Tomotaka Kushi Investigation of stepwise strain rate dependency on mechanical properties of soils Hirofumi Toyota Deformation rates of the real ground are not constant but might be changed with various rates during deformation process. It is necessary to understand the variation of mechanical properties of soils induced by deformation rate change for design of earth structure or ground improvement. A simplified Newmark method exists for calculating residual displacement during earthquakes. Although residual deformation is calculated by acceleration greater than yielding seismic intensity, the effects of soil deformation rates on mechanical properties have not been introduced yet. Therefore, the purpose of this study is to clarify the effect of shear rate on mechanical properties by conducting soil element tests with shear rates varied stepwisely. Experiments were conducted using a hollow torsional shear test apparatus under conditions of constant intermediate and average principal stresses in shearing. Stepwise loading, in which the shear strain rate was rapidly changed at a certain shear strain, was applied during shearing. The speed was changed by a factor of 10, and five different speeds were prepared (maximum change of 10,000 times). Step strain was defined as the interval strain between changes in strain rate. Three cases of interval strains were prepared: those were shear strain of 0.05%, 0.1%, and 0.2%. In the previous studies, tests were conducted using a isotropic confining stress of 300 kPa. In this study, the confining stress was changed to 150 kPa in order to investigate the effect of different stress conditions on the strain rate dependence. The following findings related to the shear rate dependence were obtained in this study. 1. For each specimen, the same trend of strain rate dependence with the previous studies was observed, even when the confining stress was changed. However, the change in deviatoric stress with strain rate was greater when the confining pressure was higher, indicating that the strain rate dependence was affected by the confining stress. 2. When the deviator stress changes were divided by the respective confining stress, almost the same values were obtained. Therefore, the results were successfully normalized by the confining stress. 3. The unique linear relation was obtained between the stress variation and logarithm of magnification of strain rate in all cohesive soils. This relation was newly designated as “logarithmic law of stress variation” during stepwise loading.