SOIL MECHANICS – CHAPTER 3
SOIL CLASSIFICATION (IS SYSTEM – IS:1498)
(SSC JE | RRB JE | OSSC JE – VERY HIGH WEIGHTAGE)
⚠️ One of the MOST SCORING chapters
👉 2–4 direct theory MCQs + 1–2 numerical MCQs almost every exam
🔹 1. ONE-PAGE PRECISE ONE-LINER NOTES (EXAM FOCUSED)
🔸 Purpose of Soil Classification
- To identify soil
- To predict engineering behavior
- To communicate soil type easily
🔸 IS Soil Classification System (IS:1498)
- Based on:
- Grain size distribution
- Plasticity characteristics
- Applicable to soils with particle size < 75 mm
🔸 Broad Classification
| Category | Symbol |
| Coarse-grained soils | G, S |
| Fine-grained soils | M, C |
| Highly organic soils | Pt |
🔸 Coarse-Grained Soils
- > 50% retained on 75 micron sieve
- Types:
- Gravel (G) → > 4.75 mm
- Sand (S) → 0.075–4.75 mm
🔸 Fine-Grained Soils
- > 50% passing 75 micron sieve
- Classified using Atterberg limits
- Types:
- Silt (M)
- Clay (C)
🔸 Plasticity Chart (Most Repeated Concept)
- A-line equation:
| Position | Soil Type |
| Above A-line | Clay (C) |
| Below A-line | Silt (M) |
🔸 Degree of Plasticity (Based on LL)
| LL (%) | Plasticity |
| < 35 | Low (L) |
| 35–50 | Intermediate (I) |
| > 50 | High (H) |
🔸 Symbols Used in IS System
| Symbol | Meaning |
| G | Gravel |
| S | Sand |
| M | Silt |
| C | Clay |
| W | Well graded |
| P | Poorly graded |
| L | Low plasticity |
| I | Intermediate plasticity |
| H | High plasticity |
🔸 Dual Symbols (Very Important)
- Used when soil lies near boundary
- Example:
- GW-GP
- CL-ML
🔸 Organic Soils
| Type | Symbol |
| Organic silt/clay | O |
| Peat | Pt |
🔹 2. IMPORTANT GRAIN SIZE LIMITS (DIRECT MCQs)
| Soil | Size Range |
| Gravel | > 4.75 mm |
| Sand | 0.075 – 4.75 mm |
| Silt | 0.002 – 0.075 mm |
| Clay | < 0.002 mm |
🔹 3. CLASSIFICATION OF COARSE-GRAINED SOILS
🔸 Gravels (G)
- 50% retained on 4.75 mm sieve
| Type | Symbol |
| Well graded gravel | GW |
| Poorly graded gravel | GP |
| Gravel with silt | GM |
| Gravel with clay | GC |
🔸 Sands (S)
- 50% passing 4.75 mm sieve
| Type | Symbol |
| Well graded sand | SW |
| Poorly graded sand | SP |
| Sand with silt | SM |
| Sand with clay | SC |
🔹 4. CLASSIFICATION OF FINE-GRAINED SOILS
| LL (%) | Silt | Clay |
| < 35 | ML | CL |
| 35–50 | MI | CI |
| > 50 | MH | CH |
🔹 5. IMPORTANT FORMULAS (NUMERICAL MCQs)
🔸 Coefficient of Uniformity
🔸 Coefficient of Curvature
🔸 Well Graded Conditions
| Soil | Conditions |
| Gravel | Cu ≥ 4 and 1 ≤ Cc ≤ 3 |
| Sand | Cu ≥ 6 and 1 ≤ Cc ≤ 3 |
🔹 6. IMPORTANT NUMERICAL EXAMPLES (SSC JE TYPE)
🔢 Example 1 (Easy)
Given:
D10 = 0.2 mm, D60 = 1.2 mm
🔢 Example 2 (Moderate)
D10 = 0.3 mm, D30 = 0.6 mm, D60 = 1.2 mm
🔢 Example 3 (Classification)
- Cu = 7, Cc = 2
✅ Well graded sand (SW)
🔢 Example 4 (Plasticity Chart)
LL = 45%
If actual PI > 18.25 → Clay
🔢 Example 5
LL = 60%, PI = 25%
- Above A-line
- LL > 50
✅ CH
🔹 7. 50 EXAM-LEVEL MCQs
(25 THEORY + 25 NUMERICAL)
🟢 THEORY MCQs (1–25)
Q1. IS soil classification is based on:
A. Color
B. Density
C. Grain size & plasticity ✅
D. Permeability
Q2. Boundary between sand and silt is:
A. 4.75 mm
B. 2 mm
C. 0.075 mm ✅
D. 0.002 mm
Q3. Soil above A-line is classified as:
A. Silt
B. Clay ✅
C. Sand
D. Gravel
Q4. Symbol for poorly graded gravel is:
A. GW
B. GP ✅
C. GM
D. GC
Q5. Highly organic soil is represented by:
A. OL
B. OH
C. Pt ✅
D. ML
🟡 NUMERICAL MCQs (26–50)
Q26. D60 = 1.8 mm, D10 = 0.3 mm. Cu = ?
A. 4
B. 5
C. 6 ✅
D. 8
Q27. Cu = 3, Cc = 1.2 (sand). Classification?
A. SW
B. SP ✅
C. SM
D. SC
Q28. LL = 30%, PI = 8%. Soil type?
A. CL
B. ML ✅
C. MI
D. CI
Q29. Gravel is well graded when:
A. Cu ≥ 6
B. Cu ≥ 4 and 1 ≤ Cc ≤ 3 ✅
C. Cc ≥ 4
D. Cu ≤ 4
Q30. Dual symbol is used when:
A. Soil is organic
B. Soil is coarse
C. Soil lies near boundary ✅
D. Soil is peat
🔥 EXAM TRICKS & QUICK TIPS
✔ Remember 75 micron sieve → coarse vs fine
✔ A-line decides Clay vs Silt
✔ LL < 35 → Low plasticity
✔ Cu & Cc → grading questions (sure-shot)
✔ Pt → only peat