Prediction of critical strains and critical support pressures for circular tunnel excavated in strain-softening rock mass

Abstract

The critical strain is a strain level at the tunnel periphery beyond which the instability and squeezing problems are likely to occur, thereby it provides helpful guide to assess the tunnel stability. Among the current studies for predicting the critical strain, the influence factors such as the required support pressure, the strain-softening behaviour of the rock mass, and the in-situ stress condition have not been comprehensively considered. This paper aims to overcome the limitation by proposing an improved tunnel critical strain classification, which consists of five typical critical strains and critical support pressures. Specifically, two critical levels correspond to the occurrences of the plastic softening and residual zones of the strain-softening rock mass at the tunnel periphery. Three tunnel strains of 2.5%, 5%, and 10% represent the squeezing potentials of the rock mass. The correlations between the input geological parameters and the geological strength index GSI are proposed on the basis of the existing empirical equations. A numerical procedure is presented to solve the five critical strains and critical support pressures. According to a large amount of the calculation result, the response surfaces for the critical support pressures in relation to the GSI and in-situ stress are obtained by the nonlinear regression method. The obtained critical strains are compared with the critical strain levels and measured tunnel strains by the current studies. The results indicate that, the reason for the unexpected secure condition with the large deformation in the rock mass can be attributed to the non-existence of the plastic residual zone; as the rock mass strength improves, the critical strain level reveals an unexpected growth, which is correlative to the considerations of the in-situ stress condition and the strain-softening behaviour of the rock mass.