Abstract: The aim of this study is to understand the structural characteristics of unfavorable geological bodies and the mechanism of mud and water inrush in the syncline structure of basalt strata. We investigated into the evolution of water and mud inrush, the disaster-causing geological structure, the basic characteristics of disaster-causing medium, the disaster-causing power and the disaster-inducing mechanism at the core of Wulongba syncline structure in Shizishan tunnel of Central Yunnan Water Diversion Project as a case study. Results demonstrated that the water inrush is resulted from the disintegration, softening and argillization of lower tuff cracks caused by the water storage structure in joint fissure dense zone of the upper syncline structure; 2) the diluted debris flow like water inrush medium carries huge energy while bursting instantaneously; 3) the water and mud inrush disaster is caused jointly by tunnel unloading collapse, groundwater action and stress release. The research finding is of guiding significance for mud and water inrush prevention for tunnels crossing fold structure.

Key words: diversion tunnel, water and mud inrush, disaster causing medium, groundwater, disaster-causing model