Experimental and numerical investigations on crack propagation characteristics of rock-like specimens with preexisting flaws subjected to combined actions of internal hydraulic pressure and shear force

Based on laboratory direct shear tests and discrete element theory, the crack propagation mechanism and numerical simulation of rock-like specimens with preexisting flaws considering combined actions of internal hydraulic pressure and shear force were carried out in this paper. We completed the filling of the internal hydraulic pressure in the rock-like specimens with preexisting flaws, and performed direct shear tests on the specimens. Furthermore, we explored the crack propagation process of rock-like specimens with flaws under internal hydraulic pressure and shear load. In the numerical analysis, the pipe domain model in the two-dimensional particle flow code (PFC2D) was modified owing to the high brittleness and low permeability of the cement mortar particles in the numerical model. We modified the calculation rules of the interaction between the fluid and cement mortar particles, and proposed an improved fluid–solid coupling model which is more suitable for the crack propagation analysis in the high brittle cement mortar specimens. Under the action of internal hydraulic pressure, a tensile region existed at the tip of the preexisting flaws of the cement mortar specimen, which can also explain the crack initiation and propagation along the horizontal shear direction during the stage of crack initiation. When cracks progressively propagated, concentrated tensile stress and internal hydraulic pressure were released, leading to the appearance of a large number of tensile cracks, and the release of transverse shear force and internal hydraulic pressure resulted in the appearance of a large number of shear cracks. In the presence of internal hydraulic pressure, the increased number of pre-existing flaws would reduce the shear strengths of cement mortar specimens. Compared with the shear strength of the specimen with single flaw, the shear strength of the specimen with double flaws was reduced by 7.6%, and the specimen with triple flaws was reduced by 10.2%.