Numerical Study of Hydraulic Fracturing Assisted Rock Breaking Based on Discrete Element Method

To improve the rock breaking efficiency of TBMs in hard rock, this research attempts to apply hydraulic fracturing technology to assist rock breaking and conduct numerical simulation of multi-step injection fracturing near the tunnel face to find suitable hydraulic fracturing scheme. The simulation results demonstrate that the initiation and propagation directions of hydraulic fracturing cracks are influenced by two key factors: the distance between the fluid injection hole and the tunnel face, as well as the injection sequence. As the initial fluid injection position becomes far ahead from the tunnel face, the propagation direction of hydraulic fracturing cracks gradually changes from vertical to horizontal. The second injection position in the vertical section should not be too close to the initial injection position to prevent the newly initiated hydraulic fracturing cracks converging with the previous cracks. Regarding the arrangement of injection holes on the cross section, when two or three injection holes undergo synchronous fracturing, there is a notable tendency for the fracturing cracks to propagate obliquely and intersect with each other. This intersection results in the rock mass being segmented into smaller blocks, significantly reducing the required fracturing time. Additionally, this process enhances the fragmentation of the rock mass, leading to more efficient tunnel boring machine (TBM) tunneling operations.