Predicting Unsaturated Soil Moisture Diffusivity Coefficient Using Slope of Swcc and Saturated Permeability Coefficient

Abstract

The diffusion coefficient is a crucial parameter which is used to evaluate the moisture movement in unsaturated soils. The diffusion coefficient can be measured in laboratory on tube soil specimens subjected to certain boundary conditions using thermocouple psychrometers. However, the testing range of thermocouple psychrometer is restricted from about 3.5 pF to 4.5 pF, while the suction range in engineering practice is much wider. This thesis presents a wider suction range estimation of diffusion coefficient determined by using slope SWCC and saturated permeability. In this research, diffusion coefficients of two clayey soils with different plastic limits were tested by using both methods. Then, the estimated diffusion coefficient at 3.5 to 4.5 pF is compared to the diffusion coefficient tested by thermocouple psychrometers. It turns out that thermocouple psychrometer result is lower than the estimated results from slope of SWCC and saturated permeability. For the low plastic clay soil, the thermocouple psychrometer testing result is 53.8% lower than the results from slope of the SWCC and saturated permeability. For the high plastic soil, the thermocouple psychrometer testing result is 12.5% lower than the result from slope of the SWCC and saturated permeability coefficient. In general, the diffusion coefficient is decreasing with the soil suction increasing. Further study on this area of using improved and advanced testing equipment might be useful to verify the wider ranges of diffusion coefficients.