Punching shear
Also called Punching failure, Local shear failure
A local failure in which a concentrated load pushes a plug of material through the element supporting it, rather than that element bending or the ground settling under it — an outrigger foot through a thin pad, or a mat through a firm crust into softer material below. It is governed by what lies directly beneath and around the loaded area, so a larger mat of the same thickness does not necessarily prevent it.
In practice
Three cases turn up on site. A foot bearing on a pad too thin or too flexible for it punches the pad. A mat standing on a thin competent layer over weak ground punches into what is underneath, which is why a surface test can reassure and still mislead — the layer that was tested is not the layer that fails. And plant standing over a suspended slab, a basement lid or a cellar arch raises a punching check on that structure, which belongs to whoever is responsible for it rather than to the mat supplier.
Where this comes from: Standard structural and geotechnical terminology. Whether a particular case is governed by punching, by bending or by general bearing failure is a design assessment for the temporary works designer, not something a mat specification answers.
Where it matters on site
The signature is local: one foot goes down while the other three do not, or a mat dishes around the foot while the ground beside it is undisturbed. Centring the foot and bedding the pad on a flat, prepared surface matters more here than plan area does, because an off-centre foot bears near an edge where there is less material to resist it. Where the ground has been assessed only at the surface, or only where the permanent foundations go, the punching case is unanswered and the lift plan should say so.