Soils with Bimodal Soil-Water Characteristic Curve

Soil-water characteristic curve (SWCC) describes the relationship between water content and suction of a soil. The common shape of the SWCC is sigmoid and is unimodal. Recently, a number of soils have been found to exhibit bimodal SWCC. Hence, commonly used SWCC equations for unimodal SWCC are not applicable to bimodal SWCC. Several SWCC equations have been proposed for bimodal SWCCs. However, to use these bimodal SWCC equations, it must first be established that a soil has a bimodal SWCC. Many hypotheses have been advanced to explain the occurrence of the bimodal SWCC. Generally, it is accepted that the cause of bimodal SWCC is the existence of both macropores and micropores in the soil. Generally, a bimodal grain size distribution is a pre-requisite for a bimodal SWCC but not all soils with a bimodal grain size distribution (bimodal soils) have a bimodal SWCC. In this paper, two methods of identifying the existence of bimodal SWCC in bimodal soils were investigated. One method uses a classification tree and the other method uses a neural network. The classification tree was found to perform better than the neural network and is simpler to implement. Hence the classification tree is proposed as the method to identify the existence of bimodal SWCC in bimodal soils.