Predicting Bimodal Soil–Water Characteristic Curves

Failure of colluvial soil deposits induced by rainfall is a common geohazard in the natural terrain. Many colluvial soils are widely and gap graded with a minimal sand fraction. In order to study the pore water pressures in such gap-graded, unsaturated soil deposits through a seepage analysis and to evaluate the stability of these soil deposits, the soil–water characteristic curves for gap-graded soils must be known. Usually, gap-graded soils exhibit bimodal grain-size and pore-size distributions. This technical note presents a theoretical continuum method for the determination of soil–water characteristic curves for soils with a bimodal or multimodal pore-size distribution. Based on the capillary law, the water content in a multimodal soil is equal to the sum of water stored in each pore series in the soil. Therefore, the bimodal or multimodal soil–water characteristic curves can be obtained by combining the unimodal soil–water characteristic curves for all components of the soil corresponding to the pore series weighted by the respective volumetric percentages. The proposed method is verified using experimental soil–water characteristics data of sand–diatomaceous earth mixtures with dual porosity.