Investigation of passive failure and deformation mechanisms due to tunnelling in clay

It is essential to prevent failures at a tunnel face during tunnel construction to protect existing structures and underground utilities. Many studies have investigated active failure in clay, but passive failure is often overlooked. The objec- tives of this study are to investigate passive failure as well as surface heave and excess pore-water pressure induced by tunnel face displacement in a geotechnical centrifuge. Long-term settlement and dissipation of excess pore-water pressure after passive failure are also investigated. For a tunnel located at a cover to diameter ratio of 2.1, soil in front of the tunnel face is displaced mainly forwards and upwards to the ground surface by the advancing tunnel face. The measured passive failure pressure is closely bounded by the best upper and lower bounds and agrees reasonably well with the cavity expansion solution. For a tunnel located at a cover to diameter ratio of 4.2, a localized failure mechanism is observed. There is a large discrepancy between the measured passive failure pressure and the upper bound solution. Both longitudinal and transverse surface heaves for tunnels located at cover to diameter ratios of 2.1 and 4.2 may be described by two-dimensional Gaussian distributions. The spherical cavity expansion solution appears to overestimate excess pore-water pressure upon passive failure for a tunnel located at a cover to diameter ratio of 2.1. An average of 90% degree of consolidation is reached at time factors of 0.9 and 1.5 for tunnels located at cover to diameter ratios of 2.1 and 4.2, respectively.

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