The Unified Soil Classification System is a method of categorizing soils for use in engineering and geological applications. Soils are classified in USCS groups that are designated by a group symbol and a corresponding group name. The governing standard is ASTM D2487.
Classification Groups
The following are the soil classifications:
| Symbol | Description |
|---|---|
| GW | Well graded, sandy gravel |
| GP | Gap graded or uniform gravel |
| GM | Silty gravel |
| GC | Clayey gravel |
| SW | Well graded gravelly sand |
| SP | Gap graded or uniform sand |
| SM | Gravelly sand |
| SC | Clayey sand |
| ML | Low plasticity silt |
| CL | Low plasticity clay |
| OL | Low plasticity organic silt/clay |
| MH | High plasticity silt |
| CL | High plasticity clay |
| OH | High plasticity organic silt/clay |
| Pt | Peat |
How to Determine USCS Classification
The USCS classification of a soil is determine via sieve tests and plasticity tests, and the following chart. Start at the left and proceed to the right.
| Major Divisions | |||
|---|---|---|---|
| Course-Grained Soils More than 50% retained on the 0.075 mm (No. 200) sieve |
Gravels 50% or more of course fraction retained on the 4.75 mm (No. 4) sieve |
Clean Gravels | GW |
| GP | |||
| Gravels with Fines | GM | ||
| GC | |||
| Sands 50% or more of course fraction passes the 4.75 (No. 4) sieve |
Clean Sands | SW | |
| SP | |||
| Sands with Fines | SM | ||
| SC | |||
| Fine-grained Soils More than 50% passes the 0.075 mm (no. 200) sieve |
Silts and Clays Liquid Limit 50% or less |
ML | |
| CL | |||
| OL | |||
| Silts and Clays Liquid Limit greater than 50% |
MH | ||
| CH | |||
| OH | |||
| Highly Organic Soils | PT | ||
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