AASHTO T 236:08(2018) Standard Method of Test for Direct Shear Test of Soils under Consolidated Drained Conditions.
The test results are applicable to field situations where complete consolidation has occurred tinder 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 13 1.
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.
• T 265, Laboratory Determination of Moisture Content of Soils
SUMMARY OF METHOD
The method consists of(a) placing the test specimen in the direct shear device. (b) applying a predetermined normal stress, (c) providing for drainage or wetting of the test specimen, (d) consolidating the specimen under the normal stress. (e) unlocking the frames that hold the test specimen, and (I) applying a shearing force to shear the specimen (Figures 1 and 2). Generally, three or more specimens are tested. each under a different normal stress to determine the effects on shear resistance and displacement. The range in normal stresses should be appropriate for the soil conditions being investigated.AASHTO T 236 pdf download.