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 (ρsk), the total specific surface (S), the fixed negative electric surface charge (σ), the Stern fraction (fStern), and by the chemico-mechanical state parameters able to quantify the solid skeleton density and fabric, i.e. the total (e) and nano (en) void ratio, the average number of platelets per tactoid (Nl, AV), and the effective electric fixed-charge concentration (csk,0). In turn, looking at saturated active clays only, the state parameters seem to be controlled by the effective stress history (SH), ionic valence (vi) and related exposure sequence of salt concentrations in the pore solution (cs). 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 (usw) 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).