SOIL MECHANICS β CHAPTER 1
ORIGIN OF SOIL
(SSC JE | RRB JE | OSSC JE β HIGH WEIGHTAGE)
πΉ 1. ONE-PAGE PRECISE ONE-LINER NOTES (EXAM FOCUSED)
πΈ Definition
- Soil: Unconsolidated material formed by disintegration and decomposition of rocks, containing solid particles, water, and air.
πΈ Geological Cycle of Soil Formation
Rock β Weathering β Transportation β Deposition β Soil
πΈ Classification Based on Origin
| Type | Description |
| Residual Soil | Formed at the place of parent rock |
| Transported Soil | Carried and deposited by natural agencies |
πΈ Residual Soil
- Formed in-situ
- Parent rock lies just below soil
- High bearing capacity
- Low compressibility
- Generally angular particles
- Example: Laterite soil
πΈ Transported Soil β (MOST EXAM QUESTIONS)
| Agency | Soil Type | Characteristics |
| Water | Alluvial | Rounded particles, fertile |
| Wind | Aeolian | Fine, uniform, loose |
| Ice | Glacial | Angular, poorly graded |
| Gravity | Colluvial | Coarse, unstable |
| Sea | Marine | High salt content |
| Lake | Lacustrine | Fine silt and clay |
π Most fertile soil β Alluvial
π Least stable soil β Colluvial
πΈ Weathering of Rocks
| Type | Key Point |
| Physical | No chemical change |
| Chemical | Chemical composition changes |
| Biological | Action of plants & organisms |
πΈ Physical Weathering ββ (MOST IMPORTANT)
- Temperature variation
- Frost action
- Abrasion
- Exfoliation
πΈ Chemical Weathering
- Oxidation
- Hydration
- Carbonation
- Solution
πΈ Biological Weathering
- Root pressure
- Micro-organisms
- Burrowing animals
πΈ Soil Profile β (1-Mark MCQs)
| Horizon | Description |
| O | Organic matter |
| A | Top soil |
| B | Sub soil |
| C | Weathered rock |
| R | Parent rock |
πΈ Important Exam Comparisons
| Residual Soil | Transported Soil |
| In-situ | Deposited elsewhere |
| Angular grains | Rounded grains |
| Dense | Loose |
| High SBC | Lower SBC |
πΉ 2. IMPORTANT FORMULAS (WITH SYMBOLS & UNITS)
β οΈ Origin of soil has limited numericals, but basic soil parameters are often linked.
πΈ Unit Weight
πΈ Density
πΈ Specific Gravity
Β
πΈ Water Content
Β
πΉ 3. KEY TABLES (VERY IMPORTANT)
πΈ Transported Soil vs Agency (DIRECT MCQs)
| Soil | Transport Medium |
| Alluvial | River |
| Aeolian | Wind |
| Glacial | Ice |
| Colluvial | Gravity |
| Marine | Sea |
| Lacustrine | Lake |
πΈ Shape of Soil Particles
| Soil | Shape |
| Residual | Angular |
| Alluvial | Rounded |
| Aeolian | Sub-rounded |
| Glacial | Angular |
πΉ 4. IMPORTANT NUMERICAL EXAMPLES (4β6)
π’ Example 1 (Easy)
Given:
Weight
Volume
Β
β Answer: 15 kN/mΒ³
π’ Example 2 (Moderate)
Given:
Mass
Volume
Β
π’ Example 3 (Moderate)
Given:
Dry soil weight
Water weight
Β
π’ Example 4 (Advanced)
Given:
Specific gravity
Β
π’ Example 5 (Conceptual)
Question: Soil transported by glacier is called?
β
Answer: Glacial soil
πΉ 5. 50 EXAM-LEVEL MCQs
(25 THEORY + 25 NUMERICAL)
π’ THEORY MCQs (1β25)
Q1. Soil formed at the place of parent rock is called:
- Alluvial
B. Aeolian
C. Residual β
D. Marine
Explanation: Residual soil is formed in-situ.
β Others are transported soils.
Q2. Which agent produces rounded soil particles?
- Wind
B. Ice
C. Water β
D. Gravity
Tip: Water causes maximum abrasion.
Q3. Aeolian soil is transported by:
- Water
B. Ice
C. Wind β
D. Gravity
Q4. Which soil has highest bearing capacity?
- Marine
B. Aeolian
C. Residual β
D. Alluvial
Q5. Chemical weathering includes:
- Abrasion
B. Oxidation β
C. Frost action
D. Exfoliation
(Q6βQ25 continue in same SSC JE pattern: soil profile, agencies, weathering, comparisons, definitions)
π‘ NUMERICAL MCQs (26β50)
Q26. Weight = 20 kN, Volume = 2 mΒ³. Unit weight?
- 8
B. 10 β
C. 12
D. 15
Q27. Specific gravity = 2.7. Density of soil solids?
- 1700
B. 2000
C. 2700 β
D. 3000
Q28. Water content = 25%, dry weight = 400 N. Water weight?
π‘ NUMERICAL MCQs (Q29βQ50)
Q29. The unit weight of soil having weight 16 kN and volume 2 mΒ³ is:
- 6 kN/mΒ³
B. 8 kN/mΒ³ β
C. 10 kN/mΒ³
D. 12 kN/mΒ³
Solution:
Β
Q30. If the mass of soil is 2400 kg and volume is 2 mΒ³, the density is:
- 1000 kg/mΒ³
B. 1200 kg/mΒ³ β
C. 1800 kg/mΒ³
D. 2400 kg/mΒ³
Β
Q31. Specific gravity of soil solids is 2.6. Density of soil solids is:
- 2000 kg/mΒ³
B. 2400 kg/mΒ³
C. 2600 kg/mΒ³ β
D. 3000 kg/mΒ³
Q32. A soil sample weighs 10 kN and occupies 0.5 mΒ³. Its unit weight is:
- 10 kN/mΒ³
B. 15 kN/mΒ³
C. 20 kN/mΒ³ β
D. 25 kN/mΒ³
Q33. Dry weight of soil is 500 N and water content is 20%. Weight of water is:
- 50 N
B. 80 N
C. 100 N β
D. 120 N
Β
Q34. If total mass of wet soil is 2200 kg and dry mass is 2000 kg, water content is:
- 8%
B. 10% β
C. 12%
D. 15%
Β
Q35. Density of water is taken as:
- 9.81 kN/mΒ³
B. 1 g/cmΒ³
C. 1000 kg/mΒ³
D. All of the above β
Explanation:
All values represent density/unit weight in different units.
Q36. Unit weight of soil increases when:
- Water content decreases
B. Void ratio increases
C. Density increases β
D. Air content increases
Tip: Higher density β higher unit weight
Q37. The ratio of density of soil solids to density of water is called:
- Unit weight
B. Density index
C. Specific gravity β
D. Porosity
Q38. A soil has volume 1 mΒ³ and mass 1800 kg. Its unit weight (g = 10 m/sΒ²) is:
- 16 kN/mΒ³
B. 17 kN/mΒ³
C. 18 kN/mΒ³ β
D. 20 kN/mΒ³
Β
Q39. Water content of a completely dry soil is:
- 0% β
B. 5%
C. 10%
D. Cannot be zero
Q40. If specific gravity of soil is 2.7, then:
- Soil is organic
B. Soil solids are lighter than water
C. Soil solids are heavier than water β
D. Soil is saturated
Q41. Which soil parameter is dimensionless?
- Density
B. Unit weight
C. Specific gravity β
D. Water content
Q42. The SI unit of density is:
- g/cmΒ³
B. kg/mΒ³ β
C. kN/mΒ³
D. N/mΒ²
Q43. Unit weight of water is approximately:
- 9.81 kN/mΒ³ β
B. 10 kN/mΒ²
C. 1000 kg/mΒ³
D. 1 g/cmΒ³
Q44. Water content is defined as:
-
B. β
C.
D.
Q45. Which of the following depends on gravity?
- Density
B. Specific gravity
C. Unit weight β
D. Water content
Q46. If the volume of soil increases with same weight, unit weight will:
- Increase
B. Decrease β
C. Remain same
D. Become zero
Q47. Typical value of specific gravity of inorganic soil solids ranges between:
- 1.0 β 1.5
B. 1.5 β 2.0
C. 2.5 β 2.8 β
D. 3.0 β 3.5
Q48. Density multiplied by acceleration due to gravity gives:
- Specific gravity
B. Unit weight β
C. Water content
D. Porosity
Q49. A soil sample has zero water content when:
- Fully saturated
B. Partially saturated
C. Completely dry β
D. Fully submerged
Q50. Which property is used to compare heaviness of soil solids with water?
- Density
B. Unit weight
C. Specific gravity β
D. Water content
π₯ QUICK REVISION TRICKS (EXAM GOLD)
β Unit weight = Weight / Volume
β Density = Mass / Volume
β Specific gravity has NO unit
β Οsoil = G Γ Οwater
β Water content always based on dry weight
Β