Theoretical and experimental research on the slip surface of unsaturated loess slopes

Based on the limit equilibrium theory and combined with the variational method, a stability analysis model for homogeneous unsaturated loess slopes is established. The variation law of the safety factor of unsaturated loess slopes and the variation characteristics of the sliding surface under rainfall conditions are determined. For verification purposes, the results from the proposed formulations are compared against those from several existing analytical solutions, it is shown that the safety factor results were in good agreement with those obtained by the Spencer, with a maximum deviation of no more than 1.9%. Considering the suction effect of unsaturated soils, the changes in the safety factor of the original dry slope and the position of the potential sliding surface throughout the entire life cycle of rainfall infiltration are obtained by simulating rainfall through transient seepage. The calculation results are further compared with the field inclinometer monitoring results of the unsaturated loess slope that had experienced slip failure due to continuous rainfall. The results show that the safety factor of the unsaturated loess slope temporarily increases and the potential slip surface deepens in position at the early stage of rainfall; as rainfall continues, the safety factor decreases rapidly, and the potential slip surface shifts to the shallow part. Continuous heavy rainfall will lead to a 28.8% reduction in the safety factor and cause significant changes in the position of the potential slip surface. The slope slip surface positions calculated by the proposed method show good agreement with field measurements. For unsaturated loess slopes that fail under intense rainfall conditions, the slip surface is closer to the logarithmic spiral.