G or S prefix
Soils with more than 50% retained on No. 200 sieve (0.075mm). Includes gravels and sands. Generally good bearing capacity and drainage characteristics. Classified by grain size distribution and plasticity.
| USCS | Soil Name | Bearing (kPa) | Settlement | Notes |
|---|---|---|---|---|
| GW | Well-graded Gravel | 600-1,200 | Low | Excellent foundation material, Cu>4, 1<Cc<3 |
| GP | Poorly-graded Gravel | 500-1,000 | Low | Good drainage, uniform size distribution |
| GM | Silty Gravel | 400-800 | Low-Medium | Fines content 5-12%, reduced permeability |
| GC | Clayey Gravel | 300-600 | Medium | Plastic fines, lower permeability |
| SW | Well-graded Sand | 400-800 | Low | Good foundation, Cu>6, 1<Cc<3 |
| SP | Poorly-graded Sand | 300-600 | Low | Uniform sand, good drainage |
| SM | Silty Sand | 200-400 | Medium | Fines reduce drainage, some plasticity |
| SC | Clayey Sand | 150-300 | Medium-High | Plastic fines, reduced strength when wet |
M or C prefix
Soils with more than 50% passing No. 200 sieve. Includes silts and clays. Behavior governed by plasticity and water content. Bearing capacity highly variable; settlement concerns common.
| USCS | Soil Name | Bearing (kPa) | Settlement | Notes |
|---|---|---|---|---|
| ML | Low Plasticity Silt | 100-200 | Medium-High | Non-plastic fines, frost susceptible |
| CL | Low Plasticity Clay | 100-250 | Medium | Lean clay, moderate compressibility |
| OL | Organic Silt/Clay | 50-150 | High | Organic content, high compressibility |
| MH | High Plasticity Silt | 75-175 | High | Elastic silt, micaceous soils |
| CH | High Plasticity Clay | 75-200 | Very High | Fat clay, high shrink-swell potential |
| OH | Organic High Plasticity | 25-100 | Very High | Peat, muck - unsuitable for foundations |
Pt
Highly organic soils with fibrous or amorphous structure. Extremely high compressibility and very low bearing capacity. Generally unsuitable for construction without ground improvement or deep foundations.
| USCS | Soil Name | Bearing (kPa) | Settlement | Notes |
|---|---|---|---|---|
| Pt | Peat | 10-50 | Extreme | Fibrous organic matter, very high water content |
| Pt | Muck | 5-25 | Extreme | Decomposed organic, semi-liquid consistency |
Various
Key parameters used in USCS classification. Sieve analysis determines coarse vs. fine; Atterberg limits (LL, PL) determine plasticity; coefficients (Cu, Cc) determine gradation quality.
| Parameter | Description | Formula/Value | Notes |
|---|---|---|---|
| No. 4 (4.75mm) | Gravel/Sand boundary | - | % retained = gravel; % passing = sand/fines |
| No. 200 (0.075mm) | Sand/Fines boundary | - | >50% passing = fine-grained soil |
| LL (Liquid Limit) | Plasticity indicator | - | LL<50 = low plasticity; LL>50 = high |
| PI (Plasticity Index) | PI = LL - PL | - | A-line: PI = 0.73(LL-20) separates CL/ML |
| Cu (Uniformity) | Cu = D60/D10 | - | Cu>6 (sand) or >4 (gravel) = well-graded |
| Cc (Curvature) | Cc = (D30)2/(D10×¹0) | - | 1<Cc<3 with high Cu = well-graded |