Surface diffusion, or migration within the electrical double layer next to mineral surfaces, is often invoked as a significant contributor to the overall diffusion coefficient in compacted clays, particularly where model predictions underestimate measured diffusion coefficients. The potential for surface diffusion of Sr2+, Ca2+ and Na+ on three clays compacted to dry bulk densities of 1.25 and 1.60 Mg/m3 was examined. The clays were a bentonite, an illite/smectite, and a glacial lake clay (composed mainly of smectite, illite, kaolinite and quartz). The clays were saturated with a Na-Ca-Cl-dominated synthetic groundwater solution with an effective ionic strength of 220 mol/m3. Total intrinsic diffusion coefficients for the cations were determined from their steady-state flux through compacted clays, and apparent diffusion coefficients were obtained from the time-lag technique. Models of diffusive transport in compacted clays, based only on diffusion in the pore solution, adequately described the diffusion data for all clays and diffusants, and there was no need to invoke other transport mechanisms, like surface diffusion. The data indicate that surface diffusion is not a significant transport mechanism in compacted clays at least to a clay density of 1.60 Mg/m3.

Hydraulic conductivity tests were conducted on thirteen compacted clayey soils being used for compacted clay liners at landfills throughout the United States. The soils were prepared to various molding water contents and then compacted and permeated in the laboratory. Results of the tests show that for all of the soils, zones exist in the compaction plane (i.e., dry unit weight vs. water content) where the hydraulic conductivity is similar. These zones fall roughly parallel to contours of constant initial saturation (degree of saturation at compaction), with lower hydraulic conductivities generally occurring for conditions corresponding to higher initial saturation. Wet of the line of optimums, lower hydraulic conductivity is also attained for soils that are more plastic and have a greater quantity of fines. A regression equation was developed from the data to estimate hydraulic conductivity given the initial saturation, compactive effort, plasticity index, and clay content.

Two different approaches used to express the tortuosity of water pathways in a solid clay body are known: (1) a static approach based on microscopic measurement in thin sections; and (2) a dynamic approach based on ‘imbibition’ speed. Each is expressed by a corresponding equation. The second method has the following advantages over the first: (a) it enables the testing of a much larger volume than that observed in a common thin section; (b) it measures the effects of all particles, including the finest fragments barely visible under optical microscope which also have influence on hydraulic conductivity or the imbibition speed of water; (c) in the case of expansible clays, the magnitude of k (tortuosity of pathways including the shape of particles) can be compared with k for non-expansible clays using a non-polar liquid, e.g. tetraline; (d) the measurement of the imbibition speed into the clay aggregate through a constant area is automated. Specific properties and processes important for sealing clays are discussed in the connection with the dynamic approach.