Loss of rate dependency on shear strength of soils
Rio Tanoue
Hiroshi Toyota

The deformation of actual ground does not always deform with a constant rate, and the deformation rate may change under progress of failure. It is beneficial to understand the mechanical property changes of soils induced by rate variations for precise design of soil structure and ground improvement. There exists the simplified Newmark method to estimate residual displacement during seismic motions. This method extracts the acceleration waveforms that exceed a predetermined yield seismic intensity and calculates the residual displacement from twice integration of the acceleration records. However, the effects of rate changes on mechanical properties of soils is not considered in this method. Therefore, this study aims to investigate the loss of rate dependency on shear strength of soils using a soil element test in which the shear rate is varied stepwise.

A hollow cylinder torsional shear apparatus was used under conditions where the intermediate principal stress and mean principal stress remained constant during shearing. The step-wise loadings were applied, in which the shear strain rate was sharply changed at each specific shear strain rage. The shear strain rate was changed by a factor of 10 four times, indicating that five different shear strain rates are used. Additionally, the shear strain in the constant rate section between each step-wise loading was set to 0.1%.

As a result, the following findings were obtained:
1. Although the relationships between shear strain ƒÃs and deviatoric stress q show rate-dependent results, the present result is lower than that of previous ones conducted last year. The reason is considered that mean effective principal stress was decreased at the beginning of shearing.
2. The monotonic loading was continuously conducted after the step-wise loadings. A loss degree of rate-dependency was defined using the average slop of shear strain and deviator stress relation from the beginning of monotonic loading to reaching a stable shear strength.