Gradation is defined as the distribution of granual material such as crushed stone and sand on geometric series of sieve sizes: i.e., each successive sieve openning is approximately double the size of the previous one. Ideally, the distribution of graine sizes should be such that the smaller grains fit into the intersticies between the larger grains, and thus provide a mass that has minimum porosity. Such a distribution is considered to be "well graded"
When cumulative percentages of material passing each sieve are plotted against sieve size, a curve of a form of a large open figure "S" results: the central part of the curve is linear, with ends flattened to simulate "S".
The flatter the curve, the more well graded is the material. In practice, the gradation of the material can be adjusted