Visualization investigation of the large deformation and instability failure of shield tunnel using transparent soil models

Reliable prediction of the large deformation and instability failure caused by shield tunnels is critical for minimizing the adverse effects that may occur during tunnel excavation. A transparent soil surrogate representing the geotechnical behavior of sand was used to investigate spatial soil deformations within the soil mass. The support pressure, soil displacement, surface settlement, and failure mechanism were captured to explore the instability and failure of the shield tunnel for six cover-to-diameter (C/D) ratios ranging from 0.5 to 3.0. The support pressure undergoes three stages: rapid decrease, minor rebound, and stabilization. The soil mass ahead of the excavation face goes through three distinct stages: an elastic-plastic state, local instability failure, and ultimate overall instability failure. The failure mechanism is identified as a logarithmic spiral failure mode in front of the tunnel face and an ellipsoid failure mode above the tunnel face. The maximum surface settlement in the longitudinal section of the tunnel occurs at a distance of 0.2D to 0.3D ahead of the excavation face, while the maximum surface settlement in the cross-section is observed at the tunnel axis. The surface settlement can be divided into three stages: no settlement, slow settlement, and accelerated settlement. Considering the lag in surface settlement relative to excavation face displacement, it is critical in practical engineering to monitor the rate of surface settlement growth and control it within the no settlement or slow settlement stages to guarantee construction safety.