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In this paper, the author performed two centrifuge model tests at the University of Science and Technology of Hong Kong for two cases where the tunnel was located at Cover over Diameter (C/D) of 1.5 and 3.3 to investigate the passive failure and deformation mechanism due to tunnelling in two-layer soils: sand and clay. The tests were carried out on a 1/100 scale miniature model. The experimental process simulated the progress of the tunnel face with a speed of 0.2 mm/s (equivalent to 15 m per day in practice), displaced to 40 mm and then stopped. Linear variable differential transformers are used to measure the ground transitions during the testing process. Particle image velocimetry technology measures the movement of soil in front of the tunnel face. The load cells were attached to one end of the tunnel face in order to obtain the passive pressure exerted on the tunnel face. As the tunnel face displaces, the ground in front of the tunnel face is shifted forward, while the ground that is far from the surface of the tunnel is pushed outward; the effect on the ground thus causes the ground to emerge, and so form a breakout. The partial failure mechanism in front of the tunnel face is similar to the localised cutting mechanism. When the observation failure mechanisms are idealised by continuous lines, the failure mechanism of the ground in front of the tunnel face is shaped like a funnel.