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).
Related ๐ IS 1498 โ๏ธ Concrete cover ๐ Concrete cover ๐ Min cover for RCC ๐ง RCC design
Coarse-Grained Soils (>50% retained on 75ฮผ sieve)
Classified by grain size distribution (Cu, Cc)
Symbol Name Description Field ID Use / SBC GW Well-graded Gravel Cu > 4, Cc = 1โ3 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 = 1โ3 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 75ฮผ sieve)
Classified by Atterberg limits (LL, PL, PI) โ 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 โ settlement issues CH Clay (High Plasticity) LL > 50, above A-line Very stiff when dry, high shrink-swell, cracks Very poor โ 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 โ 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) โ blows per 300mm penetration
Symbol Name Description Field ID Use / SBC N < 2 Very Soft Clay qu < 25 kPa Squeezes between fingers, flows SBC: 50โ75 kN/mยฒ N = 2โ4 Soft Clay qu = 25โ50 kPa Easily moulded by finger pressure SBC: 75โ100 kN/mยฒ N = 4โ8 Medium Stiff Clay qu = 50โ100 kPa Moulded by strong finger pressure SBC: 100โ200 kN/mยฒ N = 8โ15 Stiff Clay qu = 100โ200 kPa Indented by thumb, cannot mould SBC: 200โ300 kN/mยฒ N = 15โ30 Very Stiff Clay qu = 200โ400 kPa Indented by thumb nail only SBC: 300โ440 kN/mยฒ N > 30 Hard Clay qu > 400 kPa Cannot indent with thumb nail SBC: > 440 kN/mยฒ N < 4 Very Loose Sand Relative density < 15% Collapses easily, very loose SBC: < 100 kN/mยฒ N = 4โ10 Loose Sand RD = 15โ35% Easy to dig, doesn't hold shape SBC: 100โ150 kN/mยฒ N = 10โ30 Medium Dense Sand RD = 35โ65% Requires effort to dig SBC: 150โ300 kN/mยฒ N = 30โ50 Dense Sand RD = 65โ85% Difficult to dig, resists penetration SBC: 300โ440 kN/mยฒ N > 50 Very Dense Sand RD > 85% Very hard to penetrate SBC: > 440 kN/mยฒ
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 โ 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 โ dark grey/black, musty smell Detect organic Grain Size Visual Test Visible grains = coarse; smooth = fine Gravel >4.75mm, Sand 0.075โ4.75mm, Fines <0.075mm Coarse vs Fine
Notes
โข IS 1498 classification requires both grain size distribution AND Atterberg limits for fine-grained soils
โข A-line equation: PI = 0.73 ร (LL โ 20). Above A-line = Clay, Below = Silt
โข Cu (Uniformity Coefficient) = D60/D10. Cc (Curvature Coefficient) = D30ยฒ/(D60 ร D10)
โข SPT N-values need correction for overburden pressure (CN) and energy ratio (CE)
โข In India, Black Cotton Soil (CH) is extremely common in Deccan Plateau โ high shrink-swell, needs special foundation treatment
โข Laterite/Murum (GM/GC) is common in western India โ good bearing capacity when compacted
โข Marine clay in coastal areas is highly compressible โ settlement is the governing criterion
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