Flows in granular soils are complicate phenomena between fluid dynamics, geotechnics, geology and geomorphology, and they include debris flows in torrents, landslides showing flow-like motion and landslide-with-basal-flow maintaining the almost intact soil structure during motion. The mechanisms of flows are not yet well understood, though it is very important for landslide hazard assessment. This paper presents well-documented and selected case records, which are informative for understanding of the mechanism of flows in granular soils. The shear strength mobilized after failure and during motion is the most important factor to understand the mechanism of flows from the geotechnical aspect. The torquecontrol undrained ring shear apparatus was used to reproduce the shear behavior of the shear zone for each case record. Within the investigated cases, the sliding-surface liquefaction, which is explained as Ògrain crushing in the shear zone with the progress of shear displacement causes volume reduction and the resulting liquefaction in the shear zoneÓ, seemed to have taken a vital role for such flow phenomena in granular soils.
Rapid landslides do not allow people to get away and having caused catastrophic disasters over the world. Progress of urbanization and regional development accompanied by rapid population growth early 21st century will further accelerate this kind of disasters. Rapid landslides must be resulted by a large difference between shear stress and shear resistance. Rapid landslide from cliff or very steep slopes such as rock falls, debris falls can be foreseen from the geomorphology. However, some landslides in granular soils move rapidly on not-steep or even gentle slopes. This type of phenomena is called as flowslide, such as Bishop (1973), Eckersley (1990), Hungr (1995) and others. Hutchinson (1986) stated flow slide is a slope failure involving the collapse of metastable structure in a loose, cohesionless mass.