Critical support pressure of shield tunnel face in soft-hard mixed strata

Support pressure is a key factor to the stability of excavation face during shield tunneling, which further imposes a safety issue on surrounding buildings and facilities. However, several studies have been reported on critical support pressure in mixed strata regarding twin-tunnel excavation. This paper aims to obtain the critical support pressure based on minimizing the influence on the surrounding environment induced by shield tunneling in soft-hard mixed strata. A series of three-dimensional finite element models are established to analyze soil and lining responding with different support pressure caused by twin tunnels excavation. The results reveal that the ground surface settlement and lateral displacement of soil are sensitive to the change of support pressure, especially under clay-dominated conditions. Increasing support pressure slightly changes the lining movement and location of maximum stresses of the existing tunnel due to adjacent tunnel excavating. According to this study, the upper and the lower critical support pressures are 1.43 and 1.06 times of lateral earth stress at tunnel crown, respectively, which are also compared with the limit support pressure and measured results from a shield tunnel in the literature. The critical support pressure has great significance for the stability of the excavation face and minimizing the influence on the surrounding environment caused by shield tunnel excavation.