ANALYTICAL THEORY ON INSTABILITY SLIDING OF SURROUNDING ROCK BLOCKS IN TUNNEL ROOF UNDER STRESS WAVE ACTIONS

Due to the effect of geological structure and excavation surface, the surrounding rock of the tunnel is usually cut into blocks with different shapes and scales. Among them, some key blocks remain stable after excavation unloading, however, they may lose their stability and slide when disturbed by stress waves, e.g., blasting vibration or earthquakes. Based on the theory of stress wave propagation, an analytical model for the dynamic instability and sliding of rectangular key blocks in the roof of tunnel surrounding rock was derived. In the calculation of block slide displacement, different close-open modes of joints between blocks are considered, and linear elastic model and Coulomb slip model are used to describe the normal and tangential behavior of joints. The effectiveness and reliability of the analytical model were verified through UDEC numerical model. Then, the parameters that may affect the instability and slide of the block were discussed based on the analytical model. The research results indicate that the joint surfaces between the tunnel surrounding rock blocks will close and open under the action of stress waves. During the opening process, the friction force between the joint surfaces will gradually decrease or even become zero. The ultra-low friction and non friction stages of the joint surfaces between the blocks have the greatest impact on the slide displacement of the blocks. When the stress wave acts vertically on the structural surface of the surrounding rock block and its strength is sufficient to induce the opening of the joint surface, the parameters that significantly affect the slide displacement of the block are the initial normal stress of the joint surface, the amplitude and frequency of the stress wave, and the rock type of the block. The variation of the slide displacement of the block under a single period sine stress wave is between 0.659 and 1.502 mm. The influence of initial normal stiffness of joints and block size is followed, and the variation of block slide displacement is between 0.15 and 0.168 mm.