عنوان مقاله [English]
Analysis of stresses and displacements around circular openings excavated in rock masses has been one of the most important problems in tunneling. Plastic zone is formed around underground openings as a result of high stress magnitudes. The ground response curve is one of the best methods for understanding tunnel stability that describes the relationship between decreasing inner pressure and increasing 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 for this purpose, are relevant to elastic-perfectly-plastic or elastic-brittle-plastic behavior of rocks. However, 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.
In this study, it is attempted to develop the earlier methods and present a new algorithm by considering quality of rock mass and variable dilatancy, and applying the real behavior of rock mass to analyze ground response curve. The results based on the proposed analytical solution were in good agreement with the results obtained from applying the numerical method and measuring wall convergence in Ghomroud tunnel. The results indicated the effect of dilation parameter on the convergence of the tunnel wall. Moreover, the results indicated that as the rock mass quality (geological strength index) increased, the post-peak behavior of rock mass converged to elastic-brittle-plastic.
Due to loading underground excavation and redistribution of stresses, plastic zone forms around tunnel. The real behavior of plastic zone is the type of 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. Furthermore, the effect of various parameters including dilation, geological strength index, and critical softening parameter has been investigated in this paper.
Methodology and Approaches
In this study, a new proposed algorithm was used for calculating ground response curve. In order to investigate the verification of the proposed algorithm, the results of the analytical solution was compared with the results of the applied numerical method. For showing the applicability of the proposed algorithm, Ghomroud tunnel radius convergence was calculated by the proposed analytical solution and was compared with the measured tunnel wall convergence at the site. The results obtained from the proposed analytical solution were in good agreement with the measured tunnel convergence results.
Results and Conclusions
In the proposed algorithm, strain softening behavior, variable dilatancy and geological strength index were applied. The results obtained from the proposed algorithm were in good agreement with the numerical and field results. The results of calculation through the proposed algorithm indicated the importance of dilation in the estimation of tunnel convergence. Using constant dilation, an excessive estimation of the displacement of tunnel wall was obtained, and thus, the tunnel analysis results using constant dilation should be considered as conservative results.