An equation to represent grain-size distribution

The grain-size distribution is commonly used for soil classification; howev er, there is also potential to use the grain-size distribution as a basis f or estimating soil behaviour. For example, much emphasis has recently been placed on the estimation of the soil-water characteristic curve. Many metho ds proposed in the literature use the grain-size distribution as a starting point to estimate the soil-water characteristic curve. Two mathematical fo rms are presented to represent grain-size distribution curves, namely, a un imodal form and a bimodal form. The proposed equations provide methods for accurately representing uniform, well-graded soils, and gap-graded soils. T he five-parameter unimodal equation provides a closer fit than previous two -parameter, log-normal equations used to fit uniform and well-graded soils. The unimodal equation also improves representation of the silt- and clay-s ized portions of the grain-size distribution curve.