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.
Understanding Engineering: Bearing Capacity of Foundations
Soil does not behave like a solid block of concrete. It is made of separate grains, water, air, and sometimes clay minerals that hold water at their surfaces. A load makes the grains move against each other.
In dense sand or gravel, resistance mainly comes from friction and from grains locking together. In clay, resistance can come from cohesion, which is the tendency of fine particles to stick together. Loose sand can compress quickly as grains rearrange.
Soft clay may keep moving for months or years after construction. This is why two sites that look similar from the ground surface can need very different foundation designs.
Water has a major effect on soil strength. Below the water table, water fills many of the spaces between grains. The water pressure carries part of the stress, leaving less grain to grain contact to resist sliding.
Heavy rain, flooding, leaking pipes, or poor drainage can therefore reduce the soil strength near a foundation. Some clays swell when wet and shrink when dry, causing repeated ground movement.
Engineers investigate these conditions by drilling boreholes, collecting samples, measuring groundwater, and running field tests. A soil report identifies the layers below a site, since a thin weak layer beneath stronger soil can control the design.
The shape and position of a foundation change how its load enters the ground. A wider footing spreads the load over more soil, but it may reach a deeper weak layer if it becomes very large. A deeper footing can gain support from firmer material and from the weight of soil above its base.
It can still be affected by nearby excavations, slopes, utility trenches, or foundations for neighbouring buildings. Loads should act near the centre of a footing. When a column load is off centre, the pressure becomes greater on one side.
The edge with high pressure may sink more, making the structure tilt. Wind, earthquakes, vehicle braking, and machines can add sideways or changing loads that must be considered.
Settlement is often the issue people notice first in real buildings. Small, even settlement may cause little harm. Differential settlement is more serious because one part of a structure moves more than another.
It can create cracks in brickwork, jam doors, slope floors, and strain water or gas pipes. Engineers estimate immediate settlement in sands and long term consolidation settlement in saturated clays. They may use raft foundations to spread loads, piles to transfer loads to deeper ground, or ground improvement methods such as compaction and drainage.
During construction, survey points and settlement plates can track movement. Students should pay attention to the difference between soil strength and soil stiffness.
Strength concerns sudden sliding failure. Stiffness concerns how much the ground deforms under normal working loads.
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.