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There has been an increasing demand for new tunnels in respond to rapid growth in cities. Earth pressure balanced or slurry shield tunneling method is commonly used to construct tunnels in soft ground to improve stability and safety. Over the decades, many studies have been carried out to investigate active failure of tunnel face in sand and clay. However, investigation on passive failure of tunnel face is relatively rare. Failures of a compressed air section during construction of Dockland Light Railway extension and the 2nd Heinenroord Tunnel during drilling imply that a systematic investigation on passive failure at tunnel face is required. The objectives of this study are to investigate passive failure and deformation mechanisms due to tunnelling in sand and clay by carrying out centrifuge model tests and numerical simulations together with plasticity and cavity expansion solutions. Centrifuge model tests were carried out for tunnels located at cover to diameter (C/D) ratios ranged from 2.0 to 4.0 in medium dense sand and soft clay. Three-dimensional finite element analyses were carried out to back-analyse the measured results. After calibrating the numerical tool, numerical parametric study was performed to extend the investigation to consider C/D in a wider range of 1.0 to 6.0. In addition, tunnels located at C/D ratio of 2.0 in loose and dense sand were also included in the numerical parametric study to investigate effect of relative density. The measured and computed results are used to evaluate some existing upper bound and cavity expansion solutions in estimating passive failure pressure. Besides, Gaussion distributions are adopted to describe the induced surface heave. A localised failure mechanism associated with surface settlement is observed for shallow tunnel located at C/D of 2.2 in loose sand due to soil compression. As initial relative density is increased, on the contrary, a funnel-type failure mechanism associated with surface heave is induced at

Tunnelling is one of the major construction methods to sustain the increasing demand on development in cities. Although many studies have been carried out to investigate the active failure mechanism of tunnel face in sand, the study of passive failure of tunnel face is relatively rare and most of studies are analytical solutions based on the upper bound theorem. In this paper, centrifuge model tests and three-dimensional finite element analyses have been conducted to study the passive failure mechanisms of tunnel face in sand for tunnels located at cover over diameter (C/D) ratios equal to 2.2 and 4.3. Passive failure pressures of tunnel face as well as ground surface displacements were investigated in centrifuge tests. From both centrifuge and numerical investigations, it is found that for a tunnel located at C/D ratio equals to 2.2, the soil in front of the tunnel face is displaced by the advancing tunnel face while the soil further away from the tunnel face is forced upwards to the ground surface. A funnel-type failure mechanism is observed and the extent of the failure mechanism is narrower than a five-block failure mechanism commonly assumed in an existing upper bound solution. However, the calculated passive failure pressure by the upper bound solution is fairly consistent with the measured face pressure. It is observed that the funnel-type failure mechanism induces surface heaves. Both observed longitudinal and transverse heaves are well described by Gaussian distributions. For a tunnel located at C/. D ratio equals to 4.3, the displacements of soil are confined around the vicinity of an advancing tunnel face and a localised failure mechanism associated with ground settlement is observed and computed. There is a large discrepancy between the localised failure mechanism and the five-block failure mechanism. The calculated failure face pressure is higher than the corresponding measured value by 50%. However, reasonable consistency can be found between measured