A series of undrained triaxial compression tests with controlled strain-rate and strain-acceleration was performed on reconstituted clay. On the basis of the test results, isotach models using irrecoverable strain and its strain rate were developed so as to evaluate time-dependent behaviour of the clay under undrained conditions. Applicability of these isotach models was in depth examined in relation to the undrained be-havoiur of clay subjected to shearing, creep and relaxation.