FOUNDATION ENGINEERING – SHALLOW FOUNDATIONS
(DESIGN + PYQs | SSC JE | RRB JE | OSSC JE | State AE/JE)
👉 3–6 questions every exam (direct + conceptual)
👉 Design basics + settlement + depth + proportioning
👉 Very high scoring if formulas & checks are clear
🔹 1. ONE-PAGE PRECISE ONE-LINER NOTES (EXAM FOCUSED)
🔸 Foundation
- Substructure that transfers structural load to soil safely
- Must satisfy:
- Bearing capacity
- Settlement criteria
- Stability
🔸 Shallow Foundation
- Foundation with:
Where
Df = depth of foundation
B = width of footing
🔸 Types of Shallow Foundations (VERY IMPORTANT)
| Type | Use |
| Isolated footing | Single column |
| Combined footing | Two columns close |
| Strap footing | Eccentric load |
| Strip footing | Load-bearing walls |
| Raft (Mat) | Low SBC / many columns |
🔸 Criteria for Design of Shallow Foundations
✔ Bearing capacity safety
✔ Settlement within permissible limits
✔ No sliding / overturning
✔ Economical dimensions
🔹 2. DESIGN STEPS (SSC JE FAVOURITE)
🔸 Step 1: Load Calculation
P = \text{Column load} + \text{Self weight of footing (≈10%)}
🔸 Step 2: Area of Footing
🔸 Step 3: Dimensions
- Square footing:
- Rectangular footing:
🔸 Step 4: Check Bearing Pressure
🔸 Step 5: Settlement Check
- Immediate
- Consolidation (clay)
🔹 3. DEPTH OF FOUNDATION (RANKINE FORMULA – VERY IMPORTANT)
🔸 Minimum Depth
Where:
q = applied pressure
γ = unit weight of soil
🔹 4. SETTLEMENT CRITERIA (DIRECT MCQs)
🔸 Permissible Settlement (IS PRACTICE)
| Structure | Settlement |
| Isolated footing | 25 mm |
| Raft foundation | 50 mm |
| Differential | L/500 |
🔸 Types of Settlement
| Type | Soil |
| Immediate | Sand |
| Consolidation | Clay |
| Secondary | Organic clay |
🔹 5. COMBINED & STRAP FOOTING (CONCEPTUAL)
🔸 Combined Footing
- Used when:
- Columns are close
- SBC is low
- Shape:
- Rectangular
- Trapezoidal
🔸 Strap Footing
- Two isolated footings connected by strap beam
- Used for eccentric column near boundary
- Strap beam does not transfer soil pressure
🔹 6. RAFT (MAT) FOUNDATION
🔸 Used When
✔ SBC is very low
✔ Columns are closely spaced
✔ Settlement control is critical
🔸 Advantages
✔ Reduces differential settlement
✔ Load shared by entire area
🔹 7. IMPORTANT NUMERICAL EXAMPLES (SSC JE LEVEL)
🔢 Example 1 (Area of Footing)
Given:
Column load = 900 kN
SBC = 200 kN/m²
🔢 Example 2 (Square Footing Size)
🔢 Example 3 (Bearing Pressure Check)
If actual area = 2.2 × 2.2 = 4.84 m²
✔ Safe
🔢 Example 4 (Minimum Depth)
Given:
q = 180 kN/m²
γ = 18 kN/m³
φ = 30°
🔢 Example 5 (Conceptual)
Low SBC + many columns → Raft foundation
🔹 8. PYQ-LEVEL MCQs (SSC JE / RRB JE)
🟢 THEORY MCQs (1–15)
Q1. A foundation is shallow if:
- Df > B
B. Df = B
C. Df < B
D. Df ≤ B ✅
Q2. Strap footing is provided when:
- SBC is high
B. Column is near property line ✅
C. Columns are close
D. Soil is clay
Q3. Raft foundation is most suitable when:
- Loads are small
B. SBC is high
C. SBC is low & columns are close ✅
D. Soil is rock
Q4. Which footing minimizes differential settlement?
- Isolated
B. Combined
C. Strip
D. Raft ✅
Q5. Differential settlement permissible limit is:
- L/250
B. L/300
C. L/400
D. L/500 ✅
🟡 NUMERICAL / CONCEPT MCQs (16–30)
Q16. Load = 600 kN, SBC = 150 kN/m²
Area required =
A. 2
B. 3
C. 4
D. 4 m² ✅
Q17. Increasing area of footing will:
- Increase pressure
B. Reduce pressure ✅
C. Not affect pressure
D. Increase settlement
Q18. Minimum depth of foundation depends on:
- Load only
B. SBC only
C. φ & unit weight of soil ✅
D. Width only
Q19. Which footing is economical for wall loads?
- Isolated
B. Combined
C. Strip footing ✅
D. Raft
Q20. Self-weight of footing is usually assumed as:
- 2%
B. 5%
C. 10% ✅
D. 20%
(Remaining PYQs include: area traps, depth formula substitution, footing type selection, settlement concept MCQs)
🔥 EXAM TRICKS & SHORTCUTS (VERY IMPORTANT)
✔ Area = Load / SBC (most repeated)
✔ Df / B ≤ 1 → Shallow foundation
✔ Raft → settlement control
✔ Strap beam does NOT rest on soil
✔ Differential settlement = L/500
✔ Depth formula uses (1−sinφ)/(1+sinφ)