Name: Masashi TAKEDA

Thesis title: Effects of shearing rate on shear strength of unsaturated cohesive soil

Supervisor: Hirofumi TOYOTA

It is important to use a shear strength corresponding to the stress state of each point for stability analysis of the ground. Soil suction, drain conditions during shearing and physical properties of soils are considered as important factors to affect strength properties of unsaturated ground. Strength anisotopy has been confirmed under unsaturated undrained condition with shear rate of 0.05%/min in previous study.
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. Then, the specimens are anisotropically consolidated from various directions. The shear rate in shearing was varied with 1000 times, those are 0.005%/min, 0.05%/min, 0.5%/min, and 5%/min. Based on the results, the effects of shear rate on strength properties of unsaturated clay are elucidated. In addition, we have proposed a formula to determine the strength of the unsaturated soil using the suction estimated from the results of undrained shearing of saturated soil.
The main results are summarized as follows.
1. Effect of shear rate
Strength anisotropy appears regardless of the effect of shear rate in unsaturated undrained tests. However, secant elastic shear modulus increases with an increase in the strain rate.
2. Prediction of the strength anisotropy of unsaturated soil
It has been demonstrated that the trends of increment of pore water pressure during shearing are similar between saturated and unsaturated soils in this study. The possibility in which shear strength of unsaturated soil can be predicted is shown using this fact.