Case study on hydro-mechanical behavior of tunnel structure in red mudstone adjacent to water-rich fault zones

Humidity diffusion softens water-sensitive rock, increasing stress on the tunnel structure. Previous studies have mainly focused on water-rock interaction in soft rock and controlling tunnel large deformations, with little attention to tunnel stress evolution under humidification effects. This study examines the tunnel in red mudstone affected by a water-rich fault inrush event. Laboratory tests revealed the strength and expansion properties of soaked red mudstone. Field measurements were conducted to investigate changes in rock moisture content, deformation, and stress response of tunnel structures across three sections affected by fault water inrush, aiming to explore the mechanism of tunnel stress evolution under humidification. The results show that the mechanical strength and expansibility of red mudstone were negatively linearly and positively linearly related to the rock water content, respectively. Rapid swelling release mainly occurred at the initial moisture increase. Continuous seepage from water-rich fault had varying humidification effects on the surrounding rock of the three measurement sections. The tunnel foundation near the fault received water supply, resulting in a more severe hydraulic coupling of red mudstone at the tunnel invert. Moistening significantly increased rock deformation, with a saturation level of 80 % in the base mudstone triggering a deformation inflection point, presenting four deformation stages that fit with a composite function of exponential and logarithmic curves. The process of increasing moisture contributes to the long-term accumulation of tunnel structure stress. The radial pressure deviation around tunnel increases with the increase of rock water content in humidification zone, with the ratio of radial stress to maximum expansion stress ranging from 1.3 to 2.4, leading to tunnel stress concentration effects. This case study provides references for design procedure and deformation prediction of tunnel support system in red mudstone under humidify conditions.