Generally we use undrained strength for an analysis of bearing capacity of a clayey ground whose permeability is so low. Therefore, it is necessary to obtain an accurate value of the undrained strength in the ground. But the undrained strength of clayey soil depends on various conditions, for example, fabrics of soil grains, stress and strain state, stress history and so on. Now, all relations between the undrained strength of clayey soil and the factors influencing on it are not clarified. As one of approach to more detail study of the undrained strength, the authors would like to propose that clayey soils are classified into two groups from a mechanical point of view, that is, one has a true cohesion c(e) and the other has no true cohesion. The authors have tried triaxial compression tests on very many kinds of clayey soils under various conditions. Clayey soils are classified into two groups from a sedimentary environment at soil sampling. One is marine clay and the other is landslide clay. The number of marine clay is about 10 samples and landslide clay is about 20 samples. From theses tests it is found that most Japanese clayey soils have no true cohesion and the angle of shearing resistance phi' in terms of effective stress for a given clayey soil is constant under various conditions.
