Structural Effect of En-echelon Fractures on Shear Behavior of Rock Mass Under Constant Normal Load Conditions: An Experimental Study

En-echelon fractures widely exist in natural rock masses. Although the formation and occurrence features of en-echelon fractures have been extensively studied by geologists, the anisotropic mechanical behavior caused by en-echelon fractures has received little attention. In this study, a set of idealized en-echelon fractures were prefabricated and subjected to direct shear tests. The influence of structural parameters (e.g., fracture angle and persistency) of en-echelon fractures on shear strength, friction and dilation behavior, shear failure structures, and acoustic emission response was comprehensively evaluated. The test results show that the shear of en-echelon fractures involves a cracking stage and a shear-slipping stage. These two stages can be quantitatively described by three shear strength indices and three dilation indices, which present varying degrees of anisotropy under different shear stages and boundary conditions. Also, various types of failure structures evolved from en-echelon fractures can be observed. They essentially depend on different crack types and coalescence patterns of rock bridges and greatly affect the friction and dilation characteristics in the shear-slipping stage. Based on the test results, the anisotropic micro-cracking mechanism was first discussed, and then an idealized vertical slope analysis was performed. Finally, some implications were discussed for the fault structures in field and the deep fault behavior.