Macro-micro failure and crack coalescence behavior of soft-hard composite rock with three parallel joints under uniaxial compression

Jointed soft-hard composite rock mass is very common in engineering. The existence of joint and the uncoordinated deformation of soft-hard layer make the failure behavior more complicated. In this study, soft-hard composite rock with three parallel joints (two coplanar joints and one out-of-plane parallel joint) were fabricated to carry out uniaxial compression experiment. With the aid of digital imagine correlation (DIC) and acoustic emission (AE) technology, the influence of joint angle, ligament angle and the location of out-of-plane joint on crack propagation behavior of specimen is analyzed. The results show that when the hard layer contains more joints, the rock mass presents more obvious brittleness. The variation of peak strength with ligament angle also depends on the rock layer where the out-of-plane crack is located. Three failure modes and ten crack coalescence modes are summarized, which are affected by joint angle and ligament angle. The transition of crack coalescence between adjacent rock layer is mainly influenced by the space of wing cracks at joint tips. The coalescence of rock bridge in the layer with double joints is affected by ligament angle and the relative strength of layer. When the out-of-plane joint is in the hard layer, the coalescence possibility is greatly reduced.