Recovered Strength of Landslide Soils and its Relationship with Mineralogical Composition

Abstract

Strength recovery from the residual state and factors controlling the recovery rate of the strength were examined for landslide soils collected from Japan, China, and Taiwan. The samples passed through a 420-μ m sieve were subjected to large-displacement shear under the drained condition in a ring shear apparatus, and fully-softened and residual strengths were measured. They were then re-consolidated and re-sheared to measure the recovered strength. Grain size distribution was measured for the sheared samples by separation into respective fractions after complete dispersion by sonic wave treatment and adjustment to pH 10 by adding 1 M NaOH. Mineralogical composition was analyzed by X-ray diffraction for the fractions separated in the grain size analysis. The samples showed a considerable variation in strength parameters, grain size distribution, and mineralogical composition.In general, the smaller the difference between fully-softened and residual strengths was, the greater the extent of recovery from the residual strength became. Landslide soils containing high contents of silt and sand and dominating with quartz and feldspars showed a strong recovery of residual strength at effective normal stresses until 400kPa, due to the re-consolidation effect on the disturbed thin-layer still existing on the shear surface in the residual state. In landslide soils dominating with quartz and feldspars but containing considerable amounts of mica and chlorite, the recovery of strength was noticed at lower normal stresses but hardly seen at higher normal stresses. This is ascribable to the formation of residual shear surface due to the presence of preferred-oriented minerals at higher normal stresses. The recovery of residual strength was not essentially recognized in landslide soils dominated by prefered-oriented minerals such as smectite, due to the development of residual shear surface to make slickenside.