Prediction of Unsaturated Soil Diffusivity Coefficient from SWCC and Saturated Permeability Tests Yi Tian 1 and Rifat Bulut 2 1 Graduate Student, School of Civil and Environmental Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, USA 2 Associate Professor, School of Civil and Environmental Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, USA; rifat.bulut@okstate.edu ABSTRACT: The diffusion coefficient is an important unsaturated soil parameter for the estimation of the moisture movement in the soil mass. The diffusion coefficient can be measured in laboratory on Shelby tube soil specimens subjected to specified boundary conditions using thermocouple psychrometers. This parameter can also be estimated by using the soil water characteristic curve (SWCC) and saturated permeability coefficient of the soil. This research paper presents the results of the SWCC and permeability tests on an unsaturated soil and predicts the diffusion coefficient. While there is significant difference between the diffusivity parameters obtained using the SWCC and permeability test results, the difference between the column drying test and the SWCC and permeability test results is substantial. The diffusivity coefficients obtained from the psychrometer is 10 3 times higher than those from the SWCC and permeability tests. INTRODUCTION The unsaturated soil diffusion coefficient is widely utilized for the estimation of the moisture movement in the soil mass. The moisture through unsaturated soil is driven by the total suction gradient from low total suction regions to the high suction regions (Mitchell 1979). The unsaturated diffusion coefficient describes moisture flow within a soil mass at certain boundary conditions. The common methods to determine diffusion coefficient in the laboratory include horizontal infiltration method by Bruce and Klute (1956) and the Mitchell (1979) method which is based on measuring the suction profiles with time using thermocouple psychrometers. Mitchell (1979) performed drying and wetting laboratory tests to determine the unsaturated diffusion coefficient based on suction measurement of a soil column. Studies performed by Lytton et al. (2005) and Mabirizi and Bulut (2010) improved the drying and wetting diffusion coefficient measurements using thermocouple psychrometers. However, based on the Mitchell (1979) study, the diffusion coefficient also can be determined using the slope of SWCC and coefficient of saturated Page 1