Strain-degradation analysis of tunnel structures based on twin-shear unified strength criterion

Abstract This paper focuses on the stability analysis of tunnel structures with a circular hollow section based on the with a strain-degradation model using unified strength theory. A simplified numerical approach is proposed for analyzing the elasto-plastic behavior of surrounding rocks. A constitutive equation is proposed based on the unified strength theory and the strain degradation behavior was taken into account as well with deviatoric plastic strain chosen as the degradation parameter. Meanwhile, the plastic potential function can be obtained with the associated flow rule, by which the surrounding rocks can be classified as plastic zone and elastic zone. The solution in plastic zones adopted the differential method subdividing surrounding rock into infinitesimal concentric annuli, in which the radial stresses decrease monotonically with a decrease in radial coordinate. The relationship of the increment of stresses and the increment of strains in adjacent annuli can be derived from equilibrium, constitutive and geometrical equations. The numerical solution of each annulus is calculated from the outmost annulus at the elastic-plastic interface. In addition, the impact of the intermediate principal stress factor and the critical degradation parameter on the solution, and the factors of influence on plastic radius are discussed.