A large 3D laboratory test on the deformation characteristic of shallow loess tunnel under different plastic states

A large 3D laboratory model test was conducted to study the deformation characteristic of the shallow loess tunnel under the hard, soft, and flow plastic states. The ground displacement difference (GDD), pre-convergence rate, tunnel face’s extrusion displacement, and radial stress-release rates are analyzed. The result shows that the GDD of the hard, soft, and flow plastic states loess tunnel is larger at the sidewall while smaller at the vault, and the GDD of the sidewall is about five times that of the vault. The longitudinal disturbance range of excavation to the vault and sidewall is 1.5D and 1.2D (in which D is tunnel span), and the advanced reinforcement length of soft and flow plastic loess should be at least 1.5D. The tunnel face in the hard, soft, and flow plastic loess presents the deform state of vertical, drum out, and slide types, respectively, and the reinforcement length for tunnel faces in soft and flow plastic loess should be at least 0.8D ahead tunnel faces. The maximum stress-release rate of the vault under soft and flow plastic states is about 70%; the excessive stress release should be avoided to prevent structural instability in actual engineering. It is feasible to determine the reserved deformation by model test in the absence of field data in loess tunnels. The reserved deformation of the vault is about 2–4 times of sidewall; the reserved deformation between vault and sidewall should be a curved transition.