Mechanical Response of a Circular Tunnel under Anisotropic Stress Conditions Using the Hoek–Brown Strain-Softening Model

The present study attempts to examine the behavior of circular tunnel excavation in rock mass under anisotropic stress conditions with various postpeak behaviors. A brief review of the importance of the possible postpeak behavior of the rock mass and anisotropic field stress condition is made to highlight its importance in practical tunnel construction. A plane strain model is developed in a finite-element package to simulate the excavation response of a circular tunnel using the Hoek–Brown strain-softening model. The modeling scheme and the constitutive model are verified against published results for both isotropic and anisotropic stress conditions. The verified model is subsequently utilized to study the mechanical response of the tunnel under anisotropic stress conditions. The results are presented in terms of the evolution of the ground reaction curve, plastic zone, and radial and tangential stress distributions around the tunnel. For various stress conditions considered, a distinct variation in the displacement and yield zone is seen. In general, a higher tangential stress concentration is observed at the sidewalls in the case where the in situ stress ratio is 0.5. Formation of the yield zone begins at the sidewalls and propagates toward the crown as the degree of deconfinement increases. For stress ratios of 1.5 and 2.0, yielding begins at the crown and propagates toward the sidewall as deconfinement increases. An asymmetrical yield zone is formed because of stress anisotropy. The mechanism causing the observed behavior is discussed, and the findings’ practical implications are highlighted. The findings of this study indicate how crucial it is to take the in situ stress ratio into account when choosing the excavation sequence for underground excavations in rock mass.