The residual strength of smectite-dominated soils from the main slip surfaces of the Kamenose landslide, Japan, was measured by ring shear test, and the relations of the residual strength to the clay fraction content and clay particle orientation were studied. The residual angles of shearing resistance, [Formula: see text], were estimated assuming that the residual cohesion is zero. The magnitude of the residual angle of shearing resistance was dependent on the <2-μm clay fraction content, CF, and on the effective normal stress, [Formula: see text], because of curved strength lines. At [Formula: see text] equivalent to the overburden pressures, the soils had [Formula: see text] of 6–8°. These residual angles of shearing resistance were fitted on the [Formula: see text] – CF line for sand–bentonite mixtures, and exhibited the sliding shear mode.The orientation of smectite particles on shear surfaces showing slickensides was assessed by X-ray diffraction technique. The orientation of smectite particles was found to reduce the residual strength considerably, and this orientation effect was revealed more clearly at low effective normal stresses. Key words: residual strength, particle orientation, landslide, smectite, shear mode, slip surface, slickenside, clay mineral, physicochemical properties, ring shear test.
Residual strength parameters by the ring shear test using samples near slip surface of landslide