Soil contains four primary components, for engineering purposes:
- gravel
- sand
- silt
- clay
Organic material can be present in surface samples as well but this has no relevance to engineering applications.
Gravel and sand are classified as coarse-grained, whereas silt and clay are classified as fine-grained.
Various classification schemes exist which attempt to define the grain size distribution of each of the four components. A summary table of these classifications is below:
| System | Date | Sizes (mm) | |||
|---|---|---|---|---|---|
| gravel | sand | silt | clay | ||
| Bureau of Soils | 1890 | 1 – 100 | 0.05 – 1 | 0.005 – 0.05 | < 0.005 |
| Atterberg | 1905 | 2 – 100 | 0.2 – 2 | 0.002 – 0.2 | < 0.002 |
| MIT | 1931 | 2 – 100 | 0.06 – 2 | 0.002 – 0.06 | < 0.002 |
| USDA | 1938 | 2 – 100 | 0.05 – 2 | 0.002 – 0.05 | < 0.002 |
| USCS | 1953 | 4.75 – 75 | 0.075 – 4.75 | < 0.075 | |
| ASTM | 1967 | 4.75 – 75 | 0.075 – 4.75 | < 0.075 | |
| AASHTO | 1970 | 2 – 75 | 0.075 – 2 | 0.002 – 0.075 | 0.001 – 0.002 |
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