Prediction of the coefficient of permeability for unsaturated soil by considering the film flow

It has become a common practice that the hydraulic conductivity of unsaturated soils is estimated from the soil-water characteristic curve by using either empirical method or statistical method. The estimated results by the empirical method are dependent on the empirical parameters while those by the statistical method are determined based on the theory of statistics. As a result, the results by the statistical method are more reliable than those by the empirical method. It is observed that the hydroscopic water is misused as the capillary water in the conventional statistical method. Because of the adsorptive force, the water film (hydroscopic water) is attached around the soil particle, and it acts as the transferring medium for the water migration in soils. The flowing rate of the water in the adsorbed water film is a function of the thickness of the water film. Both the thickness of water film around a single soil particle and the flowing rate through the water film are firstly computed. Subsequently, the probability of the connecting between particles with different sizes is calculated by using the grain size distribution (GSD) data. Consequently, a new equation is proposed for the estimation of the hydraulic conductivity of unsaturated soils by considering the film flows in soils. The proposed method is verified using the experimental data from literatures. It is indicated that results by the newly proposed method provide better agreement with the experimental data as compared with those by the conventional statistical method.