ABSTRACT: Strain rate sensitivity is one of the typical time-dependent behaviors of soil as well as creep and stress relaxation. In particular, it is well known that a unique stress-strain curve exists for each different strain rate in clayey soil: the isotaches characteristics. Originally, the concept of isotaches was proposed in one-dimensional consolidation of clay; however, we can also observe the isotaches characteristics in the stress-strain relation obtained by the triaxial compression test with various strain rates. The purpose of the present study is to reexamine the concept of isotaches through the detailed results of triaxial tests on reconstituted Fukakusa clay and a constitutive modeling. Undrained triaxial tests both of normally consolidated and overconsolidated clay were performed with various constant strain rates and step-changed strain rates to observe the isotaches characteristics in a wide range of axial strain. The results of the step-changed strain rates test show that the phenomenon of isotaches exists in the range of low level strain. It is also seen, however, that this phenomenon is not observed in a range of high level strain, in particular, around the critical state. The results of step-changed strain rate tests were numerically simulated by an elasto-viscoplastic model for clay. It was found that the model can well simulate the trend of the isotaches behavior in the range of low level strain observed in the experiment. In addition, the trend of stress-overshooting and stress-undershooting in the range of high level strain observed in the experiment can be described adequately using the model except for the stress-overshooting of overconsolidated clay.

Key words: elasto-viscoplastic constitutive model, isotaches, normally consolidated clay, overconsolidated clay, strain rate effect, triaxial compression test (IGC: D6)