are found in the seepage water as flow rates increase. High loadings of COnhlmin2nts occur
at these times as toxic: pollutant concentrations increase along with flows.
" -
3.2
Design aDd Operatin& Factors Affecting ARD
Once the chemistry and pathways leading to acid generation are UDdelStoOd. and poteDtially
acid-generating rock is determined to be present, specific: design aud operating procedures can be
evaluated based on their effect on the generation aud traospon of acid draiDace. For eumple. during
construction of a waste pile. segnaatioD of poteIItiaIly acid-geoeratiDg waste materiaIa is increasingly
being employed as an imnwli2M measure to control acid geaeratioD. ~u~t. of imlN!Jdi!lte
measures that are discussed in this sedion include blending acid-DeUtralizina materiala
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Waste RocJc PUe Repon
result iR - wealy araded diIUibutioD throughout the height of the pile (Northwest Geod1em, 1991).
This method ia -- ~" wid1 frictional waste material such as blasted rock aDd penueable sand-
gravel ~ 10" a m...~....ation of coarse material at the base of the pile forma an UDderdrain
aDd prevema die deYeiopmeat of hydrostatic pressure (CANMET. 1977).
Pusb~ma is pedormed by dumping the waste rock near the crest of the pile with a truck
or conveyor, IIXttheD pushing the rock over the crest with a bulldozer. Although large-sized
particles still colla on the boaom of the pile, there is much less segregation of the finer particles
with puahoodwDpiq.
Wdh freKumpiDI. the waite pile ia constructed by raDdomly depositina piles of waste rock,
approv"'-"y 2 meten in hei&ftt. aaou 1 level surface of the pile. Each layer, called a lift, is
co~tct prior 10 initi-v- of the sublequem lift, resultiq in greater compacdoa but reduced
~ of partide... M 1 result, there ia lea natunJ dlmmeliq aad Itntification tbaIa with
ead- or pIIIIMiumpiDa IIId lea pnt-ntd for OXYI. aDd water tl'aDIpOIt leIdiDa 10 acid leaeraDoa.
are.. COIJIII8Ctioa IDd 1- dI-w~ aIIO reduce pore war. preuurea, tbua produc:inc 1 more
stable walle pile. F« my mecbocI of COIIIUUCtioIa, additional beDefit is provided by the routiD8 use of
dump tnICt tnftk wbicb. .... directed in 1 raudoaa ~, provjda a uniform compacdoa ICIOI8
the wortiD& Awe..., of a lift or WIIt8 pile.
3.2.2 DivOtlioollnmoff coIIttOIlO addreu Am
WheaeY. m.~ ... met piles connini1\l acid-ge:aeratia& sulfide materiala sbould be
located away from surface W8tID, spriDp, seeps, aDd wetlanda (IMAC, 1992). In additiOD, with
potenti~any acid-geaentiDa .... rock piles, water should be diverted around the waste pile with
divenioD l'it--H. DivenDa t"JI-w, should lead 10 wastewater ~ systemI that iDclude acid
neuttaliz3tion willa sIbIin8 COIIIfOIIIik. such as lime, soda ash, or mdi\Uft hydroxide, followed by
aeration and maaIa precip~ These wastewater ~ systaDI typically pIICI'Ite 1 hydroxide
and/or su1&ta sIudp reqWrinl4~. The selection of a disposal praaice sbou1d consider that
metala ar".~ ~. tllalludp may leach, especially at low pH.
Crou . .IJ. vaDey-fill, and other designs requiring the diversion of water through the waste
pile, sbouki be.L 1.~...1b8 pocenri21 exists for acid generation or lea~hing of toxic poliutaDtl
(e.g., nitrateII from reaiduJI bllltina compounds, heavy metals). In the event that diversion of water
through the pile caDDOt be avoided, it is necessary to design the pile and rock drain to address several
environme0t8 impact issues. First, it is critical to determine the acid genenDon potemiaI of waste
rock that is to be disposed in designs that require rock drains (e.g., cross-valley or valley-fill
designs). Second, allowing oxygenated water to pass through a pile via rock drains allows for the
generation and transport of acid from the waste rock. Therefore, the drainage system should be
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Waste Rock Pile Report
designed to minimi7~ the retention time of water in the pile, thereby reducing its coDtact with reactive
sites. Lastly, sufticieat flow must be ensured through the drain, otherwise ecological babitals
downstream may be compromised due to decreased flows (Das, 1990).
Snow and ice should be kept off the surface of waste rock piles, and particularly acid-
generating piles, to""prevent water infiltration in the spriDg. Infiltration rianced by snow and ice-
melt may lead to icid generation if reactive rock is present. In addition, waste rock covered by SIIOW
and ice and associated infiltration leads to weak layers in the pile that are predisposed to failures.
Section 2.4.3 of this report presents methods for preventing the vl'Ilmvlation of ice and mow on the
pile.
3.2.3 MonitoriDa for ARD
As stated earlier, chemical aiIaIyses coDducted as part of aD eavirollllll!ftt2l monitorm,
program that show iDaeaseI in sulfate aDd toW dissolved solidi may sipal the initiaIgeaentioa of
acid draiDaae. In addition to chemical lDalysis, there are other medaods that may indicate acid
geaention aDd assist in 1II!IIti~ the rates of reactioD.
For example, since pyrite oxidJtion is exothermic, monitoriDa of ~atdre may be used to
detect acid geueration and determiDe reaction rates in waste rock piles. T~Mdre profiles,
corrected for surface seasonal fluctuatioDs, identify the depths and rates of acid geoention. In order
to develop the correction fadors for atmospberic tempeiawre changes, base1iDe tempentures for each
season must initially be established. Heat generation models tate iDto accouut the beat loss to the
surface above a certain ~ and beat loss to the groUDd below that depth. TemperatUre profiles
from several boreholes are then used as input to the model. Laval (1991) iDdicates that CUITeIIt
models are oDe-dimensional, as8lnft,,,, linearity of reaction rates and beat flux with increased depth.
Additional field data would allow for the development of more complex models that CODSU water
infiltration and oxygen convectioa iDSide the waste pile. (Laval, 1991)
A second medIod for predicting acid generation rates is based on l~""ue COncemratioDS, flow
rates to c;ollectioa dird1ea, aDd reaction stoichiometry. Given the average concentration of pyrite in
the waste rock ~ the iron IDd sulfate concentration of the l~l'.hlte. and the average flow rate, an
estimMM time period for poteDtial acid generation may be calculated (Laval, 1991).
3.3
Predictive Testing
The ability to predict the long-term geochemical behavior of a waste rock pile is crucial
during design and planning stages. If the long-term geological behavior of the pile can be accurately
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Waste Rock PUe Report
prediCted tbrou&b teICiD&. measureI caD be.taken to control acid rock draiDap aDd metaIa I~hi...
The objectivea oi predictive teItiDa are: 1) to determiDe if a discrete volume of waite rock will
geaenre acid, ... 2) 10 predic:t the quality of the drainage based on the rate of acid form2tinn
measured.
Two majof- types of characterization tests are currently used in predicting the behavior of
waste rock piles: -acid-genenting potential (AGP) testa aDd 'l'2l"hue extraction tests. AGP tests are
described either as static or kinetic aDd are used in determining a waste'. pnt.....nsl for genenting aDd
neutra1izin& acid. Static AGP tests screen wastes for their net add-aenenting poteDtia1. The 1D08t
common static teIt, ~ 1CCOUIItiD&. --.ora the acid-aenenting pnt..misl of. WIIte baled on
its sulfur COIltellt, usiD& . smid1iometric coavenion of iroD sulfide 10 sulfuric acid. The neutraIiziDa
potential of the waD iI quaDtified by - r Hun.., die amouJIt of acid co""'med by . IIDIIl MnI.at of
fiDely grouDd .....
If . sipi~ acid-aeae:nt:iDa poteI1tiaI ia idemified ill . waste by acid-baIe ~.~ or
ocher Italic ~. it is paenIIy tested furdIer usm, . tiDetic metbod. lCiIwic ~ an more
comp1- tedmiqueI aimed . ,iBmI-m., tb81d1111 concIitioai of dleWllte pile over tim8. The .....~
COIDIDOD ~ tilt is . .......w.y cell tilt. The humidity cell tat apoI. .. WII88 rock ....,... . -r
alterDatiDa dry aDd moiat air fIoM, '-M~. aDd in IOID8 c:aIeI, naIIInl bacteria, OVII' . period of ~
mamba (Lawreace, 1990).
Mally 1ftMm~I. have beeR writteIa OD 1he use of st3tic: aDd tineUc t~ indudiJIc sampIiDg
protocoIa. A review oftbe8e tapa iI foUDd in EPA (1994) IDd in varioua nporII puhlHhM by
MEND IDd the BridsIa Columbia Acid Mine DraiDap Tilt Force.
l~!lte attacDoD - .. employed to predict tile mobility of heavy meWa from the rock
pile USm,1D acidic levlt,,,,..-timlll Bec:auae the chemilUy of wure rock piles d1aD&es
cousidenbly 0V8' time. tradiIioIIIIlaboraaory I~ing t.eItI (e..., EPA', TCLP tat) may not
accurately predict the loDg-tmD .eoIoIkal behavior of waste rock piles (LawreDCe, 1990). Some
testa, however, .. ~ 10 IIIimic precipitation infiltration (e.,., EPA'. MeIbod 1312, the
SyntheOc ~ ~ Pmc:edure, aDd Nevada's Meteoric W.. Mobility Test) aDd may be
more 3waua- .~.. -- met 1baD the TCLP test.. Laboratory I_t>hil\l procedures are nec-es.sary in
identifyiD& h r dlarKl8'ilCicla required by environmemal regulations. However, waste rock
pile desip, ~ IDII ~i!ltina requires additional tests thJt are designed 10 ideDtify '~t>Joa:rt1!
charaeteriltia over die life of . waste pile. As a result, researchen have developed, aDd continue to
develOP, teclmiques for cbanderiziDg mining wastes aDd predicting their long-term behavior in waste
rock piles. Examples of long-term leachability tests include lab column tests, pilot rock piles, and
teSt pads in the field.
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Waste RocIc Pile Ri!port
Results of predictive testing must be interpreted carefully. In addition, results from any one
particular test may DOt be conclusive. Predictive characterization tests may fail to consider factors
such as climate, hydrology, and the physical and geotechnical characteristics of the waste. Most cases
require multiple techniques, or a technique designed to reflect site-specific geological and atmospheric
conditions. (La~ce, 1990)
3.4
ARD ModeIi"1
The genermon of acid in a waste rock pile is governed by a complex combination of
physical, geochemical, and biological processes. With the increased power aDd speed of computers,
researchers have attempted to silPul$ the formation and JDiIntion of acid rock drainale utilizing
computerized mathematical modda. Researchers at Ohio State University completed the first
comprehensive ARD computer model in 1972 (Northwest Geochan, 1991).
ARD mode1l based primarily on the stoichiometry of acid geDentioD reactions and FICk',
Law of gaseous diffusion have been available for many years (Northwest (jIJnmMII. 1991). SiDce the
early 1970's, how~, ARD models have been expanded to Uu:oIpOOlte a IIIIIDbec of additioDal
factors that either CODttibute to or deter acid rock drainage. '
These factors include, but are DOt limited to, gas and water transport in a waste pile, climatic
conditioDs, particle size distribution in the pile, and the type of waste rock preseIIt. First, simple
stoichiometric reIatioDShipl have been expanded to incorporate waste from piles COnt2inil\g multiple or
distinct types of geological material. Models now exist for piles that comain pyrite, chalCO{lytite,
chalocite, covelite, or mixtures of these materials. Contributions from bacteria present in waste rock
piles can also be incorporated into ARD models. Second, modelling of oxygen transport through the
pile has improved. Currem mode1l simulate the diffusion of oxygen into pore clt211ftPJa, advection
(Le., bulk gaseous transport resultiDa from pressure gradients that are induced primarily by heat
generated during aciclgeoeratioD), IDd ovenll diffusion of air through the pile (Le., raDdom
movemem towards the bottom of the pile, which is the direction of lower oxygeD CODCeIIttation).
Third, the particle size distribution of materials in the pile is incorporated in many models, euabling
the model to de8IrmiDe more accurately the amount of geological material that is available for
oxidation. Nm.. improvements have been made in the modelling of water movemem through waste
rock piles. Compla pattems of water migration resulting from saturated flows (heavy water flow),
unsaturated flows (small 8trickles8 of water), and the migration of acid condensation all can be
incorporated into ARD models. Finally, surficial and climatic processes that affect the overall pile
dynamics can be addressed in ARD models (Northwest Geochem, 1991).
ARD models have improved greatly over the past several decades. Future modeling efforts
will need to address the effectiveness of various control measures designed to alter acid generation
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32
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Wam Rock Pik RqJon
and migration behavion. Mode1iD& of ARD will UDdoubtedly coDtiDue to improve in the future aDd
will play an ~ role in tile mmagemeut and coDttol of ARD.
3.5
En~~- DeiPr lor die PreYmtiOD of Add GeDeratiOD in Waste Rock PI-
As more miDeI encouoteI' sulfide waste rock during the excavation of ore grade rock, the need
grows for proven engineered desigDa for disposal of these reactive wastes in waste rock piles. The
intemaJ. reactioDS, or geochemical proc:essea, occurrina in a waste rock pile include the following:
acid geuentioD, acid neutra1izaDoD, bacterial enb~ of reaaion mea, aDd metals lead1ing.
ARD preveative tedmiquea tar,. reducdoDI in oxyaeAlw-.1raDSpOrt aad bact8iaI COUIIt ~t?ry
for acid ,eaentioa or increaIe8 in neutralizatioD or bufferina capacity. Examples of acid drainage
preveudve """iqueI iIIcI1Id8 jmpenDeIbIe CIpI .. 1m.., surface water divenioaa, aad bllIIdina
acid~........~ -- 8UdI . limaIDDe wida 1118 WIIt8 met. Several of 1bae typea of di~
desigDl/melbodlare wau-aly ia 1118 or 1IIIdIr study IIId are dilauled Wow. All of these w-Jlmq,-
are of rec:eat oriPL 1ber Ipp8r' ~..~11y 1OUDd, but their toaa-term eft'ecdv- il1IIIpftJVSL
As a reIUIt, lcma-urm vipl- iI DeC...,.
In 9dt'mn.. researchen are iDvestiptiq DOVel acid ,eaenDoD COIIIrOI medaodI aimed .
inhibiting me 'eDefltioa of acid iD die waste pile inItead of CODttOlliDa or tratiD& die acid after it has
heeD 1eDef1led. Two such mecbodJ are pboIpblt. Iddition aad electrolytic COIIttOI, wbkb aIIO are
pr~ in tbis sectioIL
En~aDoD
EncapsuI2rinD (or segregation) of sulfide-bearing waste rock within a waste pile iI an
inc:reasiDgly common design for the prevention of acid gmention in waste rock piles. Th«e are
several venionl of the eDCaplul!ltin& design, which generally iDYolv. the dispoAl of reactive waste
within one area of a -- rock pile, aurrounded by DOD-acid-ge.scltiq waste. Fipre 4-3 illustrates
a geueraiized cn8~~ view of a waste rock pile iDcorpo(atma the eocappliltioD method 10
isolate reactiw .--1108 VA'" aud water. VariatioDS on the ,enenI desip may be iDcorporated
wben require4 " dT ~ cooditioDa. For example, in some c1imate1 it may be ..easary 10
surroUDd the & I!.- -- wida low permeability materiala 10 prevent moisture from infiltrating the
isolated waste. fa ...irina, aD 1IDdedying capillary barrier may DOt be nec-:ln,y in an area of deep
groundwater aDd 'intihwl precipitaDoD (i.e., most areas of Nevada).
The effectiveness of encapsulation remains open to debate. Harris and Ritchie (1990) repon
on a completed 600 m long, 400 m wide and 15 m high waste rock pile at the Woodcutters M"me in
Australia. This pile was constrUcted to contain the most reactive waste rock (5 perceot pyrite) in the
middle of the pile. Indications suggest that the pyritic material in the pile is oxidizing and a
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Waste Roclc PUe Report
monitoring program has been established to determine the extent aud rate of oxidation. The report
did not provide information on the indicators used to determine that acid genention was occurring in
the pile.
The Me' .~J1pJin Mine in California bas employed an encapsulation medIod for the disposal of
reactive waste. To. date, monitoring of the waste rock pile UDderdrains indicates that the method bas
been effective (see Section 4.0). Similarly, the Rain mine in Nevada began encapsulating sulfide
waste rock within the larger pile by placing compacted neutral materials over, unda', aDd around the
sulfide material.
~erin2 2M Blendm, of Acid 2nd Alkaline Materials
The addition of albline materials to acid-aeaentiDc materials caD provide pH coDIIOI in waste
rock piles. MEND cita several metboda that have been SUUated aDdIor inv-"pwI for their
etJectiveaesa. Neutralization metbods cited by MEND include bleadiDg acid-conmm"-l aDd acid-
produciq wastes to effect neutralization within the waste pile; placing albliDe mareriala sud1 .
limestone upgradieut of the acid-genenting waste roc:t, aDd placiD& 2IbJine-mareriaIs in a collection.
tread1 doWDStream of the acid source. Because these control tecbniques rely on the flow of water
throu8b bo8h the acid-CO-lIII"'1 aDd lCid-aeaentiDc material, the mcC181 of the tedmiques is bigbl,
depeDdeat on site hydrology aDd the ability to predict aDd manipulate WI8IIr t10w tbrougb the system.
MEND SOU- that the volume of material required to DeUttalize biPly acidic waste met could be
probibitive (Northwest Geochem, 1991). In addition, wbile pyrite oxidation is nearly complete within
the first year aDd a half of weathering, the acid drainage may leach from the met OVa' a period of
many decades, imposing long-term trMmneat requirements on mininl companies (Ziemtiewicz, et.al.,
1990).
Entrineered Soil Covers
Waste pileslocared in - with fine.grained soil, or till, mixed with day may be l'2ftdiciltel
for closure using ~~ soil coven. Fine-grained soils, when compacted aDd applied at near-
saturated coDditioDl, caD 0. low hydraulic conductivities, which reduces the transport of dissolved
oxygen via advectioD. In addition, at near-saturated conditions, few of the pores remain filled with
air, leading to low oxypn diffusion rates. A single saturated soil layer used as a waste pile cap
would eventually des_Irate due to drainage and water loss from evaporation. Use of a thick layer
may address this problem in some climates. In addition, single layer caps do DOt protect the pile
from wind and water erosion without vegetation. (Northwest Geocbem, 1991)
In order to maint2in erosion protection, an impermeable barrier, aud long-term stability to the
cap, multiple layered caps are advisable. A top layer of vegetation or coarse gravel prevents erosion
from weathering. A second layer of medium to fine textured soils serves to retain moisture for
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Waste RocJc Pile Report
vegetation and reduce oXYleD diffusion to the waste pile. In addition, the moisture retention layer
prevents the underlying layers from cracking due to excessive dryness. A drainage layer prevents
PODding above the lower, impervious layer and prevents moisture loss from the lower layer, which
may be designed to remain moist. The infiltration barrier is comprised of fine-grained soil,
impenneable clay, compacted till,-andJor synthetic materials. For drainage to be effective, it must
m~int~in a slope of 1 percent or greater (Brodie, et.al., 1992). Once installed, the greatest advantage
of engineezed soif covers is low m~intemlnce (MEND, 1992).
Case Study for En2ineered Soil Coven
In the 1950'., the OI~ of Mines of Soud1 Africa investigated several optiona for
Stabilizing slime deposits from gold mines against wind and rain. The research team determined that
a vegetation layer would provide the Deeded stabilization if suitable plaDt species were identified.
After testing hUDdreda of species, the represematives from the ChambeI' of MiD8 concluded that
common plaDt sPecia were required in order 10 lft.8innin an Idequaae supply of seed, aod that
modif1catioDs of the soil wilhlime aod fertilizer would smnin the growth of mmmoa planaa.
In vegetating the inactive waste piles, the research team eDCOUDteI'ed, in. 1961, slime ~
of high acidity that would not sustain the growth of vegetatioD. After experimaItin& with rotavatioIl ~
and lime additioo, the research team realized that the acid was risinl from below the up.- 1ayen of
the slime deposit, probably due to upward movemeDt of soil moisture replacing near-surface moisture
that had evaporated. In addition, the research team detmnined that the acidic layer lOOVed
downwards after rain, and was confined the first 2 meters of depth. The situation was resolved by
jmnl1ing wa. sprinkler systeml OD top of the slime depolit 10 nWnnUt moisture in the upper layers
and induce a downward movemeDt of watel' 10 the lower alkaline regiaDi of the deposit. A1I a result,
the pH of deposit surface was lNIinhlinsd at the level required 10 sustain plaDt growth. The sprinklers
provided an intermittent mist so that pooling and soil consolidation did not preseat additional barriers
to plant growth. Long-term monitoring of the slime deposit also was required in order 10 lNIinnin
the balance between optimum pH and moisture content. (Marsdea, 1987)
Chemical T.....rr-w
Chemical additives used to control the bacterial population or coat particle surfaces to reduce
acid generation from the waste rock have been researched to a limited exteDt. MEND provides
references for the application of anionic surfactants to limit bacterial growth in waste rock piles at
coal mines, and the use of organic and inorganic chemical treatment to control bacterial growth and
coat particle surfaces. MEND suggests that chemical treatment may be more appropriate for sbon-
term rather than long-term treatment because the chemicals tend to solubilize in water and require
repeated applications. In addition, chemicals applied to the surface of the rock pile are unlikely to
reach acid-generating reactions that occur at greater depths within the rock pile. (MEND, 1992)
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Waste Rock Pile Report
Underwater Disnosal
Disposal of waste rock underwater reduces the influx of oxygen to espnri211y zero due to
elimination of air transport underwater, and the low aqueous diffusivity of oxygen. Concerns with
this technique include impacts on the benthic ecosystem of potential metals l~i..g from the waste
rock, and the long.:term poteDtial for the low oxygen levels presem in the water to generate acid.
Robertson (1990)recommends a thin sand layer over the waste rock pile to penetrate pores and
reduce direa contact with the water. Robertson also recommends consideration of man-made water
bodies using non-acid generation waste rock for impoundment construction. However, the proximity
of a suitable water body, site, and/or construction materials may eJimin2t~ underwater disposal as a
viable option. In addition, the waste rock materWlDDIt be kept beaead1 the -- surface at all
times.
Underwater disposaJ of old waste rock is not recom..-vled because OXid,""D at acid-
generating sites bas already occurred. When placed underwater, 1arp IIDOUDD of acid c:ouJd become
mobilized. Cose1y controlled aeatnlization operaDoDl are pouible, however, tbe euviroDmeaIal
impact of large volumes of sludge (e.g., from lime tr~) COnbini"l metal precipitatel may be
significaat. (Robertson, 1990)
PhO$phate Addition
The addition of smaU quaDtities of rock phosphate (approwim!8b!ly S ~) to . waste rock pile
creates a chemical and physical environment that inhibits the formation of acid. Iron iolll are
consumed in rapid readiolll with phosphate ions to produce an iDsoluble ferric or fenoua pbospbate
and are thus prevented from r-mng with oxygen and water to produce acid. In addition, the
resulting phosphate salts precipitate and occupy reactive sites in the waste rock, further inhibiting acid
generation. (Ziemkiewicz, et.a1., 1990, MEND, 1993)
Electrolytic Control
UtiliziDa the principle of electrolysis, iron ions and other cations are oxidized at the cathode
and precipitate iDIoIuble hydroxide floes, as hydrogen ions (H +) are reduced at the anode to release
hydrogen gas~. This proceu has a secondary benefit of removing hydrogen ions which reduces
the acidity of the pile, and consequently, the potential for metals leaching (Ziemkiewicz, et.al., 1990).
3.6
Comparison of Engineered Designs
A study was recendy conducted under the Mine Environment Neutral Drainage Program
(MEND) to compare selected ARD preventive techniques (Yanful and Payant, 1993). Samples of
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Waste RocIc PUe Report
waste rock came from the Stratmat site, Heath Steele Mines, near Newcastle, New Brunswick, and
from Lea Mines Selbaie, located near Joutel, Qu6bec. Outdoor lysimeter tests and indoor laboratory
column experimeaa were performed on the waste rock samples with covers of water, soil, aDd wood
bark, as well as OD samples amended with varying levels of either limestone or phosphate rock.
Results of tripli~ nma were compared to controls which did not receive covers or aJDP.nd~.
The Stratmat waste rock was derived from the footwall of the ore zone, ~cl1 contains
dissemin:lt"C! and massive sulfides (sphalerite-galena-pyrite and chalcopyrite); the waste rock sample
was characterized as a sericitized aDd pyritized meta-rbyolite consisting of approJ:im:ltp~y 20~ pyrite
(9~ - 10~ total sulfur). The Selbaie waste rock sample came from a regioa domin:lteel by massive
pyrite iDfilled with quartZ; the sample COnt2iJWt apptO%im:lt..dy 75~ pyrite (almost SO~ total sulfur).
The outdoor tes1I used 170 kg of each type of waste rock in each experimaIt. The c:ruabed
(1-2 iDch) waste rock aDd covera.oc mneMl'IU!IIt$ wen placed in 160-L plastic mnniNq, wida bottom
draiDt for withdrawal of analytical samples, and exposed to ambieat weather CODditioDI. The iDdoor
tests each used 20 kg of waste rock sample in a 6-iDch PVC column with bottom drain; the cohmma
were subjected to cycles of wet and dry conditions. The followin& covers aDd :alllP.ftltments were
tested with each rock type:
-
.
water (1 m deep);
.
soil: 15 em of compacted clay sandwiched between 7.~ layers of saud;
.
wood bark (15 em thick, UDCOmpacted);
.
crushed phosphate rock, added at lev~ of 1 ~ aDd 3 ~ by weight, mixed with the waste rock;
.
crushed limestoue, added at levels of 1 ~ aDd 3 ~ by weight, mixed with the waste rock.
Lear1l1tM samples were collected periodically through the bottom draiDs and were analyzed for
pH, metals, aDioDI, ad acidity (as me CaCO,). Effluent water quality had been monitored for
approxim!lt..dy 14 moD8ba as of the date of publication. and was scheduled to continue for another two
years.
The control samples (without coven or amendments) began producing acid within 5 weeks of
the start of the teSts. The Stratmat samples generated acid at a higher rate than the Selbaie samples,
despite their lower sulfur content; the reason for the difference was not determined, but could be due
to different pore structures in the rocks. Higher acid production rates were observed in the laboratory
s2IDPles, and were atttibuted to higher indoor temperatures.
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Waste RocIc PUe Repon
The experiments using a wood bark covec wece designed to test the hypothesis that the
presence of orpnic material could promote ,the growth of sulfate-reducing bacteria i:D the waste rock;
howevec, the organic material apparently promoted the growth of iron-oxidizing microorganisms more
than that 0' sulfa&e.reducing bacteri&. As a result, the drainage from the experiments with the wood
bark coven connined hlgho leve1a of acidity than the controls. Efftueot from a separate control
experiment (wood -bark without waste rock) exhibited little acidity and near-neutral pH, showing that
acid production was due to bacterial oxidation of the sulfide rock and not decomposition of the wood
bark cover.
The effectiveaesa of the CODttOI measures was determined as the peccemage reduction in add
production, compared to the coDlrOls. The phosphate rock tr-*-t lOIt effectiveaeu after initially
suppressing acid production; the initial suppression was attributed to the preseace of low levels of
carbonate miDenI (calcite) in die ~ which delayed add productioa fur some time. The soil
cover was more effective in the iDdoor tests dim in the outdoor teIII; this was aaributed to infiltration
of air aDd water arouDd the edges of the soil cover, aad to the effeda of adverse natural climatic
conditioDa (r.~ aDd thawina) which were not preseat in the labotatoty.
The most effective control for both types of rock in both the indoor aDd outdoor testa was tile
water cover, which suppressed 99~ of the acid production. Limestone am8WI~. soil cover, aDd
phosphate rock amendment fm decreasing ordec of effectiveness) were all less effective dim the water
cover in controlling acid production; this order of effectiveness held for all test conditions.
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Waste Roc/c Pile Report
4.0 CASE STUDY
Exemplary practices with respect to the environmental management of waste rock piles are
briefly presented in the following case study. This case study and other similar operations provide an
opportunity to d~ostrate the effectiveness and feasibility of environmentally sound design and
operation practicea -currently in use. Further, the study can provide insight into the geotechnical and
geochemical behaVior of waste rock piles.
4.1
McLaupIiD MiDe
The inDovative waste rock disposal system and general environmeutal managemem practiced
at the McLaughlin Mine have been ~ by the minina industry II excellem exampla of the
ongoiDa efforts by the iDdustry 10 mitipte the impacts of 1DiDiDa. In addition, the McLaughlin MiDe
bas been boDOred for their eavironllUl!llbl managemalt programs through IIUIDeIOUI awards aud
comlN8ftll~, iDcludiq the 1991 BLM 8Parmer in the Public Spirit8 award, aad a 1984 Sierra
Club Connn-d2tinn.
The eaviroomeatal manaaer at the miDe, Mr. Ray Knuu, ViII the recipiellt of the 1993 Ead8
A. Chiles award for 8the application of eDVironmP.lltsll1y sound manaaemem principIa to the mining
and extraction of minenl resources in the Intermountain West. 8 The U.S. EPA appreciates Mr.
Krauss' usistaDce (through telephone COnversatiODS aud providing ~l1ftIIJJIt2rinn of McLaughlin's
waste rock disposal aDd emironnumnl management practices) in the development of this case study.
The following case study was developed from the materials provided by Mr. Krausa: Homestake
(untt!rtM), Krausa (1993), aDd Krauss (1994).
4.1.1 Mine Background
The McLaughlin Mine is a gold mining and milling facility located in the Coast Range
appro][,1ft!rtPly 70 air mila north of San Francisco, California. The components of the mine (open
pit, freshwater tIIIn'Oir. waste rock pile, and mill and mine buildings) are distributed over private
and public 1aDdI (BLM) in three counties (see Figure 4-1).
The mine ViII developed in an historic mercury mining district in the early 19808 following
an extenSive exploration program that defined a gold ore body grading 0.113 ounces of gold per ton.
The ore body is mined using open pit surface mining methods. For every ton of ore removed from
the pit, five tons of waste rock must be removed. The estimated total ore to be mined at the
McLaughlin mine is 30 million toDS. Therefore, 150 million tons of waste rock will need to be
excavated and disposed of during mining. The mined ore is transported to a crushing and grinding
circuit where the ore is reduced
Final
39
July 19, 1995
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"T1
[
I
is SCALE.
\~ ~~ - .
II I . I ,. Mill.
~ . ~
II
CIlU.A CI.
-"11[1'.-- --
f~ "In
i ~
~ ~i
ft =:
. ~ =
I.r
ii '0
UClA..1tf1
18 ~.
~;: \~
"CII
,-..
~~7 i
\
.
"'" ~.
e. ~
'<
-
~ ~
-
~
U\ f
~
-------
Wam Rock PUe Repon
to a muddy slurry. The slurry is then transported via a pipeline to a procesa facility where the gold is
extracted from the rock. The barren slurry is then disposed in a tailings impouDdmeIIt.
4.1.2 Waste Rock Pilea
Waste rock from the pit bas occurred/is planned in three areas: the East waste rock pile, the
Site Five waste rock pile and the north pit backfill (see Figure 4-2). The East waste rock pile, now
closed aDd reclaimed, coven 6S acres and the Site Five waste rock pile, which currently receives
waste rock, coven 38S aae&. The areal extent of the north baC'lrfili pit is DOt known since this area is
not yet actively receivina waste rock.
The waste rock piles, with a combiDed total disposal capacity of 150 millioD toIII, are
constructed uaiq the ~i... COIIItrUCtioD sequence. The waste rock was iDitiaIly ead-piled in lifts
50 to 100 feet in height; curready the waste rock is beiDa piled in lifts 50 feet in heiabt- The
coDSttudioD of ftat beDd1es betweea lifts resulll in ID overall &lope ",e DOt ~~-li... 18 decrees (3
horizontal to 1 vertical). The waste rock pile slope betweea the baK:hes is araded to aD aqle DOt 10..-
exceed 22 degrees (2.S horizontal to 1 vertical). These final slope angles facilitate reclamation aod -~
assure seismic stability.
Serpentine sou (relatively low strength) UDdedie the East aDd Site FIVe waste rock piles.
FoUDdation preparation required the installation of UDderdrainl in areal of pound-water disclI2rJe.
Those areas were bladed to subsoila by bulldozers and filled with DOn-acid-aeuentiD& rock sized 8 to
10 inches. A fil~ fabric encloses the rock-filled drains and prevema the JDiantion of fiDe sediment
into the drain. WaIa' from the UDdadninage system is conveyed to a sump for use in the process
circuit. An overflow pond near the sump would receive any overflow in the eveut aignificam ftows
from the UDderdraina exceed the capacity of the sump. Underflow from the East waste rock pile
underdrains is about 1 gpm. The rate of flow from the underdrains at the Site Five waste rock pile
was not obtained.
Surface water is intercepted upstream of the waste rock piles and diverted around the piles
through ditcha. 1biI WIler and the runoff from the reclaimed benches at the Site Five and the East
waste rock pil. .. directed to the stream diversion system. This wat« does DOt come into contact
with wastes or di88DIbed grouud. Runoff from the active surfaces of the Site Five pile flows to
sediment poDds. Waf.« from these sediment ponds is discharged after settling or pumped to the
grinding circuit for reuse. The discharge from these ponds is permitted.
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Waste Rock Pile Report
FIgure 4-2.. McLaughlin Mine Layout
(Source: Homestake MiniJII Company)
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Waste RocJc PUe Report
4.1.3 Waste Roc:t Disposal Planning, Monitoring, Reevaluation aDd Plan Modification
Whea the McLaugliD Mine was in the planning stages in 1981 aDd 1982, representative waste
rock samples w.. characterized, using static testing (considered state-of-the-art at that time), for the
POtential to g~ acid rock drainage. Based on these tests, it was calculated that only eight
percent of the waste rock would have a net acid-generating potential. Therefore, the determination
was made that raiidom dumping of waste rock into the waste rock pile would not lead to the
formation of acid drainage. McLaughlin monitored the eft1ueut from the waste rock pile UDderdrains
from the startup date. The early results (analyzed usiq statistical aDd trend analysis) of this
monitoriq indicated that sulfate production was ~~iQI predicted rates. Calcium CODCeDtratioDl
were also iDaeasiD&, indicatiD& IOIDe neutralizatioD was occ:urring, however, it was insufficient to
match the rate of acid formation.
At that time, Me' ..sugtalin Mille staff reevaluated the acid geueration poteDtial of the waste
rock. 1bia reevaluation determined that up to 40 perceDt of the waste material had a net acid-
genention poteIItial. Based on this DeW information, the McLauablin staff impJemented a DeW waste
rock lDIDaIemeut proaram requiriDa ~ diffa-e.-.tiBina of waste types in tile pit - a
cl1an&e from the raDdom disposallDdbocl used . the waste rock pil..
Waste rock wu clauified . type 1 or type 2. Type 1 WIIte rock is --acid-aeaentina and
is further divided by physical charaderiItic:a iDto daya - other 1arJer-sized material. All type 2
waste is acid-geaeratin&. AD experieaced mine geologist is respoD8ib1e for .f1~. waste m.aterialJ
after vUuaIIy enmininl drill COrel for ore and waste type. The visual obIavatioD med10d used to
differentiate wastes in the pit prior 10 excavation was developed ba1ed upon the results of the
extensive testing of waste rock types for acid generation poteDtial (coDducted during the reevaluation
of waste rock at the miDe). Both typeI of flagged waste are excavated and trucked to the waste rock
piles for disposal based on a selective waste rock disposal JDaDagemeat plan.
The earlier random waste rock disposal method was dlanged 10 a waste selective program that
encapsu1atea reactive waste rock (type 2 waste) using type 1 DOn-acid geaentina clays. These DOn-
acid genen£iDI cIaJI.. used iD aD approximately 5-foot-tbick layer at the base of the waste rock
pile. The pile.. CC8ICnJd,ed by placing type 2 waste on top of the clay in so-foot~gh lifts. This
low lift heigbt .,-- dump compaction. During the constrUction of the pile, the gradient of the lift
surface is carefully c:oatrolled to eosure that precipitation flows rapidly off the lift surface. In the
event the d~ surface does DOt tighdy compact, clay is placed on the working surface. After the
type 2 waste lift is placed and compacted, the exterior slope is reduced to 2.5:1 (H:V). Type 1 waste
is then dumped over the exterior slope to provide a IG-foot-thick layer when compacted. The surface
of the lift is covered with type 1 waste and dozer spread to 5 ft thickness. When possible, truck
traffic is routed over the clay surface to provide compaction.
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Waste Rock Pile Report
A secoDd SO-foot-thick layer of type 2 waste is dumped and compacted on top. of the clay,
capping the first lift of type 2 waste. This second lift receives the same clay layer as the first lift. In
addition, the outer 150 feet of the top surface are left as a bench to provide the oven1J. waste rock
pile slope of 4:1 (H:V). An additional S-foot-thick layer of clay is placed on top of the clay bench to
increase the thictDesa to 10 feet. The mine persoDDe1 ensure that the clay layers on the slope and
surface of the c1uDIp are tied together to form a continuous cap over the type 2 waste. Figure 4-3
illustrates this meCbod of construction for the Mcl-211&J"lin waste rock piles. Final closure criteria for
the waste piles require a 2~foot-thick cover between the reactive waste rock and the surface of the
pile. This twenty foot layer consists of: 1) 15 feet of DOn-acid aenerating, low permeability clay
directly above the reactive ~ 2) three-and~oe-half feet of weathered oxidized rock above the clay,
and; 3) o~aJJd.ooe-half feet of top soil. The cIoaure of ead11ift occun . the earliest opportunity,
followed by reclamation.
4.1.4 What Weat WroDl'l
The initial testin& for acid aeneratioD poteDtiaI in the waste rock was based on static teatiD& of
composite samples. Although tbia method was state-of~ in the early 19101 (duriDa die -
developmem of the Mt"T ~ng'''in Mille), ita limitatioDl became apparem - the operation of die waste
rock piles began. Mt"T .sngftlin staff have idemified several erron in the initial teItiD& of waste rock
with respect to the maaagemeat of waste at the mine.
.
The testing of composite samples of the various geologic rock types at the miDe was fouDd to
mask or minimi7.e the acid-generatiDg poteDtiaI of specific waste types.
.
The samples were subjected to a static acidlbase a.cc:oumin& procedure that involved grinding
the materials to miDua 200 mesh. This procedure compares the total acid poteDtial to the total
alkalinity of the sample. This testing was determined DOt to be representative of the behavior
of the waste rock - fouDd in the pile where the run of mine waste rock rarely approaches the
size 200 mesh used in the static testing. In addition, the waste rock excavated aDd disposed in
the pile dispIayI fracture patterDS that are more likely to selectively expose sulfides than
carbonat&.
.
ReaaioD kinetics are likely to favor acid-forming reactions over neutralization reactions and
the neutralization products tend to form a scale on the carbonate rock, preventing further
reaction.
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Waste Rock PiU Report
4.1.5 Waste Rock Dispoaal MaoaIement and Cost
Mille pi..... . McLaughlin DOW integrate8 waste management with ore deve1opmeat.
MeT .2JJghlin baa developed a life of mine inventory for waste rock and the waste lIWLi&emeat plan
reflects the proportjoal of the various waste types. The need for double haDdliD& of the acid and DOD-
acid fonning material baa been avoided throu8h careful desip and operation of the waste rock pile
lifts, providiD& sUfficient area for disposal of each waste type. The implementation of the
McLaughlin waste rock JDaDagemeot system baa resulted in the protection of pre-mini", watu quality
and aquatic ecology doWDStream of the mine. The cost of implementinl a reactive waste rock
JDaDagemeat system . opented . the McLaughlin mine may, at first glaDCe, be conaidered excessive
by mine plamen. Mr. Krauu indicated that the Ml"(-2IIp.Iu. approach to waste rock disposal baa
been criticized by those in the mini~1 iDdu8try that believe the system is overly CODIeI'Vative aDd
costly. Be refuteI tbia opinioll, . foIIow8:
1M C06t of impleInentbI, 1M tIdd rock dralNJ,e lfIQIIQ,emtmt sy-- Lr 18t1mlNll at
uppnw-mely '*' caa per toll of on pl4ad ill 1M pike 1M CO# Lr IIIb8imiutl by
C/ITefiM pl/lMiIt,lO atlOId * 1Wd 10 double IulntIk allY MWte rock anJlto IItillu .,
MUte mpUnd 10 be 1IIOWd by 1M mlM plait, anJllfOt mlM other materl/1b to ejfect
donn. 1hl.r C06t U rdiIIlwly buiPUfaurt wheJI COIrIptInd to what 1M C06t ",;,,,, be
to corutnM:t and opert8 CI MIter tTei1IIIIe1rI p/IIIII to tmII the dUchaTIe from -
II1I1naIIIIged MUte rock pilL
~
Lookiq toward the fumre, Mr. Krmsa DOt.eI that Ho~"e now iDcorporatel, from the
exploration staae forwvct, CODIidentioD of acid rock draiDale potaItiaI iD80 the evaluation aud
planning for new mines.
-
Final
45
July 19, 1995
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Site 5 Waste Du'mp X-Section
Typical 450 Foot Wide Dump Terrace
r. ~.~. ~:.I TYPE' WASTE
I I TYPE 2 WASTE
r',::',::) OXIDIZED ROCK
Overa.ll Dump Slope -f '
Interbench Dum.p Slope .-=:::J ,
2.5
r
TYPE
2 POD
.. r...., rluoM.....
OXIDIZED ROCK
TYPE' (10' r...,.. ,.,.ha..)
('0' r...... hote""..)
Topsoil
( ~.. .' CM8. )
. .
"........ .../--
~..,~
,r
..
8CA&I
~
Figure 4-3. Inter Barrier Construction at the McLaughUn Mine Waste Rock Piles
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Waste Rock Pile Report
5.0 BIBUOGRAPHY
Bell, A.B., Riley, M.D., and Yanful, E.K. 1994. Evaluation of a Composite Soil Cover to Control
Acid Waste Rock Pile Drainage. In: Proceedings from the International Land Reclamation and
Mine Drainage Conference and Third International Conference on the Abatement of Acidic
Drainage, Volume 1 of 4: Mine Drainage, USBOM Special Publication, SP 06A-94.
Bell, A.V., L., Surges and E.K. Yanful. 1991. An Update on the Acid Waste Rock Fields Trials at
Heath Steele Mines New Brunswick. In: Proceedings of the Second International Conference
on the Abatement of Acidic Drainage, Volume 3, pp. 319-340.
Bennett, J.W. aDd Ritchie, A.I.M. 1991. Measurements of the Transport of Oxygen into Two
Rehabilitated Waste Rock Dumps. In: Proceedings of the Second International Conference on
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