Study on undrained and residual strength of clay soils having no true cohesion

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There exist many clay soils which have no true cohesion in Japan.Shear strength of such a clay soil can be determined by the angle of shear resistance phi' and pore water pressure generated during shear, that is, the coefficient of pore water pressure A_f at failure. We carried out the triaxial compression and the ring shear tests for many clay soils sampled from landslide sites and the seabeds. Also we conducted mineral analyzes to investigate influence of clay minerals on the strength parameter. Conclusions are as follows.1.The clay soil which has no true cohesion has an unique angle of shearing resistance phi' regardless of states of clay soil.2.A clay which has lower water soil than so called normally consolidated clay soil can obtained by remolding and reconsolidating soil. Such a clay soil is also normally consolidated state. A_f of this clay soil increases with consolidation pressure, though it is constant for the so called normally consolidation state. The angle of shear resistance phi' from the plain strain test is not always bigger than one from the triaxil test.3.The angles shear resistance phi' and phir corresponding to the residual strength become small if the clay soil contains clay minerals such as expansive chlorite, talc and montmorillonite. It is problem that difference between phi' and phi_r becomes big. This difference is also big when minerals mentioned above is containe.