Indian Standard soil classification system based on grain size and plasticity. Used for foundation design, earthwork, and geotechnical investigation. Similar to Unified Soil Classification System (USCS).
Coarse-Grained Soils (>50% retained on 75μ sieve)
Classified by grain size distribution (Cu, Cc)
| Symbol | Name | Description | Field ID | Use / SBC |
|---|
| GW | Well-graded Gravel | Cu > 4, Cc = 1–3 | Wide range of grain sizes, dense | Excellent foundation, fill, road base |
| GP | Poorly-graded Gravel | Does not meet GW criteria | Uniform-sized gravel, may have gap | Good foundation, drainage |
| GM | Silty Gravel | Fines are silty (below A-line) | Gravel with dusty fines, low plasticity | Fair to good foundation |
| GC | Clayey Gravel | Fines are clayey (above A-line) | Gravel with sticky fines | Fair foundation, impervious core |
| SW | Well-graded Sand | Cu > 6, Cc = 1–3 | Wide grain range, compacts well | Good foundation, fill |
| SP | Poorly-graded Sand | Does not meet SW criteria | Uniform sand, loose | Fair foundation, drainage |
| SM | Silty Sand | Fines are silty | Sandy with non-sticky fines | Fair foundation |
| SC | Clayey Sand | Fines are clayey | Sandy with sticky fines, moulds when wet | Fair foundation, embankment |
Fine-Grained Soils (>50% passing 75μ sieve)
Classified by Atterberg limits (LL, PL, PI) — use plasticity chart
| Symbol | Name | Description | Field ID | Use / SBC |
|---|
| ML | Silt (Low Plasticity) | LL < 35, below A-line | Slightly gritty, quick dilatancy, dry strength low | Poor foundation, not suitable for fill |
| MI | Silt (Medium Plasticity) | 35 < LL < 50, below A-line | Smooth, slow dilatancy | Poor foundation |
| MH | Silt (High Plasticity) | LL > 50, below A-line | Very smooth, soapy feel, no dilatancy | Very poor foundation |
| CL | Clay (Low Plasticity) | LL < 35, above A-line | Stiff when dry, medium strength | Fair to poor foundation |
| CI | Clay (Medium Plasticity) | 35 < LL < 50, above A-line | Stiff, medium dry strength, slow drying | Poor foundation — settlement issues |
| CH | Clay (High Plasticity) | LL > 50, above A-line | Very stiff when dry, high shrink-swell, cracks | Very poor — Black Cotton Soil |
| OL | Organic Silt (Low LL) | LL < 50, organic odour | Dark colour, organic smell, spongy | Not suitable for foundation |
| OH | Organic Clay (High LL) | LL > 50, organic | Very dark, organic smell, compressible | Not suitable — remove or bypass |
| Pt | Peat | Highly organic | Fibrous, spongy, very dark, lightweight | Must remove entirely |
SPT N-Value vs Soil Consistency
Standard Penetration Test (IS 2131) — blows per 300mm penetration
| Symbol | Name | Description | Field ID | Use / SBC |
|---|
| N < 2 | Very Soft Clay | qu < 25 kPa | Squeezes between fingers, flows | SBC: 50–75 kN/m² |
| N = 2–4 | Soft Clay | qu = 25–50 kPa | Easily moulded by finger pressure | SBC: 75–100 kN/m² |
| N = 4–8 | Medium Stiff Clay | qu = 50–100 kPa | Moulded by strong finger pressure | SBC: 100–200 kN/m² |
| N = 8–15 | Stiff Clay | qu = 100–200 kPa | Indented by thumb, cannot mould | SBC: 200–300 kN/m² |
| N = 15–30 | Very Stiff Clay | qu = 200–400 kPa | Indented by thumb nail only | SBC: 300–440 kN/m² |
| N > 30 | Hard Clay | qu > 400 kPa | Cannot indent with thumb nail | SBC: > 440 kN/m² |
| N < 4 | Very Loose Sand | Relative density < 15% | Collapses easily, very loose | SBC: < 100 kN/m² |
| N = 4–10 | Loose Sand | RD = 15–35% | Easy to dig, doesn't hold shape | SBC: 100–150 kN/m² |
| N = 10–30 | Medium Dense Sand | RD = 35–65% | Requires effort to dig | SBC: 150–300 kN/m² |
| N = 30–50 | Dense Sand | RD = 65–85% | Difficult to dig, resists penetration | SBC: 300–440 kN/m² |
| N > 50 | Very Dense Sand | RD > 85% | Very hard to penetrate | SBC: > 440 kN/m² |
Field Identification Tests (no lab needed)
Quick tests for soil identification at site
| Symbol | Name | Description | Field ID | Use / SBC |
|---|
| Dilatancy | Shake Test | Moist ball on palm, tap — water appears on surface | Quick reaction = Silt (ML), No reaction = Clay (CL/CH) | Silt vs Clay |
| Dry Strength | Dry Pat Test | Dry a soil pat and try to break it | Crumbles easily = Silt, Hard to break = Clay | Silt vs Clay |
| Thread Test | Plasticity Test | Roll soil into 3mm thread | Breaks at 3mm = PL reached. Many rerolls = high plasticity | Estimate PI |
| Colour/Odour | Organic Test | Dark colour + organic odour when heated | Organic soil — dark grey/black, musty smell | Detect organic |
| Grain Size | Visual Test | Visible grains = coarse; smooth = fine | Gravel >4.75mm, Sand 0.075–4.75mm, Fines <0.075mm | Coarse vs Fine |
Notes
• IS 1498 classification requires both grain size distribution AND Atterberg limits for fine-grained soils
• A-line equation: PI = 0.73 × (LL − 20). Above A-line = Clay, Below = Silt
• Cu (Uniformity Coefficient) = D60/D10. Cc (Curvature Coefficient) = D30²/(D60 × D10)
• SPT N-values need correction for overburden pressure (CN) and energy ratio (CE)
• In India, Black Cotton Soil (CH) is extremely common in Deccan Plateau — high shrink-swell, needs special foundation treatment
• Laterite/Murum (GM/GC) is common in western India — good bearing capacity when compacted
• Marine clay in coastal areas is highly compressible — settlement is the governing criterion
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