An innovative modelling technique for bimodal soil water characteristic curve under wetting process

Accurate measurement and use of an appropriate soil-water characteristics curve (SWCC) is crucial because it underpins most other estimations and analyses in unsaturated soil mechanics. Since it is difficult to measure a complete SWCC in the laboratory without combining multiple test methods, mathematical equations are subsequently used to model the SWCC that covers the entire suction range (up to 10⁶ kPa). Studies have been carried out to develop mathematical equations for modelling drying bimodal SWCC. However, no equations have been developed to model bimodal SWCC under wetting process. Ignoring the wetting SWCC could lead to results which do not represent the actual condition of the soil, especially under water infiltration due to rainfall and snowmelt. This is coupled with the fact that all available best-fitting equations are either complex and cumbersome or have parameters lacking physical meaning in relation to the variables of SWCC. In this paper, a new mathematical equation is proposed to model the bimodal SWCC under wetting process. Several experimental datasets from published literature were used to assess the performance of the proposed equation. The results of the statistical analysis indicate that the coefficient of determination (R²) is close to 1.0 and root mean square error (RMSE) is approaching 0. In addition, an approach to measure hysteresis of SWCC using high suction polymer sensor (HSPS) has been developed, although this can be made using other methods such as pressure plate. However, this method is fast and can measure the bimodal SWCC under wetting process accurately, which is a novel method on the topic. It should be noted that all the parameters in the proposed model have physical meaning, and the proposed equation has its advantage in representation of the bimodal SWCC under wetting process.