Recommendations for Selecting an Optimal Rock Joint Direct Shear Test Procedure

Evaluating the shear strength of rock joint surfaces is critical for many rock engineering applications, and the direct shear test is often used for this purpose. There are three direct shear testing techniques suggested by the ASTM D5607: Single Stage (using three individual samples), and Multistage (using a single sample) with two variations (with and without return to the original position). In addition to these, a modified version referred to as the Limited Displacement Multistage Direct Shear Test has been developed, which is often able to achieve more accurate peak shear strength values by limiting shear displacement, thus reducing damage to key asperities that control peak shear strength. Rock joint specimens exhibit a wide variety of joint characteristics and corresponding variation in shear strength behaviors. Suggestions are made regarding which variation of the direct shear test is likely to be optimal for a given specimen, based on observation of four different joint characteristics: joint interlocking and mating, asperity roughness and shape, asperity strength, and joint surface condition.