Determination of basic parameters of unimodal and bimodal soil water characteristic curves

The determination of basic parameters (such as air-entry value, residual suction, slope at transition zone, etc.) of soil-water characteristic curve (SWCC) is the most fundamental for predicting the strength, permeability and constitutive relation of unsaturated soils. In this paper a method is proposed to determine the basic parameters of unimodal and bimodal SWCCs based on the modified Fredlund and Xing’s fitting equation. Firstly, the parameters of the fitting equation can be obtained by the optimal fitting of experimental data. For the bimodal SWCC, the piecewise fitting method is adopted based on the pore size distribution characteristics of soils. Secondly, the slopes and equations of tangent lines are obtained in the transition and residual zones based on the geometric relationship of the fitting equation. Furthermore, the air-entry value and residual suction of SWCCs are determined by the geometric relationship of tangent lines. Finally, the proposed method is validated to be feasible by use of the experimental data of unimodal and bimodal SWCCs.