Abstract
One of the major problems to be clarified in landslide study is the mechanism of the formation of the slip surface in the colluvial soils derived from mudstone. For this purpose, the samples were taken at, above and below the slip surface of a landslide, and analyzed by physical, chemical and mineralogical techniques. The residual strength of them was also measured by a ring shear apparatus.
The total clay content of the colluvial-soil samples tended to be highest at the slip surface, and their main clay mineral was mica followed by kaolinite and vermiculite. The water-dispersive clay was near 30% at the slip surface and its content was clearly higher than the contents for above and below the slip surface.
At the slip surface disintegration into clay-sized particles is accelerated under the concentration of shear stress and by the supply of water through heavy rainfall. Under this condition, the reorientation of platy mica particles along the slip surface is facilitated, and the shear strength of the slip surface falls to the residual strength. The residual angle of shearing resistance of the slip-surface soil was measured to be 11° and this value was at the same level as the value expected for clay mica.
