Intrinsic and State Parameters Governing the Efficiency of Bentonite Barriers for Contaminant Control

Abstract

The osmotic, hydraulic and self-healing efficiency of bentonite based barriers (e.g. geosynthetic clay liners) for containment of polluting solutes are governed by both the chemico-physical intrinsic parameters of the bentonite, i.e. the solid density (ρ _sk ), the total specific surface (S), the fixed negative electric surface charge (σ), the Stern fraction (f _Stern ), and by the chemico-mechanical state parameters able to quantify the solid skeleton density and fabric, i.e. the total (e) and nano (e _n ) void ratio, the average number of platelets per tactoid (N _l,AV ), and the effective electric fixed-charge concentration ($\bar{c}_{sk,0}$). In turn, looking at saturated active clays only, the state parameters seem to be controlled by the effective stress history (SH), ionic valence (ν _i ) and related exposure sequence of salt concentrations in the pore solution (c _s ). A theoretical framework, able to describe chemical, hydraulic and mechanical behaviors of bentonites in the case of one-dimensional strain and flow fields, has been set up. In particular, the relationships, linking the aforementioned state and intrinsic parameters of a given bentonite with its hydraulic conductivity (k), effective diffusion coefficient ($D_{s}^{*}$), osmotic coefficient (ω) and swelling pressure (u _sw ) under different stress-histories and solute concentration sequences, are presented. The validity of the proposed theoretical hydro-chemico-mechanical framework has been tested by comparison of its predictions with some of the available experimental results on bentonites (i.e. hydraulic conductivity tests, swelling pressure tests and osmotic efficiency tests).