Effect of variable strain rate on stress–strain behaviour of saturated clay

This paper presents the results of laboratory investigation on variable strain rate effects of soils deformed under undrained conditions. Consolidated undrained triaxial tests were conducted under two categories with: (a) a constant strain rate control and (b) a variable strain rate control. In the constant strain rate control tests, the strain rate remained constant throughout the shearing, whereas in the variable strain rate control tests, the strain rate varied from 50 mm/s to 0·0007 mm/s during the shearing. Tests were carried out at initial effective confining pressures of 100 kPa, 200 kPa and 400 kPa. It was observed that the peak deviator stress increased and pore water pressure decreased with an increase in strain rate. However, this trend became reversed at a higher strain rate such as 70 mm/s. The stress–strain behaviour under variable strain rate control tests showed the tendency of soils to switch to a different stress–strain path as the strain rate changed. Two new parameters, $ku$ and $kσ$⁠, were introduced to express the change in excess pore water pressure and effective normal stress with strain rates. An approximate procedure sensitive to strain rate has been developed to predict the stress–strain behaviour of soil under undrained conditions.