A new large strain approach for predicting tunnel deformation in strain-softening rock mass based on the generalized Zhang-Zhu strength criterion

Abstract

The deformation capacity of tunnel will be overestimated by small strain theory especially in high geostress and heavily squeezing rocks, the calculated wall convergence of tunnel may even exceed the excavated radius. Based on the Generalized Zhang-Zhu (GZZ) strength criterion, a new large strain solution for tunnel excavated in strain-softening rocks under high geostress is proposed by a semi-analytical approach. During calculation, the material nodes in the plastic zone of surrounding rock are generated automatically in each unloading step. The locations of these nodes are updated after each step and the governing equations are established on the updated state. The proposed procedure allows one to predict the rock stresses, strains, and deformations in a rational and rigorous manner, and it is validated by numerical simulation. Results show that the ground displacement calculated by proposed approach is in good agreement with that obtained by GeoFBA3D, which indicates that the proposed approach is capable in predicting the large deformation of surrounding rock. As long as the unloading radial pressure of each step δp is sufficient small, i.e., the number of material nodes in the plastic zone exceeds 40, the computational error can be neglected. Compared to HB strength criterion, the GZZ criterion can take into account the intermediate principal stress, which is beneficial to tunnel stability. The applicable conditions for large strain theory and small strain theory are given by parametric studies. Parametric studies show that the calculation error produced by small strain theory can reach 30% when the deformation of surrounding rock exceeds about 10%R0, therefore, a large strain theory is necessary for calculation.