Abstract: During the excavation of Anshi Tunnel of Yunfeng Expressway Project, two large-scale water and mud inrush incidents occurred at the excavated and supported face of the tunnel (Station YK42+951.6), which was 614 m away from the exit apron of the right tunnel. Combined with engineering geology and hydrogeology conditions in the Anshi tunnel, the factors that might induce the hazards and integrated mitigation measures of water and mud inrush incidents are studied in this paper. The results show that the water inrush hazards occurred suddenly and had a strongly destructive influence with a high evolution rate. The main reason for the water inrush and mud hazard was that there was a hidden saturated fracture zone at the upper right of the right tunnel mileage post K42+955, which had not been accurately characterized during the subsurface investigation stage. The fracture zone was in the shape of an irregular weathering lens, only about 3 meters away from the tunnel arch. When the tunnel excavation face passed through the weathering rock lens, although the quartz schist of the tunnel vault was in a relatively intact state, there was no obvious sign of water inrush and mud inrush. The cracks caused by the passage of time and the disturbance of the tunnel construction gradually penetrated and the seepage channel expanded. When the strength of the surrounding rock of the tunnel vault reached the critical limit state, the first burst of water and followed the mud inrush. After the first water and mud inrush, a large number of debris material quickly accumulated in the excavated area, and the water and mud inrush could be stopped temporarily. With the continuous inflow and convergence of the supply groundwater, the potential energy increased sharply, and the pressure increased, resulting in the second water and mud inrush. After an comprehensive considering the tunnel excavation environment, an integrated mitigation system was implemented, and it was successfully applied to the practice of minimizing and controlling water inrush hazards in the hidden saturated fracture zone of the Anshi Tunnel, which can be used as a reference for similar tunnel projects.

Key words: Tunnel Excavation, Water and Mud inrush, Saturated Fracture Zone, Integrated Mitigation