Maiko MIZUISHI Effects of stress and suction history on mechanical properties of unsaturated cohesive soil Supervisor Hirofumi TOYOTA The stresses in the ground have acted from the various directions when the structures, embankments and retaining walls are constructed on the real ground. In addition, the stress conditions are different in each point. Therefore, it is important to use the shear strength corresponding to the stress state of each point for the stability analysis because the stresses of ground induced by the structures are different in the place. Then, Bjerrum et al. (1973) proposed the experimental methods to estimate the real ground strength, and it has been confirmed that the different undrained shear strength are obtained from the various shear tests. Thus, it is important to examine the practical evaluation method of undrained shear strength because it is not possible to decide strength parameters for earth stability problems from the one experiment. The evaluation method of undrained shear strength was proposed in saturated soil consolidated from various directions using the hollow cylinder torsional shear apparatus, which can be reproduce three-dimensional stress conditions. However, a lot of unsaturated cohesive soils also exist in the ground, especially near the surface. Then, the hollow cylinder torsional shear apparatus was also used for demonstrating the appearance of the strength anisotropy and clarifying its mechanism in unsaturated cohesive soils anisotropically consolidated from the various directions. The main results are summarized as follows: 1. The drained shear strengths of saturated cohesive soil were not changed by the direction of major stress, ?f. Therefore, there is not the drained strength anisotropy induced by anisotropic consolidation in saturated cohesive soil. 2. In CW tests of unsaturated cohesive soil, strength anisotropy can be confirmed when the specimen was dehydrated under low confining stress level. It is considered that this anisotropy was caused by suction. However, strength anisotropy can not be confirmed when the specimen was dehydrated after finishing anisotropic consolidation. This reason is considered that enough water was not drained from the specimen anisotropically consolidated. 3. The peak appeared in the relationships between drained strength and ?f. That is the drained strength increases with ?f whose value is up to 45 degrees. On the other hand, the drained strength decreases with ?f that is more than 45 degrees. This kind of tendency was observed in the different suction level.