ARTICLE IN PRESS
International Journal of Rock Mechanics & Mining Sciences 41 (2004) 489
SINOROCK2004 Paper 2B 34
Failure mechanisms of tunnels in weak rock with interbedded structures S.-C. Hsua,*, S.S. Chiangb, J.R. Laib a
Department of Construction Engineering, Chaoyang University of Technology, 168 Gifeng East Road, Wufeng, Taichung County, Taiwan, ROC b Chaoyang University of Technology, Taichung, Taiwan, ROC
Abstract Formations of interbedded sandstone and shale are common deposits in the northern and western parts of Taiwan. Tunnels that have been constructed in these formations along the Northern Second Highway around Taipei area have encountered some serious failures during construction. One of the tunnels, the Pitan Tunnel, that experienced squeezing failure and the resulting massive cavein, is evaluated and analyzed in this study, to reveal the actual causes of such failure. The interstratiﬁed soft rocks had a steeply dipping angle (80 ), and had weak frictional resistance between the interface of sandstone and shale. The tunnel axis was parallel to the strike of the bedding plane. To simulate and understand the causes and related mechanisms of the failed case, numerical analyses utilizing the distinct element method were used. Furthermore, the impacts of the overburden on the tunnel failure modes are analyzed to simulate the different tunnel sections, cave-in and squeezing zones. Based on the numerical simulations, the failure of the Pitan Tunnel may have initiated from the ﬂexural tensile buckling of the interbedded formation located at the right sidewall (Fig. 1). The sliding of the roof rocks and the shearing failure of the formation at the left sidewall occurred subsequently. A tunnel with a steeply dipped formation at the portal section or under a shallow overburden is prone to have cave-in failure. In the case of a tunnel under a deeper overburden, only squeezing failure occurs at both sidewalls. The roof does not collapse, due to the higher horizontal stress. The discontinuity spacing of the interbedded formation inﬂuences the width of the buckling failure. In other words, for the same thickness of the formation, a smaller spacing will have a wider buckling range. The location of the interbedded formation also greatly affects the stability of a tunnel.
Keywords: Tunnel; Squeezing; Shale; Failure mechanism; Discontinuity dip; Numerical analysis
Fig. 1. Cave-in failure develops as squeezing failures occur at both of the sidewalls for simulation of the Pitan Tunnel (the bedding spacing is 0.5 m and dip angle is 80 ).
*Corresponding author. Tel.: +886-4-23323000 x. 4242; fax: +886-4-23742325. E-mail address: [email protected]
(S.-C. Hsu). For full length paper see CD-ROM attached. doi:10.1016/j.ijrmms.2003.12.024