Engineering Geology 54 (1999) 173–183 Modelling the mechanical behaviour of expansive clays E.E. Alonso *, J. Vaunat, A. Gens UPC-DIT, c/Jordi Girona 1–3, Mo ´ dulo D-2, 08034 Barcelona, Spain Abstract A simple formalism is presented to model the behaviour of expansive clays. Two levels of structure are considered. The behaviour of the macrostructure follows the model developed for unsaturated materials by Alonso et al. [Ge ´otechnique 40 (3) (1990) 405–430]. The behaviour of the microstructure is adapted from the work of Gens and Alonso [Can. Geotech. J. 29 (1992) 1013–1032] in order to include the possibility of the micropores being partially saturated. Mechanical coupling between both levels of structure are defined through two functions, one for wetting and the other for drying. They express the change in macrostructural void ratio due to a change in microstructural void ratio, and their value depends on the state of compaction of the macrostructure. The general shape of these curves is discussed on the basis of experimental evidence. Phenomena such as the dependency of strain on stress– suction path, accumulation of expansion strain during suction cycles at low confining stress, accumulation of compression strain during suction cycles at high confining stress, strain fatigue during drying–wetting cycles, macropore invasion by expanded microstructure and development of macroporosity during strong drying can be represented. A mathematical formulation of the model is described and its performance finally assessed by comparison with laboratory tests. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Elasticity; Expansive soils and rocks; Model; Plasticity; Suction; Swelling 1. Introduction of low activity. Formulated in the framework of hardening elastoplasticity, this model extends the The mechanical behaviour of compacted expan- modified Cam–Clay model (Roscoe and Burland, sive clays is a critical factor in the design of seals 1968 ) by considering two independent sets of stress for repositories to store hazardous wastes. An variables: the net stress defined as the excess of adequate constitutive model for this type of mate- total stress over air pressure and the suction. A rial is therefore required when realistic numerical key element of the model is the dependency of the simulations of sealing systems are to be performed. yield locus on suction, which can be represented The clays are generally compacted and are conse- in the p–q–s diagram ( p=mean net stress, q= quently initially in unsaturated state. deviatoric stress and s=suction) by the LC surface Alonso et al. (1987, 1990) presented the (Fig. 1). BBM is able to reproduce most responses Barcelona Basic Model (BBM) describing the of partially saturated soils under simple loading– stress–strain behaviour of partially saturated soils unloading and wetting–drying paths. These are: $ increase of shear strength and preconsolidation * Corresponding author. Fax: +34-3-401-6504. pressure p 0 with suction; E-mail addresses: alonso@etseccpb.upc.es ( E.E. Alonso), $ development of reversible swelling strains when vvaunat@etseccpb.upc.es (J. Vaunat), gens@etseccpb.upc.es (A. Gens) suction decreases at low confining stress; 0013-7952/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0013-7952(99)00079-4