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 portable 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,By best centrifuge experiments,reasonable consistency can be found between measured and computed passive face pressures.

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