Influence of multi-perforations hydraulic fracturing on stress and fracture characteristics of hard rock mass under excavation condition

Abstract

Performing multi-perforations hydraulic fracturing (MPHF) on the rock mass is a highly effective method for increasing the fracture network, in this paper, MPHF technology was considered to assist mechanical excavation for hard rock mass in underground engineering. To end this, the influences of MPHF on the stress and fracture characteristics of hard rock mass under excavation conditions were investigated theoretically and numerically by considering different perforation spacings, perforation heights, and injection pressures. Theoretical analyses were performed based on the stress field theory of hydraulic fracturing and analytical solutions for excavation stress redistribution. A numerical model was established using a flow-coupled discrete element method, and it was verified using theoretical solutions. The results show that the local maximum principal stresses are in the horizontal direction under different MPHF parameters. The local stress differences decrease as the increasing of perforation spacing and the decreasing of perforation height and injection pressure. With an increase in perforation spacing, fewer horizontal fracture trajectories were generated between the perforation zones, and the number of microcracks initially increased and then decreased, indicating there is an optimal perforation spacing in MPHF under excavation. Moreover, the increase in perforation height, injection pressure, and injection time promote the propagation of crack trajectories and increase the number of microcracks. However, in practice, they should not increase without limits owing to efficiency and economic considerations. Therefore, when using MPHF for preconditioning the rock mass to assist mechanical excavation in the hard rock mass, the project economics and performance can be enhanced by careful selection of appropriate MPHF parameters.