Practical ground response curve considering post-peak rock mass behaviour

Abstract

Analysis of stresses and displacements around circular opening excavated in rock mass has been one of the significant issues in tunnelling. A plastic zone is formed around underground opening as a result of high induced stress magnitudes. The ground response curve is one of the best methods to investigate the tunnel stability that represent the relationship between the decreasing of inner pressure and the increasing of radial displacement of tunnel wall. In recent years, several methods have been suggested for analysis of ground response curve by many researchers, however, some of the analytical solutions that have been presented, are relevant to elastic-perfectly plastic or elastic-brittle-plastic behaviour of rock. But real behaviour of plastic zone is strain softening with variable dilatancy. In this study, it is attempted to take into account the post-peak rock mass behaviour (drop modulus and variable dilation angle) in the stress-based finite difference approximation numerical procedure, so an algorithm is suggested to analyse the tunnel stresses and convergences. In order to verify the proposed algorithm, the results were compared with FLAC numerical solution. For showing the applicability of the proposed algorithm, the Ghomroud tunnel convergence was calculated based on the proposed algorithm and compared with the monitored tunnel convergence. At the end, the effect of various parameters including dilation, critical softening parameter and rock mass quality (GSI) was studied on the tunnel convergence and plastic zone thickness. Based on the obtained results, the higher quality of the rock mass surrounding the tunnel has, the narrower plastic zone forms and the lower tunnel wall converges. Also, the rock mass with higher brittleness means more convergence and wider plastic zone. Furthermore, constant dilation in plastic zone overestimates the tunnel wall convergence.

Keywords:

• ground response curve
• plastic zone
• strain-softening behaviour
• critical softening parameter
• dilation of rock mass
• geological strength index