Passive failure and deformation mechanisms due to tunnelling in sand and clay

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