The ALTERNAT model described in this paper is a double hardening model for the mechanical behaviour of sand under alternating loading. For determination of the parameters of the ALTERNAT model, special types of triaxial tests are required, e.g., drained triaxial tests with monotonically increasing or decreasing axial strain and constant isotropic stress. These tests are not easy to be conducted in soil mechanics laboratories. It is intended here to choose a simple theoretical model to get the

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... quired stress-strain relationships for the determination of ALTERNAT model parameters. Of many theoretical models available to predict the overall response of sands, the endochronic model is chosen for this task. This model treats the sand as non-linear elastic-plastic material. Furthermore, the theory assumes inelastic changes to be caused only by the rearrangement of grains. The stress path required in the tests is applied in the computer program (ENDOCH) written by the authors. The parameters are calculated for two samples for which Ng and Dobry (1994) made experimental investigation. It is concluded that the proposed procedure for the determination of the ALTERNAT model parameters is successful. The stress-strain relations predicted are similar to those obtained by Molenkamp based on laboratory test results. INTRODUCTION-The ALTERNAT Model The model described in this paper forms the major component of a double hardening model for the mechanical behaviour of sand under alternating loading. The model was developed by Molenkamp (1987) at Delft Geotechnics. In Figure (1), the yield surfaces of both plastic models, namely the "compressive" model and the "deviatoric" model are shown in the stress space of the isotropic stress, s, and the deviatoric stress, t.