Disaster mechanism of tunnel face with large section in sandy dolomite stratum

Abstract During the construction of tunnels in sandy dolomite strata, the frequent occurrence of catastrophic instability of the tunnel face has brought great challenges and risks to the on-site tunnel construction. Therefore, the mineral composition of dolomite is analyzed, and the sanding mechanism of dolomite is explored from macro, meso and micro levels. Secondly, the triaxial test is designed through the particle gradation obtained by the screening test to obtain the sandy dolomite parameters under different sanding conditions. After parameter calibration, a three-dimensional discrete element calculation model is established to simulate the excavation of dolomite tunnels with different sanding degrees in the longitudinal, vertical and horizontal paths, and the disaster characteristics and evolution mechanism of the local disaster in front of the tunnel face are obtained. Based on the macro level, dolomite sanding mainly experiences four stages: slight sanding, medium sanding, severe sanding and disaster. When the sanding degree is less than 20%, the failure zone of the surrounding rock in front of the tunnel face is smaller, the stress field can be stabilized in a short time, and only a small range of stress unloading will be generated due to excavation. When the sanding degree reaches 20%, the failure zone in front of the tunnel face rapidly develops forward and upward, and the stress in the longitudinal, vertical and horizontal paths all changes suddenly, and the dynamic arching effect is obvious. It implies that 20% sanding degree is the mutation point of the stability of the tunnel face. As the sanding degree of the fine particle increases, the development of catastrophic sand inrush on the tunnel face gradually becomes faster, and eventually tends to be stable, indicating that the water inrush and sand inrush on the tunnel face will develop faster and faster without treatment. The research results can provide basic data for tunnel face reinforcement in sandy dolomite stratum.