Unstable collapse analysis of SPBS in saturated sand based on model experiment

Abstract

The instability of the tunnel face during shield tunnelling can lead to extensive surface subsidence in saturated sand, potentially leading to ground collapse accidents that pose a serious threat to the safety of surface infrastructure. In order to study the mechanism of ground collapse induced by face instability, a laboratory-scale slurry pressure balance shield (SPBS) machine system is employed to simulate the entire process of tunnelling. The soil pressure, pore water pressure, and surface subsidence response during unstable are revealed through multi-field monitoring. The morphological evolution characteristics of collapse pits are analyzed using 3D scanning technology. The experimental results indicate that: (1) The key to stable tunnelling is to balance the pressure in slurry chamber with tunnelling speed, which ensures the formation of filter cake in front of the cutterhead. (2) The torque of the cutterhead, soil pressure, and surface subsidence will respond significantly and synchronously when the tunnel face is unstable, whereas the soil and water pressures are relatively less noticeable. (3) Continuous tunnelling results in a gentler angle of repose and a wider range of collapse of the collapsed pit in the longitudinal direction. (4) The formula for predicting the duration of collapse is proposed which integrates the evolution patterns of the collapse pit effectively and has been well-validated by comparison with these experimental results. The study provides a reference for the safe construction of tunnel engineering in saturated sand.