AN INVESTIGATION ON THE UNIQUENESS OF THE INCREMENTAL RESPONSE OF ELASTOPLASTIC MODELS FOR VIRGIN SAND

The paper investigates the possibility of the occurrence of a multipli city of homogeneous responses of soil specimens under the same increme ntal loading, and the associated non-controllability of the imposed lo ading path. It is shown that for non-associated elastoplastic strain-h ardening constitutive laws, such a possibility exists even in the hard ening regime. Only when the stiffness matrix is positive-definite is t he incremental solution unique for any loading programme. A particular constitutive model is chosen and the shape of the loci of non-uniquen ess is calculated for various loading programmes. It is shown that the conditions for the occurrence of a shear band and of a peak of the de viator stress in a triaxial undrained test can be seen as special case s of the theory presented. Accordingly, the so-called 'instability lin e' concept in undrained conditions is generalized. (C) 1998 by John Wi ley & Sons, Ltd.