Predicting the permeability function for unsaturated soils using the soil-water characteristic curve

TLDR

Permeability of unsaturated soils is governed by pore‑size distribution and can be predicted from the soil‑water characteristic curve, which is described by a general equation valid over the full suction range. The authors use this general equation to derive a permeability function expressed as an integration of suction versus water content from zero to saturation. Using this approach eliminates the need for residual water content, and the resulting permeability predictions fit literature data excellently, allowing use of normalized water content or degree of saturation. Keywords: coefficient of permeability, soil‑water characteristic curve, unsaturated soil, water content, soil suction.

Abstract

The coefficient of permeability for an unsaturated soil is primarily determined by the pore-size distribution of the soil and can be predicted from the soil-water characteristic curve. A general equation, which describes the soil-water characteristic curve over the entire suction range (i.e., from 0 to 10 6 kPa), was proposed by the first two authors in another paper. This equation is used to predict the coefficient of permeability for unsaturated soils. By using this equation, an evaluation of the residual water content is no longer required in the prediction of the coefficient of permeability. The proposed permeability function is an integration form of the suction versus water content relationship. The proposed equation has been best fit with example data from the literature where both the soil-water characteristic curve and the coefficient of permeability were measured. The fit between the data and the theory was excellent. It was found that the integration can be done from zero water content to the saturated water content. Therefore, it is possible to use the normalized water content (volumetric or gravimetric) or the degree of saturation data versus suction in the prediction of the permeability function. Key words : coefficient of permeability, soil-water characteristic curve, unsaturated soil, water content, soil suction.