ASTM D6243/D6243M-20

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D6243/D6243M − 16 D6243/D6243M − 20
Standard Test Method for
Determining the Internal and Interface Shear Strength of
Geosynthetic Clay Liner by the Direct Shear Method
This standard is issued under the fixed designation D6243/D6243M; the number immediately following the designation indicates the
year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last
reapproval. A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers a procedure for determining the internal shear resistance of a Geosynthetic Clay Linergeosynthetic
clay liner (GCL) or the interface shear resistance between the GCL and an adjacent material under a constant rate of deformation.
1.2 This test method is intended to indicate the performance of the selected specimen by attempting to model certain field
conditions.
1.3 This test method is applicable to all GCLs. Remolded or undisturbed soil samples can be used in the test device. See Test
Method D5321/D5321M for interface shear testing of non-GCL geosynthetics. See Guide D7702/D7702M for a summary of
available information related to the evaluation of direct shear results obtained using this test method.
1.4 This test method is not suited for the development of exact stress-strain relationships within the test specimen due to the
nonuniform distribution of shearing forces and displacement.
1.5 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each
system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used
independently of the other, and values from the two systems shall not be combined.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.7 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D653 Terminology Relating to Soil, Rock, and Contained Fluids
3 3
D698 Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort (12,400 ft-lbf/ft (600 kN-m/m ))
D1557 Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort (56,000 ft-lbf/ft (2,700
kN-m/m ))
D2435/D2435M Test Methods for One-Dimensional Consolidation Properties of Soils Using Incremental Loading
D2487 Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)
D3080/D3080M Test Method for Direct Shear Test of Soils Under Consolidated Drained Conditions
D3740 Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in
Engineering Design and Construction
D4439 Terminology for Geosynthetics
D5321/D5321M Test Method for Determining the Shear Strength of Soil-Geosynthetic and Geosynthetic-Geosynthetic
Interfaces by Direct Shear
D6072/D6072M Practice for Obtaining Samples of Geosynthetic Clay Liners
This test method is under the jurisdiction of ASTM Committee D35 on Geosynthetics and is the direct responsibility of Subcommittee D35.04 on Geosynthetic Clay
Liners.
Current edition approved Jan. 1, 2016March 15, 2020. Published January 2016March 2020. Originally approved in 1998. Last previous edition approved in 20132016 as
D6243/D6243MD6243/D6243M – 16.–13a. DOI: 10.1520/D6243_D6243M-16.10.1520/D6243_D6243M-20.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6243/D6243M − 20
D6496/D6496M Test Method for Determining Average Bonding Peel Strength Between Top and Bottom Layers of
Needle-Punched Geosynthetic Clay Liners
D7005/D7005M Test Method for Determining the Bond Strength (Ply Adhesion) of Geocomposites
D7466/D7466M Test Method for Measuring Asperity Height of Textured Geomembranes
D7702/D7702M Guide for Considerations When Evaluating Direct Shear Results Involving Geosynthetics
3. Terminology
3.1 Definitions—For definitions of terms relating to soil and rock, refer to Terminology D653. For definitions of terms relating
to GCLs, refer to Terminology D4439.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 adhesion, c , n—the y-intercept of the Mohr-Coulomb strength envelope.
a
3.2.2 atmosphere for testing geosynthetics, n—air maintained at a relative humidity of between 50 and 70 % and temperature
of 21 6 2°C2 °C [70 6 4°F].4 °F].
3.2.3 GCL, n—a manufactured hydraulic barrier consisting of clay bonded to a layer, or layers, of geosynthetic materials.
3.2.4 Mohr-Coulomb friction angle, δ, n—(angle of friction of a material or between two materials, degrees) the angle defined
by the least-squares, “best-fit” straight line through a defined section of the shear strength-normal stress failure envelope; the
component of the shear strength indicated by the term δ, in Coulomb’s equation, τ = C + σ * tan (δ) (see 13.6).
a n
3.2.4.1 Discussion—
The end user is cautioned that some organizations (for example, FHWA, AASHTO, along with state agencies who use these
documents) are currently using the Greek letter, Delta (δ), to designate wall-backfill interface friction angle and the Greek letter,
3,4
Rho (ρ), to designate the interface friction angle between geosynthetics and soil.
3.2.5 Mohr-Coulomb shear strength envelope, n—(angle of friction between two materials) (degrees) the angle whose tangent
is the slope of the line relating limiting value of the shear stress that resists slippage between two solid bodies and the normal stress
across the contact surface of the two bodies. Limiting value may be at the peak shear stress or at some other failure condition
defined by the user of the test results. This is commonly referred to as interface friction angle. D653
3.2.6 secant friction angle, δ , n—(angle of friction of a material or between two materials, °) the angle defined by a line drawn
sec
from the origin to a data point on the shear strength-normal stress failure envelope. Intended to be used only for the normal stress
on the shearing plane for which it is defined.
3.2.7 shear strength, τ,n—the shear force on a given failure plane. In the direct shear test it is always stated in relation to the
normal stress acting on the failure plane. Two different types of shear strengths are often estimated and used in standard practice:
3.2.7.1 peak shear strength—the largest value of shear resistance experienced during the test under a given normal stress.
3.2.7.2 post-peak shear strength—the minimum, or steady-state value of shear resistance that occurs after the peak shear
strength is experienced.
3.2.7.3 Discussion—
The end user is cautioned that the reported value of post-peak shear strength (regardless how defined) is not necessarily the residual
shear strength. In some instances, a post-peak shear strength may not be defined before the limit of horizontal displacement is
reached.
3.2.8 shear strength envelope, n—curvi-linearcurvilinear line on the shear stress-normal stress plot representing the combination
of shear and normal stresses that define a selected shear failure mode (for example, peak and post-peak).
4. Summary of Test Method
4.1 The shear resistance internal to the GCL or between a GCL and adjacent material, or between any GCL combination selected
by the user, is determined by placing the GCL and one or more contact surfaces, such as soil, within a direct shear box. A constant
normal stress representative of design stresses is applied to the specimen, and a tangential (shear) force is applied to the apparatus
so that one section of the box moves in relation to the other section. The shear force is recorded as a function of the horizontal
displacement of the moving section of the shear box.
4.2 To define a Mohr-Coulomb shear strength envelope, it is recommended that a test points be performed at different normal
stresses, selected by the user, to model appropriate field conditions. However, there may be instances where fewer test points are
LRFD Bridge Design Specifications, 5th Edition, American Association of State Highway and Transportation Officials (AASHTO), Washington, D.C., 2010.
“Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Design and Construction Guidelines,”FHWA GEC 011, FHWA-NHI-10–024FHWA-NHI-10-024, Vol
1 and FHWA-NHI-10–025FHWA-NHI-10-025, Vol II, U.S. Department of Transportation, Federal Highway Administration (FHWA), Washington, DC,D.C., 2009.
D6243/D6243M − 20
desired (see Note 1). The peak shear stresses, or shear stresses at some post-peak displacement, or both, are plotted against the
applied normal stresses used for testing. The test data are generally represented by a best fit best-fit straight line through the peak
strength whose slope is the Mohr-Coulomb friction angle for peak strength between the two materials where the shearing occurred,
or within the GCL. The y-intercept of the straight line is the cohesion intercept for internal shearing or adhesion intercept for
interface shearing. A straight line fit for shear stresses at some post-peak displacement is the post-peak interface strength between
the two materials where the shearing occurred, or the post-peak internal strength within the GCL. If the post-peak shear stresses
have reached a constant value less than the peak strength, the post-peak strength is the interface residual strength or the internal
residual strength.
NOTE 1—There may be some investigative cases where only a single test point is desired. If the field design conditions will experience a range of
normal stresses, it is standard industry practice to bracket the normal-stress normal stress range with tests on both sides of the range, as it is unconservative
to extrapolate results outside of the normal-stress normal stress range tested. When defining a Mohr-Coulomb shear strength envelope over a range of
normal stresses, standard industry practice is to use a minimum of three test points. Attempting to define a single linear Mohr-Coulomb shear strength
envelope over too-large too large of a normal-stress normal stress range may prove to be problematic in many cases because most failure envelopes exhibit
significant curvature over such a large range, particularly at low normal stresses on the shearing plane.
5. Significance and Use
5.1 The procedure described in this test method for determination of the shear resistance for the GCL or the GCL interface is
intended as a performance test to provide the user with a set of design values for the test conditions examined. The test specimens
and conditions, including normal stresses, are generally selected by the user.
5.2 This test method may be used for acceptance testing of commercial shipments of GCLs, but caution is advised as outlined
in 5.2.1.
5.2.1 The shear resistance can be expressed only in terms of actual test conditions (see Notes 2 and 3). The determined value
may be a function of the applied normal stress, material characteristics (for example, of the geosynthetic), soil properties, size of
sample, moisture content, drainage conditions, displacement rate, magnitude of displacement, and other parameters.
NOTE 2—In the case of acceptance testing requiring the use of soil, the user must furnish the soil sample, soil parameters, and direct shear test
parameters. The method of test data interpretation for purposes of acceptance should be mutually agreed to by the users of this standard.
NOTE 3—Testing under this test method should be performed by laboratories qualified in the direct shear testing of soils and meeting the requirements
of Practice D3740, especially since the test results may depend on site-specific and test conditions.
5.2.2 This test method measures the total resistance to shear within a GCL or between a GCL and adjacent material. The total
shear resistance may be a combination of sliding, rolling, and interlocking of material componentscomponents.
5.2.3 This test method does not distinguish between individual mechanisms, which may be a function of the soil and GCL used,
method of material placement and hydration, normal and shear stresses applied, means used to hold the GCL in place, rate of
horizontal displacement, and other factors. Every effort should be made to identify, as closely as is practicable, the sheared area
and failure mode of the specimen. Care should be taken, including close visual inspection of the specimen after testing, to ensure
that the testing conditions are representative of those being investigated.
5.2.4 Information on precision between laboratories is incomplete. In cases of dispute, comparative tests to determine whether
a statistical bias exists between laboratories may be advisable.
5.3 The test results can be used in the design of GCL applications, including but not limited to, the design of liners and caps
for landfills, cutoffs for dams, and other hydraulic barriers.
5.4 The displacement at which peak strength and post-peak strength occursoccur and the shape of the shear stress versus shear
displacement curve may differ considerably from one test device to another due to differences in specimen mounting, gripping
surfaces, and material preparation. The user of results from this standard is cautioned that results at a specified displacement may
not be reproducible across laboratories and that the relative horizontal displacement measured in this test at peak strength may not
match relative shear displacement at peak strength in a field condition.
6. Apparatus
6.1 Shear Device—A rigid device to hold the specimen securely and in such a manner that a uniform shear force without torque
can be applied to the tested interface. The device consists of both a stationary and moving container, each of which is capable of
containing dry or wet soil and areis rigid enough to not distort during shearing of the specimen. The traveling container must be
placed on firm bearings and rack to ensure that the movement of the container is only in a direction parallel to that of the applied
shear force.
NOTE 4—The position of one of the containers should be adjustable in the normal direction to compensate for vertical deformation of the GCL, soil,
and adjacent materials.
6.1.1 Square or rectangular containers are recommended. They should have a minimum dimension that is the greatest of 300
mm [12 in.], 15 times the d of the coarser soil used in the test, or a minimum of five times the maximum opening size (in
plan)plane) of the geosynthetic tested. The depth of each container should be at least 50 mm [2 in.] or six times the maximum
particle size of the coarser soil tested, whichever is greater.
D6243/D6243M − 20
NOTE 5—The minimum container dimensions given in 6.1.1 are guidelines based on requirements for testing most combinations of GCLs and adjacent
materials. Containers smaller than those specified in 6.1.1 can be used if it can be shown that data generated by the smaller devices contain no bias from
scale or edge effects when compared to the minimum size devices specified in 6.1.1 for specific materials being tested. The user should conduct
comparative testing prior to the acceptance of data produced on smaller devices. For direct shear testing involving soils, competent geotechnical review
is recommended to evaluate the compatibility of the minimum and smaller direct shear devices.
6.2 Normal Stress Loading Device, capable of applying and maintaining a constant uniform normal stress on the specimen for
the duration of the test. Careful control and accuracy (62 %) of normal stress is important. Normal force loading devices include,
but are not limited to, weights, pneumatic or hydraulic bellows, or piston-applied stresses. For jacking systems, the tilting of
loading plates must be limited to 2° from the shear direction during shearing. The device must be calibrated to determine the
normal force delivered to the shear plane.
6.3 Shear Force Loading Device, capable of applying a shearing force to the specimen at a constant rate of displacement. The
horizontal force measurement system must be calibrated, including provisions to measure and correct for the effects of friction and
tilting of the loading system. The rate of displacement should be controlled to an accuracy of 610 % over a range of at least 6.35
mm/min [0.25 in./min] to 0.025 mm ⁄min [0.001 in./min]. The system must allow constant measurement and readout of the applied
shear force. An electronic load cell or proving ring arrangement is generally used. The shear force loading device should be
connected to the test apparatus in such a fashion that the point of the load application to the traveling container is in the plane of
the shearing interface and remains the same for all tests. (See Note 6).
NOTE 6—The operating range of normal and horizontal shear stresses for a device should be limited to between 10 and 90 % of its calibrated range.
If a device is used outside this range, the report shall so state and give a discussion of the potential effect of uncertainties in normal stress on the measured
results.
6.4 Displacement Indicators, for providing continuous readout of the horizontal shear displacement, and if desired, vertical
displacement of the specimen during the consolidation or shear phase, or both. Displacement indicators, such as dial indicators,
or linear variable differential transformers (LVDTs), capable of measuring a displacement of at least 75 mm [3 in.] for shear
displacement and 25 mm [1 in.] for vertical displacement are recommended. The sensitivity of displacement indicators should be
at least 0.02 mm [0.001 in.] for measuring shear displacement and 0.002 mm [0.0001 in.] for measuring vertical displacement.
6.5 GCL Clamping Devices, required for fixing GCL specimens to the stationary section or container, the traveling container,
or both, during shearing of the specimen. Clamps and grips shall not interfere with the shearing surfaces within the shear box and
must keep the GCL specimens flat during testing. Gripping surfaces must develop sufficient shear resistance to prevent
non-uniformnonuniform displacement of the GCL and adjacent geosynthetics. Gripping surfaces must develop sufficient shear
resistance to prevent tensile failure within any geosynthetics material outside the specimen area subjected to normal stress. Flat,
jaw-like clamping devices are normally sufficient. Textured surfaces or soil must be used to support the top and/or bottom top,
bottom, or both of the geosynthetic. Where the internal shear resistance of the GCL is to be measured, rough (textured) surfaces
must be used on the top and bottom of the GCL to force internal shearing within the GCL. These surfaces must permit flow of
water into and out of the test specimen. Work is still in progress to define the best type of textured surfaces. Selection of the type
of texture surface should be based on the following criteria:
6.5.1 The gripping surface should be able to mobilize fully mobilize the friction between the gripping surface and the outside
surfaces of the GCL:GCL. The rough surfaces must be able to prevent slip between the GCL and the gripping surface to prevent
tensile failure in the geotextile. This requirement also applies to any geosynthetics used to determine interface shear strength of
the GCL.
6.5.2 The gripping surface must be able to completely transfer the applied shear force through the outside surfaces into the
inside of the GCL:GCL. A textured steel gripping surface made of rasps, truss plates, nail boards, or machined angled spikes 1
to 2 mm tall mounted on a rigid substrate have been found to work. Truss plates with teeth ground down so they extend 1 to 2
mm into the GCL with at least 1 point per cm are the preferred gripping surface for this standard, and should be used unless
specific factors dictate a different gripping surface. Indicate the gripping surface type, spacing, and height on the test report. Gluing
of the GCL to a substrate may influence the strength behavior of the GCL and may not be used.
6.5.3 The gripping surface must not extend into the failure plane for internal shear of the GCL. The resulting failure surface
for internal shear of GCL should be entirely within the GCL.
NOTE 7—The selection of specimen substrate may influence the test results. For instance, a test performed using a rigid substrate, such as a wood or
metal plate, may not simulate field conditions as accurately as that using a soil substrate. However, use of compressible soils as a substrate is not
recommended due to the possibility that these soils may compress under the applied normal load to the extent that the intended shear plane is no longer
level with the gap between the two halves of the shear box. The user should be aware of the influence of substrate on direct shear resistance data. Accuracy,
reproducibility, and relevance to field conditions should be considered when selecting a substrate for testing.
NOTE 8—Gripping and clamping systems vary widely and can be different based on the geosynthetic material being tested. Several authors have
successfully used a multitude of systems.
Fox et al., 1997,; Pavlik, 1997,; Trauger et al., 1997,; Fox et al., 1998,; Zanzinger and Alexiew, 2000,; Olsta and Swan, 2001,; Triplett and Fox, 2001,; Marr, 2002,;
Koerner and Lacy, 2005,; Fox et al., 2006,; and Allen and Fox, 2007.
D6243/D6243M − 20
6.6 Soil Preparation Equipment, for preparing or compacting bulk soil samples, as outlined in Test MethodsMethod D698,
D1557, or D3080/D3080M.
6.7 Miscellaneous Equipment, as required for preparing specimens. A timing device and equipment required for maintaining
saturation of the geosynthetic or soil samples, if desired.
7. GCL Sampling
7.1 Lot Sample—Divide the product into lots, and for any lot to be tested, take the lot sample as directed in Practice
D6072/D6072M (see Notes 6 and 7).
7.2 Laboratory Sample—Consider the units in the lot sample as the units in the laboratory sample for the lot to be tested. For
a laboratory sample, take a sample extending the full width of the GCL production unit and of sufficient length so that the
requirements of 7.3 can be met. Take a sample that will exclude material from the outer edge.
7.3 Test Specimens—From each unit in the laboratory sample, remove the three specimens (or fewer if specified by the user)
as outlined in 7.3.1.
7.3.1 Remove specimens for shearing in a direction parallel to the machine, or roll, direction of the laboratory sample and three
specimens for shearing in a direction parallel to the cross-machine, or cross-roll,cross-roll direction, if required (see Notes 9 and
10). All the specimens should be sufficiently large to fit snugly in the container described in 6.1.1, and they should be of sufficient
size to facilitate clamping. All specimens should be free of surface defects, etc., that are not typical of the laboratory sample. Space
the specimens along a diagonal of the unit of the laboratory sample. Take no specimens nearer the edge of the GCL production
unit than ⁄10 the width of the unit.
NOTE 9—Lots for GCLs usually are designated by the producer during manufacturing. While this test method does not attempt to establish a frequency
of testing for the determination of design-oriented data, the lot number of the laboratory sample should be identified. The lot number should be unique
to the raw material and manufacturing process for a specific number of units, for example, rolls, panels, etc., designated by the producer.
NOTE 10—The shear strength characteristics of some GCLs may depend on the direction tested. In many applications, it is necessary to perform shear
tests in only one direction that matches the direction of shear in the installation. In addition, it is often necessary to perform shear tests against a specific
side of the geosynthetic that matches the installation. The direction of shear and the side of the GCL specimen(s) must be noted clearly in these cases
NOTE 11—To understand the shear characteristics of the GCL specimen(s), it may be useful to conduct peel strength tests on the material adjacent to
the location the shear test specimen was taken from the laboratory sample. Peel strength testing should be conducted in general accordance with Test
Method D6496/D6496M, but the number of test specimens is at the discretion of the user.
NOTE 12—To understand the shear characteristics of the geocomposite specimen(s), it may be useful to conduct ply adhesion tests on the material
adjacent to the location the shear test specimen was taken from the laboratory sample. Ply adhesion testing should be conducted in general accordance
with Test Method D7005/D7005M, but the number of test specimens is at the discretion of the user.
NOTE 13—To understand the shear characteristics of the textured geomembrane specimen(s), it may be useful to conduct asperity height tests of the
shear test specimen(s) prior to shearing (and potentially after shearing). Asperity height testing should be conducted in general accordance with Test
Method D7466/D7466M, but the number of measurements taken is at the discretion of the user.
8. Shear Device Calibration
8.1 The direct shear device must be calibrated to measure the internal resistance to shear inherent to the device. The inherent
shear resistance is a function of the geometry and mass of the traveling container, type and condition of the bearings, and type of
shear loading system, and the applied normal stress. The calibration procedure described in this section is applicable to certain
devices. Other procedures may be required for specific devices. Refer to the manufacturer’s literature for recommended calibration
procedures. (See Note 1114).
NOTE 14—Calibration of electronic equipment used in this method and calibration for device friction should be performed at least once per year using
traceable reference materials.
8.2 Assemble the shear device completely without placing a specimen inside it. If the device permits, apply a normal stress
equal to that for which friction is being measured. If applying a normal stress, some low friction low-friction mechanism such as
rollers must be used to resist the normal stress without creating a shear resistance. Some boxes do not permit calibration with a
normal stress. Adjust the gap between the upper and lower box to the value used in shear testing. Apply the shear force to the
traveling container at a rate of 6.35 mm/min [0.25 in./min]. Record the shear force required to sustain movement of the traveling
container for at least 75 mm [3 in.] total shear displacement. Record the applied shear force at 1 mm [0.05 in.] intervals. Determine
the average shear force over 75 mm [3 in.] of displacement. Variations in shear force of more than 25 % of the average value may
indicate damaged or misaligned bearings, an eccentric application of the shear force, or a misaligned box. The equipment must
be repaired if the measured shear force varies by more than 25 % of the average value.
8.3 The maximum shear force recorded is the internal shear correction to be applied to shear force data after the testing of the
specimens. The internal shear correction for device friction should not exceed 10 % of the measured peak strength.
9. Conditioning
9.1 Maintain samples at the as-received moisture content until ready to cut specimens for testing.
9.2 For tests on GCL without soil, test specimens at the temperature specified in the standard atmosphere for testing
geosynthetics. Humidity control normally is not required for direct shear testing.
D6243/D6243M − 20
9.3 When soil is included in the test specimen, the method of conditioning is selected by the user or mutually agreed upon by
the user and the testing agency. Material required for the specimen shall be batched by thoroughly mixing soil with sufficient water
to produce the desired water content. Allow the soil to stand prior to compaction in accordance with the following guide:
Classification (by Practice D2487) Minimum Standing Time, h
SW, SP No Requirement
SM 3
SC, ML, CL 18
MH, CH 36
9.3.1 In the absence of specified conditioning criteria, as described in 9.4, the test should be performed at the temperature
specified in the standard atmosphere for testing GCLs. Relative humidity control should be performed when specified by the user.
9.4 The minimum user specified user-specified test conditioning criteria include the following:
9.4.1 The test configuration, including all components from the top to bottom (supporting substrates, soil, geo
...