Large tunnels for transporation purposes and face stability of mechanically driven tunnels in soft ground
With the advent of the large diameter tunnel boring machine (TBM), mechanically driven large diameter tunnel is becoming a more attractive option. During operation, a large diameter tube allows for stacked deck configuration with shafts dropped to platform level (no station caverns). The extensive information has been compiled on innovative TBM tunneling projects such as the Barcelona Line 9, where the concept of continuous station has been used for the first time, stormwater management and roadway tunnel in Malaysia, where the floodwater bypass tunnel and the road tunnel are incorporated in a single bore tunnel. The decision making process that led to the construction of large bore tunnel is also presented. A detailed study has been carried out to determine the necessary face support pressure in drained conditions (with ideal membrane), and undrained conditions. The effects of tunnel diameter, cover-to-diameter ratio, at-rest lateral earth pressure coefficient, and soil shear strength parameters on the local and global stability of the excavation face of mechanically-driven tunnels have been investigated. The relation between the face support pressure and the calculated tunnel face displacement gave the minimum face support pressure that should be applied on the tunnel face to avoid abrupt movement of the tunnel face. Simple expressions have been developed for the support pressure as a function of tunnel diameter, cover depth, lateral earth pressure coefficient, and soil strength parameters. The required face support pressures are compared to the analytical solutions available from the literature. It has been found that analytical stability solutions generally underestimate the required face support pressure and excessive deformation will take place in the ground near the tunnel heading when these solutions are used. By using plastic limit analysis, a rigid and deformable prism-and-wedge model has been developed; in undrained conditions, upper bound solutions against collapse load are derived for face pressure. Deformable blocks enabled to take into account the effect of non-uniform support pressure due to the unit weight of the supporting medium. The upper bound solution derived as a function of tunnel diameter and cover depth, normalized undrained shear strength ratio, and unit weight of the ground and the supporting medium was compared with a solution available from the literature. Largest face support pressure was obtained when the uniform face support pressure was applied and it was smallest when identical unit weight was used for the ground and the supporting medium.