Time-Dependent Behavior of the Tunnels in Squeezing Ground: An Experimental Study

The presence of squeezing ground conditions often poses significant challenges in predicting tunnel response over time and to the design of an adequate support system to stabilize the tunnel. Many analytical, empirical, observational, and numerical models have been proposed over the years for the design of tunnels in squeezing ground conditions, but they all have some limitations. The study presented in this paper focused on improving the understanding of tunnel squeezing via a unique physical model test that simulated tunnel boring machine (TBM) excavation in squeezing clay-rich rocks. The physical model included a large true-triaxial cell, a miniature TBM, a laboratory-prepared synthetic test specimen having properties similar to natural mudstone, and instrumentations to monitor deformations around the tunnel boundary during and after the excavation. Experiments were conducted at realistic in-situ stress levels to study time-dependent tunnel convergence in three-dimensions. A tunnel was excavated using the miniature TBM in a cubical rock specimen loaded in the true-triaxial cell; then the confining stress was increased in stages to values above the rock unconfined compressive strength. Strain gauges embedded in the rock specimen and a digital borehole caliper monitored tunnel wall deformations with time. The degree of tunnel squeezing was characterized using a classification system based on tunnel radial strain. A model for time-dependent tunnel longitudinal displacement profile (LDP) was proposed using measurements of the tunnel convergence at different times and different stress levels.