United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-87/008 May 1987
&EPA Project Summary
Evaluation of Retorted Oil
Shale as a Liner Material for
Retorted Shale Disposal Sites
William J. Culbertson, Jr., Charles H. Habenicht, and James D. Mote
This study has considered the pos-
sibility of using a spent oil shale itself as
a water barrier or "liner" beneath a
spent oil shale waste embankment.
Pertinent properties of unburned Tosco
II spent shale and an average mixture of
Lurgi spent shale have been measured.
Materials consisting of 1, 20, and 30%
burned spent Tosco shale admixed into
unburned Tosco II shale have also been
considered. Two autoclave mellowed
materials admixed into their respective
unmellowed spent shales have also been
studied.
This work indicates the difficulty of
having both easy self healing and low
permeability of the unmellowed Tosco
materials and mixtures thereof, as well
as perhaps the unmellowed Lurgi spent
shale. Autoclave mellowing of the
burned Tosco material, however, pro-
duced a high plasticity index material
that may be blended with the silty un-
burned Tosco II spent shale to produce
a liner having (at least in the short term)
both low permeability and good self-
healing possibilities. A similar attempt
with the Lurgi spent shale was not
successful due to the high permeability
produced in. the short term aging
experiment.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that Is fully docu-
mented In a separate report of the same
title (see Project Report ordering In-
formation at back).
Objectives
An experimental program was con-
ducted to determine the efficacy of using
spent oil shale itself as a barrier between
a waste pile of spent oil shale and the
surrounding aquifer or country rock. The
objective of the program was to produce
a liner material which had much of the
frictional characteristics and volume
stability of silt with the impermeability of
clay without a tendency for eventual
cementation on the one hand or leach-
ability and partial soil skeleton loss on
the other. In other words the liner material
should be highly impermeable yet have
sufficient plasticity to accommodate sub-
sidence without rupturing or, if ruptered,
to self heal.
The objectives stated above are difficult
criteria to meet and certainly require some
modification and admixing of various
materials to produce a liner with the
desired properties.
Approach
The routine testing approach was cen-
tered around the study of rather highly
consolidated specimens of various mix-
tures of spent oil shale at two moisture
contents: one at the optimum water con-
tent for maximum dry density, and the
other at a somewhat wetter than optimum
water content. Often wetter than optimum
material is used for small dam cores and
around abutments for increased flexibility,
lower brittleness, and sometimes lower
permeability. A vertical consolidation
pressure of around 280 psi* was produced
by spring loaded oedometers.
* 1 psi = 0.0703 kg/cm2
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To simulate a liner placement technique
that is sometimes proposed, the speci-
mens were compacted in spring oedome-
ter sheaths to 100% of standard proctor
or 100% of modified proctor. These
compactions allowed a small additional
consolidation in the oedometers. The
oedometer consolidation approximately
models burial of the material under an
embankment of moderate height and al-
lows a standard and somewhat realistic
environment for subsequent aging/curing
and/or cementation processes in the
specimen. The immediate application of
the full 280 psi consolidation pressure
just after compaction does not simulate
real conditions, however, since some time
is needed for construction of full embank-
ment height.
After permeability testing a specimen,
it was transferred from the spring
oedometer to a rubber membrane in a
triaxial chamber for torsion testing under
a confining water pressure generally
selected to produce a value of 0.5 to 0.7
for the ratio of the lateral stress to the
vertical stress, K0. A K0 of 0.7 is higher
than corresponds to a two dimensionally
normally consolidated silty material but
may be about right for certain specimens.
In this way the tendency for swelling in
the diameter of a specimen as it is ex-
truded from the oedometer sheath to the
rubber membrane in the triaxial chamber
is mitigated. Such swelling might break
the bonding of cemented specimens.
Without the confining pressure even some
stiff, partially cemented specimens were
crushed when only moderate vertical
pressures were applied prior to torquing.
It is desirable to perform shear strength
tests on undisturbed specimens. The
properties of specimens may be altered
by swelling which softens them; by over
consolidation during extrusion which
hardens them; or by breaking cementation
which softens them or fractures them
prematurely. In order to mitigate these
problems a special triaxial test apparatus
was designed and constructed.
The triaxial torsion machine was de-
signed to produce strains under triaxial
loads that simulate conditions expected
for liners of spent shale piles. In general
the expected strains exceed the capacity
of most compressive triaxial machines.
Hence the machine used in this program
was designed to accommodate the antici-
pated large shear strains. Additionally
the fixtures allowed the transfer of
samples from the spring oedometer cells
used for the consol idation experiments to
the triaxial testing apparatus. This pre-
vented either swelling or further densifi-
cation of soft clays being tested at over-
consolidation ratios of unity or above.
Also, for the case where cementation
may have occurred in the specimen,
transfer of the sample while maintaining
the confining stresses was possible, thus
eliminating any longitudinal or laterial
strains that would disrupt the cementa-
tion. Hence, the properties of specimens
exhibiting cementation could be deter-
mined accurately.
The well mixed moistened spent shale
batches were compacted by a miniature
proctor system to either standard or
modified proctor and consolidated under
a spring force equivalent to around 280
psi vertical pressure. Data for secondary
consolidation curves were obtained during
curing of the specimens until they were
tested (still under the same 280 psi
vertical soil skeleton pressure) for per-
meability under a water pressure of
usually 20 psi. Lower water pressures
were found to sometimes give unusually
low permeation rates or erratic rates.
This may have been due to the "oily"
hydrophobic nature of some specimens.
At 20 psi hydraulic pressure the per-
meability values seemed internally con-
sistent with each other. Some specimens
were not subjected to water permeation
in order to compare them with permeated
ones in later shear strength testing.
Several modifications of the testing
procedures were required during the pre-
liminary experimental methods develop-
ment in order to produce consistent
results. These procedures were sub-
sequently used to produce the data
required for the torsion stress vs strain
curves.
All specimens were transferred from
their individual spring oedometers to a
triaxial confining water pressure ap-
paratus in a way to minimize disturbance
causing overconsolidation or breaking the
bond between specimen and pore stones
and vanes. After the transfer, each 1 in.*
high by 2.5 in. diameter specimen was
retained between drained pore stones
and its cylindrical surface was covered by
a thick gum rubber membrane. Brass
vanes embedded in the pore stones aided
torsioning the specimen for obtaining
peak shear strength, "residual" shear
strength, initial stiffness, and twist to
peak strength.
Availability of the peak and "residual
strength" of a specimen allowed com-
putation of a brittleness index. Since the
normal or vertical pressure on the ends
of the specimen is known (in some tests
*1 in. = 2.54 cm.
this was made equal to the prior con-'
solidation pressure during torsioning),
angles of internal friction for peak and
residual strengths corresponding to the
high overburden load of around 280 psi
were calculated.
After obtaining the torsion stress/strain
curve, the specimens were removed from
the triaxial container and their enclosing
gum rubber membranes were cut away
so previously transferred longitudinal
acrylic paint stripes on the specimens
could be examined and the specimens
photographed.
Some physical and chemical properties
of the specimens were determined on
dried fragments left from the torsion test,
including evolved gas analysis (EGA) for
hydrate water of species formed during
curing. Some of these species are of a
cementing or potentially leachable nature.
X-ray diffraction scans for confirmation
or identification of these species were
also made.
The Atterberg limits of beginning
materials and blends of spent shale
materials are a parameter important in
soil mechanics correlations. These were
obtained for some raw material spent
shale and mellowed spent shale. Atter-
berg limits on cemented specimens were
not made.
Spent Shale Treatment
Both Lurgi and Tosco II spent shales
were evaluated as potential liner mate-
rials. The evaluation included the fol-
lowing material: (a) unburned Tosco II, (b)
10% burned Tosco, 90% unburned Tosco
II; (c) 20% burned Tosco, 80% unburned
Tosco II; (d) 30% burned Tosco, 70%
unburned Tosco II; (e) 75% mellowed
burned Tosco, 25% burned Tosco; (f) 50%
mellowed burned Tosco, 50% unburned
Tosco II; (g) Lurgi; (h) 75% mellowed
Lurgi, 25% unmellowed Lurgi; and (i)
50% mellowed Lurgi, 50% unmellowed
Lurai.
The two component samples were ad-
mixed uniformly prior to compacting.
The mellowed material was produced
by treatment in an autoclave at various
steam pressures (and therefore at various
temperatures). This treatment reduced
the cementation tendency of the material.
Samples with both optimum moisture
and "wet of optimum" moisture were
tested. The optimum moisture was deter-
mined as that moisture content necessary
to produce the maximum dry density in
the compacted sample. For the wet of
optimum samples the moisture contents
ranged from 3 to 9% above the optimum
moisture.
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The aging times for the various speci-
mens ranged from 2 weeks to 9 months.
Results and Conclusions
The Lurgi and burned Tosco material,
including all the two-component mixtures
containing burned Tosco material, was
too brittle to be considered as serious
candidates for liner material. The Lurgi/
mellowed -Lurgi mixtures had the highest
permeabilities; therefore, they also were
not suitable as liner materials.
The Tosco II unburned material and the
50% mellowed burned Tosco, 50% un-
burned Tosco II both exhibited some
plasticity while retaining relatively low
permeability. Hence both of these mate-
rials are candidate liner materials since
they have some self-healing characteris-
tics and the low permeability necessary
to prevent contamination of the ground
water outside the spent shale pile.
The samples which were wet of
optimum showed the higher plasticity.
In all cases the permeability decreased
with time as confirmed by the long term
tests.
There was little change in plasticity
with time.
The mineral grain density tended to
continue to decrease with time.
Note that only two varieties of shale,
with modifications and mixtures of them,
were included in this investigation. Of
the nine variations studied, only two are
considered as candidates for liner
material.
Further work is necessary to determine
the efficacy of other shales as liner mate-
rial. For example, Union shale was not
included in this study. Furthermore no
optimization investigations were carried
out.
W. Culbertson, Jr., C. Habenicht, andJ. Mote are with Denver Research Institute,
Denver, CO 80208.
Edward R. Bates is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of Retorted Oil Shale as a Liner
Material for Retorted Shale Disposal Sites," fOrder No. PB 87-165 270/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:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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Environmental Protection Information
Agency Cincinnati OH 45268
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