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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                                                                                                  U.S.OFFICIALM*
United States
Environmental Protection
Agency
                                   Center for Environmental Research
                                   Information
                                   Cincinnati OH 45268
Official Business
Penalty for Private Use S300


EPA/600/S2-87/025
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