Face stability analysis of opposite-excavation tunnels: influences of face distance and pore water pressure

Abstract

Appropriate support pressure is crucial for maintaining the face stability of tunnels constructed using the opposite-excavation method. In this study, two failure mechanisms are constructed to determine the appropriate support pressure on the faces of opposite-excavation tunnels, considering the effects of face distance and pore water pressure. Two theoretical models for the limit support pressure of tunnel faces are developed based on the limit equilibrium method, and then validated by simulation results and previous studies. The results show that as the face distance between the opposite-excavation tunnels decreases, the limit support pressure initially increases and then decreases. A critical distance of 20 m is identified, with the corresponding critical limit support pressures of 160 kPa and 325 kPa without and with the influence of pore water pressure, respectively. Additionally, the limit support pressure is found to decrease with cohesion but increase with unit weight. When the face distance exceeds 30 m and without the influence of pore water pressure, a larger support pressure is required to maintain the face stability of tunnel in strata with a larger internal friction angle; conversely, when the face distance is less than 30 m, an opposite trend is observed. Considering the influence of pore water pressure, maintaining the face stability of tunnel in strata with a larger internal friction angle requires a smaller limit support pressure.