# 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 (with the exception of Pt):
| First and/or second letters | Second letter |
| Letter Definition G gravel S sand M silt C clay O organic | Letter Definition P poorly graded (many particle of about the same size) W well-graded (many different particle sizes) 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 | Major divisions | Major divisions | Group symbol | Group name |
| Coarse grained soils more 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 |
| Coarse grained soils more 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 | GP | poorly graded gravel |
| Coarse grained soils more 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 | gravel with >12% fines | GM | silty gravel |
| Coarse grained soils more 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 | gravel with >12% fines | GC | clayey gravel |
| Coarse grained soils more than 50% retained on or above No.200 (0.075 mm) sieve | sand ≥ 50% of coarse fraction passes No.4 (4.75 mm) sieve | clean sand | SW | well-graded sand, fine to coarse sand |
| Coarse grained soils more than 50% retained on or above No.200 (0.075 mm) sieve | sand ≥ 50% of coarse fraction passes No.4 (4.75 mm) sieve | clean sand | SP | poorly graded sand |
| Coarse grained soils more than 50% retained on or above No.200 (0.075 mm) sieve | sand ≥ 50% of coarse fraction passes No.4 (4.75 mm) sieve | sand with >12% fines | SM | silty sand |
| Coarse grained soils more than 50% retained on or above No.200 (0.075 mm) sieve | sand ≥ 50% of coarse fraction passes No.4 (4.75 mm) sieve | sand with >12% fines | SC | clayey sand |
| Fine grained soils 50% or more passing the No.200 (0.075 mm) sieve | silt and clay liquid limit < 50 | inorganic | ML | silt |
| Fine grained soils 50% or more passing the No.200 (0.075 mm) sieve | silt and clay liquid limit < 50 | inorganic | CL | lean clay |
| Fine grained soils 50% or more passing the No.200 (0.075 mm) sieve | silt and clay liquid limit < 50 | organic | OL | organic silt, organic clay |
| Fine grained soils 50% or more passing the No.200 (0.075 mm) sieve | silt and clay liquid limit ≥ 50 | inorganic | MH | elastic silt |
| Fine grained soils 50% or more passing the No.200 (0.075 mm) sieve | silt and clay liquid limit ≥ 50 | inorganic | CH | fat clay |
| Fine grained soils 50% or more passing the No.200 (0.075 mm) sieve | silt and clay liquid limit ≥ 50 | organic | OH | organic clay, organic silt |
| Highly organic soils | Highly organic soils | Highly organic soils | PT | peat |

### ASTM D-2487

| Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests | Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests | Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests | Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests | Soil Classification | Soil Classification |
| Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests | Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests | Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests | Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests | 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 | Cu ≥ 4 and 1 ≤ Cc ≤ 3 | GW | Well-graded gravel |
| 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 | Cu < 4 and/or Cc < 1 or Cc > 3 | GP | Poorly graded gravel |
| COARSE-GRAINED SOILS More than 50% retained on No.200 Sieve | Gravels More than 50% of coarse fraction on No. 4 Sieve | Gravels with Fines More than 12% fines | Fines classify as ML or MH | GM | Silty Gravel |
| COARSE-GRAINED SOILS More than 50% retained on No.200 Sieve | Gravels More than 50% of coarse fraction on No. 4 Sieve | Gravels with Fines More than 12% fines | Fines classify as CL or CH | GC | Clayey gravel |
| COARSE-GRAINED SOILS More than 50% retained on No.200 Sieve | 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 |
| COARSE-GRAINED SOILS More than 50% retained on No.200 Sieve | Sands 50% or more of coarse fraction passes No.4 sieve | Clean Sands Less than 5% fines | Cu < 6 and/or Cc < 1 or Cc > 3 | SP | Poorly graded sand |
| COARSE-GRAINED SOILS More than 50% retained on No.200 Sieve | Sands 50% or more of coarse fraction passes No.4 sieve | Sands with Fines More than 12% fines | Fines classify as ML or MH | SM | Silty sand |
| COARSE-GRAINED SOILS More than 50% retained on No.200 Sieve | Sands 50% or more of coarse fraction passes No.4 sieve | Sands with Fines More than 12% fines | 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 |
| FINE-GRAINED SOILS 50% or more passes the no. 200 Sieve | Silts and Clays Liquid limit less than 50 | Inorganic | PI < 4 and plots below "A" line | ML | Silt |
| FINE-GRAINED SOILS 50% or more passes the no. 200 Sieve | Silts and Clays Liquid limit less than 50 | Organic | Liquid limit—oven dried < 0.75 | OL | Organic clay |
| FINE-GRAINED SOILS 50% or more passes the no. 200 Sieve | Silts and Clays Liquid limit less than 50 | Organic | Liquid limit—not dried | OL | Organic silt |
| FINE-GRAINED SOILS 50% or more passes the no. 200 Sieve | Silts and Clays Liquid limit 50 or more | Inorganic | PI plots on or above "A" line | CH | Fat clay |
| FINE-GRAINED SOILS 50% or more passes the no. 200 Sieve | Silts and Clays Liquid limit 50 or more | Inorganic | PI plots below "A" line | MH | Elastic silt |
| FINE-GRAINED SOILS 50% or more passes the no. 200 Sieve | Silts and Clays Liquid limit 50 or more | Organic | Liquid limit—oven dried < 0.75 | OH | Organic clay |
| FINE-GRAINED SOILS 50% or more passes the no. 200 Sieve | Silts and Clays Liquid limit 50 or more | Organic | Liquid limit - not dried | OH | Organic silt |
| HIGHLY ORGANIC SOILS | HIGHLY ORGANIC SOILS | HIGHLY ORGANIC SOILS | HIGHLY ORGANIC SOILS | PT | Peat |