Sinking or settling concrete
A slab section sits lower than its neighbours, creating a lip at the joint. Common on driveways near the garage apron, on walkways over backfilled trenches, and on patios adjacent to a foundation.
VerdictCosmetic — does not compromise the slab
What causes it
The slab did not fail — the ground under it moved. Uncompacted backfill settles under its own weight and under load. Water washing through or under the base erodes it, which is why sinking often follows a plumbing leak, a downspout discharging at the slab edge, or poor drainage. Expansive clay soils move seasonally with moisture, lifting and dropping slabs on a cycle.
How it is repaired
A sound slab that has simply settled can be lifted rather than replaced. Mudjacking pumps a cement slurry through drilled ports to fill the void and raise the slab; polyurethane foam injection does the same with expanding foam, weighs far less, and cures in minutes. Both cost a fraction of replacement and take hours. A slab that is cracked into several pieces is usually past lifting.
How to prevent it
Compact the base in lifts rather than dumping and grading. Fix the water first — downspout extensions, positive drainage away from the slab, and any leak — because a lifted slab over an unresolved water path settles again.