Daiki Morita

Effects of stepwise changed strain rate on mechanical properties of different soils

Hirofumi Toyota

Deformation rate of real ground is not constant but might be changes various rates during deformation process. It is important to understand the mechanical property’s change 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.

In this study, the effects of stepwise change of shear rate on the mechanical properties of soil were investigated. Experiments were conducted using a hollow torsional shear test apparatus, and “STEP loading”, in which the shear strain rate is rapidly changed at a certain shear strain under the condition of constant intermediate and mean principal stresses during shearing. Three types of shear strain rate changes were conducted: 1. Four-time 10-fold changes (designated as five-step changes), twice 100-fold changes (three-step changes), and one-time 10,000-fold change (two-step changes). Moreover, three kind of shear strain, which keeps a constant shear strain rate between STEP loadings, are set as follows: 0.05%, 0.1%, and 0.2%. New Zealand (NZ) kaolin and bentonite blended samples were used to examine the effects of shear rate on soil properties. 

The following findings related to the isostatic law in shearing were obtained from the study:
1. In the STEP loading, the shear rate became greater and smaller, the deviator stress increased and decreased, respectively.
2. In NZ kaolin, the deviator stress changes induced by STEP loading decreased at greater shear strain. However, this trend was unapparent in bentonite blended sample.
3. When shear strain of constant rate became greater, the changed deviator stress gradually diminished in NZ kaolin. However, no clear disappearance of this kind was observed in the bentonite sample. 
4. In both samples, the deviator stress changes induced by STEP loading are independent on the current shear rate. The deviator stress changes are determined by the magnifying power of the shear strain rate prior to that STEP loading and are proportional to the logarithm of the magnifying power.