Water infiltration characteristics in loess associated with irrigation activities and its influence on the slope stability in Heifangtai loess highland, China

The water infiltration characteristics associated with irrigation activities which contribute to the slope stability failures of Heifangtai loess highland of China from the mid-1980s are still not well understood. For this reason, infiltration characteristics of loess were studied from 1D column test by adding water at daily intervals for a long period of time. Experimental results of the 1D column test were numerically simulated along with 2D simulation studies for a typical Heifangtai slope for studying the water infiltration characteristics and its influence on the slope stability for a 33 years period. Numerical simulation results highlight that the infiltration characteristics in the 1D column and 2D slope are similar and can be explained by dividing the unsaturated zone into three different zones. Simulation results of the 2D slope were valuable to understand how the irrigation activities affect the hydraulic behavior and the slope stability. The initial hydraulic state with low water content and no GWT condition transforms into a new hydraulic equilibrium state with a constant water content in the unsaturated zone and a stable GWT. This process can be divided into four different stages. The safety factor (Fs) of the slope decreases during the first three stages; however, it is stable in the fourth stage when a hydraulic equilibrium condition is reached. Slopes in Heifangtai can fail during any of the first three stages due to different triggering factors. The most feasible way to control the occurrence of landslides is to control or reduce the total amount of irrigation water per year. The results summarized in the present study are useful to better understand the cumulative effects of infiltration events on stability of slopes.