Improved prediction of water retention characteristic based on soil gradation and clay fraction

Abstract

As an important function for determining both the hydraulic and mechanical properties of soils, the soil–water characteristic curve (SWCC) based on the particle size distribution underestimates the water content at high suction head. In this study, the quantitative relationship between the clay content and residual pore fraction is established based on soil gradation parameters (uniformity coefficient Cu and curvature coefficient Cc); the van Genuchten equation is employed to describe the soil–water characteristic curve; and then a semi-physical and semi-statistical method based on clay content is proposed to predict water-holding of unsaturated soils, especially for the soil with higher clay contents or low water contents (high matric suctions). Eighty-seven soil samples containing nine texture classes from the unsaturated soil hydraulic database (UNSODA) are used to verify the proposed approach. Two traditional models proposed by Hwang et al. (2011) and Meskini-Vishkaee et al. (2014) are introduced to compare the predictive effect with the improved method. Although three methods all show good performance, the improved prediction method based on uniformity coefficient (Cu=d60/d10) has a better estimation accuracy than the two others according to statistical parameters, including root-mean-square error (RMSE), sum of square errors (SSE), and determination coefficient (R2). The ranking order of average uniformity coefficients in the three soil classes (sandy, silty, and clayey) agrees well with the ranking order of clay contents. Meanwhile, various dy/dx are used to characterize soil gradation and have similar goodness of fitting to residual pore fraction. In summary, the improved prediction model can effectively predict soil–water characteristic curves, especially for soil with low water contents (high matric suctions) or high clay contents. The model benefits the predictions of mechanical properties, permeability coefficient, shear strength and slope stability of unsaturated soil at low water contents.