Abstract: In order to obtain the meso-damage evolution law and mechanical response of tunnel surrounding rock in unloading failure process, the numerical simulation of the internal meso-damage of the surrounding rock during unloading was carried out by the particle discrete element method, and the effect of the initial stress on the failure and mechanical properties of the surrounding rock was analyzed by combining the failure characteristics of thick-walled cylindrical surrounding rock specimens. The results show that: (1) The cracks induced by unloading were distributed around the inner wall of the specimen. Under the influence of unloading stress adjustment, the cracks accumulated and gradually diverged and expanded to the outer wall, showing an "hourglass-type" damage failure. (2) The number of cracks generated by unloading increases exponentially with the increase of stress, and the growth rate of the number of cracks after unloading was significantly higher than that of the number of cracks during unloading. (3) When the unloading stress level was lower than 80% of the uniaxial peak strength of the surrounding rock, the stress was fully adjusted during unloading and remained stable after unloading. When the unloading stress level was higher than the uniaxial peak strength of the surrounding rock, the stress was not sufficiently adjusted in unloading stage and continued to be adjusted after unloading, resulting in "V-shaped" destruction of the surrounding rock. (4) The initial stress level has a significant impact on the excavation unloading-induced damage failure and mechanical properties of the surrounding rock. The larger the stress level, the earlier the time of damage rupture of the surrounding rock after unloading.

Key words: excavation unloading, discrete element method, surrounding rock, meso-damage, mechanical behavior