ORIGIN OF SOIL #SOIL MECHANICS - ℙℕ 𝕃𝕖𝕒𝕣𝕟 𝔸𝕝𝕖𝕣𝕥

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 and decomposition 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 and deposited by natural agencies

🔸 Residual Soil

Formed in-situ
Parent rock lies just below soil
High bearing capacity
Low compressibility
Generally angular particles
Example: Laterite soil

🔸 Transported Soil ⭐ (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

📌 Most fertile soil → Alluvial
📌 Least stable soil → Colluvial

🔸 Weathering of Rocks

Type	Key Point
Physical	No chemical change
Chemical	Chemical composition changes
Biological	Action of plants & 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 ⭐ (1-Mark MCQs)

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 are often linked.

🔸 Unit Weight

$\displaystyle \gamma =\frac{W}{V}(\text{kN/}{{\text{m}}^{3}})$

🔸 Density

$\displaystyle \rho =\frac{M}{V}(\text{kg/}{{\text{m}}^{3}})$

🔸 Specific Gravity

$\displaystyle G=\frac{{{{\rho }_{{soil}}}}}{{{{\rho }_{{water}}}}}$

🔸 Water Content

$\displaystyle w=\frac{{{{W}_{w}}}}{{{{W}_{s}}}}\times 100\%$

🔹 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
Volume

$\displaystyle \gamma =\frac{{18}}{{1.2}}=15\text{ }\!\!\backslash\!\!\text{ kN/}{{\text{m}}^{3}}$

✅ Answer: 15 kN/m³

🔢 Example 2 (Moderate)

Given:
Mass
Volume

$\displaystyle \rho =\frac{{1800}}{1}=1800\text{ }\!\!\backslash\!\!\text{ kg/}{{\text{m}}^{3}}$

🔢 Example 3 (Moderate)

Given:
Dry soil weight
Water weight

$\displaystyle w=\frac{{40}}{{200}}\times 100=20\%$

🔢 Example 4 (Advanced)

Given:
Specific gravity

$\displaystyle {{\rho }_{{soil}}}=G\times {{\rho }_{{water}}}=2.65\times 1000=2650\text{ }\!\!\backslash\!\!\text{ kg/}{{\text{m}}^{3}}$

🔢 Example 5 (Conceptual)

Question: Soil transported by glacier is called?
✅ Answer: 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:

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?

Wind
B. Ice
C. Water ✅
D. Gravity

Tip: Water causes maximum abrasion.

Q3. Aeolian soil is transported by:

Water
B. Ice
C. Wind ✅
D. Gravity

Q4. Which soil has highest bearing capacity?

Marine
B. Aeolian
C. Residual ✅
D. Alluvial

Q5. Chemical weathering includes:

Abrasion
B. Oxidation ✅
C. Frost action
D. Exfoliation

Q6. The topmost layer of soil rich in organic matter is called:

A) Horizon B
B) Horizon C
C) Horizon A
D) Horizon D

Answer: C

Q7. Which soil horizon is known as the zone of accumulation (illuviation)?

A) O-horizon
B) A-horizon
C) B-horizon
D) C-horizon

Answer: C

Q8. The process by which soil particles are removed from the upper horizon and deposited in lower horizon is called:

A) Eluviation
B) Illuviation
C) Leaching
D) Weathering

Answer: B

Q9. Which of the following is not an agent of weathering?

A) Wind
B) Water
C) Temperature
D) Cement

Answer: D

Q10. Mechanical weathering mainly results in:

A) Chemical decomposition
B) Increase in particle size
C) Decrease in particle size
D) Formation of new minerals

Answer: C

Q11. Chemical weathering of feldspar produces mainly:

A) Quartz
B) Mica
C) Kaolinite
D) Calcite

Answer: C

Q12. Which type of weathering is dominant in hot and humid climates?

A) Physical weathering
B) Chemical weathering
C) Biological weathering
D) Frost weathering

Answer: B

Q13. Laterite soil is formed due to:

A) Mechanical weathering
B) Chemical weathering with leaching
C) Frost action
D) Volcanic activity

Answer: B

Q14. The soil horizon consisting of partially weathered parent rock is:

A) A-horizon
B) B-horizon
C) C-horizon
D) O-horizon

Answer: C

Q15. Residual soil is:

A) Transported by wind
B) Transported by water
C) Formed in place
D) Deposited by glacier

Answer: C

Q16. Which of the following is an example of transported soil?

A) Black cotton soil
B) Laterite soil
C) Alluvial soil
D) Residual soil

Answer: C

Q17. The process of breaking down rocks by plant roots is called:

A) Mechanical weathering
B) Chemical weathering
C) Biological weathering
D) Thermal weathering

Answer: C

Q18. Exfoliation is associated with:

A) Frost action
B) Chemical weathering
C) Thermal expansion
D) Oxidation

Answer: C

Q19. Which soil has the highest permeability?

A) Clay
B) Silt
C) Sand
D) Loam

Answer: C

Q20. Which soil has the highest capillarity?

A) Gravel
B) Sand
C) Silt
D) Clay

Answer: D

Q21. Humus mainly improves soil:

A) Density
B) Strength
C) Fertility
D) Plasticity

Answer: C

Q22. Which of the following has the lowest bearing capacity?

A) Gravel
B) Sand
C) Silt
D) Clay

Answer: D

Q23. Weathering differs from erosion because weathering:

A) Involves transportation
B) Forms sediments
C) Acts in situ
D) Is caused only by wind

Answer: C

Q24. The disintegration of rocks without chemical change is called:

A) Chemical weathering
B) Physical weathering
C) Biological weathering
D) Oxidation

Answer: B

Q25. Which of the following is not a product of chemical weathering?

A) Kaolinite
B) Laterite
C) Bauxite
D) Gravel

Answer: D

🟡 NUMERICAL MCQs (26–50)

Q26. Weight = 20 kN, Volume = 2 m³. Unit weight?

8
B. 10 ✅
C. 12
D. 15

$\displaystyle \gamma =\frac{{20}}{2}=10$

Q27. Specific gravity = 2.7. Density of soil solids?

1700
B. 2000
C. 2700 ✅
D. 3000

Q28. Water content = 25%, dry weight = 400 N. Water weight?

$\displaystyle {{W}_{w}}=0.25\times 400=100\text{ }\!\!\backslash\!\!\text{ N}$

🟡 NUMERICAL MCQs (Q29–Q50)

Q29. The unit weight of soil having weight 16 kN and volume 2 m³ is:

6 kN/m³
B. 8 kN/m³ ✅
C. 10 kN/m³
D. 12 kN/m³

Solution:

$\displaystyle \gamma =\frac{W}{V}=\frac{{16}}{2}=8\ kN/m3$

Q30. If the mass of soil is 2400 kg and volume is 2 m³, the density is:

1000 kg/m³
1200 kg/m³ ✅
C. 1800 kg/m³
D. 2400 kg/m³

$\displaystyle \rho =\frac{{2400}}{2}=1200\ kg/m3$

Q31. Specific gravity of soil solids is 2.6. Density of soil solids is:

2000 kg/m³
2400 kg/m³
C. 2600 kg/m³ ✅
D. 3000 kg/m³

$\displaystyle {{\rho }_{s}}=G\times {{\rho }_{w}}=2.6\times 1000=2600$

Q32. A soil sample weighs 10 kN and occupies 0.5 m³. Its unit weight is:

10 kN/m³
B. 15 kN/m³
C. 20 kN/m³ ✅
D. 25 kN/m³

$\displaystyle \gamma =\frac{{10}}{{0.5}}=20$

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 ✅
D. 120 N

$\displaystyle {{W}_{w}}=0.20\times 500=100\ \text{N}$

Q34. If total mass of wet soil is 2200 kg and dry mass is 2000 kg, water content is:

8%
B. 10% ✅
C. 12%
D. 15%

$\displaystyle w=\frac{{2200-2000}}{{2000}}\times 100=10\%$

Q35. Density of water is taken as:

9.81 kN/m³
B. 1 g/cm³
C. 1000 kg/m³
D. All of the above ✅

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 ✅
D. Air content increases

Tip: Higher density → 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 ✅
D. Porosity

Q38. A soil has volume 1 m³ and mass 1800 kg. Its unit weight (g = 10 m/s²) is:

16 kN/m³
B. 17 kN/m³
C. 18 kN/m³ ✅
D. 20 kN/m³

$\displaystyle W=mg=1800\times 10=18000\ \text{N}$

Q39. Water content of a completely dry soil is:

0% ✅
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 ✅
D. Soil is saturated

Q41. Which soil parameter is dimensionless?

Density
B. Unit weight
C. Specific gravity ✅
D. Water content

Q42. The SI unit of density is:

g/cm³
B. kg/m³ ✅
C. kN/m³
D. N/m²

Q43. Unit weight of water is approximately:

9.81 kN/m³ ✅
B. 10 kN/m²
C. 1000 kg/m³
D. 1 g/cm³

Q44. Water content is defined as:

B. ✅
C.
D.

Q45. Which of the following depends on gravity?

Density
B. Specific gravity
C. Unit weight ✅
D. Water content

Q46. If the volume of soil increases with same weight, unit weight will:

Increase
B. Decrease ✅
C. Remain same
D. Become zero

Q47. Typical value of specific gravity of inorganic soil solids ranges between:

1.0 – 1.5
B. 1.5 – 2.0
C. 2.5 – 2.8 ✅
D. 3.0 – 3.5

Q48. Density multiplied by acceleration due to gravity gives:

Specific gravity
B. Unit weight ✅
C. Water content
D. Porosity

Q49. A soil sample has zero water content when:

Fully saturated
B. Partially saturated
C. Completely dry ✅
D. Fully submerged

Q50. Which property is used to compare heaviness of soil solids with water?

Density
B. Unit weight
C. Specific gravity ✅
D. Water content

🔥 QUICK REVISION TRICKS (EXAM GOLD)

Unit weight = Weight / Volume
Density = Mass / Volume
Specific gravity has NO unit
ρsoil = G × ρwater
Water content always based on dry weight

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