SEEPAGE & FLOW NETS #SOIL MECHANICS

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

4

🔹 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:

1. Flow lines only
   B. Equipotential lines only
   C. Flow lines & equipotential lines ✅
   D. Seepage lines

Q2. Flow lines and equipotential lines intersect at:

1. 45°
   B. 60°
   C. 90° ✅
   D. 120°

Q3. Discharge through each flow channel is:

1. Different
   B. Zero
   C. Same ✅
   D. Maximum at exit

Q4. Exit gradient is maximum at:

1. Upstream end
   B. Middle
   C. Downstream exit ✅
   D. Base center

Q5. Piping occurs when:

1. 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 =

1. 0.1 m
   B. 0.2 m ✅
   C. 0.4 m
   D. 0.5 m

Q28. Exit gradient is given by:

1. h/Nf
   B. h/Nd
   C. h/(Nd·d) ✅
   D. k·i

Q29. Critical gradient depends on:

1. Permeability
   B. Grain size
   C. Void ratio & specific gravity ✅
   D. Water content

Q30. Factor of safety against piping should be:

1. 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:

9. 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:

1. 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:

1. 0.5
   B. 0.8
   C. 1.0 ✅
   D. 1.5

Solution:

Q36. If exit gradient equals critical gradient, the soil is:

1. Safe
   B. Unsafe
   C. At failure (piping starts) ✅
   D. Over-safe

Q37. Factor of safety against piping is defined as:

1. B. ✅
   C.
   D.

Q38. Given and . Factor of safety is:

1. 2
   B. 3
   C. 4 ✅
   D. 5

Solution:

Q39. Piping is most likely to occur when:

1. B.
   C. ✅
   D.

Q40. In a correct flow net, the number of:

1. 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:

1. B. ✅
   C.
   D.

Q42. If number of flow channels doubles (all else same), discharge will:

1. Halve
   B. Remain same
   C. Double ✅
   D. Become zero

Q43. If number of potential drops doubles (all else same), discharge will:

1. Increase
   B. Remain same
   C. Double
   D. Reduce to half ✅

Q44. Which head component is usually neglected in seepage analysis?

1. Elevation head
   B. Pressure head
   C. Velocity head ✅
   D. Total head

Q45. Uplift pressure below a dam is:

1. Uniform
   B. Zero at upstream
   C. Maximum at downstream
   D. Maximum at upstream face ✅

Q46. Which parameter does NOT affect critical hydraulic gradient?

1. Void ratio
   B. Specific gravity
   C. Permeability ✅
   D. Soil density

Q47. Flow net is a graphical solution of:

1. 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:

1. 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:

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

Q50. Which statement is TRUE?

1. 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: