Study on Fracture Mechanism of Soft Rock with Different Prefabricated Fractures during Drilling

The primary fractures in rocks have an important influence on the rock breaking efficiency. In order to understand the fracture mechanism of broken soft rock strata more deeply, this paper carried out a series of discrete element particle flow numerical simulations with different prefabricated fractures (different number of fractures and fracture angles) based on laboratory rock mechanics tests. The results show that: the overall failure of rock mass is mainly tensile and compressive failure. And, when the crack reaches a certain depth, the influence of primary cracks in rock mass on rock breaking effect will be significantly weakened. Furthermore, in the process of rock breaking, the rock mass basically shows brittle fracture, and the crack initiation stress of rock mass has a good positive correlation with the degree of rock fragmentation. The crack initiation location is often accompanied by stress concentration, which is the key to rock fragmentation. And, the angle of prefabricated cracks will affect the crack initiation position. When the prefabricated crack angle approaches the top angle of the drill, the crack initiation stress will increase. Finally, the fracture mode and initiation mechanism of rock mass are revealed in the process of rock breaking. This study can provide some reference for the optimization of drilling parameters.