SOIL CLASSIFICATION SCHEMES
Description:
A classifi cation scheme provides a method of identifying soils in a particular group that would likely exhibit similar characteristics. In Chapter 2, Section 5, you were introduced to how some of these schemes classify soils based on particle size only. Soil classifi cation is used to specify a certain soil type that is best suited for a given application. Also, it can be used to establish a soil profi le along a desired cross section of a soil mass. There are several classifi cation schemes available. Each was devised for a specifi c use.
For example, the American Association of State Highway and Transportation Offi cials (AASHTO) developed one scheme that classifi es soils according to their usefulness in roads and highways, while the Unifi ed Soil Classifi cation System (USCS) was originally developed for use in airfi eld construction but was later modifi ed for general use.
Unifi ed Soil Classifi cation System:
The USCS is neither too elaborate nor too simplistic. The USCS uses symbols for the particle size groups. These symbols and their representations are G—gravel, S—sand, M—silt, and C—clay. These are combined with other symbols expressing gradation characteristics—W for well graded and P for poorly graded—and plasticity characteristics—H for high and L for low, and a symbol, O, indicating the presence of organic material. A typical classifi cation of CL means a clay soil with low plasticity, while SP means a poorly graded sand. The fl owcharts shown in Figures 4.9a and 4.9b provide systematic means of classifying a soil according to the USCS.
Classifi cation System:
The American Society for Testing and Materials classifi cation system (ASTM-CS) is nearly identical to the USCS. ASTM-CS uses the same symbols as USCS but provides a better scheme for mixed soils, i.e., soils consisting of mixtures of, for example, sand, gravel, and clay. Soils are classifi ed by group symbols and group names. For example, we can have a soil with a group symbol, SW-SM, and group name, which describes the soil, as “well-graded sand with silt” if the gravel content is less than 15%. Flowcharts to classify soils based on the ASTM-CS are shown in Figures.
Unifi ed Soil Classifi cation System fl owchart for coarse-grained soils.
AASHTO Soil Classifi cation System:
The AASHTO soil classifi cation system is used to determine the suitability of soils for earthworks, embankments, and road bed materials (subgrade—natural material below a constructed pavement; subbase—a layer of soil above the subgrade; and base—a layer of soil above the subbase that offers high stability to distribute wheel loads). According to AASHTO, granular soils are soils in which 35% or less are fi ner than the No. 200 sieve (0.075 mm). Silt-clay soils are soils in which more than 35% are fi ner than the No. 200 sieve.