Abstract: The pore size structure of soil specimen can be divided into unimodal, bimodal and even multimodal pore size distribution. The soil specimens with bimodal pore size structure usually show bimodal soil-water characteristic curve (SWCC). Many existing bimodal SWCC models are obtained through the method of segmented or superimposed based on the unimodal SWCC model. It is often difficult to accurately determine the relevant parameters at the intersection of two SWCCs, and the influence of void ratio on water characteristics of bimodal soil is seldom considered. In this paper, the characteristic of unimodal SWCCs and bimodal SWCCs with different initial void ratios in a wide suction range are analyzed and discussed in detail. Secondly, based on the above analysis of soil water characteristics and the two different mechanisms of adsorbed water and capillary water, the adsorption term equation is derived in the form of exponential function with the combination of Kelvin equation, and the capillary term equation is based on the modified Fredlund and Xing SWCC model. Finally, a bimodal SWCC model considering the effect of initial void ratio is proposed, and the proposed model is applicable to various soil types ranging from sands to silt and clayey soils.

Key words: unsaturated soil, soil-water characteristic curve, unimodality and bimodality, initial void ratio