![ To classify a soil using AASHTO, you typically need:
• Grain size analysis
• Liquid Limit (LL)
• Plasticity Index (PI)
Group Index (GI):
Used to further evaluate the quality of fine-grained soils (A-2 to A-7):
GI=(F−35)[0.2+0.005(LL−40)]+0.01(F−15)(PI−10)
Where:
F = % passing No. 200 sieve
LL = Liquid Limit
PI = Plasticity Index
GI = 0 for good subgrade soil
GI > 20 for poor subgrade soil](https://image.slidesharecdn.com/classificationofsoil-250612050331-dbf16896/85/Classification-of-soils-Soil-Mechanics-19-320.jpg)
Classification of soils: Soil Mechanics
- 1. Soil Identifications and Classifications Er. Madhav Prasad Guragain Pathivara Engineering College Purbanchal University
- 2. Soil classification organizes different soil types into groups based on their engineering or agricultural properties. This helps determine their suitability for construction projects like dams, highways, or foundations. Classification Systems for Engineering Purposes Particle Size Classification Textural Classification Highway Research Board (HRB) Classification Unified Soil Classification System (USCS) Indian Standard Classification System (ISCS)
- 3. 1. Particle Size Classification Soils are classified based on grain size. Terms like gravel, sand, silt, and clay describe particle sizes, not natural soil types. Common classification systems: U.S. Bureau of Soil and Public Road Administration (PRA) International Soil Classification M.I.T. Classification Indian Standard Classification (IS: 1948–1970)
- 5. 2. Textural Classification Based on percentages of sand, silt, and clay in a soil sample. Best suited for coarse-grained soils (clay properties depend less on particle size). Triangular Classification Chart (U.S. PRA) is commonly used. How to Use the Chart: Determine percentages of sand, silt, and clay. Draw lines parallel to the triangle’s sides based on these percentages. The intersection point identifies the soil type (e.g., Clay, Sandy Clay, Loam). Example: 30% sand, 30% silt, 40% clay Classified as Clay. →
- 7. 3. Unified Soil Classification System (USCS) Soils are divided into 4 major groups: Coarse-grained (retained on No. 200 sieve 75 Micron) Fine-grained (passes No. 200 sieve=75 Micron) Organic soils (Moderate (20–75%) organic matter mixed with mineral soil) Peat (Very high (>75%) organic matter, almost entirely decomposed vegetation) Group Symbols (Prefix + Suffix) Prefix Meaning Suffix Meaning G Gravel W Well-graded S Sand P Poorly-graded M Silt C Clayey fines C Clay L Low plasticity (wL < 50%) O Organic H High plasticity (wL > 50%) Pt Peat
- 8. Classification Criteria: Coarse-grained (>50% retained on No. 200 sieve): Gravel (G) if >50% coarse fraction is >4.75 mm. Sand (S) if >50% coarse fraction is <4.75 mm. Fines content: <5%: GW, SW, GP, SP >12%: GM, GC, SM, SC 5–12%: Dual symbols (e.g., GW-GM) Fine-grained (>50% passes No. 200 sieve): Classified using Plasticity Chart (Fig 4.3). A-line equation: IP = 0.73(wL – 20) separates clays (above) from silts/organics (below). Subdivided by wL: Low plasticity (L): wL < 50% High plasticity (H): wL > 50%
- 12. 4. Indian Standard Classification System (ISCS) Based on USCS but with modifications for fine-grained soils. Key Differences from USCS: Fine-grained soils divided into 3 groups (vs. 2 in USCS): Low compressibility (L): wL < 35 Medium compressibility (I): 35 ≤ wL ≤ 50 High compressibility (H): wL > 50 Soil Groups in ISCS: Coarse-grained (>50% >75 µm) Gravel (G) or Sand (S) based on 4.75 mm sieve. Subgroups: W (Well-graded), C (Clayey), P (Poorly-graded). Fine-grained (>50% <75 µm) Inorganic silts (M), Clays (C), Organics (O). Classified using Plasticity Chart (Fig 4.4). Highly organic soils (Peat, etc.)
- 17. Majority of Indian Black cotton soils lie along a band above the A-line. The plot of some of the blackcotton soils is also found to lie below the A-line. Care should be taken in classifying such soils.
- 18. 5. AASTHO Classification it is a widely used system in civil engineering to classify soils based on their suitability for highway subgrade, subbase, and base construction. AASHTO stands for American Association of State Highway and Transportation Officials. Soils are classified into seven major groups: A-1 to A-7, with A-1 being the best for road construction and A-7 the worst. Group Description Typical Soil Type A-1 Best quality granular materials Gravel and sand with good gradation A-2 Good quality granular materials Silty or clayey gravel/sand A-3 Fine sand with limited silt or clay Clean fine sand A-4 Fair to poor silty soils Silty soils, non-plastic A-5 Poor silty soils High silt content, elastic silts A-6 Fair to poor clayey soils Plastic clays A-7 Worst quality soils Highly plastic clays
- 19. To classify a soil using AASHTO, you typically need: • Grain size analysis • Liquid Limit (LL) • Plasticity Index (PI) Group Index (GI): Used to further evaluate the quality of fine-grained soils (A-2 to A-7): GI=(F−35)[0.2+0.005(LL−40)]+0.01(F−15)(PI−10) Where: F = % passing No. 200 sieve LL = Liquid Limit PI = Plasticity Index GI = 0 for good subgrade soil GI > 20 for poor subgrade soil
- 20. Group % Passing No. 200 LL PI GI Use in Roadwork A-1 < 35% — — 0-1 Excellent subgrade A-2 < 35% Var. Var. 0–10 Good subgrade A-3 < 10% — — 0 Acceptable subgrade A-4 > 35% < 40 < 10 Low Fair to poor subgrade A-5 > 35% > 40 < 10 High Poor subgrade A-6 > 35% < 40 > 10 High Poor subgrade A-7 > 35% > 40 > 10 Very high Unsuitable subgrade
- 21. 1. Agriculture & Crop Production Soil Fertility Management: Classification helps identify nutrient deficiencies and organic matter content, guiding fertilizer application. Irrigation Planning: Knowing soil texture (e.g., sandy, clayey, loamy) helps determine water retention and drainage needs. Crop Suitability: Certain crops thrive in specific soil types (e.g., rice in clayey soils, grapes in well-drained loamy soils). 2. Geotechnical & Construction Engineering Foundation Design: Engineers classify soils (e.g., Unified Soil Classification System - USCS) to assess load-bearing capacity and settlement risks. Slope Stability & Erosion Control: Clay-rich soils may expand when wet, while sandy soils are prone to erosion; classification helps in stabilization. Road & Pavement Design: Subgrade soil type (e.g., AASHTO classification) affects road durability and material selection.
- 22. 1. Agriculture & Crop Production Soil Fertility Management: Classification helps identify nutrient deficiencies and organic matter content, guiding fertilizer application. Irrigation Planning: Knowing soil texture (e.g., sandy, clayey, loamy) helps determine water retention and drainage needs. Crop Suitability: Certain crops thrive in specific soil types (e.g., rice in clayey soils, grapes in well-drained loamy soils). 2. Geotechnical & Construction Engineering Foundation Design: Engineers classify soils (e.g., Unified Soil Classification System - USCS) to assess load-bearing capacity and settlement risks. Slope Stability & Erosion Control: Clay-rich soils may expand when wet, while sandy soils are prone to erosion; classification helps in stabilization. Road & Pavement Design: Subgrade soil type (e.g., AASHTO classification) affects road durability and material selection. Practical Implication of Soil Classification System
- 23. 3. Environmental Management & Land Use Planning Contaminant Retention & Remediation: Clay soils retain pollutants, while sandy soils allow faster leaching -> classification guides cleanup strategies. Wetland Delineation: Hydric soil classification helps in identifying and protecting wetlands. Urban Development: Soil surveys prevent construction on unstable or expansive soils. 4. Water Management & Hydrology Drainage Systems: Soil permeability (from classification) determines drainage requirements. Flood Risk Assessment: Low-permeability soils (e.g., clay) increase runoff and flood risks. 5. Forestry & Ecosystem Management Reforestation Efforts: Soil type affects tree species selection for afforestation. Erosion Control in Sloped Areas: Soil classification helps in selecting appropriate vegetation for stabilization. 6. Disaster Risk Reduction Landslide Prediction: Clay-rich soils are prone to landslides when saturated. Earthquake Liquefaction Risk: Sandy, loose soils are more susceptible to liquefaction.
- 24. Key Soil Classification Systems & Their Uses USDA Soil Taxonomy (Agriculture, Ecology) Unified Soil Classification System (USCS) (Geotechnical Engineering) AASHTO Classification (Road Construction) World Reference Base (WRB) for Soil Resources (Global Soil Mapping)
- 25. (Q1.) For a given soil, the following are known: • Total mass of soil sieved= 200gm • Cumulative mass retained on 4.75mm sieve=30gm • Cumulative mass retained on 75 micron sieve=150gm • Liquid limit =38% • Plastic limit = 28% • Natural water content = 32% Classify the soil using the Unified Soil Classification System.
- 26. Step 1: Determine % Finer than 75 µm Sieve Given: - Total mass of soil sieved = 200 gm - Cumulative mass retained on 75 µm sieve = 150 gm % Passing 75 µm sieve = ((200 - 150) / 200) × 100 = 25% Since only 25% passes the 75 µm sieve, the soil is a coarse-grained soil. → Step 2: Determine Type of Coarse Soil — Sand or Gravel - Cumulative mass retained on 4.75 mm sieve = 30 gm - % retained on 4.75 mm = (30 / 200) × 100 = 15% - Therefore, % passing 4.75 mm = 85% More than 50% of coarse fraction passes 4.75 mm sieve, so the soil is sand (not → gravel).
- 27. Step 4: Use Atterberg Limits to Differentiate between SM and SC Given: - Liquid Limit (LL) = 38% - Plastic Limit (PL) = 28% - Plasticity Index (PI) = LL PL = 38 28 = 10% − − Using Plasticity Chart Criteria: - 0.73(LL 20) = 0.73 × (38 20) = 13.14% − − - Since PI = 10% < 13.14% Silty Sand (SM) → Final Soil Classification (USCS) USCS Group Symbol: SM — Silty Sand
- 28. Question (Q2): Given: - Total mass of soil sieved = 500 gm - Mass retained on 4.75 mm (No. 4) sieve = 320 gm - Mass retained on 75 micron (No. 200) sieve = 450 gm - Liquid Limit (LL) = 42% - Plastic Limit (PL) = 24% Classify the soil using USCS.
- 29. 1. Calculate % Gravel, Sand, and Fines: % Gravel = (Mass on 4.75 mm sieve / Total mass) × 100 = (320 / 500) × 100 = 64% % Sand = ((Mass on 75 micron sieve) - (Mass on 4.75 mm sieve)) / Total mass × 100 = (450 - 320) / 500 × 100 = 26% % Fines = 100 - (% Gravel + % Sand) = 100 - (64 + 26) = 10% Since % Fines (10%) < 50% → Coarse-grained soil. 2. Determine Primary Component (Gravel or Sand): % Gravel (64%) > % Sand (26%) → Primary Symbol = G (Gravel)
- 30. 3. Check Plasticity of Fines (if % Fines > 5%): Plasticity Index (PI) = LL - PL = 42 - 24 = 18% A-line equation: PI = 0.73(LL - 20) = 0.73(42 - 20) = 16.06% Since PI (18%) > A-line (16.06%) → Fines are Clay (C). 4. Final Classification: - % Fines = 10% (5% < Fines < 12%) → Dual symbol not mandatory (PI clearly indicates clay). - Soil Type = GC (Clayey Gravel). Final Classification: GC Step Calculation Result % Gravel (320 / 500) × 100 64% % Sand (450 - 320) / 500 × 100 26% % Fines 100 - (64 + 26) 10% PI 42 - 24 18% A-line PI 0.73(42 - 20) 16.06% Classification Gravel (G) + Clay (C) GC
- 31. References