AASHTO T 236-22 PDF

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, 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—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.