| Summary: | In Japan, a multitude of slope disasters occur annually, resulting in often severe loss of human life. On occasion, disasters that occur at very gentle slopes and disasters that flow long distance, associated with destructive energy, are observed. In such cases, the trigger layer often consists of volcanic soils with extremely high void ratios and external triggers, including earthquake or heavy rainfall, increase the risk of slope disasters. These volcanic soils are often characterised by an extremely loose structure and are either retained mainly by weak cementation or are composed of crushable particles having intra voids. Although the cause of destructive long-distance flow has been explained by liquefaction, there are some observed disasters in which the trigger layer was not fully saturated, i.e. liquefaction should not be considered as a major cause. Focusing on the fact that long-distance flow or gentle slope flow, disasters are triggered by volcanic soil layers of extremely loose structure, artificial loose soil samples were prepared in the laboratory. These samples were prepared with cementation between particles and with crushable particles. In this study, deformation and strength characteristics of the extremely loose soils were investigated by conducting CD triaxial tests. It is revealed that such extremely loose soils can be vulnerable after the weakly cemented structure collapses as a result of external factors such as earthquakes.
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