Pore water distribution in the unsaturated soil can be classified into two forms. Those are "bulk water" and "meniscus water". Though it is known that the suction greatly affects on behavior of unsaturated soil, its effects depend on the forms of the pore water. When only the bulk water exists, the effects of suction act like Terzaghi's effective stress, the role of which is the same as negative pore water does. However, when only the meniscus water exists, the effects of suction act as cohesion at contact points of soil particles, and it increases stiffness of soil skeleton. We must take not only suction but also pore water distribution into consideration in order to predict behavior of unsaturated soil. In this paper, we carried out triaxial compression test on kaolin specimen under undrained condition. From the test results it is found out that the pore water pressure increased with compression and decreased with expansion, however that when the expansion increased, the pore water pressure increases again regardless of volume change. We can explain this phenomenon with the different effects caused by the bulk water and the meniscus water. According to shear strength, it is found out that, by application of the suction stress, we could use similar failure criterion with that for the results of drained shear test.
As a result of unstabilization by rainfall, collapse of natural slope frequently occurred. The rainwater seepage characteristics as well as shape of the slope and strength of the soil affect on the unstabilization of the slope. Because most natural slopes are under unsaturated state, mechanics of the unsaturated soil should be applied to both of the strength and infiltration characteristic. Some constitutive models of unsaturated soil have been proposed (Alonso et al. 1990, Karube et al. 1997).