Countermeasures of water and mud inrush disaster in completely weathered granite tunnels: a case study

This paper presents a case study of the countermeasures of water and mud inrush disasters in the Junchang Tunnel in Guangxi, China. This tunnel was constructed in a weathered granite formation, and four serious water and mud inrush disasters occurred during construction. The geological and hydrological conditions are first reviewed to understand their effects on the water and mud inrush disasters. Four potential causes of the serious water and mud inrush disasters in the Junchang Tunnel were proposed, including the poor stability of the completely weathered granite, the high groundwater level and large groundwater flow rate, insufficient pre-reinforcement of the surrounding rock and the disturbances caused by tunnel construction. Water-sealing and ground reinforcement measures were suggested before excavation to improve the stability of the surrounding rock and plug the water outflow channels, and full-face curtain grouting was considered as the first choice. Furthermore, several improvements, including the combined utilization of various grouting materials, supplementary grouting with steel pipes in collapsed boreholes and dynamically optimized grouting methods, were implemented to solve the serious water inflow and collapse problems experienced during the drilling and grouting work. A detailed curtain grouting scheme was proposed and applied in practice. A comprehensive evaluation based on inspection borehole investigations and P–Q–t analysis indicated that the curtain grouting method yielded satisfactory performance in water sealing and ground reinforcement. In addition, the groundwater level and settlement data collected in the field validated the effectiveness of the proposed countermeasures.