Hydro-mechanical response of excavating tunnel in deep saturated ground

Abstract

Excavating a tunnel in a deep and saturated ground affects the short- and long-term hydro-mechanical (H-M) response in the ground surrounding the opening. However, the interactions between transient pore pressure behavior and the corresponding deformation and stresses in the ground ahead of and behind the tunnel face are still not well understood. This paper investigates the transient H-M response of excavating a tunnel in a deep and saturated ground using a two-dimensional axisymmetric coupled model in the computer code Fast Lagrangian Analysis of Continua (FLAC). The tunnel was advanced in a stepwise excavation procedure consisting of undrained excavation and drained consolidation until the final tunnel face was reached. The final excavated face was then left to consolidate toward the steady-state condition. The main results of the paper are as follows: (1) when simulating a tunnel excavation in deep saturated ground using the convergence-confinement method, the unloading factors should be nonlinear and should consists of the mechanical unloading factor in the form of excavation force and the hydraulic unloading factor in the form of excavation pore pressure. These two unloading factors are necessary because the induced H-M response near the tunnel face is a rather transient response instead of an initial or final response. Moreover, it is observed that the pore pressure dissipation is not linear either with time or with distance to the tunnel face, (2) a relationship between the unloading factors and the distance to the tunnel face should then be established. This relationship is vital because it will provide the timing for tunnel support installation, and (3) the extrusion and the convergence of the advance core could be related through the proposed equations capturing the linear relationships between the face extrusion and its convergence as well as between the core extrusion and its pre-convergence. Through these relationships, the tunnel engineer may be able to estimate the magnitude of the deformation ahead of the face, which will subsequently allow control of the deformation behind the face.