Mechanism investigation on water and mud inrush disasters when Wangjiazhai tunnel passing through the Tertiary water-rich sandstone strata

The Tertiary water-rich sandstone has characteristics of poor consolidation, low strength, high water content and susceptibility to softening in contact with water. Unfavorable geological disasters, such as water and mud inrush, are frequently encountered during tunnel construction in Tertiary water-rich sandstone regions. However, the existing research on water and mud inrush disasters in tunnels through the Tertiary water-rich sandstone lacks a thorough analysis of the causal mechanisms and influencing factors. Taking the Wangjiazhai tunnel as an example, the evolution process, triggering factors and disaster modes of water and mud inrush disasters in Tertiary water-rich sandstone areas are investigated combined with the landform, stratigraphic lithology and hydrological conditions. A numerical analysis model considering physical and mechanical parameters is established based on COMSOL Multiphysics software to further reveal the disaster mechanism. The results show that factors contributing to the water and mud inrush disaster include the properties of the water-rich sandstone, groundwater conditions, excavation disturbance and recharge of rainfall. The disaster process can be divided into water-rich zone activation stage, transformation mass seepage stage, seepage-stress failure stage and post-disaster stage. Both the permeability and water pressure head increase as excavation progresses in water-rich zone activation stage. The internal porosity and the rate of change of seepage velocity increase as the excavation distance approaches the water-rich zone in transformation mass seepage stage. The effective plastic zone expands and reaches into the interior of the water-rich zone and the lower bench surrounding rock in seepage-stress failure stage. Based on the engineering geological condition and actual construction situation in the Tertiary water-rich sandstone area, sub-horizontal drainage and advance grouting methods can be adopted to mitigate the risk of water and mud inrush disasters. The research results can provide theoretical support for the development of predictive analysis for early warning systems, the improvement of the finite element model for fluid–structure coupling, and the investigation of new reinforcement materials for water and mud inrush disasters in areas with similar engineering geological conditions.