Strength reduction finite element analysis of a stability of large cross-river shield tunnel face with seepage

The strength reduction FEM was adopted to quantify the face stability of a large cross-river shield tunnel. Two centrifuge model tests were conducted to obtain the relationship of support pressure and displacement of the centre-point of the tunnel face. After validation with centrifuge tests, elasto-plastic finite element analysis was adopted to calculate the bounds of the support pressure required for tunnel face stability, and the influences of friction angle, tunnel cover-to-diameter ratio, and water level were investigated. The safety factor of the tunnel face was assessed by strength reduction FEM. When a tunnel face is supported by a designed pressure which is smaller (larger) than the earth pressure at-rest, the obtained safety factor under seepage conditions is smaller (larger) than that under non-seepage conditions. The relationship between safety factor and support pressure shows a peak; the pressure corresponding to the peak is the best value to stabilize the tunnel face. The seepage force increases the best support pressure. The proposed strength reduction method was used in the Qianjiang shield tunnel in Zhejiang, China; the bounds of the required support pressure and the best support pressure were obtained.