Abstract
Decomposed granite soil is widely distributed in the western part of Japan. This soil is often used as the material for embankment construction. These natural and man-made slopes, however, fail easily due to rainfall. Direct shear tests were conducted on undisturbed and compacted soils by impact and static compaction under unsoaked and soaked conditions. Strength anisotropy was investigated by direct shear tests in which failure planes are normal (H specimen) or parallel (V specimen) to the direction of depth and of compaction. The result shows that compacted soil has less shear strength than undisturbed soil. The shear strength of compacted soil by impact compaction is greater than compacted soil by static compaction. Concerning the strength anisotropy for both undisturbed and compacted soils, the anisotropy of the V specimen is greater than that of the H specimen, irrespective of unsoaked or soaked condition, method of compaction, water content or density. The decrease in shear strength of compacted soil due to soaking is more significant than that of undisturbed soil. This behavior is attributable to the apparent cohesion, not to the angle of shearing resistance. The authors propose a schematic diagram of soil structure in order to explain this behavior.
