Polyaxial strength criterion and closed-form solution for squeezing rock conditions

In this paper, a nonlinear strength criterion is proposed using the average of intermediate (σ₂) and minor (σ₃) principal stresses in place of σ₃ in Ramamurthy (1994)'s strength criterion. The proposed criterion has the main advantages of negligible variation of strength parameters with confining stress and ability to link with conventional strength parameters. Additionally, a new closed-form solution based on the proposed criterion is derived and validated for Chhibro Khodri tunnel. Further, analytical solutions including Singh's elastoplastic theory, Scussel's approach, and closed-form solutions based on conventional and modified Ramamurthy (2007) criteria are compared with the results of proposed approach. It is shown that the in situ squeezing pressure predictions made by the proposed approach are more accurate. Also, a parametric study of the present analytical solution is carried out, which displays explicit dependency of tunnel stability on internal support pressure and tunnel depth. The influence of tunnel geometry is observed to be dependent on the applied support pressure.