Most of the sunken concrete we see is still good concrete. It's sitting lower than it used to because something changed underneath it. Knowing which cause is at work tells you how to fix it and how to keep it from happening again.
1. Water washes the base out
Concrete is poured in sections with joints between them. When those joints aren't sealed, rain, snowmelt and downspout water run straight down through them. Moving water carries fine sand and silt with it. Over a few years, that leaves a hollow under the slab edge, and the slab drops into it.
Look at where your downspouts empty. A downspout dumping onto a driveway or patio edge is the most common single cause of a sunken corner.
2. Freeze-thaw keeps loosening the soil
Clay-heavy soils common around Green Bay hold water. When that water freezes it expands and can lift the slab slightly; when it thaws, the soil underneath is wetter and softer than before. Repeat that every winter and the base gradually loosens, especially at joints and edges where water gets in.
3. Backfill settles next to the house
When a house is built, the basement hole is dug wider than the foundation, then the extra space is filled back in. That fill is rarely compacted as well as undisturbed soil. It settles for years, and anything sitting on it, like the garage apron, front walk, stoop or a patio along the house, settles with it. That's why so many homes under ten years old already have a drop at the garage.
4. Other causes
- Tree roots that pull water out of the soil in dry summers, or pushed a slab up (that one needs grinding or replacing, not lifting).
- Leaking water lines or broken drain tile under a slab.
- Burrowing animals under patios and stoops.
- A thin or missing gravel base when the concrete was poured.
Why the house doesn't sink but the slabs do
Wisconsin's building code puts house footings at least 48 inches below grade, below the frost line (SPS 321.16). Driveways, walks, patios and most stoops sit on a few inches of gravel on top of the soil, so they ride every freeze, thaw and washout that the house is built below. That's why a sunken slab next to the house is rarely a foundation problem.
What stops it
- 1Lift what sankFoam lifting fills the voids and raises the slab without adding weight to soft soil.
- 2Seal the jointsJoint sealing closes the path water uses to wash the base out.
- 3Move the waterExtend downspouts away from slabs and keep the ground sloping away from the house.
How the lift itself works, step by step: how foam concrete lifting works.