Koki Takahashi Effects of shear strain rate variation on stress-strain relationships of soils Hirofumi Toyota There are various rates when the ground deforms. For example, some slopes are suddenly collapsed during rainfall, others are deformed only several centimeters in a few days such as sluggish landslides which fluctuate according to season. Although reproduction of an actual displacement rate is difficult in laboratory element tests, we can grasp tends of mechanical property changes induced by strain rate. This knowledge will be necessary to clarify the collapse mechanism and the prediction for future collapse. Therefore, in this study, saturated undrained tests were conducted with changing the shear strain rate under conditions of a constant intermediate principal stress coefficient and a constant mean stress during shearing using a hollow torsional shear apparatus. The purpose of the study is to consider the effects of change in shear strain rate on the stress-strain relationships of soils. One sample used in the experiment were New Zealand kaolin clay containing bentonite, which has a large liquid limit, as a highly plastic clay sample. Others are New Zealand kaolin clay as a clay sample, and Toyoura sand as a sand sample. By using those three samples, which have different liquid limits and different particle size, the effects of change in shear rate on the stress-strain relationship were investigated. The following three findings were obtained from the study. 1. Rate dependence is small on the undrained stress-strain relationship of sand. 2. The undrained stress-strain relationship of cohesive soils has rate dependence in the small shear strain region. 3. The range of shear strain in which this rate dependence is observed increases with the liquid limit.