Shear behaviour of a rock bridge sandwiched between incipient joints under the influence of hydraulic pressures

The rock bridges sandwiched in incipiently jointed rock mass were considered as barriers that block the fluid seepage, and provide certain shear strength reservation. For better revealing the influence of hydraulic pressure on the failure behaviour of rock bridges, direct shear tests were carried out through a newly proposed method on rock samples that contain two parallel incipient joints. By developing the gypsum-silicone pad coupling samples, a conventional triaxial test system was qualified to implement direct shear tests with satisfied sealing capability. The results showed that the rock bridges could be failed through the tensile failure, shear failure and mixed failure mechanism. The hydraulic pressure would facilitate the tensile failure mechanism and induce rougher fracture surfaces; while the normal stress would facilitate the shear failure mechanism and induce less rough fracture. The hydraulic pressure reduced the global shear strength of the rock block through reducing the efficient normal stress applied on the rock bridge area, which was highly dependent on the joint persistence, k. Moreover, because of the iterating occurrence of the hydraulic pressure lag with the fracture propagation, the rock bridge failure stage in the shear stress-shear displacement curves displayed a fluctuation trend.