STRENGTH AND FAILURE MECHANISM OF BRITTLE ROCKS UNDER TENSILE-COMPRESSIVE STRESS STATE WITH PARTICLE FLOW SI MU LA TION

The excavation of underground rock masses in high-stress region often produces the tensile-compressive stress state in surrounding rock, which leads to the tensile failure hazard of rock masses. The traditional parallel bond model in PFC, however, cannot simulate the high ratio between uniaxial compressive strength and tensile strength of brittle rocks. To solve this problem, a new parallel bond failure criterion considering two tensile strength parameters is established and a series of numerical simulation tests under tensile-compressive stress condition are conducted in this paper. The simulated tensile-compressive strengths are close to that tested by physical experiments. A high ratio between uniaxial compressive strength and tensile strength is achieved. The failure mechanism is analyzed in detail. With the increase of confining pressure, the inclination angle of fracture surface increases gradually and the fracture transforms from tensile fracture to tensile-shear fracture. Echelon cracks are found at the fracture region under tensile-compressive stress state. The mechanical properties of fracture surfaces are revealed according to the mesoscopic particle displacement fields. The tension property decreases and shear property increases gradually with the increase of confining pressure(i.e. with the increase of the inclination angle of fracture surface). The evolution process of rock damage under tensile-compressive stress state can be divided into the following four stages. They are elastic deformation stage, stable rupture development stage, unstable rupture development stage and overall rupture stage(post-peak stress drop and residue stage). When the confining pressure is larger, the elastic deformation stage and stable rupture development stage are more short, the unstable rupture development stage is more long and intensive, the fracture surface friction is stronger, and the stress fluctuation is greater at post-peak residue stage.