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.