MOISTURE MOVEMENT IN A HORIZONTAL SOIL COLUMN UNDER THE INFLUENCE OF AN APPLIED PRESSURE

When moisture moves into a dry horizontal soil column under the influence of an applied pressure, there will be two distinct regions in the soil-water system: a saturated region and an unsaturated one. Both of these regions grow larger with time. A theoretical solution of the problem is obtained by combining Darcy's equation with a modified diffusion equation that accounts for the moving interface between the two regions. The nonlinear partial differential equation of diffusion is transformed to a nonlinear ordinary differential equation, and a numerical procedure for its solution is presented. A set of experiments was conducted at various applied pressures using silica flour as the soil sample. Moisture content as a function of time and position was determined by use of γ-ray attenuation equipment. The experimental results compare favorably with the results given by numerical procedures based on theory. It is also shown that an analysis based on the assumption of a soil fully saturated to the wetting front with capillary forces acting at the air-water interface gives very good results provided that the proper capillary suction is selected.