Study on the development mechanism of fault rockburst under different excavation directions in deep TBM tunnels

During the excavation of deep tunnels, the advancing direction of the working face relative to a fault (advancing from the hanging wall to the footwall, denoted as HTF, or from the footwall to the hanging wall, denoted as FTH) is a critical factor affecting the risk of fault rockburst. This study investigates six fault rockburst cases encountered in a deep tunnel excavated by a tunnel boring machine (TBM). By integrating microseismic (MS) monitoring, numerical simulations, and theoretical analysis, the development mechanisms of fault rockburst under the HTF and FTH conditions are systematically revealed. The results indicate that: (1) Compared to the HTF condition, rockbursts under the FTH condition occurred more frequently, involved larger failure volumes, and exhibited greater suddenness. (2) The evolution of MS parameters indicates that the FTH condition is characterized by more abrupt energy release and a lower proportion of shear fractures. The fault is significantly disturbed at approximately three and two times the tunnel diameter ahead of the working face under the FTH and HTF conditions, respectively. (3) Numerical simulations clarify the mechanical mechanism. The FTH condition induces an earlier initiation and larger magnitude of principal stress rotation on the fault plane, thereby generating a higher additional shear stress increment that more readily drives the fault toward macroscopic slip instability. Furthermore, an evaluation model centered on the slip potential function and energy release rate is established based on fracture mechanics theory. This model fundamentally reveals the directional control mechanism by which the FTH condition synergistically enhances both shear stress and normal unloading, thereby systematically increasing the rockburst risk. The research findings provide an important theoretical basis and practical guidance for the directional targeted warning and mitigation measures of fault rockbursts in deep TBM tunnels.