The effect of irrigation on slope stability in the Heifangtai Platform, Gansu Province, China

Abstract

Agricultural irrigation in the Loess Plateau induces many loess landslides. Long-term flood irrigation is frequent on the top platform of loess slopes, where many landslides occur. To confirm the relationship between irrigation and landslides, we monitored the soil moisture content and groundwater level in Heifangtai Platform, and performed tests on unsaturated soil, including TRIM (Transient Release and Imbibition Method) and triaxial shear test. Then, we simulated the changes of the seepage field on slopes for flood irrigation at different locations and analyzed the effect of flood irrigation on slope stability. Monitoring results show that flood irrigation affects soil moisture content far more than rainfall. With the increasing depth, the response time of the change of soil moisture content increases gradually. The lag effect is obvious. During irrigation, at a depth of 0.1–0.2 m, the soil moisture content increases sharply; at a depth of 0.5–3 m, it increases gradually under different degrees; and at a depth of 5 m, it increases little. After irrigation, the soil moisture content decreased rapidly at a depth of 0.1 m and 0.2 m and returned to the average level nearly a month later. However, the soil moisture content continued to increase slowly over a period at a depth of 0.5–3 m. During the monitoring period, the cumulative rainfall was <110 mm, and the depth affected by rainfall is not >0.5 m. These data show the change of groundwater level is out of sync with irrigation activities and has a visible lag effect. According to the results of seepage and stability analysis, short-term irrigation has little impact on the groundwater level. However, it affects the flow field and causes the matric suction of the superficial loess to decrease. Long-term irrigation causes the groundwater level to increase locally. Compared with single irrigation of high intensity, the reduction of slope stability is more evident under long-term irrigation of low intensity. Changing the irrigation methods to reduce deep infiltration might be one useful precaution to minimize sliding. The closer the distance of the irrigation location to the slope edges is, the higher the effect of flood irrigation on the distribution of pore pressure and matric suction will be. Irrigation near potential slide sites has an apparent cumulative effect and lag effect on slope stability. The longer the irrigation time and the closer to the edge of the slope the irrigation is, the higher the effect on slope stability will be. Avoiding flood irrigation near or at the potential sliding region may reduce the frequency of landslides.