Suction Controlled shear strength behavior of collapsible soil.

Shemsu Kemal, K. Shoichi, A. Shigeyasu and K. Akira.

Graduate School of Agricultural Science, Department of Agricultural Facilities Engineering, Kyoto University, Japan (all authors).

Mailing address: Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502.

Position: Postgraduate student (corresponding author).

E-mail address: shemsu@relief.kais.kyoto-u.ac.jp. Or shemsuk@yahoo.com

Tel/Fax: +81-75-753-6346

Proposed session theme: Geotechnical Engineering.

Abstract:

 Volume change and shear strength are the two important topics of unsaturated soils. It is well known that collapsible soils are found in unsaturated state, and the gradual change from the unsaturated state to saturated state due to suction reduction result in total volume change of soil mass, which is termed as collapse. On the other hand, the shear strength is said to be dependent on the initial void ratio before shear (i.e., the degree to which the soil has been “pre-densified” by some past action).

The objectives of this study are investigating the influence of suction on volume change and the after collapse shear behavior of a collapsible soil that can be experimentally verified using suction controlled triaxial test.

A modified conventional triaxial apparatus for testing of large sized soil is used in this study. The major modification involves the ability to control the pore-water and pore-air pressures separately. Two soil samples and a total of four specimens (two from each) were prepared at dry-of-optimum water content and lower dry unit weight. This condition results in meta-stable structure, which leads to collapsible soil. Consolidated undrained (CU), and consolidated drained (CD) test were carried out before and after collapse on soil sample 1 and 2, respectively. The volume change behavior during collapse and the shear strength behavior of the soil sheared before and after collapse were investigated.

In general, collapse experienced soils is found to be less compressible, result in higher strength and shows less development of pore pressures during shear.

This study reveals that the shear strength of unsaturated soil is a function of not only the magnitude of the suction in the soil before shear, the stress history in relation to matric suction also shows to play an important role in the shear strength of unsaturated soil.

Key words: Collapse, Shear strength, Suction recovery, Matric suction, Collapsible soil, Modified triaxial cell.