Experimental study on active failure modes of slurry shield-driven tunnel faces in sand

To study active failure modes and the limit supporting pressure of a slurry shield tunnel face through sand, a stability control model test system of a slurry-shield tunnel face was designed, which can imitate the slurry pressure imbalance across a slurry shield tunnel face by reducing the water pressure of a flexible pressure chamber and can simulate the failure of the tunnel face based on the displacement and the shear strain field collected by the PIV system and the water pressure monitored using a hydraulic gauge. A series of model tests were carried out to investigate the failure modes of the tunnel face and the limit supporting pressure with three different cover-to-diameter ratios of 0.5, 1.0 and 2.0. The test results show that the deformation of the face can be divided into four phases as the support slurry pressure is reduced. At the end of the third phase, the tunnel face is in the limit equilibrium state and the corresponding slurry supporting pressure is the limit slurry supporting pressure. As the slurry support pressure decreases, the deformation in front of the face would gradually expand to the ground surface. It is also observed that, when the covered depth is relatively small(C/D = 0.5), the critical failure zone reaches the ground, and that, while the covered depth is greater(C/D = 1.0 or 2.0), the critical failure zone remains underground. The failure modes and the limit supporting pressure calculated by Soubra's and Mollon's theoretical models are closer to those obtained from the model tests. © 2019, Science Press. All right reserved.