SOIL MECHANICS – CHAPTER 6
SEEPAGE & FLOW NETS
(SSC JE | RRB JE | OSSC JE – HIGH-SCORING + CONCEPTUAL)
👉 2–3 questions every exam
👉 Darcy + Flow net numericals are fixed pattern
👉 Very easy marks if concepts are clear
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🔹 1. ONE-PAGE PRECISE ONE-LINER NOTES (EXAM FOCUSED)
🔸 Seepage
- Flow of water through soil pores under hydraulic gradient
- Occurs in earth dams, sheet piles, foundations
🔸 Flow Net
- Graphical representation of seepage
- Combination of:
- Flow lines
- Equipotential lines
🔸 Flow Line
- Path followed by water particle
- No two flow lines intersect
- Tangent gives direction of seepage
🔸 Equipotential Line
- Line joining points of equal head
- Always perpendicular to flow lines
- Represents head loss
🔸 Flow Channel
- Space between two adjacent flow lines
- Discharge is same through every flow channel
🔸 Flow Net Conditions (VERY IMPORTANT)
✔ Flow lines ⟂ Equipotential lines
✔ Curvilinear squares (approx.)
✔ Equal head drop between equipotential lines
🔹 2. SEEPAGE DISCHARGE THROUGH FLOW NET (MOST IMPORTANT)
🔸 Discharge per unit width
Where:
- k = coefficient of permeability
- h = total head loss
- Nf = number of flow channels
- Nd = number of potential drops
🔹 3. HEAD, PRESSURE & UPLIFT (FAVOURITE)
🔸 Total Head
(Velocity head usually neglected)
🔸 Pressure Head
🔸 Uplift Pressure
- Acts upward below hydraulic structures
- Maximum at upstream side
- Decreases along seepage path
🔸 Exit Gradient (VERY IMPORTANT)
Where d = depth of last field
🔸 Critical Hydraulic Gradient
🔸 Factor of Safety against Piping
✔ Safe if FOS ≥ 4 (exam value)
🔹 4. QUICK CONCEPT TABLES
Flow Line vs Equipotential Line
| Flow Line | Equipotential Line |
| Path of seepage | Constant head |
| Tangent = velocity direction | ⟂ to flow line |
| No head loss | Head loss occurs |
Effect of Seepage
| Aspect | Effect |
| Effective stress | Decreases |
| Uplift | Increases |
| Stability | Reduces |
🔹 5. IMPORTANT NUMERICAL EXAMPLES (SSC JE LEVEL)
🔢 Example 1 (Direct Flow Net Discharge)
Given:
k = 5 × 10⁻⁵ m/s
h = 6 m
Nf = 4
Nd = 12
🔢 Example 2 (Head Loss per Drop)
Total head = 8 m
Nd = 16
🔢 Example 3 (Exit Gradient)
h = 6 m
Nd = 12
d = 0.5 m
🔢 Example 4 (Critical Hydraulic Gradient)
Gs = 2.65
e = 0.65
🔢 Example 5 (Factor of Safety)
❌ Unsafe (needs ≥ 4)
🔹 6. 50 EXAM-LEVEL MCQs
(25 THEORY + 25 NUMERICAL)
🟢 THEORY MCQs (1–25)
Q1. Flow net consists of:
- Flow lines only
B. Equipotential lines only
C. Flow lines & equipotential lines ✅
D. Seepage lines
Q2. Flow lines and equipotential lines intersect at:
- 45°
B. 60°
C. 90° ✅
D. 120°
Q3. Discharge through each flow channel is:
- Different
B. Zero
C. Same ✅
D. Maximum at exit
Q4. Exit gradient is maximum at:
- Upstream end
B. Middle
C. Downstream exit ✅
D. Base center
Q5. Piping occurs when:
- i < ic
B. i = 0
C. i > ic ✅
D. i < 1
🟡 NUMERICAL MCQs (26–50)
Q26. k = 10⁻⁴ m/s, h = 10 m, Nf = 5, Nd = 10
Discharge = ?
A. 5×10⁻⁴
B. 10⁻⁴
C. 5×10⁻⁵
D. 5×10⁻⁴ m³/s/m ✅
Q27. If Nd = 20 and total head = 4 m, head per drop =
- 0.1 m
B. 0.2 m ✅
C. 0.4 m
D. 0.5 m
Q28. Exit gradient is given by:
- h/Nf
B. h/Nd
C. h/(Nd·d) ✅
D. k·i
Q29. Critical gradient depends on:
- Permeability
B. Grain size
C. Void ratio & specific gravity ✅
D. Water content
Q30. Factor of safety against piping should be:
- 1
B. 2
C. 3
D. ≥ 4 ✅
🔥 EXAM TRICKS & SHORTCUTS
✔ Q = k h (Nf/Nd) → no area needed
✔ Flow net → graphical Darcy’s law
✔ Exit gradient always checked at downstream
✔ ic = (Gs−1)/(1+e) → memorize
✔ Unsafe if i ≥ ic
(Q31–Q50 include: uplift pressure numericals, piping condition traps, flow net counting, quick substitutions)
SEEPAGE & FLOW NETS (SSC JE | RRB JE | OSSC JE)
focused on uplift pressure numericals, piping condition traps, flow-net counting, and quick substitutions — exactly exam-pattern.
🟡 NUMERICAL & CONCEPTUAL MCQs (31–50)
Q31. Total head causing seepage is 9 m and number of potential drops is 18.
Head loss per drop equals:
A. 0.25 m
B. 0.5 m ✅
C. 1.0 m
D. 1.5 m
Solution:
Q32. If pressure head at a point below a dam is 3 m, uplift pressure equals:
- 3 kN/m²
B. 9.81 kN/m²
C. 29.4 kN/m² ✅
D. 98.1 kN/m²
Explanation:
Q33. Exit gradient is maximum at:
- Upstream face
B. Middle of base
C. Downstream exit point ✅
D. Centerline
Q34. Given:
Total head = 6 m, Nd = 12, depth of last field d = 0.5 m.
Exit gradient is:
A. 0.5
B. 0.8
C. 1.0 ✅
D. 1.5
Solution:
Q35. For a soil with and , critical hydraulic gradient is:
- 0.5
B. 0.8
C. 1.0 ✅
D. 1.5
Solution:
Q36. If exit gradient equals critical gradient, the soil is:
- Safe
B. Unsafe
C. At failure (piping starts) ✅
D. Over-safe
Q37. Factor of safety against piping is defined as:
- B. ✅
C.
D.
Q38. Given and . Factor of safety is:
- 2
B. 3
C. 4 ✅
D. 5
Solution:
Q39. Piping is most likely to occur when:
- B.
C. ✅
D.
Q40. In a correct flow net, the number of:
- Flow lines equals equipotential lines
B. Flow channels affects discharge
C. Potential drops affects discharge
D. Both B and C ✅
Q41. Discharge through a flow net is proportional to:
- B. ✅
C.
D.
Q42. If number of flow channels doubles (all else same), discharge will:
- Halve
B. Remain same
C. Double ✅
D. Become zero
Q43. If number of potential drops doubles (all else same), discharge will:
- Increase
B. Remain same
C. Double
D. Reduce to half ✅
Q44. Which head component is usually neglected in seepage analysis?
- Elevation head
B. Pressure head
C. Velocity head ✅
D. Total head
Q45. Uplift pressure below a dam is:
- Uniform
B. Zero at upstream
C. Maximum at downstream
D. Maximum at upstream face ✅
Q46. Which parameter does NOT affect critical hydraulic gradient?
- Void ratio
B. Specific gravity
C. Permeability ✅
D. Soil density
Q47. Flow net is a graphical solution of:
- Bernoulli’s equation
B. Darcy’s law ✅
C. Navier–Stokes equation
D. Continuity equation only
Q48. For a given flow net, discharge is independent of:
- Head loss
B. Permeability
C. Width of dam (per m considered)
D. Shape of flow channels ✅
Q49. If seepage force increases, effective stress in soil will:
- Increase
B. Remain same
C. Decrease ✅
D. Become zero
Q50. Which statement is TRUE?
- Exit gradient is checked at upstream end
B. Factor of safety against piping should be < 1
C. Flow lines can intersect equipotential lines at any angle
D. Safe design requires against piping ✅
🔥 QUICK EXAM RECAP (DON’T MISS)
✔ Discharge:
✔ Head per drop:
✔ Exit gradient:
✔ Critical gradient:
✔ Piping starts when:
✔ Safe when:
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