Concrete answers
Why does concrete sink and settle?
The short answer
Concrete sinks when the ground underneath it moves or disappears. The usual causes are water washing soil out from under the slab, clay soil shrinking in dry weather, and fill that was not compacted properly before the pour.
The usual causes
- Washout: downspouts and runoff carry soil away from under the slab edge.
- Clay shrinkage: clay soils shrink when they dry out, leaving voids.
- Poor compaction: loose fill settles under the weight of the slab, often next to a foundation.
- Decaying roots or debris under the slab leaving gaps as they rot.
Where it shows up
At the garage door, where the drive meets the street, along walkways and on patios beside the house, often wherever roof water lands.
What can be done
If the slab is still in one piece, it can often be leveled. If it is broken, the section is usually replaced. Either way, fixing the water source is what keeps it from sinking again.
This page is general information. Every slab and every yard is different, so the right answer for your concrete comes from looking at it in person.
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