Foundation Engineering / How a footing fails
Three ways to fail
Only one of them looks like failure.
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
- Removed the hidden cap from the friction-angle sensitivity 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.
- Friction-angle selection and general/local failure assumptions require ground-specific assessment.
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
Textbook bearing failure is dramatic: the ground heaves up beside the footing and the foundation tips. Some foundations develop a progressive load–settlement response with no distinct peak. Knowing which sort of ground gives which behaviour tells you whether your calculated ultimate load means anything.
By the end of this lesson you should be able to
- Distinguish general, local and punching shear failure
- Say which soil conditions produce each
- Explain what 'ultimate bearing capacity' means when there is no peak
General shear failure happens in dense sands and stiff clays. A continuous slip surface develops from the edge of the footing right up to the ground surface. Load rises to a clear peak and then falls; the ground beside the footing lifts visibly; the footing usually rotates. This is the mode the classical bearing capacity equation describes.
Local shear failure is the intermediate case. The slip surface starts to form but dies out before reaching the surface. There is some heave and a poorly defined peak — often no true peak at all, just a knee in the load–settlement curve.
Punching shear failure happens in loose sands and soft clays. No slip surface reaches the surface at all. The footing drives a column of soil vertically downwards, compressing it. The load–settlement curve simply steepens; there is no peak, no heave, and no obvious failure.
The practical consequence is that the mode you are in tells you which calculation governs.
In dense sand or stiff clay, general shear applies, the bearing capacity equation describes a real mechanism, and the calculated ultimate load means something physical.
In loose or soft ground, punching applies. The footing will have settled unacceptably long before it reaches the calculated capacity, so the bearing calculation is not the one that decides the design — and applying the classical equation without reducing the strength parameters overestimates capacity, because it assumes a mechanism that never forms.
Check yourself
A plate loading test on loose sand produces a load–settlement curve that steepens steadily with no peak, and no heave is seen at the surface. What is happening, and what does it mean for design?