Prediction of swelling pressure – dry density relationship for compacted Bentonites and Bentonite – sand mixtures

Abstract

The mechanical stability of compacted bentonites and bentonite-sand mixtures under isochoric conditions is widely studied due to their potential application in nuclear waste repositories in the form of buffer and backfill material. The knowledge of swelling pressure variation with compaction density for different bentonites and bentonite-sand mixtures is thus important for the design of such repositories. The available predictive models require the knowledge of surface properties of clay particles and complex interactions of montmorillonite clay particles with other particles during swelling process. Thus, the applicability of the existing diffuse double layer based theoretical models is limited due to the influence of non-montmorillonite minerals, non-clay content, and multi-valent exchangeable cations on the mechanical behavior. A predictive model based on the linearization of the swelling pressure data using a normalization-dry density was proposed in this paper. The influence of percentage of montmorillonite content and surface properties on the swelling pressure variation with density was captured by a single empirical parameter proposed in this study. The proposed model required a single experimentally measured swelling pressure data point of a given bentonite or bentonite-sand mixture in the dry density ranging between 1.45 – 1.7 Mg/m³ for establishing the swelling pressure curves over a wide range of dry densities. The model does not require the knowledge of the surface properties, unlike the theoretical models. The predictive model was successfully validated on the laboratory measured data of several bentonites and bentonite – sand mixtures from the present and literature studies. Furthermore, its ability to generate swelling pressure curves over a wide dry density range enhances its versatility and utility in engineering applications such as designing nuclear waste repositories.