Unified Soil Classification System

The Unified Soil Classification System (USCS) is a soil classification system used in engineering and geology to describe the texture and grain size of a soil. The classification system can be applied to most unconsolidated materials, and is represented by a two-letter symbol. Each letter is described below:

First and/or second letters Second letter Letter Definition G gravel S sand M silt C clay O organic PT peat Letter Definition P poorly graded (many different particle sizes) W well-graded (many particle of about the same size) H high liquid limit L low liquid limit

If the soil has 5–12% by weight of fines passing a #200 sieve (5% < P#200 < 12%), both grain size distribution and plasticity have a significant effect on the engineering properties of the soil, and dual notation may be used for the group symbol. For example, GW-GM corresponds to "well-graded gravel with silt."

If the soil has more than 15% by weight retained on a #4 sieve (R#4 > 15%), there is a significant amount of gravel, and the suffix "with gravel" may be added to the group name, but the group symbol does not change. For example, SP-SM could refer to "poorly graded SAND with silt" or "poorly graded SAND with silt and gravel."

Symbol chart Major divisions Group symbol Group name Coarse grained soilsmore than 50% retained on or above No.200 (0.075 mm) sieve gravel> 50% of coarse fraction retained on No.4 (4.75 mm) sieve clean gravel <5% smaller than No.200 Sieve GW well-graded gravel, fine to coarse gravel GP poorly graded gravel gravel with >12% fines GM silty gravel GC clayey gravel sand≥ 50% of coarse fraction passes No.4 (4.75 mm) sieve clean sand SW well-graded sand, fine to coarse sand SP poorly graded sand sand with >12% fines SM silty sand SC clayey sand Fine grained soils50% or more passing the No.200 (0.075 mm) sieve silt and clayliquid limit < 50 inorganic ML silt CL lean clay organic OL organic silt, organic clay silt and clayliquid limit ≥ 50 inorganic MH elastic silt CH fat clay organic OH organic clay, organic silt Highly organic soils PT peat

ASTM D-2487

Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests Soil Classification Group Symbol Group Name COARSE-GRAINED SOILS More than 50% retained on No.200 Sieve Gravels More than 50% of coarse fraction on No. 4 Sieve Clean Gravels Less than 5% fines ≥ 4 and 1 ≤ ≤ 3 GW Well-graded gravel Cu < 4 and/or Cc < 1 or Cc > 3 GP Poorly graded gravel Gravels with Fines More than 12% fines Fines classify as ML or MH GM Silty Gravel Fines classify as CL or CH GC Clayey gravel Sands 50% or more of coarse fraction passes No.4 sieve Clean Sands Less than 5% fines Cu ≥ 6 and 1 ≤ Cc ≤ 3 SW Well-graded sand Cu < 6 and/or Cc < 1 or Cc > 3 SP Poorly graded sand Sands with Fines More than 12% fines Fines classify as ML or MH SM Silty sand Fines classify as CL or CH SC Clayey sand FINE-GRAINED SOILS 50% or more passes the no. 200 Sieve Silts and Clays Liquid limit less than 50 Inorganic PI > 7 and plots on or above "A" line CL Lean clay PI < 4 and plots below "A" line ML Silt Organic Liquid limit—oven dried < 0.75 OL Organic clay Liquid limit—not dried OL Organic silt Silts and Clays Liquid limit 50 or more Inorganic PI plots on or above "A" line CH Fat clay PI plots below "A" line MH Elastic silt Organic Liquid limit—oven dried < 0.75 OH Organic clay Liquid limit - not dried OH Organic silt HIGHLY ORGANIC SOILS PT PeatASTM Standard D2487, 2000, "Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)," ASTM International, West Conshohocken, PA, 2000, DOI: 10.1520/D2487-00, www.astm.org.

Unified Soil Classification System

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