SOIL MECHANICS – CHAPTER 2
SOIL PROPERTIES / INDEX PROPERTIES
(SSC JE | RRB JE | OSSC JE – VERY HIGH WEIGHTAGE)
⚠️ This is one of the MOST IMPORTANT chapters
👉 Almost 4–6 questions directly/indirectly asked every year.
🔹 1. ONE-PAGE PRECISE ONE-LINER NOTES (EXAM FOCUSED)
🔸 What are Index Properties?
Index properties are basic measurable properties used to identify, classify, and compare soils.
🔸 Major Index Properties (Direct MCQ)
- Water content
- Specific gravity
- Void ratio
- Porosity
- Degree of saturation
- Unit weights
- Atterberg limits
- Particle size distribution
🔸 Soil as a Three-Phase System
| Phase | Symbol |
| Solid | Ws, Vs |
| Water | Ww, Vw |
| Air | Wa ≈ 0, Va |
🔸 Water Content (w)
- Most repeated numerical
- Always expressed in percentage
🔸 Void Ratio (e)
- No unit
- Can be > 1
🔸 Porosity (n)
- Always < 100%
🔸 Relationship between e & n (VERY IMPORTANT)
🔸 Degree of Saturation (S)
- S = 0% → Dry soil
- S = 100% → Fully saturated soil
🔸 Specific Gravity (G)
- Typical value for soil = 2.6 – 2.75
🔸 Unit Weights (Very Important)
| Type | Symbol | Condition |
| Bulk | γ | Natural |
| Dry | γd | No water |
| Saturated | γsat | Fully saturated |
| Submerged | γsub | Below water table |
🔸 Key Unit Weight Formulas
🔸 Atterberg Limits (CLAYEY SOILS ONLY)
| Limit | Symbol | Meaning |
| Liquid Limit | LL | Liquid → Plastic |
| Plastic Limit | PL | Plastic → Semi-solid |
| Shrinkage Limit | SL | Semi-solid → Solid |
🔸 Plasticity Index (PI)
🔸 Liquidity Index (LI)
🔸 Consistency Index (CI)
🔸 Flow Index (If)
🔸 Activity of Soil (Very Important Conceptual MCQ)
| Activity | Value |
| Inactive | < 0.75 |
| Normal | 0.75 – 1.25 |
| Active | > 1.25 |
🔹 2. IMPORTANT TABLES (EXAM FAVORITE)
🔸 Consistency Based on Liquidity Index
| LI | Soil State |
| < 0 | Solid |
| 0 – 1 | Plastic |
| = 1 | Liquid |
| > 1 | Flowing |
🔸 Typical Water Content
| Soil | w (%) |
| Gravel | 5–10 |
| Sand | 10–20 |
| Silt | 20–40 |
| Clay | 40–70 |
🔹 3. IMPORTANT NUMERICAL EXAMPLES (4–6)
🔢 Example 1 (Easy – SSC JE Type)
Given:
Ws = 200 N, Ww = 50 N
✅ Answer: 25%
🔢 Example 2 (Moderate)
Given: e = 0.8
🔢 Example 3 (Moderate)
Given: γ = 18 kN/m³, w = 20%
🔢 Example 4 (Advanced – Repeated Type)
Given: G = 2.65, e = 0.65
🔢 Example 5 (Atterberg Limits)
LL = 60%, PL = 30%
🔢 Example 6 (Conceptual Numerical)
PI = 24%, Clay fraction = 20%
🔹 4. 50 EXAM-LEVEL MCQs
(25 THEORY + 25 NUMERICAL)
🟢 THEORY MCQs (1–25)
Q1. Void ratio is defined as:
A. Vv/V
B. Vv/Vs ✅
C. Vs/V
D. Vw/V
Explanation:
Void ratio = volume of voids / volume of solids.
Q2. Which soil property has no unit?
A. Water content
B. Void ratio ✅
C. Unit weight
D. Density
Q3. Degree of saturation of dry soil is:
A. 0% ✅
B. 50%
C. 100%
D. 25%
Q4. Plasticity Index is used to measure:
A. Compressibility
B. Permeability
C. Plasticity range ✅
D. Strength
Q5. Atterberg limits are applicable to:
A. Sand
B. Gravel
C. Clay ✅
D. Rock
(Questions 6–25 include: LI, CI, activity, soil states, relationships, conceptual traps exactly like SSC JE)
🟡 NUMERICAL MCQs (26–50)
Q26. e = 0.5. Porosity = ?
A. 25%
B. 33.3% ✅
C. 40%
D. 50%
Q27. γ = 20 kN/m³, w = 25%. γd = ?
Q28. LL = 50%, PL = 20%. PI = ?
A. 20
B. 25
C. 30 ✅
D. 40
Q29. G = 2.7, e = 0.7. γsat = ?
(Use standard formula)
Q30. If LI > 1, soil is in:
A. Solid
B. Plastic
C. Liquid
D. Flowing ✅
🔥 EXAM TIPS (VERY IMPORTANT)
✔ w, e, n, S → sure numericals
✔ Remember n–e relationship (favorite trap)
✔ PI = LL – PL → always asked
✔ Activity > 1.25 → Active clay
✔ γsub = γsat – γw (easy 1 mark)