The objective of this study is to experimentally investigate the rate-dependent shear strength of rock fractures by performing triaxial shear testing under various shear velocities and confinements. A polyaxial load frame is used to perform the shear tests on tension-induced fractures prepared in 50×50×87 mm3 rectangular blocks. The fracture area is 50×10 mm2. The specimens are prepared from granite, marl and sandstone. The confining pressures vary from 1, 3, 7, 12 to 18 MPa. The axial stresses are applied under constant rates equivalent to the shear velocities on the fractures from 8.7×10−6 to 8.7×10−3 mm/s. The asperity amplitudes on the fracture planes are measured from laser-scanned profiles along the shear direction and used to estimate the joint roughness coefficients (JRC) of the fracture. The JRC's are averaged as 15, 8 and 6 for the granite, marl and sandstone. The test results indicate that the peak shear strengths under each confinement increases with shear velocities. The Barton's criterion is modified here to explicitly incorporate the shear velocity and confining pressure.

1. Introduction

Understanding the nature behavior of rock mass is important in many geotechnical applications. The performance of engineering structures constructed in rock is concerned with the presence of fractures in rock when subjected to forces and displacements (Curran and Leong, 1983; Li et al., 2012). In rock masses, properties such as roughness, separation and joint aperture have considerable effects on shear strength of rock fractures. The shear behavior of rock fractures is usually estimated through direct shear tests (e.g., ASTM D5607–08) to determine the peak and residual strengths of the rock fractures. Its test configurations however pose some disadvantages that the magnitudes of the applied normal stress are limited by the uniaxial compressive strength of the rock and that the fractures are sheared under unconfined conditions. The triaxial shear testing (Brady and Brown, 2006; Jaeger et al., 2007) has been developed to simulate the frictional resistance of rock fractures under confinements. The normal stress at which the shear strengths are measured can be controlled by the applied axial stress and confining pressure.

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