This study investigates the failure mechanism of a gallery tunnel. The case tunnel, built for No. 8 highway in the 1990s in eastern Taiwan, was damaged successively in 2008 and 2009. The slope above the tunnel collapsed and destroyed the tunnel more than 20 m. A great number of rock wedge with debris piled upon the damaged tunnel. Field investigation based on the ISRM suggested method regarding discontinuity in rocks indicated that there are three dominant discontinuity sets near the tunnel. However, the slope above the tunnel is steep and high, and the variation on attitudes of discontinuities were concerned for subsequent analysis. As such, a ground-based LiDAR was utilized to acquire global coordinates of the slope near the tunnel, and high precision and high density point clouds were obtained accordingly. The attitudes of fracture planes on the slope were analyzed and the stereo geographic projection analysis was used to determine the number of dominant discontinuity sets and their attitudes. A statistically hypothesis testing method was applied to examine the representativeness of the interpreted attitudes of discontinuity sets. After that, the spatial locations of wedges observed on the slope were determined accordingly. Based on the attitudes and spatial locations of rock wedge above the tunnel, the mechanism and possible influence factors that are associated with the failure of tunnel are discussed. Stability of the tunnel is also evaluated accordingly.

1. Introduction

The tunnel is a most durable structure in all kinds of geological engineering structures, which often has a hundred years or more in service life cycle. However, the gallery tunnel near 168k of No. 8 highway was damaged successively twice. The date of first damaged was near September 24th, 2008, the landslide volume was about 2,500 m3 (Fig. 1, red area) and it caused gallery tunnel damaged about 20 m; the date of second damaged was near October 9th, 2009, the landslide volume was about 54,000 m3 (Fig. 1, blue area) and it caused gallery tunnel damaged again.

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