Modelling the mechanical behaviour of expansive clays

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. [Géotechnique 40 (3) (1990) . 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 stresssuction 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.

A coupled THC model of the FEBEX in situ test with bentonite swelling and chemical and thermal osmosis

“…Their model, however, is restricted to isothermal free swelling/shrinking. Swelling can be computed with elastoplastic models (Gens and Alonso, 1992;Thomas and He, 1998) such as the Barcelona Basic Model (BBM) (Alonso et al, 1990) and the Barcelona Expansive Model (BExM) (Alonso et al, 1999). The elastoplastic model has been further extended to account for macro-and micro-structures of expansive clays (Alonso et al, 1999;Thomas and Cleall, 1999;Sánchez et al, 2005).…”

Use of a Dual-Structure Constitutive Model for Predicting the Long-Term Behavior of an Expansive Clay Buffer in a Nuclear Waste Repository

“…The extension of BBM to a dual structure model enables simulating the behavior of expansive soils, such as the dependency of swelling strains and swelling pressures on the initial stress state and on the stress path, strain accumulation upon suction cycles and secondary swelling. It is believed that such behavioral features are mainly related to the existence of coupled chemical-hydrogeological-mechanical phenomena between distinct levels of structure within the material (Alonso et al, 1999).…”

“…Conceptually, in a dual structure model, as described by Alonso et al (1999) and Sánchez et al (2005), the total volume, …”

“…To rigorously model these characteristics of expansive soils, it is necessary to use a dual structure constitutive model that acknowledges the existence of two porosity levels (Gens and Alonso, 1992;Alonso et al, 1999;Gens et al, 2006;Sánchez et al, 2005;. Expansive clays present a bimodal or trimodal pore size distribution (Figure 1) in which the pores having a diameter smaller than 0.1 μm correspond to voids inside clay aggregates and the larger pores correspond to pores between clay aggregates (Dixon et al, 1999;Villar, 1999;Delage et al, 2006;Wang et al, 2013b).…”

“…In this section, the development and implementation of a dual structure model for expansive soils into TOUGH-FLAC is presented. First, an overview of the basic equations in the dual structure model following (in part) the developments by Alonso et al (1999) and Sánchez et al (2005) is presented. Finally, the implementation of this model into TOUGH-FLAC is summarized.…”