Numerical analyses using distinct element method (UDEC) are used to study the effective ways of stabilization for tunnels located in steeply inclined discontinuities. Impacts of rock bolt position, rock bolt angle, and bolt spacing for tunnels in the soft rock with interlayered structures are analyzed in this study. The Pitan Tunnel fails under the suggested types of support since the dead weight of the roof rock is too heavy. It causes subsequent massive caving and sliding. Based on the numerical simulations, the failure of these tunnels may initiate from the flexural tensile buckling of the interbedded formation located at the sidewall. If the joints located at the right hand side of the tunnel, the most effective bolting position is close to the tunnel crown. Thus, the displacements around the tunnel can be controlled. As the joints locate at the middle of a tunnel, most failures are due to the sliding of the joints above the crown and causing squeezing at the sidewalls. For these kinds of joints, the most effective support is to install rock bolts at about 30 degrees to the joints at the crown. The tunnel will be more stable under higher lateral earth pressure.
The failure mechanism and support for the tunnels experienced squeezing deformation has not been clearly understood and explored, especially for the stratum with interbedded structures. There are four possible failure forms of the surrounding rocks of tunnels in squeezing rocks: complete shear failure, local shearing failure, buckling failure, as well as tensile splitting, shearing, and sliding failure, as described by Aydan et al. (1993, 1996) and Wang (2003), based on the cases in Japan and Taiwan. Squeezing rock is frequently encountered in schists, argillaceous schists, shales, claystones, marls, or tectonically altered gneiss.