What is Bearing Capacity?
Bearing capacity is the maximum load per unit area that soil can safely support without failure or excessive settlement. It is a critical factor in foundation design, determining whether the ground can withstand the weight of buildings and structures.
Bearing capacity is the safe load per unit area that soil can carry: q = P/A. It depends on soil cohesion, friction angle, foundation depth and width.
- 1↓Soil cohesion (c)Adhesion between soil particles; increases bc
- 2↓Friction angle (φ)Angle of internal friction; increases Nq
- 3↓Foundation depth (Df)Deeper foundations → higher bearing capacity
- 4Foundation width (B)Wider footprints → better load distribution
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Step-by-step worked examples
A 2 m × 3 m foundation supports a 600 kN load. Calculate bearing capacity.
A = 2 × 3 = 6 m² q = P / A = 600 / 6 = 100 kPa
A square footing 1.5 m × 1.5 m must support 450 kN. What is the bearing capacity?
A = 1.5 × 1.5 = 2.25 m² q = 450 / 2.25 = 200 kPa
A circular foundation (diameter 2.5 m) carries 800 kN. Find bearing capacity.
A = π × (1.25)² ≈ 4.91 m² q = 800 / 4.91 ≈ 163 kPa
Flashcards
Quick quiz
Q1.Foundation 2.5 m × 2.5 m, load 1000 kN. Bearing capacity?
Q2.Bearing capacity formula:
Q3.Which increases bearing capacity most?
Q4.For 500 kN load, Area 8 m². Bearing capacity in kPa?
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Common mistakes
Using gross load instead of net load above foundation. — Correct: Always use the actual load transferred to soil.
Ignoring soil type and assuming same bearing capacity everywhere. — Correct: Bearing capacity varies greatly with soil properties—test and design per site.
Forgetting to apply a factor of safety. — Correct: Safe bearing capacity = ultimate capacity / FS (typically FS=2–4).
Assuming rectangular foundations are always optimal. — Correct: Foundation shape, depth and width all affect load distribution and capacity.
FAQ
What is bearing capacity in civil engineering?
The maximum safe load per unit area that soil can support without failure, settlement, or bearing capacity failure.
How do I calculate bearing capacity?
Use q = P/A (basic formula) or Terzaghi's formula for detailed analysis: q = c·Nc + γ·Df·Nq + 0.5·γ·B·Nγ.
What factors affect bearing capacity?
Soil cohesion, friction angle, foundation depth, width, water table, load inclination, and soil density.
Why is bearing capacity important?
It ensures the foundation is safe—prevents settlement, tilt, or catastrophic failure of the structure.