<p>Landslides with deep-seated clay-rich shear zones are highly susceptible to intense rainfall infiltration. Such hydrological disturbances promote pore-pressure generation within the shear zone, reducing effective stress and potentially triggering unexpected sliding. This study investigates a rainfall-induced catastrophic landslide in the Three Gorges Reservoir area through laboratory tests and numerical simulations. A series of direct shear tests were conducted under constant shear stress paths, where increased back pressure was applied to induce pore pressure and trigger instability. The results show that the shear-zone soils are prone to instability under hydrological perturbations, with the response strongly influenced by mobilised stress levels. Numerical simulations, based on a hypoplastic model calibrated with test data, further reveal that crack infiltration accelerates saturation, promotes the development of a continuous basal sliding surface. The close agreement with field evidence confirms that pore-pressure buildup is the primary driver of rainfall-induced landslide initiation and underscores the critical role of crack infiltration in accelerating failure.</p>