Where Voids Come From
Concrete does not need to sink for trouble to be underway. In Minnesota, spring melt and heavy rain wash fine soil out from under slabs a little at a time, erosion follows water paths along pipes and footings, and disturbed backfill keeps settling for years after construction. The result is a hollow layer between the concrete and the ground. A slab can bridge a void for a while, sounding hollow underfoot, until a heavy load or one more freeze-thaw cycle cracks it through.
What We Fill
Anywhere water or settling soil has left empty space under concrete, the foam can reach it through dime-sized holes. Common void-fill work includes:
- Hollow spots under driveway and sidewalk slabs that have not dropped yet
- Washed-out space under and behind concrete steps
- Erosion voids around culverts and drain pipes
- Gaps under garage, basement, and shop floors
- Loose zones under footings that need stabilizing
Stabilize Now or Lift Later
Filling a void is cheaper and simpler than lifting a slab that has already settled into one, and far cheaper than replacing concrete that cracked over hollow ground. The foam expands to fill the entire cavity, compacts the loose soil around it, and cures waterproof in about 15 minutes, so the next washout event has nothing left to carve away. Because the material is lightweight, it stabilizes weak ground without adding the load that a slurry fill would put on it.
Why homeowners pick us for void filling
- Stops settling before slabs drop or crack
- Compacts and stabilizes the loose soil around the void
- Waterproof foam that washout cannot erode away
- Lightweight fill that adds almost no load to weak ground
- Reaches under culverts, steps, and slabs through dime-sized holes
- Surfaces stay in service; most fills are done in hours
How the job runs
- Step 1 of 4
Find the voids
We check for hollow-sounding sections, settlement clues, and washout paths to map where the empty space sits.
- Step 2 of 4
Drill access holes
Dime-sized holes are drilled through the slab or alongside the structure so the foam can reach the full cavity.
- Step 3 of 4
Fill and stabilize
Expanding foam packs the void, presses into loose soil, and cures in about 15 minutes without lifting anything out of place.
- Step 4 of 4
Confirm and patch
We verify the cavity is full, patch the holes, and flag any drainage issues that created the void in the first place.
What does void filling cost in the Twin Cities?
The volume of the void is the biggest driver, since material scales with the space being filled. Depth, access, and the number of separate cavities matter too. A hollow spot under one sidewalk panel is a quick visit, while washout running the length of a culvert takes more time and foam. The useful comparison is with what the void leads to if it is left alone: a cracked or settled slab that then needs lifting or replacement. Filling early is the smaller bill. We scope it on site and quote before drilling.
Fair questions, straight answers
How do I know if there is a void under my concrete?+
The classic sign is a hollow sound when you tap or walk on the slab, especially near edges and joints. Others include soil washing out beside the concrete, new cracks over one area, and water disappearing under a slab during rain. We can check suspect areas during an assessment and map what is under there.
Should a void be filled even if the slab has not settled?+
Yes, that is the ideal time. A slab bridging a void is carrying loads it was never designed to span, and one heavy vehicle or hard freeze can crack it. Filling the cavity while the concrete is still at grade avoids both the crack and the eventual lift, at lower cost than either.
Can you fill washout around a culvert?+
We can. Water moving along a culvert pulls soil away from the pipe over time, leaving voids that let the surface above sag or the pipe shift. The foam packs those cavities, stabilizes the surrounding soil, and cures waterproof, so the flow path through the pipe stays where it belongs.
Will the foam wash out like the soil did?+
No. Cured polyurethane is waterproof and permanent, so the water that carved out the original void has nothing to carry away. That is a key difference from slurry-based fills, which can erode again. Pairing the fill with better drainage keeps new voids from forming beside the repair.