Hydromechanical Responses of a Decommissioned Backfilled Tunnel Drilled into a Poro-Viscoelastic Medium

Abstract

This paper is an extension of earlier papers (Wong et al., 2008a, b) which sought to investigate the poroelastic and poro-viscoelastic behaviour of a ground medium following the closing of a spherical cavity, whereas the focus here is for a long horizontal tunnel with a circular section. It presents Laplace transform analytic solutions for the time dependent hydromechanical responses of a decommissioned and backfilled deep tunnel drilled into a poro-viscoelastic host medium. Both the backfill, which is assumed as poroelastic, and the host medium are supposed saturated with water. The solutions developed and inverted numerically to obtain the responses in time domain are used to study the effects of lining support deterioration, the backfill stiffness and of the host medium viscosity. The convergence of the tunnel leads to the expulsion of water from the backfill into the host medium resulting in corresponding transient responses. Each of the key parameters studied can have the effect of either attenuating or accentuating the transient and long term hydromechanical responses by delaying or shortening the dissipation period depending on the direction of change taken by the parameter value. The solutions presented in this paper permit an assessment of the relative importance of these parameters. Comparisons are also drawn between a tunnel and a spherical cavity of identical radius. Generally speaking, the implications of lining support, backfill stiffness and viscosity of ground medium are more significant in terms of the resulting hydromechanical responses, for the tunnel than for the spherical cavity.