Study on Determining Safe Distance and Optimization of Supporting Methods for Adjacent Tunnels in Fault Fracture Zone

The design and construction of tunnels and underground structures in deformed rock masses, such as fault fracture zones, pose significant challenges in geotechnical engineering. This study proposes an optimized excavation method for adjacent tunnels in fault fracture zones, integrating in-situ testing, numerical simulation, and field verification. First, the physical and mechanical properties of the surrounding rock mass were characterized using a Point Load Tester (PLT) and an elastic wave velocity measuring instrument (ZBL-U5200), and the rock mass strength weakening law was quantified. The rock mass strength degradation coefficient was calibrated based on field monitoring data to reflect the actual geological conditions. Subsequently, FLAC3D numerical simulation was employed to evaluate the stability of adjacent tunnels in the weak rock mass. A technical scheme combining sequential excavation and gradual cross-sectional enlargement was proposed for the weak rock sections, accompanied by a parallel simultaneous construction strategy. The effectiveness of the proposed method was verified through numerical simulation and field application. Advanced excavation and support technologies were successfully implemented, significantly improving the stability of adjacent tunnels. This study provides a reliable technical approach for tunnel excavation and underground structure construction in highly fractured and weak rock masses, particularly in fault fracture zones.