United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-87/025 July 1987
&EPA Project Summary
Prediction/Mitigation of
Subsidence Damage to
Hazardous Waste
Landfill Covers
Paul A. Gilbert and William L Murphy
This report describes the causes and
effects, prediction methods, and tech-
nologies that may be applied for the
prevention of subsidence in hazardous
landfills. The information should be of
assistance to those involved in evalua-
ting landfill permit applications. The
goal is to help prevent damage to, and
resulting leaks through, landfill covers
caused by subsidence-induced
stresses.
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).
Background
Section 3004 of the Resource Conser-
vation and Recovery Act (RCRA) of 1976
requires the Administrator of the Envi-
ronmental Protection Agency (EPA) to
establish standards applicable to owners
and operators of hazardous waste treat-
ment, storage, and disposal (TSD) facil-
ities. Among the standards are require-
ments for "treatment, storage, or
disposal of all such waste received by
the facility pursuant to such operating
methods, techniques, and practices as
may be satisfactory to the Administra-
tor." The implementing regulations for
landfill covers are found in 40 CFR
264.310, "Closure and postclosure
care," which states that the final cover
must be designed and constructed to (1)
provide long-term minimization of migra-
tion of fluids through the closed landfill;
(2) function with minimum maintenance;
(3) promote drainage and minimize
erosion or abrasion of the cover; (4)
accommodate settling and subsidence so
that the cover's integrity is maintained;
and (5) have a permeability less than or
equal to the permeability of any bottom
liner system or natural soils present.
Monitoring and maintenance, includ-
ing necessary cover repairs, are also
required throughout the postclosure
period. The postclosure period is desig-
nated in 40 CFR 264.117 as 30 years
after completion of closure.
EPA recognizes the need to provide
guidance in implementing the cover
requirements. This document addresses
the fourth requirement listed above
regarding settlement and cover subsi-
dence.
Purpose
The final report summarized herein,
presents technical guidance directed at
predicting, reducing, and preventing
landfill settlement and related cover
damage by subsidence. The final report
is intended to be used by regulatory
personnel and by operators of hazardous
waste landfills.
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Scope
The information presented in the final
report pertains to hazardous waste
landfills designed, constructed, and
operated within the United States under
the RCRA regulations. Landfills con-
structed and capped before the passage
of RCRA in 1976 may not meet RCRA's
relatively stringent waste placement,
liquid waste limitations, liner specifica-
tions, and leachate collection and control
requirements, and thus may not be
amenable to the analytical, construction,
and remedial guidance presented in the
final report.
Conclusions and
Recommendations
Hazardous waste landfills meeting
RCRA requirements have physical char-
acteristics that influence their potential
for settlement and subsidence. Attention
to those characteristics can minimize
postclosure subsidence damage.
Data on physical properties of real and
simulated hazardous waste are available
to assist the landfill operator or permit-
ting agency in assessing long- and short-
term settlement potential.
Landfill subsidence results from pri-
mary consolidation and secondary com-
pression of the waste mass, and from
collapse of voids or cavities in the fill and
around containers by corrosion, oxida-
tion, combustion, or biochemical decay
of landfilled materials.
Rarely, a landfill may be a monofill; that
is, it may contain uniform layers of
drummed wastes or uniformly placed
bulk wastes. More often, the landfill
consists of different types of wastes
placed nonuniformly across the landfill
in layers separated by intermediate
covers of soil. The potential for differen-
tial settlement is greater in landfills with
nonuniform wastes and waste place-
ment procedures.
Bulk wastes behave differently from
containerized (e.g., drummed) wastes in
settlement characteristics. Bulk wastes
behave relatively predictably, much like
soils, becoming increasingly consoli-
dated with time, but at a decreasing rate.
Containerized wastes may remain rela-
tively underformed until the containers
degrade and collapse, at which time voids
will be created, and consolidation will
begin.
Settlement by consolidation and
secondary compression of bulk waste
landfills in which drainage layers are
provided will probably be essentially
complete before final closure. Compac-
tion of waste materials and installation
of drainage layers are recommended to
lessen the potential for postclosure
settlement and cover subsidence.
The approximate time required for
primary consolidation to occur can be
estimated for a waste or soil layer if the
liquid limit is known for the material and
if the shortest distance to a drainage path
(e.g., a drain layer) is known. Time, for
any degree of consolidation, can be
computed more precisely if the compres-
sibility or coefficient of consolidation has
been determined for the material.
Of the controlling factors, the distance
to a drainage path has the most pro-
nounced effect on consolidation time for
a waste layer. This fact indicates the
desirability of including frequent drain-
age layers and of removing liquid from
the landfill mass so that most of the
consolidation will occur before closure.
The time required for ultimate settle-
ment of containerized (drummed) waste
to occur cannot be computed without
knowledge of the drum deterioration
time. The time cannot be determined,
although it is expected to be several
years, perhaps several decades, if water
infiltration is prevented by an impervious
cap and liner system.
The void space around drums or other
containers in a landfill can be a major
contributor to total postclosure settle-
ment and should be filled with solidifying
agents or a free-flowing backfill to
minimize the void component of total
settlement.
The surest way of avoiding problems
associated with postclosure deteriora-
tion of drums and the delayed settlement
and cover subsidence associated with it
may be to ban drums from landfills.
Instead, drums can be emptied and
crushed or reclaimed. Drum contents can
be treated and disposed as bulk waste.
Equations for calculating settlement
time should be used more to identify
operational landfilling and waste treat-
ment procedures that will minimize
settlement time than to predict precise
values from theory.
Differential settlement across rela-
tively short distances that may occur
within subcells comprising a larger
landfill cell is more threatening than
relatively uniform settlement across
longer distances that may occur across
targe monofills. For the former, tensions!
stresses may be sufficient to cause
cracks in the cover resulting in leakage
of water into the landfill. Those tensional
stresses may not develop over longei
distances, but ponding of water mai
occur on the cover barrier, weakening it!
ability to repel water.
Similarly, tensional stresses are antic
ipated to cause few or no problems wit)'
flexible membrane barriers over large
subsidence areas. Locally severe differ
ential subsidence can cause strair
sufficient to rupture a flexible membrane
or otherwise cause its premature failure
Two or more central column models
for analyzing landfill deformation (settle
ment) can be used to predict differentia
settlement between columns and there
by to determine the effect of differentia
subsidence on the final cover.
Expressions for analyzing the deflec
tion of a beam can be used to identif
parameters controlling the deformatioi
of a landfill cover subjected to differentia
settlement. Once identified, the parame
ters can be adjusted by cover design am
construction procedures to minimizi
distress to cover components.
Differential settlement can be mini
mized by compacting wastes durini
placement, eliminating void space withii
the landfill, stabilizing liquids befon
placement, and other considerations
The length of the cover (represented a
a beam) subjected to subsidence can b<
reduced by placing wastes as uniform!
as possible to provide uniform suppor
to the cover. The cover soil component
can be made more resistant to distres
by compacting the cover barrier soils we
of optimum water content.
Final cover components will stretc
under differential settlement and mus
be constructed to withstand tensil
strain. The average tensile strain in th
cover can be computed, and the maxi
mum value of the differential settlemer
that can be tolerated by the cover soil
can be estimated from that computatior
Plastic soils (soils with high plasticit
indexes) should be selected for use a
cover components to produce a cove
resistant to tensile strain.
Laboratory investigations by other
indicate the flexible membrane liner
(FML's) (components of the barrier laye
in covers) may fail at lower strains tha
would be expected from manufacturer!
data. Every effort should be made t
reduce differential settlement potentii
of the landfill and to design the covt
to resist tensile strain.
Landfilled wastes should be con
pacted or treated where possible 1
reduce potential settlement. Compactic
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methods include standard compaction
techniques, vibratory rollers, and pre-
compression (preloading and surcharg-
ing). Waste treatment methods include
addition of fixative agents to render the
wastes permanently less compressible.
The stabilization of liquid wastes with
pozzolanic materials has been shown to
increase compressive strength and
lessen settlement potential. Such stabil-
ization could be especially beneficial for
containerized wastes.
Paul A. Gilbert and William L Murphy are with the U.S. Army Waterways
Experiment Station, Vicksburg, MS 39180.
Robert P. Hartley is the EPA Project Officer fsee below).
The complete report, entitled "Prediction/Mitigation of Subsidence Damage
to Hazardous Waste Landfill Covers." (Order No. PB 87-175 378/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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Environmental Protection
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Center for Environmental Research
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EPA/600/S2-87/025
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