Soil Classification

Any system of soil classification involves grouping the different soil types into categories that possess similar properties and, in so doing, providing the engineer with a systematic method of soil description. Casagrande (1948) advanced one of the first comprehensive engineering classifications of soil. In the Casagrande system, the coarse-grained soils are distinguished from the fine on the basis of particle size. Gravels and sands are the two principal types of coarse-grained soils and, in this classification, both are subdivided into five subgroups on the basis of grading (Table 5.1). Well-graded soils are those in which the particle size distribution extends over a wide range without excess or deficiency in any particular sizes, whereas in uniformly graded soils, the distribution extends over a very limited range of particle sizes. In poorly graded soils, the distribution contains an excess of some particle sizes and a deficiency of others. A plasticity chart is used when classifying fine-grained soils, that is, silts and clays (see Table 5.2).