Abstract：In order to study the viscoplastic properties of clay，triaxial compression tests by using computer controlled triaxial apparatus with local displacement transducer(LDT)，and one dimensional compression tests were performed on undisturbed and reconstituted specimens of three types of Japanese Pleistocene clay(Kitan clay，Ohi-machi clay，and Fujinomori clay). In the tests，the axial (vertical) strain rate was changed stepwise many times，and drained creep tests were performed several times during monotonic loading at constant strain rate. Global unloading (and also reloading in some tests) was applied，during which creep loading tests were performed several times. Results of triaxial and uniaxial compression tests on the different types of Pleistocene clay are presented in this paper. The results show that viscoplastic properties of the clay are significant in all tests. Analyses on the results show that the stress change upon a step change in the strain rate is always proportional to the instantaneous stress in both triaxial and uniaxial compression tests. In order to describe the viscoplastic property of clay，a new parameter β，called viscoplastic parameter，is introduced，which can be obtained with traditional triaxial or uniaxial compression tests by changing the axial strain rate stepwise many times. The viscoplastic properties of a given clay specimen can represented by the b value. A larger b value means a larger viscosity. Test results also show that the β value is generally smaller with undisturbed clay than with reconstituted clay and in drained triaxial compression than in uniaxial compression.
