Face stability and required support pressure for TBM driven tunnels with ideal face membrane – Drained case

Assuming that an ideal membrane develops at the face, three-dimensional finite element simulations are employed to investigate the effects of tunnel diameter, cover-to-diameter ratio, lateral earth pressure coefficient, and soil strength parameters on the stability and displacements of the excavation face of mechanically driven tunnels in drained conditions. 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. An equation is given for the minimum support pressure as a function of friction angle, cohesion, lateral earth pressure coefficient, tunnel diameter, and tunnel depth. The minimum support pressures are compared to the analytical solutions available from the literature. For cohesionless ground, the face support pressures obtained from the finite element analysis shows a good agreement with the values from the analytical methods published in the literature when cover-to-diameter ratio is 1. However, as the cover-to-diameter ratio increases, the values from the finite element analysis are greater than suggested by the analytical method. For cohesive ground, the pressure from finite element analysis is found to be almost always equal to or greater than the values obtained with analytical solutions.