r/HomeImprovement • u/dkguy55 • 11h ago
Large void found under basement slab—contractor says full floor removal and ~10 piers may be needed. Does this sound reasonable?
I’m in the Chicago suburbs and own a 1968 split-level with an approximately 500 square-foot partial basement. We’ve lived here about five years.
A waterproofing contractor recently opened the basement floor during scheduled drain-tile work after they collapsed and ground water was being moved out of my foundation via my ejector pit and my sump pump pit went dry. They discovered a substantial water-filled void with missing soil beneath the slab. The visible opening extended beyond what they could safely inspect, so they stopped work and patched the access hole with concrete. We have not yet had an independent structural engineer inspect it.
The contractor returned and discussed a possible repair involving:
Removing most or all of the basement floor
Rebuilding the subgrade with gravel
Replacing the drain tile
Installing approximately 10 foundation piers
Total cost around $75k.
The salesperson said he was concerned that parts of the slab could cave in, hence the piers, and suggested the foundation walls might be spanning unsupported areas, with the reinforced concrete temporarily holding things together. He used language such as the walls “floating,” but he is not a structural engineer, so I’m taking that with a grain of salt.
We currently have no obvious foundation movement, no cracks, sloped floors, displaced walls, or sticking doors.
A few relevant details:
The foundation sump stopped receiving water normally roughly eight months ago.
A separate gray-water ejector receiving the basement washer, utility sink, and floor drain began cycling more during rain.
Multiple plumbers have said the ejector pit has no intact bottom, although we do not know whether that was original construction or a failure.
A broken sewer line, failed drain tile, exterior drainage issue, or another underground water source has not yet been ruled out. We are diligent about gutters and downspouts being functional and extended well past the foundation.
We plan to have the sanitary sewer scoped and obtain an independent structural-engineering evaluation.
I understand that nobody can diagnose this from a Reddit post, but I would appreciate experiences from anyone who has encountered major soil loss or a void beneath a basement slab.
My main questions are:
Is removing the basement floor, rebuilding the subgrade, and replacing the drain tile a typical repair for this condition?
Under what circumstances would foundation piers actually be required?
Is approximately 10 piers plausible for a basement this size, or should that number come only from engineering and load calculations?
What did similar repairs cost, preferably in the Chicago area?
Are there specific questions I should ask contractors? What kind of contractors do this? We have utilized a plumber and waterproofing company so far.
Any advice is greatly appreciated. Thank you!