Plastic-Zone Prediction for Squeezing Large Deformations in a Noncircular Tunnel Using the Slip-Line Method

Squeezing large deformations in deeply buried tunnels is commonly observed in strongly altered surrounding rock. In this paper, a slip-line method for predicting the slip system and stress distribution in the plastic zone of a noncircular tunnel is proposed. The balance equation and rigid-plastic models, such as the Hoek–Brown and Mohr–Coulomb models, are combined to obtain the differential equations for the two slip lines. The effect of the supporting system is taken into account by introducing confining stress into the rigid-plastic model. The finite-difference method is introduced to solve the equations and obtain the slip system and yield-stress distribution. Numerical results are compared with analytical solutions to verify the proposed slip-line method. The results show that the slip-line method is effective in predicting the slip system and stress distribution in the plastic zone of a noncircular tunnel with a supporting system. Enhancing the supporting system is effective in reducing the size of the plastic zone, thereby controlling the possibility of squeezing large deformations. For the same supporting system, the plastic-zone size decreases nonlinearly as the uniaxial compressive strength σ ci , geological strength index (GSI), and m i increase, whereas it increases nonlinearly as the damage parameter D increases.