Water movement through a swelling material

Abstract

The effect of the drag of a fluid on the porous material through which it flows is examined for a swelling material. It is shown that, when equilibrium is established between drag and swelling in films of oriented sodium montmorillonite, there is a decrease in void ratio in the material in the direction of flow. This effect of drag increases with decreasing electrolyte concentration of the permeating solution, in accord with double-layer theory, and with increasing drop in hydrostatic pressure across the film. Hydraulic conductivity was found to increase with decreasing electrolyte concentration of the permeating solution, contrary to usual experience. The cause of this is considered to be the increase in spacing corresponding to decrease in concentration. Hydraulic conductivity of unconfined film at equilibrium was found to decrease with increasing drop in hydrostatic pressure across the film. An increase in the pressure drop causes a decrease in the spacing of the clay, particularly towards the base of the film, so that the hydraulic conductivity of the whole film is lowered. Consequently Darcy's law does not hold for these films. Implications for less ordered swelling systems are considered briefly. The effect of drag on soils under field conditions will ordinarily be negligible.