Abstract

Deep-seated gravitational slope deformation (DGSD) is characterized by progressive movement on and beneath ground surface. Such large, slow and complex deformation is usually difficult to define. A tunnel crosses through a deep-seated creeping slope deforms with the slope, demonstrating lasting damages. The patterns and transformation of these damages vary from section to section. To decide a legitimate maintenance strategy, the deformation characteristics of each section in the tunnel should be clarified first.

The case tunnel is located on a deep-seated moving slope in southeastern Taiwan. From 1991, two years after the tunnel is built, until now, damages are continually reported. According to investigations of ground surface displacements, it is very likely that more than one boundaries of slope moving masses pass through the tunnel. Within the possible areas in the case tunnel influenced by the moving slope, this study set monitoring profiles every 5 m for 150 m long. About every half a year in 2009–2012, an integrated geodetic survey method having 3–5 mm accuracy – the micro-displacement monitoring technology is employed to measure the absolute three dimensional positions of the monitoring points. Consider each tunnel profile as a unit, the displacements of the monitoring points on a profile are decomposed into the rigid body motion and the deformation of the corresponding profile. The rigid body motions are further separated to in-plane and out-of-plane translation.

The exquisite geodetic survey results indicate that the amount of translation descended from the western portal to the east. At the two distinct locations where more than 150 mm dislocation can be visually identified (about 33 m and 100 m from the western portal), the translation has a considerable drop. Between August 2009 and April 2010 after the tremendous Typhoon Morakot, the displacements of every profile increased markedly, and decreased to the usual amount in the next half year. The tunnel is divided by six zones, three deformation zones, two seriously deformation zones and a transition zone based on the monitoring data.

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