In soil science, soil gradation is a classification of a coarse-grained soil that ranks the soil based on the different particle sizes contained in the soil. Soil gradation is an important aspect of soil mechanics and geotechnical engineering because it is an indicator of other engineering properties such as compressibility, shear strength, and hydraulic conductivity. In a design, the gradation of the in situ (on site) soil often controls the design and ground water drainage of the site. A poorly graded soil will have better drainage than a well graded soil, if it is not high in clay quality. Soil is graded as either well graded or poorly graded. Soil gradation is determined by analyzing the results of a sieve analysis or a hydrometer analysis. The process for grading a soil is in accordance with either the Unified Soil Classification System or the AASHTO Soil Classification System. Gradation of a soil is determined by reading the grain size distribution curve produced from the results of laboratory tests on the soil. Gradation of a soil can also be determined by calculating the coefficient of uniformity, , and the coefficient of curvature, , of the soil and comparing the calculated values with published gradation limits. Soil gradations Soil gradation is a classification of the particle size distribution of a soil. Coarse-grained soils, mainly gravels or sands, are graded as either well graded or poorly graded. Poorly graded soils are further divided into uniformly-graded or gap-graded soils. Fine-grained soils, mainly silts and clays, are classified according to their Atterberg limits. Well graded A well-graded soil is a soil that contains particles of a wide range of sizes and has a good representation of all sizes from the No. 4 to No. 200 sieves. A well-graded gravel is classified as GW, while a well-graded sand is classified as SW. Poorly graded A poorly-graded soil is a soil that does not have a good representation of all sizes of particles from the no. 4 to no. 200 sieve. A poorly-graded gravel is classified as GP, while a poorly-graded sand is classified as SP. Poorly-graded soils are more susceptible to soil liquefaction than well-graded soils. A gap-graded soil is a soil that has an excess or deficiency of certain particle sizes or a soil that has at least one particle size missing. An example of a gap-graded soil is one in which sand of the no.