SUSAMI Ayaka

Study on strength anisotropy of unsaturated soil induced by suction

TOYOTA Hirofumi

Study about anisotropy of soils has been experimentally and theoretically conducted for long years. In particular, it is known that cohesive soils show an anisotropy of strength and deformation characteristics, which is caused by orientation of soil particles during sedimentation and anisotropic stress conditions. However, studies for anisotropy of unsaturated soils little exist.
Objective of the study is that existence of strength anisotropy for an unsaturated soil is confirmed using the hollow cylinder torsional shear apparatus, which can be reproduced three-dimensional stress conditions. About testing methodology, unsaturation of specimens is carried out by the pressure plate method applying 400 kPa of suction. And the specimens are anisotropically consolidated from various directions. Shearing was done at different shear rate, where high shear rate is ten times faster than low shear rate.
The main results are summarized as follows:
1.The strength anisotropy of unsaturated cohesive soil appeared under high shear rate, whereas there was the strength anisotropy of saturated cohesive soil under undrained condition. Shear strength of saturated cohesive soil largely decreases with the increase of the difference of major principal stress directions between consolidation and shearing. On the other hand, shear strength of unsaturated cohesive soil relatively decreases in the range that the difference of major principal stress directions is from 0 to 45 degrees. Then the strength shows constant value between 45 and 90 degrees in the difference of major principal stress direction. It is confirmed that the anisotropy of strength is significant even in consideration of the potential effect of measuring error. Moreover, when the initial suction increases by 400 kPa, shear strength increases by 140 kPa.
2.There is anisotropy of the elastic shear modulus in both saturated and unsaturated cohesive soils. The elastic shear modulus increases with increase of suction, and it decreases linearly with the increase of the difference of major principal stress directions. Moreover, the rate of the decrease is approximately constant between saturated and unsaturated soils.