Every building, bridge, and retaining wall must transfer its weight safely into the ground. Bearing capacity is the ability of soil to support a foundation load without sudden shear failure or excessive sinking. Engineers use this idea to choose footing sizes, depths, and soil improvement methods.
It matters because a foundation can look strong in concrete but still fail if the soil beneath it is overstressed.
For a shallow strip footing, Terzaghi modeled the soil below the footing as a failure mechanism with a central wedge pushed downward and curved shear zones spreading outward. Engineers estimate an ultimate bearing capacity, then divide by a factor of safety to get an allowable bearing pressure. They also check settlement, because a footing may be safe against collapse but still move enough to crack walls or damage equipment.
Good foundation design satisfies both strength limits and serviceability limits.
Key Facts
- Bearing pressure is q = P / A, where P is applied load and A is footing contact area.
- Ultimate bearing capacity, qult, is the pressure at which soil is expected to fail in shear.
- Allowable bearing capacity is qall = qult / FS, where FS is the factor of safety.
- For a strip footing in Terzaghi theory, qult = cNc + qNq + 0.5γBNγ.
- Surcharge at footing depth is q = γDf, where γ is soil unit weight and Df is embedment depth.
- A design is acceptable only if qapplied ≤ qall and predicted settlement ≤ allowable settlement.
Vocabulary
- Bearing capacity
- Bearing capacity is the maximum pressure soil can support under a foundation before failing or deforming too much.
- Ultimate bearing capacity
- Ultimate bearing capacity is the foundation pressure that causes a shear failure mechanism to form in the soil.
- Allowable bearing capacity
- Allowable bearing capacity is the reduced design pressure obtained by applying a factor of safety to the ultimate capacity.
- Failure wedge
- A failure wedge is the block of soil directly under a footing that moves downward as surrounding soil shears outward.
- Settlement
- Settlement is the downward movement of a foundation caused by compression or distortion of the soil beneath it.
Common Mistakes to Avoid
- Using ultimate bearing capacity as the design pressure is wrong because foundations need a factor of safety for uncertainty in soil strength, loads, and construction conditions.
- Checking shear failure but ignoring settlement is wrong because a footing can avoid collapse yet still sink enough to damage the structure.
- Forgetting to use the actual footing contact area in q = P / A is wrong because bearing pressure depends on the load spread over the soil, not just the column size.
- Assuming deeper footings are always better is wrong because embedment can increase surcharge resistance, but drainage, weak layers, groundwater, and excavation effects may control the design.
Practice Questions
- 1 A square footing carries a service load of 900 kN and has dimensions 2.0 m by 2.0 m. Calculate the applied bearing pressure in kPa.
- 2 A soil has an estimated ultimate bearing capacity of 450 kPa. If the required factor of safety is 3.0, find the allowable bearing capacity. Is a footing pressure of 180 kPa acceptable for shear strength?
- 3 A footing has an applied pressure lower than its allowable bearing capacity, but settlement analysis predicts 65 mm while the project limit is 25 mm. Explain whether the design is acceptable and what an engineer might change.