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Description / Abstract:
This method describes procedures for determining the
consolidated drained shear strength of a soil material in direct
shear. The test may be conducted in either a single shear or in a
double shear. The direct shear test is well suited to a
consolidated drained test because the drainage paths through the
test specimen are short, thereby allowing excess pore pressures to
be dissipated fairly rapidly. The test can be made on all soil
material,2 and on undisturbed or remolded samples.
The test results are applicable to field situations where
complete consolidation has occurred under the existing overburden
and failure is reached slowly so that excess pore pressures are
dissipated. The test is also useful in determining the shearing
resistance along recognizable weak planes within the soil
material.
Note 1-If failure is forced to occur on or near
a horizontal plane at the middle of the specimen, it may not
necessarily occur along the weakest plane, thereby overestimating
shear strength parameters. Only when weak plane(s) are recognizable
within the soil mass or interfaces between dissimilar materials are
being tested, and the plane or interface at question is placed
within the limits of the forced failure zone, can the shear
resistance along these planes or interfaces be evaluated. The
usefulness of direct shear test results was discussed in the
Symposium on Direct Shear Testing of Soils; the proceedings appear
in ASTM Special Technical Publication 131.
The test is not suited to the development of exact stress–strain
relationships nor for evaluating any other associated quantities
such as moduli within the test specimen because of the non-uniform
distribution of shearing stresses and displacements. The slow rate
of displacement provides for dissipation of excess pore pressures,
but it also permits plastic flow of soft cohesive soils. Care
should be taken that the testing conditions represent those being
investigated.
The values stated in SI units are to be regarded as the
standard.
2 See Section 5.4 for specimen/particle-size
relationship.