Numerical Study of Anisotropic Weakening Mechanism and Degree of Non-Persistent Open Joint Set on Rock Strength with Particle Flow Code

The strength of a rock cut by non-persistent open joint set is controlled by complex interactions of the joint network and intact rock bridges. An extensive series of uniaxial numerical tests of the particle models are carried out under different joint fabrics and rock compression-tension strength ratios. The test results show joint dip angle is the main factor controlling the overall anisotropic strength characteristics. With joint dip angle increasing, the main failure modes successively appear as splitting failure, block rotation failure, step-path failure, planar failure, and splitting failure. Accordingly, the strength generally shows a trend of decreasing first and then increasing, and the joint dip angle corresponding to the minimum strength is biased towards 0°, usually at 15°–30°. The damage of rock bridges is mainly controlled by tensile failure, except in planar failure mode. Joint spacing has an impact on the local anisotropic strength characteristics. The joint spacing and persistence ratio jointly control the degree of penetration difficulty of the rock bridges. The compression-tension strength ratio of intact rock uniformly affects the absolute magnitude of the jointed rock strength and does not affect its anisotropic characteristics. Based on the weakening mechanism, an empirical strength formula is established to realize the practical equivalent characterization of the anisotropic weakening degrees of the non-persistent open joint set on the rock strength.