Estimation Of Permeability Functions Research Articles

Effects of residual suction and residual water content on the estimation of permeability function

Soil-water characteristic curve (SWCC), which defines the relationship between water amount in the soil and matric suction, contains key information for the application of unsaturated soil mechanics principles to engineering practice. Best fit equations are commonly used for representation of the SWCC for unsaturated soils. Normally, these best fit equations are mathematically continuous and governed by a few fitting parameters. Either volumetric water content, θw or normalized volumetric water content, Θ, is adopted in the different best fit equations. If θw is used to establish SWCC, the parameter related to residual suction, Cr needs to be defined prior to fitting process of SWCC data. On the other hand, if Θ is used to develop SWCC, the residual volumetric water content, θr, needs to be defined prior to fitting process of SWCC data. Results of analyses in this study indicate that the performance of the best fit equation is not affected by the value of Cr, but it is significantly affected by the value of θr. As a result, the performance on the estimation of the permeability function is also affected by the value of θr. Different types of soils are used to investigate the effect of Cr and θr on the performance of the best fit equation and the estimation of the permeability function.

Estimation of permeability function from the soil–water characteristic curve

Because direct measurement is time-consuming and costly, the permeability function of unsaturated soil is commonly determined by estimation using the soil–water characteristic curve (SWCC). Various prediction models (i.e., indirect methods) for the permeability function have been proposed by different researchers. Mualem (1986) categorized these prediction models into three groups: empirical, macroscopic, and statistical models. Of these, the statistical model is the most rigorous and provides the most accurate results (Leong and Rahardjo 1997). In this paper, a new equation for the calculation of the permeability function is proposed in which the permeability function of unsaturated soil is correlated with the fitting parameters of SWCC. In other words, unsaturated hydraulic conductivity is computed using an electronic spreadsheet with input parameters (i.e., the fitting parameters of SWCC). The proposed equation is shown to be the general form of the equations by Marshall (1958) and Kunze et al. (1968). In the equation, suction is considered as a variable and SWCC, in the form of degree of saturation, S, is adopted as a probability function. Soil volume change is also incorporated in this equation. Lastly, the proposed equation is verified using experimental data from the literature.

Effect of range of soil–water characteristic curve measurements on estimation of permeability function

The most commonly used indirect method to determine unsaturated permeability of soil is to estimate the unsaturated permeability function from soil–water characteristic curve and saturated permeability. The suction range that soil–water characteristic curve (SWCC) can be measured in the laboratory depends on the type of equipment used for measurement. As the estimation models use the available measured SWCC data to obtain the unsaturated permeability function, the estimated permeability function can be affected by the number of measured data of SWCC and the suction range over which the SWCC data are measured. Therefore, there is a need to investigate the effect of the range of SWCC measurements on the estimation of unsaturated permeability function. In this study, the effect of the range of SWCC measurements is investigated through different estimation models that are based on four different best-fit soil–water characteristic curve equations and three different relative permeability equations. It was found that the range of SWCC measurements greatly affect the estimated permeability functions. It was also found that the effect of the range of SWCC measurements is more significant than the selected best-fit SWCC equation used.