Foundation Engineering / What a foundation has to do
Two requirements, not one
Why a foundation that cannot fail can still be wrong.
Lesson review: expert verification remains open
This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.
Corrections checked in this pass
- Replaced the settlement influence approximation with the flexible rectangular solution and a separately identified rigid-square factor. Corrected the net-load comparison.
Checks needed for project use
- A geotechnical reviewer must confirm drainage and stress basis, bearing-factor formulation, correlations and settlement influence factors. Partial factors and project-specific ground conditions are outside these teaching checks.
- Account for excavation and foundation weight when establishing actual net load; surface half-space comparison excludes embedment and soil layering.
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesWhy this matters
A foundation transfers the structure’s loads into the ground. Its design must address bearing failure and movement, including differential settlement. A strength check alone cannot establish whether the supported structure will tolerate that movement.
By the end of this lesson you should be able to
- State the two requirements and the limit each is checked against
- Explain why settlement usually governs on clay
- Recognise that the two checks use different soil properties
Every foundation has to satisfy two separate requirements.
It must not collapse. The ground beneath must be able to carry the load without a shear failure — soil squeezing out sideways and the foundation punching down. This is the ultimate limit state, and the property that governs it is the soil's strength.
It must not move too much. Even a foundation far from collapse settles, and the structure above has to tolerate it. This is the serviceability limit state, and the property that governs it is the soil's stiffness.
Strength and stiffness are different properties. A soil can be strong and soft, which is common in overconsolidated clay, and a foundation on it will pass the first check comfortably and fail the second.
Consider a 2.5 m square pad founded 1.2 m below ground in clay with undrained shear strength 75 kPa and unit weight 19 kN/m³. With the stated undrained square-footing model and no depth factor, the net ultimate pressure is approximately 461 kPa. A traditional global factor of 3 gives 154 kPa. This is an illustrative allowable-pressure calculation, not a complete Eurocode 7 design.
At a net working pressure of 150 kPa, the net load is 937.5 kN. An elastic homogeneous-ground estimate with E = 25 MPa, Poisson’s ratio 0.4 and the stated shape factor gives 14.1 mm settlement. Whether that is acceptable depends on differential movement, the supported structure and the ground model.
Increase the square footing width from 2.5 m to 6 m while keeping pressure at 150 kPa. The net load rises from 937.5 kN to 5,400 kN. In this undrained model, the net ultimate pressure remains approximately 461 kPa; the pressure margin does not improve. The elastic settlement estimate rises to 33.9 mm because, at fixed pressure and otherwise unchanged assumptions, settlement is proportional to width.
That comparison does not hold total load constant. For similar square footings under a fixed total load, pressure varies as 1/B² and this elastic estimate of settlement varies as 1/B. A wider footing therefore reduces settlement in that particular model. Layered ground, foundation rigidity and changes in load require separate analysis.
Check yourself
A pad foundation on firm clay is found to have a factor of safety of 3.1 against bearing failure, and a predicted settlement of 45 mm against a structural limit of 25 mm. What does the design need?