The Yan'an area, located on the Loess Plateau in Northern Shaanxi Province, China, experienced heavy and prolonged rainfall in July 2013, which induced 8,135 slope failures (loess landslides and loess flow-slides), destroying approximately 10,000 cave dwellings and killing 45 people. Field investigations have indicated that the occurrence of loess flow-slides is closely related to infiltration depth, slope angle, slope morphology, rainfall intensity, and loess strength. The results from this study showed that (a) most of the loess flow-slides in Yan'an were small scale, ranging in volume from tens of cubic meters to hundreds of cubic meters; (b) the loess flowslides only occurred at a depth of\2 m, corresponding to a surface layer of completely saturated loess; (c) most slope failures of loess flow-slides occurred on loess slopes with angles between 35°and 50°; and (d) a reduction in the soil strength of saturated loess was a primary cause of the occurrence of loess flow-slides. However, tree roots may reinforce deeper soil mass ([2 m) and can effectively prevent loess flow-slides.

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