SOIL CLASSIFICATION – DETAILED EXPLANATION
1. Meaning of Soil Classification
Soil classification is the process of grouping soils based on their:
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Grain size
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Plasticity
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Engineering behavior
It helps engineers to:
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Predict soil performance
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Choose foundation type
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Design roads, embankments, and dams
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Communicate soil properties easily
2. Need for Soil Classification
Soil classification helps to:
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Identify soil quickly
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Predict strength & compressibility
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Decide drainage characteristics
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Select suitable construction method
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Reduce risk of foundation failure
3. MAJOR SOIL CLASSIFICATION SYSTEMS
In civil engineering, the most commonly used systems are:
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USCS – Unified Soil Classification System
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AASHTO Classification System
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IS System (Indian Standard – IS:1498)
4. UNIFIED SOIL CLASSIFICATION SYSTEM (USCS)
Basic Principle
Classification is based on:
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Grain size
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Plasticity
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Gradation
A. Major Soil Groups
1. Coarse-Grained Soils (More than 50% retained on 75 μm sieve)
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Gravel (G)
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Sand (S)
2. Fine-Grained Soils (More than 50% passing 75 μm sieve)
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Silt (M)
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Clay (C)
3. Organic Soils
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O → Organic silt/clay
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Pt → Peat
B. Group Symbols
| Symbol | Meaning |
|---|---|
| GW | Well graded gravel |
| GP | Poorly graded gravel |
| GM | Silty gravel |
| GC | Clayey gravel |
| SW | Well graded sand |
| SP | Poorly graded sand |
| SM | Silty sand |
| SC | Clayey sand |
| ML | Low plastic silt |
| CL | Low plastic clay |
| MH | High plastic silt |
| CH | High plastic clay |
5. AASHTO SOIL CLASSIFICATION
Used mainly for highway and pavement design.
Soils are grouped from:
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A-1 (best) → good subgrade
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A-7 (worst) → poor subgrade
6. INDIAN STANDARD CLASSIFICATION (IS:1498)
Similar to USCS but uses:
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LL = 35% as plasticity boundary
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Uses plasticity chart for fine soils
7. CLASSIFICATION PROCEDURE (STEP-BY-STEP)
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Find % passing 75 μm sieve
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Decide:
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Coarse soil or Fine soil
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Perform:
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Sieve analysis
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Atterberg limits
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Use:
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Plasticity chart
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Gradation criteria
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Assign group symbol
8. WORKED EXAMPLES
EXAMPLE 1 – Coarse-Grained Soil
Given Data
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% passing 75 μm sieve = 20%
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80% retained → Coarse soil
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Of coarse fraction:
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Gravel = 60%
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Sand = 40%
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Atterberg limits:
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LL = 30%
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PL = 22%
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PI = 8%
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Step 1 – Soil Type
Since >50% is coarse → Coarse-grained soil
Step 2 – Gravel or Sand
Gravel = 60% → Gravel soil
Step 3 – Fines Content
Fines = 20% (>12%) → use plasticity
PI = 8% → low plastic → Silty
Final Classification
GM – Silty Gravel
EXAMPLE 2 – Fine-Grained Soil
Given Data
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% passing 75 μm sieve = 65%
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LL = 55%
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PL = 25%
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PI = 30%
Step 1 – Fine Soil
Since >50% passing → Fine-grained soil
Step 2 – Plasticity Chart
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LL = 55
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PI = 30
Point lies above A-line → Clay
LL > 50 → High plasticity
Final Classification
CH – High Plastic Clay
9. PRACTICE PROBLEMS (WITH ANSWERS)
Problem 1
A soil sample has:
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% passing 75 μm sieve = 40%
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Gravel = 20%
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Sand = 40%
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LL = 28%
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PL = 22%
Solution
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Coarse fraction = 60% → Coarse soil
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Sand > Gravel → Sand
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Fines = 40% (>12%)
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PI = 6% → Silty
Classification: SM – Silty Sand
Problem 2
Given:
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% passing 75 μm sieve = 70%
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LL = 40%
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PL = 30%
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PI = 10%
Solution
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Fine soil
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LL < 50 → Low plastic
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PI below A-line → Silt
Classification: ML – Low Plastic Silt
Problem 3 (AASHTO)
Given:
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% passing 75 μm sieve = 55%
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LL = 45%
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PI = 20%
Solution
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Fines > 35% → A-7 group
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PI > LL – 30
→ A-7-6
10. IMPORTANCE OF SOIL CLASSIFICATION
Soil classification is essential to:
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Select foundation type
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Design pavements
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Estimate settlement
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Predict drainage
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Reduce construction failure
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Ensure safety and economy