Influence of the Inclined Angle of Fault Fracture Zone on Limit Support Pressure of the Excavation Face of Shield Tunnel in Composite Stratum

In order to analyze the influence of large-diameter shield tunnel crossing the fault fracture zone on the stability of the excavation face, the instability characteristics of the excavation face before and after the shield tunnel crossing the fault fracture zone are summarized. The wedge-prism model of the excavation face of the composite stratum including the fault fracture zone is improved, and the theoretical calculation formula of the limit support pressure of the excavation face of the composite stratum defined as "soft on top and hard on bottom" is deduced by considering the influence of the fault fracture zone. Combined with engineering examples, this paper carries out relevant theoretical calculations, and reveals the changing rule of the limit support pressure influenced by inclined angle of fault fracture zone during the process of large-diameter shield tunnel crossing the front and back interfaces of the fault-bearing fractured zone. The results show that: Through sensitivity analysis of the parameters of cohesion and internal friction angle, it was found that increasing the cohesion and internal friction angle of each soil layer within the excavation surface can effectively reduce the size of the limit support pressure, which effectively verifies the reliability of the improved limit equilibrium analysis model. The limit support pressure of the excavation surface shows an increasing trend with the increasing inclination angle of the fault fracture zone. Corresponding to the inclination angles of the fracture zone of 60°, 70°, 80°, and 90°, the beginning and end of the critical length range of the front interface respectively resulted in an increase of 88.4%, 91.2%, 92.9%, and 93.6% in the limit support pressure, and that of the rear interface resulted in a decrease of 51.1%, 53.1%, 55.6%, and 56.3% in the limit support pressure respectively. The researchresults have a certain reference value for the adjustment of the controlled construction measures for large-diameter shield crossing composite formations containing fault fractured zones.