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Clayton Group Services UNIFIED SOIL CLASSIFICATION SOP -12 Soils from borings and other field work need to be identified and classified, generally according to the Unified Soil Classification System (USCS). The system classifies soils based on their grain size (coarse-grained, fine-grained, or highly organic) and subdivided based on soil properties that generally are measured in the laboratory, but often can be determined in the field. Soils are classified according the attached chart. Soils are represented by a letter symbol: G (Gravels) and S Sands) are the coarse-grained soil symbols; M (Silts), C (Clays), and O (Organic silts or clays) are the fine-grained symbols; and, Pt represents the highly organic soils. These major group divisions are modified by a second letter: W (Well -sorted), P (Poorly sorted), M (Silty), and C (Clayey). Silts and Clays are modified by L (Liquid limit <50) and H (Liquid limit >50) as described below under Atterberg Limits. The boundary between coarse-grained soils and fine-grained soils is placed at the No. 200 sieve (0.003 -inch) which is the particle size visible to the naked eye. If more than 50% of the soil is retained on the No. 200 sieve, it is coarse-grained. C M —wi nEw'".. lei II.Y�71 /ly Coarse-grained soils are either gravelly or sandy. Gravel -sized particles range in size from 3 inches to 1/5 inch (No. 4 sieve). Particles greater than 3 inches are not classified in the USCS, but their presence should be noted in the field logbook and on the boring log. Sand -sized particles are between 1/5 inch (No. 4 sieve) and 3/1000 inch (No. 200 sieve). Sands and gravels are further subdivided based on the percentage of fine particles by volume and the degree of grading. Approximately equal amounts of all grain sizes indicate a well -graded soil (or poorly sorted). If a certain particle size dominates or fi the total size range is small (3 or fewer sieve sizes), the soil is classified as poorly graded (well sorted). Well -graded (GW or SW) or poorly -graded (GP or SP) symbols are used only when fines are less than 5% of the total volume. The symbols for clayey fines (GC or SC) and silty fines (GM or SM) are used when the clay or silt exceeds 12% of the total volume. If the fines are between 5 and 12%, a dual classification is used.