Fissure-controlled preferential infiltration and hydro-mechanical response of a red-bed landslide under contrasting rainfall patterns

Introduction Rainfall-induced landslides are among the most common and destructive geological hazards in mountainous and hilly regions. Rainfall infiltration modifies the hydraulic state and effective stress of slope materials, thereby contributing to slope instability. Fissures can provide preferential infiltration pathways, but the hydro-mechanical responses of fissure-developed landslides under different rainfall patterns remain insufficiently understood. Methods This study investigates the Wangjiangao red-bed landslide in Jishou City, Hunan Province, China. Based on geological investigations and field monitoring data, the responses of pore-water pressure, degree of saturation in shallow soil layers, and surface displacement were analyzed under three representative rainfall events: the first low-intensity rainfall after the dry season, short-duration intense rainfall, and prolonged rainfall. A two-dimensional coupled saturated-unsaturated seepage-stress model was subsequently established in FLAC3D. The mapped fissure-affected areas were represented as equivalent high-permeability zones to compare the evolution of pore-water pressure, degree of saturation, and displacement among the three rainfall conditions. Results Field observations showed that low-intensity rainfall was accompanied by minor pore-water-pressure fluctuations and only small displacement variations. Short-duration intense rainfall rapidly increased pore-water pressure to 34.32 kPa, followed by dissipation within two days, with a maximum surface displacement rate of 11.65 mm/day. The displacement-calibrated simulations reproduced the general deformation trends and indicated that the degree of saturation at a deep observation point within an equivalent high-permeability zone increased from approximately 45% to nearly 99% during intense rainfall. Under prolonged rainfall, simulated high-saturation zones expanded toward the deep sliding mass and modeled sliding zone, accompanied by broader reductions in suction-related equivalent cohesion. Removing the preferential hydraulic contrast of the equivalent high-permeability zones attenuated the rapid deep hydraulic response and reduced displacement accumulation under the three rainfall conditions. Discussion Together, the field observations and equivalent-continuum simulations support the interpretation that preferential infiltration associated with fissure-affected areas contributed to the rainfall-induced hydraulic and deformation responses of the Wangjiangao landslide.