Hydro-mechanical analysis of expansive clays : constitutive and numerical modelling.

Abstract

Bentonite-based materials are being currently considered in several countries as a backfill component in the multi-barrier concept for deep geological disposal of radioactive waste. The bentonite barrier fulfils several important functions: i) high swelling capacity to fill gaps and compress the excavation damaged zone and ii) very low hydraulic conductivity and important retention capacity which retards significantly radionuclides transport. Small-scale testing in geotechnical laboratories and in-situ experiments in underground research laboratories (URL) have demonstrated that initial state, water supply conditions and volume constrictions are the main aspects affecting the behaviour of bentonites. In this context, the main objective of the present study is the numerical simulation of the hydro-mechanical behaviour of expansive clays. For this purpose, a constitutive model has been developed to characterise the bentonite-based materials. The modelling of these materials is a quite challenging task. They exhibit a marked double-porosity system in which the swelling/shrinkage mechanism occurs at clay aggregate level and the collapsible behaviour comes from granular-like skeleton formed by the aggregates. In addition, several material configuration, with even more intricate fabric, have been proposed for the emplacement works of seals and plugs. The explicit consideration of two structural levels for the constitutive model seems to be suitable. Mechanical interaction and water mass exchanges between them can explain the short- and long-term behaviour. The model has been formulated using concepts of elasto-plasticity for strain hardening materials and generalized plasticity theory. The formulation has been implemented in the finite element code program CODE-BRIGHT and has been used to solve a variety of problems. The results provide relevant insights into the hydro-mechanical behaviour of double structure porous media, and they indicated the main aspects affecting the responses of expansive barriers. In particular, the relevance of the structural levels interaction has been demonstrated.