Back Issues 11-20

LANDSLIDE PROBLEMS CAUSED BY EXCAVATION OF SHEAR BAND TUNNELS INTO GRABENS

Tse-Shan Hsu

President, Institute of Mitigation for Earthquake Shear Banding Disasters

Professor, Feng-Chia University, Taiwan, R.O.C., This email address is being protected from spambots. You need JavaScript enabled to view it.

Yan-Ming Wang

Director, Institute of Mitigation for Earthquake Shear Banding Disasters, Taiwan, R.O.C.

Yu-Chien Wu

Ph.D. Student, Ph.D. Program for Infrastructure Planning and Engineering, 

Feng-Chia University, Taiwan, R.O.C.

Zong-Lin Wu

Assistant Professor, National Chin-Yi University of Technology, Taiwan, R.O.C.

Si-Min Hsu

Member, Institute of Mitigation for Earthquake Shear Banding Disasters, Taiwan, R.O.C.

Abstract

The Zhongliao Tunnel of Formosa Highway No. 3 was completed in 2000. The tunnel passes through shear bands, but the engineers overlooked the tilting effects of shear banding during design, resulting in ongoing uplift issues at the north portal of the tunnel. Following this, the consulting firm responsible for the tunnel design proposed mitigating the uplift by excavating the shear band section at the north portal of the tunnel into a graben. A similar section of the national highway was destroyed by a landslide induced by shear band graben slope sliding near Keelung in 2010. If the north portal of the Zhongliao Tunnel is excavated into a shear band graben, it is likely to replicate this landslide disaster, which shocked the international community. For this reason, the authors of this paper conducted stability analysis on the slope of the shear band graben excavated at the north portal of the Zhongliao Tunnel and drew the following three conclusions. (1) A shear zone tunnel presents only local instability problems and does not induce overall tunnel instability. (2) After the shear zone tunnel is excavated into a cutting, the slope of the shear zone cutting is prone to sliding failure. (3) The primary factors that induce the instability of the graben slope in a shear band is the infiltration groundwater, which fills the pore space in the shear band, and the softening of shear resistance due to shear banding. Based on these conclusions, the authors recommend that future stability analyses of retaining wall slopes must account for shear banding effects and thus avoid the excavation of faults or shear zones in shear band graben slopes. These measures can help mitigate sliding damage of the graben slope and ensure the safety of the public.

Keywords: fault, shear band, highway, tunnel, graben.