Analysis of Ground Response Curve in Tunnel Considering Strain Softening Behavior and variable Dilatancy

Analysis of stresses and displacements around circular opening excavated in rock mass has been one of the most important problems in tunneling. Plastic zone is formed around underground opening as a result of high stress magnitudes. The ground response curve is one of the best methods for understanding of tunnel stability that describes 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, most of the analytical solutions that have been presented, are relevant to elastic-perfectly-plastic or elastic-brittle-plastic behavior of rock. But, the real behavior of plastic zone is strain-softening with dilation. For strain-softening rock masses the attempts at elasto-plastic analysis are limited. This may be due to the difficulty in defining the material behavior and in obtaining the closed-form solutions.
Summary: In this study, it is attempted to develop the earlier methods and present a new algorithm with considering quality of rock mass and variable dilatancy, the real behavior of rock mass be applied in analysis of ground response curve. The results based on the proposed analytical solution was in agreement with Numerical method and measured wall convergence in the Ghomroud tunnel. The results indicated effect of dilation parameter on convergence tunnel wall. With the increasing of rock mass quality (geological strength index),post-peak behavior of rock mass converge to elastic-brittle-plastic.
Due to loading of underground excavation and redistribution of stresses, the plastic zone form around tunnel. The real behavior of plastic zone is strain-softening with dilation. This paper proposes a new algorithm to calculate the distribution of displacements and stresses around tunnel excavated in strain softening rock masses with variable dilatancy. Also, Effect of various parameters including dilation, geological strength index, and critical softening parameter was studied on proposed algorithm.
Methodology and Approaches: In this study, a new proposed algorithm was used for calculating ground response curve. In order to investigate the verification of proposed algorithm, the results of this analytical solution was compared with numerical method. For showing the applicability of the proposed algorithm, part of Ghomroud tunnel radius convergence was calculated by this proposed analytical solution and was compared with measured tunnel wall convergence at site. The results based on the proposed analytical solution was in agreement with the measured tunnel convergence.
Results and In proposed algorithm, strain softening behavior, variable dilatancy and geological strength index applied through algorithm, which the results indicated that proposed algorithm was in good agreement with numerical and field results. The results of calculation via proposed algorithm shown importance of dilation in estimation of tunnel convergences. Using of constant dilation estimate the displacement of tunnel wall excessive. So, Analysis with constant dilation is conservative.