Theoretical assessment of fabric and permeability changes in clays affected by organic contaminants

Abstract

Permeability of clay is known, from laboratory tests, to increase over four orders of magnitude during low effective stress permeation with some concentrated organic liquids. On an engineering scale, in a one-dimensional purely advective flow through a finite layer, such evolution is shown numerically to cause a substantial acceleration of the progress of the contamination front, compared with a constant permeability case. For a 100-fold increase in intrinsic permeability for a 100% replacement of pore water with an organic, a two times shorter advective transit time is predicted. A series of quantitative fabric models based on interpretations and concepts proposed in the past are reformulated in quantitative terms. These models are associated with adsorbed water withdrawal, particle migration, and deposition leading to pore clogging, changes in connectivity or flocculation. Permeability is derived from the flow through a simplified structure formed by dominant pores in the actual pore size distribution. The formulation is applied to interpret previously published experimental data. It is found that a series connection of the dominant pores must be postulated rather than a parallel one, with the smaller pore acting as a bottle neck. A simple pore enlargement model is found not to be realistic. The numerically most consistent results are obtained for the flocculation model involving a particle rotation generating wedge-shaped channels. Resume : On sait, sur la base d'essais de laboratoire, que la permeabilite d'une argile peut etre augmentee de quatre ordres de grandeur pendant la permeation, de certains liquides organiques concentres sous contraintes effectives basses. A l'echelle de l'ingenieur, si on considere un ecoulement unidimensionnel purement advectif a travers une couche d'epaisseur finie, on peut montrer numeriquement qu'une telle evolution cause une acceleration substantielle de l'avancee du front de contamination, compare au cas d'une permeabilite constante. Pour une augmentation de permeabilite intrinseque de 100 fois avec