In Hokkaido prefecture, Japan, a number of road mountain tunnels constructed with conventional support systems are still in use. In one of the tunnels, the progressive damage evolution and large deformation of the tunnel wall were observed. The present study investigates its mechanism and the effectiveness of the conventional support system. In order to simulate the time-dependent behavior of the target tunnel, a variable-compliance-type constitutive equation is employed and implemented into FLAC3D. A 3D numerical model reproducing the actual ground surface topography is constructed. Using the numerical model and constitutive equation, the time-dependent damage evolution and resultant deformational behavior are simulated whilst considering combinations of the conventional support system members, namely steel sets, concrete lining and invert concrete. The analysis results show that concrete invert installation is the most effective measure to suppress and control the damage evolution and deformation of the tunnel wall. The concrete lining is the second effective, alleviating the deformation taking place on the tunnel wall and crown. It is then revealed that steel sets do not significantly contribute to suppressing the damage evolution. The analysis result also indicates that axial stresses originally acting on the steel sets are re-distributed to the concrete lining and invert concrete, proving that the two support members can work more effectively than steel sets in the aspect of controlling the time-dependent damage evolution of the surrounding rock mass.

1. Introduction

More than 130 road mountain tunnels using conventional support system are still in use in Hokkaido prefecture, Japan, although NATM has already become common in Japan. In one of the tunnels, crack initiation was observed at the crown on concrete lining during its construction (hereafter, this tunnel is referred to as "target tunnel"). Occurrences of serious deformation on its side wall and concrete lining rupture after completion of the tunnel construction were also reported. It is conjectured that these were caused by the time-dependent expansion of a damage zone within a weak rock formation found in the target tunnel subjected to large horizontal stress. Comprehending the damage evolution process and its mechanism is imperative for stability assessment of not only the target tunnel but also other tunnels using similar conventional support system.

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