Differential settlement
Also called Uneven settlement, Out-of-level settlement
Settlement that differs from one bearing point to another, so a machine or structure goes out of level rather than settling evenly. For a crane on outriggers it is the more dangerous of the two: the machine was levelled before the lift, and a foot that settles further than its neighbours tilts it while it is loaded.
In practice
The causes are usually differences the set-up never looked for — one foot over a backfilled service trench or a soakaway and the rest on undisturbed ground, one on a slab and one on soil, one mat bedded flat and one bridging a high spot. What makes tilt worse than settlement is that it feeds itself: going out of level pushes the load radius out on the low side, which increases the reaction at that outrigger, which settles it further. Load charts are written for a machine standing level, so a machine that is not level is no longer working to the chart it was planned against.
Where this comes from: Standard geotechnical and lift planning practice. The out-of-level a particular machine may work at comes from that machine’s own documentation, and the decision on the day belongs to the appointed person and the crane supervisor.
Where it matters on site
This is a during-the-lift check, not only a set-up check. Levels taken before rigging say nothing about what happens once the ballast is on and the load is picked, so movement during the lift is a stop rather than bedding in. Matching the mats across all four feet is usually worth more than upsizing one of them — equal areas on unequal ground still settle unequally — and where one foot bears on a structure and another on soil, expect different stiffness even when both are adequate in capacity.