DIRECT SHEAR TEST OF SOIL
ASTM D3080, BS EN ISO 17892
1. Scope*
1.1 This test method covers the determination of the consolidated
drained shear strength of one specimen of a soil
material under direct shear boundary conditions. The specimen
is deformed at a controlled rate on or near a single shear plane
determined by the configuration of the apparatus.
1.2 Shear stresses and displacements are nonuniformly distributed
within the specimen. An appropriate height cannot be
defined for calculation of shear strains. Therefore, stress-strain
relationships or any associated quantity such as the shear
modulus, cannot be determined from this test.
1.3 The determination of strength envelopes and the development
of criteria to interpret and evaluate test results are left
to the engineer or office requesting the test.
1.4 The results of the test may be affected by the presence of
coarse-grained soil or rock particles, or both.
1.5 Test conditions, including normal stress and moisture
environment, should be selected to represent the field conditions
being investigated. The rate of shearing must be slow
enough to ensure drained conditions.
1.6 Generally, three or more tests are performed on specimens
from one soil sample, each under a different normal load,
to determine the effects upon shear resistance and displacement.
Results from a test series are combined to determine
strength properties such as Mohr strength envelopes. Interpretation
of multiple tests requires engineering judgment and is
beyond the scope of this test method. This test method pertains
to the requirements for a single test.
Significance and Use*
5.1 The direct shear test is suited to the relatively rapid
determination of consolidated drained strength properties because
the drainage paths through the test specimen are short,
allowing excess pore pressure to dissipate more rapidly than
other drained stress tests. The test can be made on any type of
soil material. It is applicable for testing intact, remolded, or
reconstituted specimens. There is however, a limitation on the
maximum particle size (see 6.2).
5.2 The test results are applicable to assessing strength in a
field situation where complete consolidation has occurred
under the existing normal stresses. Failure is reached slowly
under drained conditions so that excess pore pressures are
dissipated. The shear rate must meet the requirements of 9.10.
The results from several tests may be used to express the
relationship between consolidation stress and drained shear
strength.
8. Calibration*
8.1 Calibration is required to determine the deformation of
the apparatus when subjected to the consolidation load, so that
for each normal consolidation load the apparatus deflection
may be subtracted from the observed deformations. Therefore,
only deformation due to specimen consolidation will be
reported for completed tests. Calibration for the equipment
load-deformation characteristics need to be performed on the
apparatus when first placed in service, or when apparatus parts
are changed. The following series of steps provide one method
of calibrating the apparatus. Other methods of proven accuracy
for calibrating the apparatus are acceptable.
9. Procedure
9.1 Assemble the shear box and shear box bowl in the load
frame.
9.1.1 Intact Specimen—Place moist porous inserts over the
exposed surfaces of the specimen in the shear box, place the
shear box with the intact specimen and porous inserts into the
shear box bowl and align the bowl in the load frame.
NOTE 10—The decision to dampen the porous inserts or use dry inserts
depends on the problem under study. For intact samples obtained below
the water table, the porous inserts are usually dampened. For swelling
soils, the sequence of consolidation, wetting, and shearing should prevent
swelling until the specimen is equilibrated under the final normal stress.
9.1.2 Reconstituted Specimen—Place and align the assembled
shear box, specimen, porous inserts and bowl into the
load frame.
NOTE 11—For some apparatus, the top half of the shear box is held in
place by a notched rod which fits into a receptacle in the top half of the
shear box. The bottom half of the shear box is held in place in the shear
box bowl retaining bolts. For some apparatus, the top half of the shear box
is held in placed by an anchor plate.
9.2 Connect and adjust the position of the shear force
loading system so that no force is imposed on the shear load
measuring device. Record the zero value of the shear load
measuring device.
9.3 Position and adjust the shear displacement measurement
device. Obtain an initial reading or set the measurement device
to indicate zero displacement.
9.4 Place the load transfer plate and moment break on top of
the porous insert.
9.5 Place the normal force loading yoke into position and
adjust it so the loading bar is aligned. For dead weight lever
loading systems, level the lever. For pneumatic or motor drive
loading systems, adjust the yoke until it sits snugly against the
recess in the load transfer plate, or place a ball bearing on the
load transfer plate and adjust the yoke until the contact is snug.
9.6 Apply a small seating normal load to the specimen.
Verify that the components of the normal loading system are
seated and aligned. The top porous insert and load transfer
plate must be aligned so that the movement of the load transfer
plate into the shear box is not inhibited. The specimen should
not undergo significant compression under this seating load.
NOTE 12—The seating normal load applied to the specimen should be
sufficient to assure all the components are in contact and alignment but not
so large as to cause compression of the specimen. For most applications,
a load resulting in approximately 1 lbf/in.2 [5 kPa] will be adequate but
other values meeting the objective are acceptable.
9.7 Attach and adjust the normal displacement measurement
device. Obtain an initial reading for the normal displacement
measurement device along with a reading of the normal load
(either weights or measurement device).
9.8 Consolidation—The final consolidation normal load
may be applied in one increment or in several intermediate
increments depending on the type of material, the stiffness of
the specimen, and the magnitude of the final stress. Load
increments must be small enough to prevent extrusion of the
material from around the porous inserts. For stiff cohesive or
coarse grained material a single increment is normally acceptable.
For soft materials, it may be necessary to limit the load
increment ratio to unity as described in Test Method D2435
(11.4) and apply a number of intermediate load increments.
Based on the above considerations and instructions of the
requesting agency, calculate and record the normal force
required to achieve each intermediate normal stress level
progressing the specimen from the seating load to the final
consolidation normal stress.
9.8.1 Apply the first load increment and, if required, fill the
shear box bowl with test water, and keep it full for the duration
of the test. In the absence of specification, the bowl should be
filled with potable water.
NOTE 13—Flooding the specimen with water eliminates negative pore
pressure due to surface tension and also prevents evaporative drying
during the test. If and when to inundate the specimen as well as the water
chemistry is part of the test specification which should be provided by the
requesting agency.
9.8.2 For each intermediate stress level, apply the load as
quickly as practical. Maintain each load level until primary
consolidation is essentially complete based on either a) interpretation
of time versus normal deformation, b) experience
with the material or c) a default value of 24 h. Record the
normal deformation at the end of each increment and the
increment duration.
9.8.3 For the maximum normal stress level and for the final
normal stress level, apply the normal load to the specimen as
quickly as practical and immediately begin recording the
normal deformation readings against elapsed time. Test
Method D2435 provides details of the loading procedure and
suggestions for appropriate time recording schedules. For these
load increments, verify completion of primary consolidation
before proceeding to the next stage of the test by interpreting
either the plot of normal displacement versus log of time or
square root of time (in min). Test Method D2435 provides
interpretation details of both methods.
9.8.4 If the test specification requires consolidation to a
specific stress and then rebounding to a lower stress prior to
shearing, then the maximum stress should be maintained for at
least one cycle of secondary compression.
9.8.5 If the material exhibits a tendency to swell under the
maximum normal stress, the soil must be inundated with water
and must be permitted to achieve equilibrium (essentially stop
swelling) under this normal stress before continuing on to the
next stage of the test.
9.9 Just before shearing and after consolidation of the final
increment is completed, record the preshear normal displacement
and then remove the alignment screws or pins from the
shear box. Use the gap screws to separate the shear box halves
to approximately the diameter of the maximum sized particle in
the test specimen or 0.025 in. [0.64 mm] as a minimum default
value for fine grained materials. Back out the gap screws after
creating the gap