Comparison between the Permeability and Diffusivity Function Derived from Different Methods of Unsaturated Expansive Soils

Permeability and diffusivity function of expansive subgrade soils are critical parameters in the design of pavements. They govern the moisture flow in and out of the soil and thus the amount of movement the subgrade experiences. Several models have been proposed to estimate the both functions from the soil water characteristic curve (SWCC) and the saturated coefficient of permeability. However, these methods reveal a noticeable difference in the determination of both functions. The permeability coefficient can also be measured from the diffusion coefficient and the slope of the SWCC, or experimentally by the instantaneous profile method. In this paper, a comparison study in the determination of both parameters from the most frequently used SWCC models, diffusivity test, and instantaneous profile method is carried out. It is found that SWCC models reveal significantly less measurements than those derived from the other two methods. However, the results from the diffusivity test manifest good agreement with those derived from the instantaneous profile method.