Mud inrush flow mechanisms: a case study in a water-rich fault tunnel

Fifteen serious inrushing disasters caused by mud flow occurred in the excavation of Yonglian Tunnel with a length of 2500 m in the water-rich fault fracture zone. By taking Yonglian Tunnel as a case study, the causes of inrushing and catastrophic mechanisms were determined according to the hydrogeological features of mud inrush with a series of field tests. In addition, based on the fluid-solid coupling effect, the evolution laws of mud inrush were revealed with numerical simulations. The results show that the excavation disturbances and the F2 fault with abundant fault mud, which is easy to soften and disintegrate in groundwater, are the key triggering factors of the mud inrush. Also, its evolution process, which is facilitated by the coupled effects of damage and seepage, consists of three stages: prepared stage, latent stage and explosive stage. The latent stage has especially important significance for early warnings. The engineering case provides a reference for similar engineering.