Study On Face Deformation And Collapse Of Earth Pressure Shield Tunnel

During shield tunnelling, support pressure which is equal to the original stress at excavation surface is usually applied at excavation surface to prevent tunnelling-induced soil deformation. However, the design and control of appropriate support pressure is made difficult by the large variation of soil conditions and uncertainties in actual filed construction.In the field, the support pressure is applied through the excavated soil mass with or without additives in working chamber of earth pressure (EP) shield. Therefore, the actual support pressure is strongly influenced by the property of the mixture at the open face. Furthermore, support pressure should vary with overburden continuously. The above factors in all raise the concern on the risk of face stability. Thus it is of practical significance to study the effect of support pressure on soil deformation and face stability.In this paper, a three-dimensional Fast Lagrangian finite difference program (FLAC~3D), which can simulate large-strain deformation and progressive failure, is adopted to study the soil deformation and failure mechanism in EP shield tunnelling. The main contents can be summarized as follows:(1) The relation between face support pressure and soil deformation indicates that the influence of support pressure can be figured as three stages. Studies on the variation of stress relax around the open face due to the low support pressure show that the relaxation ratio varies with the position at the same support pressure. The determination of stress relaxation ratio is suggested to be in relation with support pressure.(2) The failure mechanism of the excavation surface and the development of post-failure deformation for both sand and clay are investigated. It is suggested that the soil Young's modulus be considered in deciding whether to apply stress control or strain control in EP tunnelling. Study on failure development at excavation surface reveals that the extent of post-failure development is strongly related to the over cut in EP tunnelling.(3) Sensitivity studies of soil parameter, overburden and tunnel diameter for the face stability are carried out numerically. The results show that: i) the tunnel can be bored safely in the open mode with the increase of soil strength; ii) the larger the overburden, the less minimized support pressure and more invisible of deformation on ground surface due to face collapse; iii) the minimized support pressure varies with the tunnel diameter approximate linearly.(4) Based on numerical simulations of trap-door tests, the pressure of the loose soil cover, one of the most important parameters in wedge stability model for limiting support pressure, is studied. The formula which takes into account the allowable displacement is given. Finally, refinement of this wedge model is proposed from findings of face failure mechanism.