THEORETICAL ASSESSMENT OF FABRIC AND PERMEABILITY CHANGES IN CLAYS AFFECTED BY ORGANIC CONTAMINANTS

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-d imensional purely advective flow through a finite layer, such evolutio n is shown numerically to cause a substantial acceleration of the prog ress of the contamination front, compared with a constant permeability case. For a 100-fold increase in intrinsic permeability for a 100% re placement of pore water with an organic, a two times shorter advective transit time is predicted. A series of quantitative fabric models bas ed on interpretations and concepts proposed in the past are reformulat ed in quantitative terms. These models are associated with adsorbed wa ter withdrawal, particle migration, and deposition leading to pore clo gging, changes in connectivity or flocculation. Permeability is derive d from the flow through a simplified structure formed by dominant pore s in the actual pore size distribution. The formulation is applied to interpret previously published experimental data. It is found that a s eries connection of the dominant pores must be postulated rather than a parallel one, with the smaller pore acting as a bottle neck. A simpl e pore enlargement model is found not to be realistic. The numerically most consistent results are obtained for the flocculation model invol ving a particle rotation generating wedge-shaped channels.