Estimation of the cohesion in shear strength under unsaturated state is an important problem when stability analysis is undertook for some natural slopes. In this paper, one simple method to estimate the cohesion is proposed. This method is based on the unconfined compression test in which suction of the specimen is measured. The unconfined compression test was carried out on compacted silty clay specimen in various water contents. Effects of the cohesion on the behavior of unsaturated soil studied through "suction stress" that corresponds to the cohesion. It is found out that relationships between the unconfined compressive strength and the suction stress are arranged uniquely, and that a prediction for the relationships between suction and the suction stress using the water characteristic curve show similar tendency with the relationship using the unconfined compression test results. These results mean an applicability of the unconfined compression test for estimating the relationship between the suction stress and suction.
When a stability of slope is analyzed, the slices method is widely used. This method is based on shear strength equation that is adaptable for saturated soil. On the other hand, some of natural slopes are under unsaturated state. If shear strengths of these unsaturated soils become problems in analysis, the triaxial compression tests are conducted in which the stress states, the drained condition and the water content of the specimens are reproduced as same as these conditions in situ. And based on the shear strengths obtained from the test results, stability analysis of the slope under unsaturated state is carried out. This method is typical one that has been used in practice. But when water content in situ varies, we must carry out many tests in which the conditions are adjusted for them.
