Field monitoring of rainfall infiltration in a loess slope and analysis of failure mechanism of rainfall-induced landslides

A full-scale field experiment involving artificial rainfall has been conducted in an instrumented loess cut slope in an expressway in a loess plateau, Northwest China. Instrumentations including soil moisture probes, tensiometers, piezometers, vacuum sensors, water level sensors, and rain gauge were installed in the slope. After monitoring for about 2 years, artificial rainfall was induced at the site in April 2007. This paper presents the results from soil moisture probes, tensiometers, and water levels to reveal the surface infiltration process in an unsaturated soil. The results show that the top 0.7 m in loess is an active zone under the process of infiltration and evaporation, when subjected to a maximum rainfall intensity of 40 mm/day. The depth of the “wetting front” during the wet season was limited to the top 2 m of soil. During high rainfall intensity of 120 mm/day, the depth of the “wetting front” is about 3 m. Based on the variation of volumetric water content and matric suction in soil, a method to analyze the rainfall infiltration is proposed. By means of Gravity-Predominant Flow (GPF) concept, the infiltration rate is analyzed in the deeper soil. The results explain well the observations that loess slopes could be stable in the wet season, but it may tend to slide about 3–6 months later in the dry season. The surface infiltration has a very limited effect on the permanent groundwater table, which is at a greater depth.