A Numerical Study of the Mechanical Behavior of Jointed Soft Rocks under Triaxial Loading Using a Bonded Particle Model

In order to master the strength and deformation characteristics, including the macro–micro failure mechanism of soft rock samples with penetrating joints under triaxial loading, a series of numerical triaxial tests have been carried out. The strength and deformation characteristics, failure modes, crack propagation, distribution of force chains, and the influences of joint dip angles and confining pressures have been analyzed and compared with the laboratory test results. The results show that (1) the residual strength ratio of jointed rock samples generally increases first and then decreases with the increase in joint dip angles under the same confining pressure and reaches the maximum value around 23–24°. Poisson’s ratio increases with the increase in the confining pressure or the joint dip angle. The elastic modulus increases with the increase in the confining pressure and decreases with the increase in the joint dip angle. (2) The jointed rock samples with different joint dip angles compact with relatively small volumetric strains and then dilate up to failure with relatively large volume expansions. Lower confining pressure and smaller dip angles will lead to a more pronounced dilation phenomenon and less obvious volume shrinkage rules. (3) The low-angle jointed rock samples all exhibit the X-type shear failure. The jointed rock samples with a joint dip angle of 45° exhibit hybrid failure with both slippage and shearing, which are controlled by both the matrix and the joint. (4) The change in the number of cracks includes three stages: the slow crack initiation stage, rapid growth stage, and crack coalescence stage. The total number of shear or tensile cracks all decrease with an increase in the joint dip angles, with the number of tensile cracks being approximately twice that of shear cracks. The tension cracks are mostly horizontal, and the shear cracks are mostly vertical. (5) The number of force chains shows a decreasing trend after the cracks begin to grow. The jointed rock samples for the intact, 15° and 30° cases all form a main force chain during the failure process, while there is no main force chain for the 45° case.