Soil Structure
Coarse-Grained Soils
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.
Soil Types
8 types
Bearing Range
150-1,200 kPa
Coarse-Grained Soil Classification
ASTM D2487| 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 |
Design Notes
- • Bearing capacity: Values are allowable (FS=2-3); ultimate capacity 2-3×¹igher
- • Compaction: GW/SW compact to 95-100% maximum density (Proctor)
- • Drainage: Excellent for GW/GP/SW/SP; use as drainage layers
- • Frost: Generally non-frost-susceptible if fines <3%
Soil Structure
Fine-Grained Soils
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.
Soil Types
6 types
Bearing Range
25-250 kPa
Fine-Grained Soil Classification
ASTM D2487| 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 |
Design Notes
- • Consolidation: CL/CH require consolidation analysis; settlement over years
- • Shear strength: Undrained strength (Su) critical for short-term stability
- • Shrink-swell: CH soils expand when wet, shrink when dry - foundation damage risk
- • Improvement: May require preloading, surcharge, or soil stabilization
Soil Structure
Peat & Highly Organic
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.
Soil Types
2 types
Bearing Range
5-50 kPa
Peat & Organic Soils
ASTM D2487| 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 |
Design Notes
- • Remove: Best practice is complete excavation and replacement
- • Deep foundations: Piles must penetrate through peat to competent stratum
- • Settlement: Long-term consolidation (10-50+ years) under load
- • Ground improvement: Geotextiles, lightweight fill, or vertical drains
Soil Structure
Classification Parameters
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.
Soil Types
6 types
Bearing Range
N/A
Classification Parameters
ASTM D2487| 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 |
Design Notes
- • Sieve analysis: ASTM D6913 - determines grain size distribution
- • Atterberg limits: ASTM D4318 - LL (Casagrande cup), PL (thread rolling)
- • Plasticity chart: A-line separates clays (above) from silts (below)
- • Gradation: Well-graded = good interlocking; poorly-graded = uniform, higher permeability