Experimental study on effect of spacing and inclination angle of joints on strength and deformation properties of rock masses under uniaxial compression

For the gypsum specimens containing a set of preexisting open flaws with fixed joint continuity factor, the influences of inclination angle and spacing of joints on the strength and deformation of the jointed specimens under uniaxial compression are systematically investigated through experiments. It is found that: (1) With the increase of joint spacing, the axial stress-axial strain curve changes from single-peak curve to multi-peak one, indicating that the ductility of the specimens increases. Four types of stress-strain curves are observed, i.e., single-peak, multi-peak during softening stage, softening after multi-peak yield platform and hardening after multi-peak yield platform. (2) For the specimens with constant joint spacing, the curves of the unified Young’s modulus, the unified strength and the first peak strain with the inclination angle of joints are V-shaped, and the minimum occurs at the inclination angle of 45 degrees, while the ratio of the residual strength to the strength and the last peak strain have inverse tendency. (3) For specimens with constant joint inclination angle, with the increase of the jointing index (the reciprocal of the joint spacing), the unified Young’s modulus, the unified strength and the first peak strain decrease, while the ratio of residual strength to strength and the last peak strain increase. (4) For each joint inclination angle, the relation between the unified Young’s modulus and the jointing index and that between the unified strength and the jointing index can be expressed by the power functions. (5) The macroscopic behavior of the jointed specimens is correlated to the closure of pre-existing joints and the cracking process of the specimen matrix. The anisotropic influence of joint spacing on the strength and deformation of jointed rock masses is significant.