Shohei Niino

Effects of stepwise changed strain rate on stress-strain relationships of soils

Hirofumi Toyota

The ground deforms with various rates, e.g. some slopes are suddenly collapsed during heavy rainfall, others are sluggish landslides that deforms only a few centimeters in a few days with seasonal fluctuation. 
Although reproduction of an actual displacement rate is difficult in laboratory element tests, we can grasp changes of mechanical properties induced by strain rate. These findings will be able to use to clarify the collapse mechanism and the prediction for the future collapse.

In this study, the stepwise changed strain rate tests, in which the shear strain rate was changed stepwisely, were conducted under a constant intermediate principal stress coefficient and a constant mean principal stress conditions during shearing using a hollow torsional shear apparatus. The objectives of the study are to discuss the effects of shear strain rate on the stress-strain relationships of soils and to evaluate the application of the Isotach raw to the shearing.

The samples used in the tests were New Zealand kaolin clay as a clay sample, Toyoura sand as a sand sample, and Yoneyama silt as a cohesive soil sample. By using those three samples having different plasticity index and different particle size, the effects of shear strain rate on the stress-strain relationship were investigated.

The following three findings were obtained from the study.
1. Rate dependence was small in the undrained stress-strain relationship of sand.
2. A relationship between shear strain that loss the Isotach law and its plasticity index was observed in clayey soils.
3. Isotach law was observed at all shear strains in the stepwise changed strain rate test of the clayey soils. However, the Isotach law gradually vanished in the section of a constant shear strain rate.