The effects of stress history and drainage condition on the mechanical properties of unsaturated cohesive soil were examined using triaxial compression and torsional shear tests. The main results are summarized as follows:
1. When arranged through the values at failure of A-type and B-type specimens, there is a strength increment depending on matric suction, which is independent of the stress path due to unsaturation and drainage condition. This means that the failure line at constant matric suction on the pnet - q plane is parallel to the CSL and is independent of the stress path and drainage condition.
2. In the statically compacted unsaturated soil (C-type specimen), the result as mentioned above is not obtained. The reason for this is that in this case, the soil does not reach the final state because of the very loose initial state of the soil.
3. When arranged through the values at ultimate (residual) state after peak, a failure line which is independent of matric suction on the pnet - q plane is obtained. This line is the boundary, which distinguishes the strain softening and hardening features of stress-strain curves.
4. The slope of the fs(s) - s relationship becomes smaller as matric suction increases and is different depending on the experimental condition, i.e., triaxial compression or torsional shear. Hence, there are effects of matric suction and stress state on the strength increment due to matric suction.