Module 3 · Lesson 3.1
Three ways to fail
Only one of them looks like failure.
Why this matters
Textbook bearing failure is dramatic: the ground heaves up beside the footing and the foundation tips. Most real foundations that are loaded to destruction do nothing of the kind. They just go down, and keep going down, and there is no moment you could point at and call failure. 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?
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint