A stress point algorithm for an elastoplastic model in unsaturated soils

Two stress fields, combination of total stresses, liquid pressure and gas p ressure have to be considered to explain the deformational behaviour of uns aturated media. Elastoplastic models developed for. these materials conside r generally two yield surfaces, each one associated to a stress field, and whose intersection produces a corner in the space of generalised stress com ponents. In this paper, a stress point algorithm is proposed to cope with t he integration of such constitutive laws, which can be seen as non smooth m ultisurface plastic models in the space of the two stress fields. The basic model developed by Alonso et al. (Alonso, E.E., Gens, A., 1990. A constitu tive model for partially saturated soils. Geotechnique 40 (3), 405-430), wh ich will be used to test the algorithm, is first described. Generalised str ess and strain variables are then defined. Implementation of the return map ping algorithm, based on an implicit integration scheme, is presented with special attention devoted to the problem of mixed control imposed by the F. E. formulation generally used to analyse the hydromechanical behaviour of u nsaturated media. Validation results on distinct generalised stress paths a re given at the end. (C) 2000 Published by Elsevier Science Ltd. All rights reserved.