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.

The results from an intensive experimental investigation on a loess that was retrieved from a typical silty loess zone in the northwestern Chinese Loess Plateau are presented and interpreted. Triaxial and oedometer tests were performed on intact and reconstituted samples in a saturated condition. The soil behavior was found to be strongly affected by structure. The compression paths of the intact samples crossed the intrinsic compression line of the reconstituted soil and reached well-defined gross yield points, after which the compression paths converged towards the intrinsic compression lines. Two critical state lines were defined for the intact and reconstituted soils in the volumetric plane as a result of a robust element of natural structure. Comparisons were made with a structured clayey loess retrieved from the southeastern Loess Plateau. It was found that the effects of structure on the behavior of the two loess soils are similar though they are very different in natural properties. This indicates that their natural structures might have experienced similar forming processes, perhaps related to their common origin. Catastrophic flowslides commonly occur in the study area, and the patterns of behavior observed in the laboratory tests may create some new insights into mechanisms of landslide initiation and subsequent movement as discussed in the study.

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