Shear strength of discontinuities in sedimentary rock masses based on direct shear tests

Accurate estimation of the shear strength of rock discontinuities is of great importance in many rock engineering practices, especially for the stability assessment of dam foundations and rock slopes. This study was conducted through both in-situ and laboratory direct shear tests on rock discontinuities, under the constant normal load (CNL) boundary condition, focusing on the influence of the length of discontinuities on the shear strength. The results of more than 109 direct shear tests carried out on rock discontinuities at the Bakhtiary dam site were analyzed to consider the size effect on rock discontinuities strength. The surface roughness of discontinuities was described by fractal dimension using a modified divider method. Moreover, numerical simulations of direct shear tests were carried out on discontinuities with different lengths in order to establish an empirical relationship that can link the shear strength of discontinuities of laboratory scale to in-situ scale. From the experimental and numerical results, it was confirmed that the peak shear strength of the discontinuity decreases as a function of the discontinuity length.