Squeezing ground conditions in tunnels are often associated with rock mineralogy, strength, ductility, excavation sequence, and the magnitude of in situ stresses. Numerous methodologies and empirical correlations have been proposed in the past to determine the level of ground squeezing conditions in tunnels, but most of them are problem specific and limited in scope. This paper presents a fundamental study of tunnel squeezing using a novel experimental approach to simulate tunnel boring machine (TBM) excavation in squeezing ground conditions. The proposed experimental setup employs a cubical specimen of synthetic mudstone, with each dimension being 300 mm and the six faces subjected to a true-triaxial state of stress, with different magnitudes of principal stresses and stress levels that correspond to realistic in situ conditions. A miniature TBM was designed, fabricated, and used to excavate a tunnel into the host rock (specimen) while the rock was subjected to a true-triaxial state of stress. Embedded strain gauges and acoustic emission sensors, which were coupled on the surface of the rock specimen, were also used to monitor the tunnel’s response during the excavation stage. The results from the experiment confirmed the capability of the physical model to provide a better understanding of tunnel squeezing and to delineate the damage and the deformation that occurs during instant stress release and creep behavior of rock around the tunnel boundary during tunnel excavation.
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