SOLUTION AND ANALYSIS OF CIRCULAR TUNNEL FOR THE STRAIN-SOFTENING ROCK MASSES CONSIDERING THE AXIAL IN SITU STRESS AND SEEPAGE FORCE

This paper presents an analysis for the distributions of stresses and displacements considering the effect of seepage force and the axial in-situ stress for strain-softening rock masses by using Mohr-Coulomb yield criterion. The plastic region is divided into a finite number of concentric rings. Using the stress and strain of the elastic portion of elastic-plastic interface as the initial value and the known differential increments of radial stress, the stress-strain increment can be obtained. Then, the stress and strain on each ring can be reduced. The displacement and plastic radius of surrounding rock can be gained. The derivations of the stress, strain and displacement solutions in strain-softening rock masses considering the influence of the axial in-situ stress and seepage force can be obtained by the use of reconstructed step-by-step solution. The theoretical results can be simplified into Wang's theory as well. The results calculated and analyzed by the approach of Matlab programming showed that the present theory developed in this paper is correct. This study found that the plastic radius and displacement of tunnel wall decrease while seepage force increases with the axial in-situ stress. And when the axial in-situ stress is the minor principal stress, the influence of seepage force is much more apparent. It can be found that the displacement of tunnel wall, the plastic radius and the distribution of stresses and strains are influenced by the consideration of seepage force.