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Compacted
Clay Liners (CCLs), often used in conjunction with geosynthetics,
are used as low hydraulic conductivity barriers in liner
and cover systems for landfills.
By
Henry Mock, Barry Sigmon, and Jim Daly
The EPA has
published guidelines for CCLs in the Technical Guidance
Document, EPA 600/R-93/182. Low hydraulic conductivity
CCLs are intended to impede leachate generated by waste
over time, thus protecting our groundwater resources.
For soils
proposed for use in low hydraulic conductivity CCLs,
this article describes the role of the geotechnical
laboratory during the material selection and qualifications
phases. Particular emphasis is placed on testing of
laboratory-compacted hydraulic conductivity samples,
which are especially useful in the borrow-source characterization
phase of a project, as they can provide an indication
of the CCL performance under field conditions.
Each American
Society for Testing and Materials (ASTM) testing procedure
is written as a stand-alone method; however, engineers
use data from a variety of procedures to determine the
suitable conditions to achieve low hydraulic conductivity
CCLs. The following list of ASTM test methods includes
those most commonly used during the borrow source characterization
phase.
ASTM C127
Test Method for Specific Gravity and Absorption of Coarse
Aggregate
ASTM D422
Test Method for Particle-Size Analysis of Soils
ASTM D698
Test Methods for Laboratory Compaction Characteristics
of Soil Using Standard Effort
ASTM D854
Test Methods for Specific Gravity of Soil Solids by
Water Pycnometer
ASTM D1557
Test Methods for Laboratory Compaction Characteristics
of Soil Using Modified Effort
ASTM D2216
Test Method for Laboratory Determination of Water (Moisture)
Content of Soil and Rock by Mass
ASTM D2487
Practice for Classification of Soils for Engineering
Purposes (Unified Soil Classification System)
ASTM D4318
Test Methods for Liquid Limit, Plastic Limit, and Plasticity
Index of Soils
ASTM D4718
Practice for Correction of Unit Weight and Water Content
for Soils Containing Oversize Particles
ASTM D5084
Test Methods for Measurement of Hydraulic Conductivity
of Saturated Porous Materials Using a Flexible Wall
Permeameter
Soil samples
from a potential borrow source are obtained for laboratory
testing prior to actual excavation associated with low
hydraulic conductivity liner construction. In addition
to field classification, typical tests—ASTM D422,
Grain Size Analysis; ASTM D4318, Atterberg Limits; and
ASTM D2216, Moisture Content—are performed on representative
samples. From these tests ASTM D2488, Classification
is assigned. Typically, low hydraulic conductivity clay
liners will classify as a CH, CL, ML, MH, or SC. Where
the base soil alone does not satisfy the project hydraulic
conductivity specification, consideration is given to
additives, such as bentonite.
If the classification
testing indicates the material might be suitable as
a source of low hydraulic conductivity soil, a laboratory
compaction test is conducted either by ASTM D698 or
D1557. Consistent with ASTM test requirements, soil
retained on sieves—qualifying as oversize particles—are
not included in the compaction specimens.
For samples
that contain more than 5% oversized material (gravel),
mathematically corrected (rock) compaction curve may
need to be calculated according to ASTM D4718. This
is important to the project as the corrected compaction
curve represents the moisture-density relationship of
the material (whole soil mass) that is placed in the
field and that is measured either by nuclear density
gauge, Shelby tube (if possible), drive cylinder, sand
cone, dielectric methods, and/or balloon method. The
geotechnical laboratory should provide both the corrected
and uncorrected compaction curves to the engineer, to
be used at their discretion. Using ASTM D4718, a specific
gravity (ASTM C127) is required to correct the compaction
curve.
With classification
and compaction testing completed, laboratory-compaction
criteria for hydraulic conductivity testing can be established.
If the engineer specified laboratory-compaction criteria
and deviation from optimum moisture content, typically,
the laboratory hydraulic conductivity sample (assuming
the normal 70 mm diameter sample) should be compacted
based on the uncorrected compaction curve. Where a larger
diameter laboratory-compacted hydraulic conductivity
sample is used (100, 150, 300 mm, etc.), the test method
might allow inclusion of the oversize fraction. However,
on most projects, large specimens are not practical
due to their increased testing costs. Low hydraulic
conductivity borrow soils will often have gravel present.
ASTM D5084 informs the user that the maximum size of
the particle in a laboratory-compacted hydraulic conductivity
sample should be one-sixth of the test specimen diameter
(or 12.5 mm for a typical 70 mm specimen). ASTM D5084
does not address the matter of compaction based on corrected
versus uncorrected compaction curve but rather states
in Section 8.3 “The material to be tested shall
be prepared and compacted inside a mold in a manner
specified by the requester.”
What needs
to be considered when establishing the laboratory-compaction
criteria for a hydraulic conductivity sample is the
effect that moisture has on the clay fraction material.
By compacting samples using the corrected compaction
curve data, you may be on the dry side of the uncorrected
compaction curve. It is very important that the clay
is moist enough so that the kneading action during compaction
will eliminate voids between the clods (no honeycombing),
which decrease the pathways of flow through the sample.
One has to
make sure that the voids between the gravel particles
are filled with clayey material so as to block the flow
paths through the sample. The gravel particles should
be embedded in a clay matrix that controls the hydraulic
conductivity. The rock content can vary considerably
(various literature reports from 20% to 50%) and still
give similar hydraulic conductivities; however, the
density will be different.
CCLs must
achieve the hydraulic conductivity required in the project
specifications, typically less than 1E-07 cm/sec. While
the percent compaction (relative to maximum dry density)
and relative moisture content (deviation from the optimum
moisture content) are important, one should not get
lost in a single requirement such that these conditions
often change during the construction. There are several
other ways to define this low hydraulic conductivity
in addition to the above such as the “line of curves”
(Mitchell et. al. 1965), “acceptable zone”
(Daniel & Benson 1990), Bentonite amended, increasing
the confining pressure, using a geosynthetic clay liner
(GCL), and saturation (Knitter et. al. 1993).
There appears
to be a very good correlation to the initial percent
degree of saturation and measured hydraulic conductivity.
Often, if a laboratory-compacted sample has an initial
percent saturation of 85% or greater, it has a favorable
chance of achieving a hydraulic conductivity of 1E-07
cm/sec or less. If you have the specific gravity (ASTM
D854), moisture content, and the density, you can determine
the percent initial saturation. You can adjust the moisture
content, the density, or both to see the effects that
initial saturation has on the measured permeability
of a sample. These variables can be changed to meet
the requirement of the low hydraulic conductivity CCL.
Conclusion
It is fascinating how all of the different ASTM
procedures and resulting data are used to achieve a
final outcome. There are many variables that interact
with each other to produce a measured hydraulic conductivity.
The role of the geotechnical laboratory continues to
be important in its meaningful test data as an indication
of the compacted clay liner performance.
Henry
Mock, Barry Sigmon, and Jim Daly are with Golder Assoc.
Inc. Laboratory in Atlanta, GA.
MSW
- November/December 2005
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