Analysis of the excavation damaged zone around a tunnel accounting for geostress and unloading

• A constitutive accounting for geostress and unloading process are adopted here.
• The difference of constitutive curve between loading and unloading state are shown in this paper.
• Effects of initial microcracks on the evolution of damage zone are discussed.

A simplified analysis method based on the damage evolution mechanism of rock mass is proposed to predict the possible excavation damaged zone around a tunnel. The formation of an excavation damaged zone is dependent on three main aspects: time-dependent macroscopic stress field during excavation, constitutive model under unloading condition, and initial microcracks. The excavation process is simplified as a time-dependent increase of the radius. For the tunnel problem, the tangential stress is under loading, while the radial stress is under unloading condition. As the macroscopic stress redistribution, the constitutive relationships which can take unloading state into account are coupled in this model. Then, in order to simulation the damage evolution process of surrounding rock mass, the sliding crack model and equivalent crack method are adopted. The different constitutive curve between loading and unloading state is discussed through examples. The efficiency of the proposed model is verified with the experimental and numerical simulation data available. The results of the proposed analytical model are in good agreement with the test results and, therefore, this model can be used in the preliminary tunnel design to evaluate tunnel stability, especially for the tunnel engineering at high geostress zone.