The properties of the coarse-grained soil are significantly affected by its gradation. The gradation of the soil is usually descripted by the particle size distribution curve (PSD)

and the gradation equation is a quantitative description for the PSD. A continuous gradation equation was developed in this paper

and its applicability was verified by many PSDs of the rockfill material

transition material and filter material used in earth-rock dams. Furthermore

the range of gradation parameters of the coarse-grained soil was summarized

and the applicability of the gradation equation was analyzed. The main conclusions are as follows: The commonly used gradation parameters of coarse-grained soils are in the range of -2