Abstract：Stress redistribution induced by excavation results in the tensile zone in parts of the surrounding rock mass. A micromechanics-based model is proposed for brittle rock undergoing irreversible changes of microscopics structures due to microcrack growth. The influences of all microcracks with different sizes and orientations are introduced into the overall compliance tensor by using the statistical average method. Overall compliances of damaged brittle rock are nonsymmetric and anisotropic. Micromechanical kinetic equations for microcrack growth characterizing the ‘process domains’of active microcracks are introduced. These ‘process domains’together with ‘open microcrack domains’domcom pletely define the integration domains of ensemble averaged constitutive equations relating macro-strain and macro-stress. Special attention is paid to the transition from structural rearrangements on the microscale to the macroscopic inelastic strain. Analyses are made on the localization of strain and damage. Results show that the onset of localization is very sensitive to the details of a constitutive law. The complete stress-strain relation including linear elasticity，non-linear hardening，rapid stress drop and strain softening is established. The behaviour of rapid stress drop and strain softening are due to localization of strain and damage. The constitutive model to analyse the localization of strain and damage is distinct from the conventional model. An illustrative example is worked out to show the capability of the presented model to predict experimentally observed reponse of brittle rock. It is emphasized that no fitted phenomenological material parameter is employed in the proposed damage model.
