Abstract: To investigate the mechanisms of water and mud inrush and roof collapse when tunneling through water-rich tertiary semi-diagenetic strata, a case study is conducted on Wangjiazhai tunnel of the Linze-Qingshui highway in Yunnan, China, and the mechanical properties, action mechanisms, and water inrush risk of tertiary semi-diagenetic strata are examined through indoor mechanical tests and fuzzy analytic hierarchy process. The results reveal the following: (1) The tertiary semi-diagenetic strata in the Wangjiazhai tunnel significantly soften upon water exposure, with cohesion initially increasing and then decreasing as water content rises, while the internal friction angle continuously decreases. At 18% water content, cohesion and internal friction angle are 12.53 kPa and 7.53°, respectively, indicating rock mass destabilization. (2) Weak tertiary semi-diagenetic strata, high groundwater level, and tunnel excavation disturbance are the primary factors influencing water and mud inrush disasters in the Wangjiazhai tunnel. (3) The tertiary semi-diagenetic strata section of the tunnel can be divided into four parts (A, B, C, and D) based on engineering geological and hydrogeological conditions, with disaster risks ranked as B=C>A>D. The estimated mud inrush volume in section A closely matches the actual disaster situation.

Key words: highway tunnel, tertiary semi-diagenetic rock, water and mud inrush mechanism, risk assessment, fuzzy analytic hierarchy process