Seepage and Stability Analysis of Loess Landslides Under the Coupled Effects of Long-Term Irrigation and Fissures

Long-term agricultural irrigation in the loess platform region of Northwest China has raised the groundwater level and triggered numerous irrigation-induced loess landslides. The widely developed fissures in loess provide preferential pathways for irrigation water infiltration and serve as key factors that control the hydrological evolution and stability of landslides. The Jiaojiayatou landslide in the Heifangtai platform, Gansu Province, was selected as the study case. A coupled saturated-unsaturated seepage–stress numerical model incorporating fissure structures was established to systematically investigate the effects of fissure depth, location, and number on the seepage field evolution, stability, and deformation characteristics of loess landslides under long-term irrigation. The results show that fissures significantly accelerate the advance of the wetting front, enlarge the high-water-content zone, increase pore water pressure, and reduce the factor of safety. Among these parameters, the effect of fissure depth is the most significant: for fissure depths of 5 m and 10 m, the simulated average annual rise in groundwater level is 0.63 m/a and 1.21 m/a, respectively. When the fissure depth increases to 15 m, irrigation water directly recharges the groundwater, leading to landslide instability (factor of safety drops to 0.97). The displacement at the slope shoulder increases by 54% compared with that in the no-fissure case, and the displacement pattern shifts from predominantly horizontal sliding to vertical settlement. Furthermore, the closer the fissure is to the platform edge and the greater the number of fissures, the lower the stability becomes and the larger the soil displacement at the slope shoulder.