Paper No. 7
Presentation Time: 8:00 AM-6:00 PM
PLATY LAYER SILICATE MINERALS FOR CONTROLLING RESIDUAL STRENGTH IN LANDSLIDE SOILS OF DIFFERENT ORIGINS AND GEOLOGY
NAKAMURA, Shinya1, GIBO, Seiichi
1, EGASHIRA, Kazuhiko
2 and KIMURA, Sho
3, (1)Faculty of Agriculture, University of the Ryukyus, Senbaru 1, Nishihara-cho, Nakagami-gun, 903-0213, Japan, (2)Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan, (3)Faculty of Agriculture, Universitty of the Ryukyus, Senbaru 1, Nishihara-cho, Nakagami-gun, 903-0213, Japan, s-naka@agr.u-ryukyu.ac.jp
Various studies have been conducted to determine the factors affecting the residual strength Φ
r of landslide soils and to correlate the residual strength to the geotechnical properties of these factors such as plasticity limits and clay fraction. These studies aim to facilitate residual strength estimation without requiring its measurement. However, few studies have analyzed the relationship between the mineralogical properties of landslide soils and the residual strength measured with a torsional ring-shear apparatus. Thus, in this study, we examine the residual strengths and mineralogical compositions of soils collected from different landslides and find that the residual strength is related to the total content of smectite, vermiculite, chlorite, and mica in the bulk soil. These minerals are 2:1-type layer silicate minerals, which are lamellar to platy in shape and apt to undergo particle reorientation after large-displacement shear, leading to a decrease in shear strength of the residual state.
In this context, the total content of smectite, vermiculite, chlorite, and mica in the sub-425-mm 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.
