SOIL MECHANICS – CHAPTER 5
COMPACTION OF SOIL (PROCTOR TESTS)
(SSC JE | RRB JE | OSSC JE – VERY HIGH WEIGHTAGE)
👉 3–4 sure questions every exam
👉 Direct numericals + theory traps
👉 Highly scoring chapter
4
🔹 1. ONE-PAGE PRECISE ONE-LINER NOTES (EXAM FOCUSED)
🔸 Compaction
- Densification of soil by expelling air
- Achieved by mechanical energy
- Used in embankments, pavements, earth dams
🔸 Objectives of Compaction
✔ Increase shear strength
✔ Reduce compressibility
✔ Reduce permeability
✔ Increase bearing capacity
🔸 Compaction vs Consolidation (VERY IMPORTANT)
| Compaction | Consolidation |
| Expulsion of air | Expulsion of water |
| Immediate process | Time-dependent |
| Mechanical energy | Static load |
| Unsaturated soil | Saturated soil |
🔸 Dry Density (γᵈ)
Where:
γ = bulk unit weight
w = water content
🔸 Compaction Curve
- Plot between γᵈ vs w
- Peak point gives:
- Maximum Dry Density (MDD)
- Optimum Moisture Content (OMC)
🔸 Zero Air Void Line (ZAV)
- Theoretical maximum dry density
- 100% saturation
🔹 2. FACTORS AFFECTING COMPACTION (DIRECT MCQs)
| Factor | Effect |
| Water content ↑ | γᵈ ↑ then ↓ |
| Compactive effort ↑ | MDD ↑, OMC ↓ |
| Soil type | Clay → low MDD |
| Method of compaction | Vibratory best for sand |
🔹 3. PROCTOR COMPACTION TESTS (VERY IMPORTANT)
🔸 Standard Proctor Test
| Parameter | Value |
| Rammer weight | 2.6 kg |
| Drop height | 310 mm |
| Layers | 3 |
| Blows/layer | 25 |
| Energy | Standard |
🔸 Modified Proctor Test
| Parameter | Value |
| Rammer weight | 4.9 kg |
| Drop height | 450 mm |
| Layers | 5 |
| Blows/layer | 25 |
| Energy | 4.5× Standard |
🔸 Comparison (FAVOURITE)
| Test | MDD | OMC |
| Standard | Lower | Higher |
| Modified | Higher | Lower |
🔹 4. IMPORTANT FORMULAS (EXAM READY)
🔸 Bulk Density
🔸 Dry Density
🔸 Relative Compaction
🔸 Degree of Saturation
🔹 5. IMPORTANT NUMERICAL EXAMPLES (SSC JE LEVEL)
🔢 Example 1 (Easy – Dry Density)
Given:
γ = 18 kN/m³
w = 10%
🔢 Example 2 (Moderate – Relative Compaction)
γᵈ(field) = 16.5 kN/m³
γᵈ(lab) = 18 kN/m³
🔢 Example 3 (MDD–OMC Identification)
From compaction curve:
Peak dry density at w = 12%
→ OMC = 12%, corresponding γᵈ = MDD
🔢 Example 4 (ZAV Line – Conceptual)
Actual compaction curve never crosses ZAV line
→ If crosses → data is wrong
🔢 Example 5 (Tricky Concept)
Which soil needs maximum water for compaction?
👉 Clay soil (high surface area)
🔹 6. 50 EXAM-LEVEL MCQs
(25 THEORY + 25 NUMERICAL)
🟢 THEORY MCQs (1–25)
Q1. Compaction is the process of:
- Expelling water
B. Expelling air ✅
C. Compression of soil grains
D. Increase in voids
Q2. Maximum dry density occurs at:
- Maximum water content
B. Zero water content
C. Optimum moisture content ✅
D. Saturation
Q3. Modified Proctor test gives:
- Lower MDD
B. Higher OMC
C. Higher MDD ✅
D. Same result
Q4. Best compaction method for sand:
- Tamping
B. Kneading
C. Rolling
D. Vibratory compaction ✅
Q5. ZAV line represents:
- 0% saturation
B. 50% saturation
C. 100% saturation ✅
D. Field compaction
🟡 NUMERICAL MCQs (26–50)
Q26. γ = 19 kN/m³, w = 15%. γᵈ = ?
- 14.2
B. 15.6
C. 16.5 ✅
D. 18.0
Q27. Field dry density = 17 kN/m³, Lab MDD = 18 kN/m³.
Relative compaction = ?
A. 90%
B. 94%
C. 95%
D. 92% ✅
Q28. Increase in compactive effort causes:
- MDD ↓, OMC ↑
B. MDD ↑, OMC ↓ ✅
C. Both ↑
D. No change
Q29. Standard Proctor rammer weight:
- 4.9 kg
B. 3.5 kg
C. 2.6 kg ✅
D. 5 kg
Q30. Compaction is NOT effective for:
- Sand
B. Silt
C. Clay
D. Rock fragments ✅
🔥 EXAM TRICKS & SHORTCUTS
✔ γᵈ = γ/(1+w) → fastest
✔ Modified Proctor → MDD ↑, OMC ↓
✔ Compaction ≠ Consolidation
✔ ZAV line = theoretical maximum
✔ Sand → vibratory, Clay → kneading
(Q31–Q50 include: dry density traps, ZAV, Proctor comparison, field control numericals)
COMPACTION OF SOIL (SSC JE | RRB JE | OSSC JE)
focused on dry density traps, ZAV line, Proctor comparison, and field control numericals, exactly as asked.
🟡 NUMERICAL & CONCEPTUAL MCQs (31–50)
Q31. Bulk unit weight of soil = 20 kN/m³, water content = 25%.
Dry unit weight is:
A. 14.8 kN/m³
B. 15.5 kN/m³
C. 16.0 kN/m³ ✅
D. 18.0 kN/m³
Solution:
Q32. If water content is zero, dry density will be:
- Zero
B. Equal to bulk density ✅
C. Maximum
D. Minimum
Explanation:
Q33. For a given soil, compaction curve lies:
- Above ZAV line
B. On ZAV line
C. Below ZAV line ✅
D. May cross ZAV line
Explanation:
ZAV line is theoretical maximum (100% saturation).
Q34. If a compaction curve crosses ZAV line, it indicates:
- High compaction effort
B. Saturated soil
C. Experimental error ✅
D. Clayey soil
Q35. Increase in water content beyond OMC causes dry density to:
- Increase
B. Remain constant
C. Decrease ✅
D. Become zero
Q36. Modified Proctor test as compared to Standard Proctor gives:
- Lower MDD & lower OMC
B. Higher MDD & higher OMC
C. Higher MDD & lower OMC ✅
D. Same MDD & OMC
Q37. Field dry density = 17.2 kN/m³
Lab MDD = 18.5 kN/m³
Relative compaction is:
A. 90%
B. 92%
C. 93% ✅
D. 95%
Solution:
Q38. Which soil shows maximum OMC?
- Gravel
B. Sand
C. Silt
D. Clay ✅
Explanation:
Clay has high surface area → more water required.
Q39. Degree of saturation at ZAV line is:
- 0%
B. 50%
C. 75%
D. 100% ✅
Q40. If compactive effort is increased, the compaction curve shifts:
- Downward & right
B. Upward & left ✅
C. Only upward
D. Only right
Q41. Best method of field compaction for clayey soil is:
- Vibratory
B. Rolling
C. Kneading ✅
D. Tamping
Q42. For the same soil, Modified Proctor energy is approximately:
- 2 times Standard
B. 3 times Standard
C. 4.5 times Standard ✅
D. 6 times Standard
Q43. Which of the following does NOT affect compaction?
- Water content
B. Soil type
C. Compactive effort
D. Permeability ✅
Q44. At OMC, soil has:
- Maximum void ratio
B. Minimum dry density
C. Maximum dry density ✅
D. Zero air voids
Q45. Zero air void line is drawn using:
- Bulk density
B. Dry density
C. Saturated density
D. Dry density at S = 100% ✅
Q46. Field compaction control is generally specified in terms of:
- Bulk density
B. Water content
C. Relative compaction ✅
D. Degree of saturation
Q47. Which soil shows least change in dry density with water content?
- Clay
B. Silt
C. Sand ✅
D. Organic soil
Q48. Compaction of soil results in:
- Increase in compressibility
B. Increase in permeability
C. Decrease in shear strength
D. Increase in shear strength ✅
Q49. If field compaction achieved is 95% of MDD, it is:
- Poor
B. Unsatisfactory
C. Acceptable for embankments ✅
D. Not usable
Q50. Which statement is TRUE?
- OMC increases with compactive effort
B. MDD decreases with compactive effort
C. ZAV line represents field condition
D. Modified Proctor gives higher MDD than Standard ✅
🔥 QUICK EXAM SUMMARY (VERY IMPORTANT)
✔ γᵈ = γ / (1 + w) → most repeated trap
✔ Compaction curve always below ZAV line
✔ Modified Proctor → MDD ↑, OMC ↓
✔ Relative compaction = field γᵈ / lab γᵈ
✔ Clay → high OMC, Sand → low OMC