The present study aims to quantify the normal contact stiffness between the concrete lining of a mountain tunnel and the surrounding weak rock mass, which has been never seriously investigated in the previously-performed numerical simulations. For the purpose, a numerical model was constructed that reproduces a previously-conducted physical test investigating tunnel lining damage due to the degradation of the surrounding weak rock mass. Elasto-plastic analyses were then performed while changing the normal stiffness kn of interface elements representing the contact. The analysis result indicated that when kn is less than 1 MPa/m, the horizontal displacement of the lining is much smaller than the experimental result. As kn is increased, the displacement becomes larger, and it was found that the analysis result agrees with the experiment the most closely when kn = 1 GPa/m. For kn exceeding the value, both the horizontal displacement and the magnitude of floor heave deviate from the experimental result. It has been also clarified that the magnitude of the horizontal displacement is hardly affected by the internal friction angle, implying the validity of the estimation. Furthermore, the effect of strain-softening behaviour was examined, implementing the most noticeable strain softening curve observed among uniaxial tests conducted in the previous experiment. The analysis result demonstrated that the strain-softening behaviour produces significantly large floor heave, indicating that in the physical test, such severe strain softening did not occur. It was then concluded that the normal contact stiffness is on the order of 1 GPa/m, especially for the weak rock mass investigated. This study contributes to a more accurate estimation of damage to tunnel lining caused by severe deformation of the surrounding rock mass.

1. Introduction

In the case of exceptionally adverse mountain conditions, mountain tunnel lining could be subjected to the external force arising from the large deformation of the rock mass undergoing severe degradation due to weathering. As a result, the lining undergoes severe deformation, producing sidewall convergence and roof sag whilst generating visible cracks (JSCE 2003). According to the study (JSCE 2003), When deformation occurs due to plastic ground pressure, compressive failure, open crack, shear crack, side wall extrusion, and floor heaving occur. And when loosing pressure occurs continuously in the longitudinal direction, open cracks in the tunnel longitudinal direction may occur at the top, and compressive failure and shear cracks may occur near both sides of the arch.

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