The unsaturated seepage process and mechanism of internal interfaces in loess-filled slopes during intermittent rainfall

•A model test is carried out to discover unsaturated infiltration of interface.•The permeability at the interface under intermittent rainfall are identified.•The unsaturated seepage process and mechanism of the interface are revealed. In the “Gully Land Reclamation” project conducted in Yan’an, China, shallow failure in the middle of loess-filled slopes is caused by the unsaturated seepage of rainwater along the internal interface between the original loess slopes and the loess-filled slopes. Although the phenomenon was observed through model test, the particular nature of the unsaturated seepage mechanism remains unclear. To explore this mechanism, we carried out an intermittent rainfall model test on a loess-filled slope, and the volume water content in different areas was continuously monitored. Based on the monitored data, the unsaturated seepage process and mechanism in the internal interface was analyzed by adopting the Fredlund and Xing model and the Childs and Collis-Geroge model. The results indicate that a water accumulation phenome occurs in the middle area of loess-filled slope because of low matric suction in the middle area and high matric suction in the bottom area. In addition, the dynamic process of pore growth and shrinkage at different areas prevents seepage to the bottom area. Furthermore, the regularity of pores at the interface during intermittent rainfall is summarized from a macroscopic perspective based on the matric suction and permeability coefficient. The unsaturated seepage process and mechanism affected by Knudsen diffusion are revealed. Because of the presence of unsaturated seepage, a shallow failure appears in the middle area of a loess-filled slope. The results have a significant impact on the field applications of interfacial impermeable prevention measures.