Shinya Nakamura

Platy layer silicate minerals for controlling residual strength in landslide soils of different origins and geology

We use a ring-shear apparatus to examine the relationship between the residual strength and mineralogical properties of landslide soils and find a significant relation between the residual strength parameter, φr, and the total content of layer silicate minerals prone to preferred orientation within the bulk soil. In this context, the total content of smectite, vermiculite, chlorite, and mica in the sub-425-μm soil fraction is a suitable mineralogical parameter for estimating the magnitude of φr. Plotting φr as a function of total mineral content forms a chair-shaped curve, based on which we classify landslide soils into three groups. In the first group of soils, sliding appears to be controlled by minerals such as quartz, feldspar, calcite, dolomite, and layer silicate minerals other than smectite, vermiculite, chlorite, and mica and their φr is almost constant at ∼32°. In the second group of soils, the controlling mineralogical factor for sliding quickly shifts from nonpreferred-orientation minerals to preferred-orientation minerals and φr decreases from 30° to 10°. In the third group of soils, sliding is well controlled by preferred-orientation layer silicate minerals and φr gradually decreases from 10° to 5°. The results show that the relationship can be used for predicting φr of a wide range of landslide soils that differ in geology, soil type, mineralogical properties, and shear strength.

Shearing rate effect on residual strength of landslide soils in the slow rate range

In the present study, we investigate the influence of shearing rate on the residual strength parameters, ϕr, in landslide soils using ring-shear tests at two shearing rates (0.01 and 0.5xa0mm/min) under selected effective normal stresses. The landslide soil samples used for this study cover a wide range of soil types and properties. Relationships between the ϕr and soil properties, such as liquid limit, plasticity index, and the clay fraction show that the ϕr decreases with increasing soil LL, Ip and CF. The differences in the ϕr at two shearing rates, ϕr (0.5)u2009−u2009ϕr (0.01), under different effective normal stresses are either negative or positive values of which the maximum magnitude is generally about 1.0°. The relationships between the ϕr (0.5)u2009−u2009ϕr (0.01) and the soil properties (LL, Ip, and CF) are not exhibited in a regular pattern. On the other hand, the ϕr (0.5) and ϕr (0.01) determined at each effective normal stress suggest that the tendency for increased negative effect of the ϕr with decreasing effective normal stress is more noticeable in the slow shearing rate range. The absolute value of ϕr (0.5)u2009−u2009ϕr (0.01) at lower effective normal stress is found to be greater than 1.0°, with the maximum of about 4.0°. The negative shearing rate effect on ϕr at low effective normal stress is affected by the undulating shear behavior due to poor or no slickenside development.

Correlation of large displacement drained shear strength of landslide soils measured by direct shear and ring shear devices

Based on a laboratory study of drained direct shear tests of remoulded, pre-cut and polished specimens and drained ring shear tests of uncut and remoulded specimens of slip surface materials of five landslide soils, significant correlations of the mobilized shear strength parameters, cohesion (c, cr) and internal friction angle (Φ, Φr), are proposed. The investigated soils consisted of the slip surface material belonging to tuffaceous clay, mudstone, loess and siltstone. Most of the previous studies on residual shear strength measured by reversal direct shear and ring shear devices have obtained significantly different results using the two devices, even when pre-cut and polished specimens were used in the direct shear device. In this study, the residual shear strength values of remoulded specimens measured by a ring shear device are shown to significantly correlate with the drained large displacement shear strength values of remoulded specimens, which were measured using pre-cut and polished specimens in a direct shear device. The correlation between the cohesion measured in the two shear devices is expressed by the linear relationship,

Influence of effective normal stress in the measurement of fully softened strength in different origin landslide soils

{c_{text{r}}} = 0.{7394}c - {6}.{6857}

Shear Strength Characteristics of Landslide Soils, Asato Landslide, Okinawa, Japan

, while the correlation between the friction angle measured in the two devices is expressed by the linear relationship,

Landslide disasters induced by the Northern Kyushu heavy rainfall in July 2017

{Phi_{text{r}}} = {1}.0{852}Phi - {6}.0{247}