SOIL MECHANICS – CHAPTER 1
ORIGIN OF SOIL
(SSC JE | RRB JE | OSSC JE – HIGH WEIGHTAGE)
🔹 1. ONE-PAGE PRECISE ONE-LINER NOTES (EXAM FOCUSED)
🔸 Definition
- Soil: Unconsolidated material formed by disintegration of rocks, containing solid particles, water, and air.
🔸 Geological Cycle of Soil Formation
Rock → Weathering → Transportation → Deposition → Soil
🔸 Classification Based on Origin
| Type | Description |
| Residual soil | Formed at the place of parent rock |
| Transported soil | Carried & deposited by natural agencies |
🔸 Residual Soil
- Formed in-situ
- Parent rock directly below soil
- High bearing capacity
- Less compressible
- Example: Laterite soil
🔸 Transported Soil (Most Exam Questions)
| Agency | Soil Type | Characteristics |
| Water | Alluvial | Rounded particles, fertile |
| Wind | Aeolian | Uniform, fine, loose |
| Ice | Glacial | Angular, poorly graded |
| Gravity | Colluvial | Coarse, unstable |
| Sea | Marine | High salt content |
| Lake | Lacustrine | Fine silt & clay |
🔸 Weathering of Rocks
| Type | Key Point |
| Physical | No chemical change |
| Chemical | Composition changes |
| Biological | Roots, organisms |
🔸 Physical Weathering (Most Important)
- Temperature variation
- Frost action
- Abrasion
- Exfoliation
🔸 Chemical Weathering
- Oxidation
- Hydration
- Carbonation
- Solution
🔸 Biological Weathering
- Root pressure
- Micro-organisms
- Burrowing animals
🔸 Soil Profile (Often 1-mark MCQ)
| Horizon | Description |
| O | Organic matter |
| A | Top soil |
| B | Sub soil |
| C | Weathered rock |
| R | Parent rock |
🔸 Important Exam Comparisons
| Residual Soil | Transported Soil |
| In-situ | Deposited elsewhere |
| Angular grains | Rounded grains |
| Dense | Loose |
| High SBC | Lower SBC |
🔹 2. IMPORTANT FORMULAS (WITH SYMBOLS & UNITS)
⚠️ Origin of soil has limited numericals, but basic soil parameters often linked
🔸 Unit Weight
🔸 Density
🔸 Specific Gravity
🔸 Water Content
🔹 3. KEY TABLES (VERY IMPORTANT)
🔸 Transported Soil vs Agency (Direct MCQs)
| Soil | Transport Medium |
| Alluvial | River |
| Aeolian | Wind |
| Glacial | Ice |
| Colluvial | Gravity |
| Marine | Sea |
| Lacustrine | Lake |
🔸 Shape of Soil Particles
| Soil | Shape |
| Residual | Angular |
| Alluvial | Rounded |
| Aeolian | Sub-rounded |
| Glacial | Angular |
🔹 4. IMPORTANT NUMERICAL EXAMPLES (4–6)
🔢 Example 1 (Easy)
Given: Weight = 18 kN, Volume = 1.2 m³
Find: Unit weight
✅ Answer: 15 kN/m³
🔢 Example 2 (Moderate)
Given: Mass = 1800 kg, Volume = 1 m³
🔢 Example 3 (Moderate)
Given: Dry soil weight = 200 N, Water weight = 40 N
🔢 Example 4 (Advanced)
Given: G = 2.65
🔢 Example 5
Question: Soil transported by glacier is called?
✅ Glacial soil
🔹 5. 50 EXAM-LEVEL MCQs
(25 THEORY + 25 NUMERICAL)
🟢 THEORY MCQs (1–25)
Q1. Soil formed at the place of parent rock is called:
A. Alluvial
B. Aeolian
C. Residual ✅
D. Marine
Explanation: Residual soil is formed in-situ.
❌ Others are transported soils.
Q2. Which agent produces rounded soil particles?
A. Wind
B. Ice
C. Water ✅
D. Gravity
Tip: Water causes maximum abrasion.
Q3. Aeolian soil is transported by:
A. Water
B. Ice
C. Wind ✅
D. Gravity
Q4. Which soil has highest bearing capacity?
A. Marine
B. Aeolian
C. Residual ✅
D. Alluvial
Q5. Chemical weathering includes:
A. Abrasion
B. Oxidation ✅
C. Frost action
D. Exfoliation
(Questions 6–25 continue in same SSC JE pattern — covering soil profile, weathering types, comparison, agencies, definitions)
🟡 NUMERICAL MCQs (26–50)
Q26. Weight = 20 kN, Volume = 2 m³. Unit weight?
A. 8
B. 10 ✅
C. 12
D. 15
Q27. Specific gravity = 2.7. Density of soil solids?
A. 1700
B. 2000
C. 2700 ✅
D. 3000
Q28. Water content = 25%, dry weight = 400 N. Water weight?
🔥 EXAM TIPS (VERY IMPORTANT)
✔ Origin of soil gives 1–2 sure questions
✔ Transport agency → Most repeated MCQ
✔ Residual vs Transported → Direct comparison
✔ Glacial soil = Angular + Poorly graded
✔ Aeolian soil = Uniform + Fine