United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-87/039 Nov. 1987
Project Summary
Design, Construction, and
Maintenance of Cover
Systems for Hazardous
Waste: An Engineering
Guidance Document
R. J. Lutton
This document emphasizes the spe-
cial characteristics of solid waste
management as they bear on cover
systems. Cover systems typically con-
sist of two to four layers of soil and
other materials and resemble a highway
pavement system in many respects.
This similarity facilitates the prepara-
tion and use of construction and
maintenance specifications. Example
specifications are in the appendices of
the full report.
Criteria and methods are provided for
engineering analyses of percolation,
erosion, stability, flooding, freezing,
and settlement. Techniques are avail-
able for compensation for each of these
threats, again using recognized design
and construction methods. Finally,
guidance is included on methods of
maintenance and repair over the unusu-
ally long design life of a solid and
hazardous waste disposal facility.
This Project Summary was devel-
oped by EPA's Hazardous Waste Engi-
neering Research Laboratory. Cincin-
nati, OH, to announce key findings of
the research project that is fully doc-
umented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Regulations for the management of
hazardous waste disposal facilities have
been promulgated by the Environmental
Protection Agency (EPA) as directed by
the Resource Conservation and Recovery
Act (RCRA). New hazardous waste dis-
posal facilities must meet the require-
ments of Title 40, Code of Federal
Regulations, Part 264 (40 CFR 264) in
order to receive a permit. RCRA also
dictates actions for correcting old sites
presenting endangerment. As a part of
its response to RCRA, the EPA has issued
technical documents to assist in the
complex and important task of planning
adequate disposal facilities.
The full document of this project
provides technical guidance on design,
construction, and maintenance of cover
for hazardous waste facilities based
largely on waste management practice,
concepts in soil construction, and inno-
vations to encourage new designs and
practice developed by creative and
competent engineers within the limits of
RCRA regulations. Suitable designs are
ultimately translated into construction
therefore, and some of the guidance is
formulated directly as specifications.
Basis of Technical Design
The physical characteristics of the site
and the configuration of its surface are
among the most important factors upon
which cover design is based. Sometimes,
site characteristics directly influence
design of the entire disposal unit or the
assemblage of disposal units; for exam-
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pie, it is necessary to limit the size of
disposal units or waste cells in hilly
terrain, and their orientation and ar-
rangement may also be restricted by
operational considerations. Broad, flat
sites present fewer restrictions on size,
orientation, and arrangement of the
disposal units.
The character of the waste can have
long-term impacts on the cover and its
performance in relation to subsidence,
differential offset, and soil particle
migration. It is even helpful conceptually
to regard the disposal unit as the lower
component or foundation of the cover
with a transition to the backfill soil within
the waste cells. Influences follow either
from the voids and other physical con-
dition of the waste at the time of burial
or from chemical-related changes that
take place over a long period. Charac-
terizing the waste focuses attention on
unfavorable eventualities that in some
cases may be avoidable. Certain worth-
while limitations on waste character and
form as well as placement procedures
may be incorporated in the plan of
operations.
The chemical constitution and the
reactivity of bulk and containerized waste
always deserve careful consideration in
cover design and the design of the
disposal unit. A listing of wastes to go
into each cell is already required in
Subpart B, 40 CFR 270. In large disposal
facilities, this undertaking may be com-
plicated or simplified by any operational
plan to segregate wastes into subcells
according to waste type. The eventual
deterioration and breakdown of initially
rigid elements such as steel barrels, bale
straps, and boxes will have to be eval-
uated as to their expected histories and
the ultimate effects. Although it is
frequently found that metals and some-
times even paper are little affected by
burial in the short term (e.g., at the
Savannah River Plant), the long-term
effects over tens of years and to the
design life of the facility must be
addressed to the highest degree available
with state-of-the-art knowledge and
practice.
The potential for generation of gases
or liberation of volatiles should also be
considered in cover and disposal unit
design. It may be necessary to provide
diversions and vents for gases blocked
from their upward pathways by the cover.
Vents to the atmosphere may be ade-
quate for toxic components of low
concentration that will be quickly
dispersed in the air to acceptable levels.
In extreme cases where the gas or
volatile component may reach a harmful
concentration, it may be necessary to
provide on line or contingency features
for absorption filters or other means of
reducing concentration of toxic compo-
nents. General categories of reactive
wastes that can conceivably affect the
performance of cover are volatile organic
chemicals, inorganic acids, materials
capable of reacting with others in the
waste cell, materials capable of under-
going volume change, and saturated
materials.
The hydrological environment of a
disposal site constitutes the most
influential combination of related exter-
nal factors in cover design and mainte-
nance. The several aspects of climate,
particularly the precipitation regime have
direct impacts on the performance of the
cover in blocking percolation into the
waste cell. Besides being foremost
among general constraints to cover
design, the climate enters into detailed
analysis and evaluation of proposed
cover designs.
The importance of the hydrological
system can be illustrated by considering
two extreme cases. It is believed that
most of the low rainfall of a typical year
at the arid Hanford and Richland,
Washington, LLRW disposal facilities
penetrates no more than a few meters
of depth. Since the ground water is at
much greater depth, most radionuclides
carried downward by percolating water
stop well above the water table. Thus,
these facilities and another hazardous
waste facility to the south near Arlington,
Oregon, benefit directly from the dry
regime. On the other hand, uncontrolled
waste sites in the relatively damp
midwestern and eastern United States
frequently exhibit ground-water contam-
ination plumes of serious dimensions
and concentrations despite the presence
of cover over the waste in some cases.
Major storm events must also be con-
sidered, since even an arid region can
be subjected to infrequent but major
storms that cause anomalous ground
saturation and percolation to a depth
ordinarily not reached. Accordingly, a
rather complete review of expectable
storm events and their frequencies
should be required in preparing the
background on the hydrological system.
Other aspects of the hydrological
system that need to be reviewed are the
evapotranspiration history, the water-
retention characteristics of the near-
surface soil, and surface runoff parame-
ters as well as available measurements
at gaging stations. In the broader sense
of hydrology, the ground-water system
constitutes an important part. Familiar-
ization with ground-water configuration,
flow directions, and velocities is essen-
tial, if for no other reason than to plan
and to anticipate contingencies for
unexpected performance or even failure
of the containment system.
General Design Concepts
Once the functions of the cover have
been established or constrained by
regulation or standard or by clearcut
technical objectives such as reduction of
percolation to insignificant levels, ele-
ments of the design are defined and
integrated using conventional design
methods.
A key task of cover design is the
selection of suitable materials. The cover
usually will include a synthetic mem-
brane and a large volume of soil or soil-
like material, but other materials are
sometimes included also: i.e., Portland
cement, bituminous concrete, seal coats,
and geotextiles. Guidance on suitability
of various types of soil for performing
cover functions and for general long-
term service ranking for various engi-
neering functions has been developed
based on considerable experience in the
Bureau of Reclamation, Corps of Engi-
neers, and other construction and engi-
neering organizations. Classification is
according to the Unified Soil Classifica-
tion System (USCS).
Emphasis on the best soil for the cover
will occasionally be unfeasible, since the
soils available at or near the site will be
chosen for reasons of economy. The
strategy of cover design targets the most
effective use of those available materials,
usually in a layered construction. The
availability of material may extend off-
site. Fly ash available in the near vicinity
continues to be an attractive possibility
for use because of its relatively fine gram
size which, by careful compaction or
addition of reactive chemicals, can be
brought to a strength and permeability
that may be suitable for fulfilling cover
functions.
Example Designs
The appendix in the full report provides
guidance on preparing specifications for
four example designs. The preparation of
plans and specifications demands careful
attention to detail, and sometimes the
choice of wording can have costly legal
ramifications. Therefore, the specifica-
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tions in the appendix as well as the
designs themselves are intended for
guidance rather than for direct applica-
tion at any facility.
Richard J. Lutton is with the U.S. Army Engineering Waterways Experiment
Station, Vicksburg, MS 39180.
Robert P. Hartley is the EPA Project Officer (see below).
The complete report entitled "Design. Construction, and Maintenance of Cover
Systems for Hazardous Waste: An Engineering Guidance Document," {Order
No. PB 87-191 656/AS; Cost: $18.95. subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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