Stability Analysis of Excavation Surface of Shield Tunnel in Upper-Soft and Lower-Hard Composite Strata

Based on the interval shield tunnel engineering between Zhouzhuang Station and Qiligou Station of the first-stage project of Xuzhou Rail Transit Line 2, the deformation characteristics and destruction mode analyses of the excavation surface of the shield tunnel in the upper-soft and lower-hard composite strata are carried out by numerical simulations. Assuming the instability mode of the excavation surface of the shield tunnel in the upper-soft and lower-hard strata is a partially trapezoidal wedge, the calculation formula for the limit support pressure of the excavation surface based on limit equilibrium theory is derived and that for the overlying soil pressure in isotropic soil and stratified soil is established based on loose earth pressure theory. The results show that the instability of the excavation surface of the shield tunnel reveals to be local failure in the upper-soft strata when the ratio of the elasticity modulus of the upper-soft and lower-hard strata is larger; while overall failure will occur when the ratio is smaller. The instability failure of the tunnel excavation surface will occur when the thickness of the upper-soft strata of the supporting project exceeds 1.5 m. On-site construction safety of the supporting project can be ensured by using grouting consolidation for treatment measures. © 2022, Editorial Department of China Railway Science. All right reserved.