Two houses on the same street, built the same year by the same builder. One driveway is still flat. The other has a slab by the garage that drops a little more every spring, and a dip in the front walk that runs in a straight line toward the street.
Same clay, same winters. The difference is what happened in the top few feet of soil under each slab: how it was dug, how it was filled back in, and where the water goes now.
Our guide to how concrete lifting works covers why slabs sink in one paragraph. This is the long version, written for northeast Wisconsin ground.
The Slab Is Just Along for the Ride
Concrete weighs roughly 140 to 150 pounds per cubic foot, so a 4-inch slab comes to about 50 pounds per square foot. A 10-by-12-foot driveway section weighs close to 3 tons. It can't float and it doesn't bend much. When the support under it changes, it goes along.
Every case of settling comes down to one of four things the soil can do:
| What the soil does | What's happening | Typical trigger |
|---|---|---|
| Squeezes | Loose soil packs tighter under weight, water, and time | Foundation backfill, trench fill, an uncompacted base |
| Washes | Moving water carries fine soil out from under the slab | Downspouts, sump lines, open joints |
| Rots | Buried wood and roots decay and leave empty space | Old stumps, removed trees, construction scraps |
| Softens | Saturated or thawing soil loses strength for a while | Spring thaw, soaked clay, standing water |
Most sunken slabs have two or three of these going at once.
What's Under Your Slab Around Green Bay
- Red clay till. The Green Bay Lobe of the last glacier left reddish, fine-grained till across the area. The state geological survey groups it in the Kewaunee Formation, and some of its layers test at more than a third clay.
- Old lake bottom. When ice blocked drainage toward Green Bay, Glacial Lake Oshkosh filled the Fox River valley and laid down layers of very fine sand, silt, and clay.
- Slow drainage. USDA descriptions of the reddish clay-till soils common across eastern Wisconsin list 35 to 60 percent clay in the subsoil and slow water movement. One, the Manawa series, has a seasonal perched water table 1 to 2.5 feet down, not far below the gravel under a driveway.
What that means for concrete: clay holds water for days or weeks after a rain or thaw, loses strength when soaked, and, along with silt, is the most frost-susceptible kind of soil. It also shrinks as it dries, so a long dry spell near big trees can pull it down and away from slab edges.
And there's plenty of water. Green Bay's 1991–2020 climate normals show about 31.6 inches of precipitation a year, with roughly 55 inches of snowfall. March through June averages more than 12 inches of precipitation, arriving just as a winter of snow melts and the frost comes out.
The Six Ways Slabs Settle Here
1. The overdig around your basement
Basements are dug wider than the walls so crews have room to work. Once the walls are up, the gap is filled back in, and that backfill goes in looser than the undisturbed ground around it. Building-trade guidance treats its settling as a given.
It's also deep, which is why small changes add up. If a 7-foot column of loose backfill compresses by just 1 percent, the surface drops about 7/8 of an inch. That's a real trip edge from a change you'd never see in the soil.
The stoop, the walk along the foundation, the AC pad, the patio by the back door, and the garage apron all sit on that zone. Backfill settles fastest in the first few years and slows after that, but every soaking speeds it up. A slab dropping along the house usually tips toward it, steering more roof water into the same backfill. See caulking where concrete meets the house and our sinking AC pad guide.
2. Utility trenches across the yard
Your water service and sewer lateral both run from the house to the street, below frost. The Green Bay Water Utility has said its mains sit about 6 feet down, roughly a foot below where frost usually reaches, and your service line's trench crossed the yard to meet one.
Trench backfill settles in a strip that follows the pipe, which gives it a signature: a dip that runs in a straight line through the walk or across the driveway, pointed at the street.
- The clock restarts with every dig. A lateral replaced three years ago is a brand-new trench, even under a 30-year-old driveway.
- The lateral may be yours. In many Wisconsin communities, the property owner owns the sewer lateral, in some places all the way to the main. If a trench line keeps sinking and drains are slow, have a plumber camera the line. A leaking lateral washes soil out from below.
3. A base nobody compacted
Under the concrete there should be firm ground and usually a layer of compacted gravel. When the gravel was dumped and raked but never compacted in thin layers, or the slab went over soft clay, leftover topsoil, or fill with sod in it, the base keeps packing down after the truck leaves.
The national model residential code's rules for floor slabs make a fair benchmark: fill free of vegetation and debris, compacted for uniform support, and no more than 8 inches of earth fill unless the building official approves more.
On clay, a base can also become a bathtub. Gravel surrounded by clay can collect yard runoff with nowhere to drain, which keeps the clay below it soft and feeds frost heave every winter.
The tell: a newer slab that dropped across most of its area, not at one corner, with no downspout nearby. See new concrete cracking for what to ask your installer.
4. Water carrying soil away
At Green Bay's normal precipitation, a 1,000-square-foot section of roof sheds close to 20,000 gallons a year. When one downspout delivers all of it to the corner of the stoop, every storm carries a little more fine soil out from under the slab. Sump lines and open joints do the same, and so does the gap at the house.
Silt washes out first, so the void under a washout is usually bigger than the gap you see at the edge. Our voids article shows how to check for one, and how to extend downspouts covers the fix.
5. Things buried under there that rot
Wood breaks down underground slowly and leaves empty space behind. The usual sources are roots from a removed tree (plenty of ash trees have come down around here), construction scraps like form boards and stakes left in backfill, and topsoil or sod left under a base. Rot is slow, so it shows up under older concrete or years after a tree came down. See tree roots lifting concrete.
6. Frost and thaw working the soil loose
Green Bay averages about 152 days a year with a low of 32°F or colder. Wet clay under a plowed slab grows ice, heaves, and then thaws from the top down, trapping meltwater above the soil that's still frozen. For a few weeks each spring, the ground right under your slab is at its softest. WisDOT's spring weight limits, which average about March 9 to May 9, exist for the same reason.
A slab rarely comes down from a heave exactly where it started, and a car or loaded trailer on it during those soft weeks can press it into the mud. Our frost heave vs. settling guide shows how to tell a winter rise from settling that's there to stay.
Where It Sank Tells You Why
| Where the drop is | Most likely cause | Clues | What to do first |
|---|---|---|---|
| Next to the house (stoop, patio, walk, AC pad, apron) | Foundation backfill plus roof water | Tips toward the house, gap opening at the wall, downspout nearby | Fix the downspout and grade, then lift and seal the house gap |
| A straight strip toward the street | Utility trench backfill | Lines up with the water meter, sewer cleanout, or curb box | Ask about past utility work. Slow drains? Camera the lateral before lifting |
| At the street end | Street or curb work backfill, the deepest part of utility trenches, melting plow piles | Recent road project, joint opening at the curb, water standing at the end of the drive | In Green Bay, aprons and public walks are right-of-way. Check permit rules first |
| Near a living tree | Usually heave (up), sometimes dry-summer clay shrinkage | One edge higher, facing the tree | Lifting can't lower it. See the tree roots guide |
| Where a tree used to be | Rotting roots | Sinking began a few years after removal | A lifting job. Foam fills root voids as it lifts |
| Under a downspout or sump outlet | Washout | Gully in the mulch, gap under the edge, hollow sound | Move the water, then lift |
| Middle of a newer slab | Uncompacted base | Whole slab dropped evenly in its first few years | Call the installer first. It may be covered |
| Every slab, a little more each spring | Frost and thaw on wet clay | Worse after the thaw, water ponding nearby | Fix drainage, re-measure in late May, then decide |
The Timeline: Years 1 to 3 vs. Year 10 and Beyond
| Age of the concrete, or of the dig under it | What's usually behind it | What it looks like |
|---|---|---|
| Years 1–3 | Foundation backfill, fresh trench fill, an uncompacted base | Stoops and aprons dropping, slabs pulling away from the house, a dip over a new lateral |
| Years 3–10 | Water patterns: grade flattened as backfill settled, worn-out sealant, extensions kicked aside | A low corner by a downspout, a slab tipping toward the foundation |
| Year 10 and beyond | Slow washouts, rotting roots and wood, years of frost in wet clay | Hollow corners, rocking slabs, a slab near a long-gone tree dropping |
| Any age, right after a dig | New trench, removed tree, addition, moved sump outlet | Settling within a year or two of the work |
Early settling is mostly about how the ground was built. Later settling is mostly about water. That's the good news, because water is the part you can control.
What You Can't Change, and What You Can
You can't swap out Brown County clay or shorten winter. You can control where roof and sump water land, whether runoff gets down through joints and the gap at the house, whether the soil slopes away from the house (see soil grading around concrete), and how much weight sits on slab edges during the thaw. We ranked those by payoff in how to prevent concrete settling.
When Settling Is a Lifting Job, and When It Isn't
If a slab sank, it's still in one or a few solid pieces, and you've found the water source, foam lifting is built for it. We drill a few 5/8" holes, about the size of a dime, and inject high-density structural foam that fills the voids first, then raises the slab in small, controlled increments. The foam weighs a few pounds per cubic foot, so it doesn't load soil that already gave way, and it's waterproof once cured. Walk on it right away and drive on it the same day.
It isn't a lifting job when the slab went up (foam can only raise concrete), when it's broken into many pieces or crumbling (see is your concrete too damaged to lift?), when a pipe is still leaking underneath, or when the ground is frozen. Our lifting season runs from spring thaw to freeze-up.
One honest caution: if the trench or backfill under a slab is less than a year old, it may not be done settling. Foam supports the slab, but soft fill deeper down can still compress. We may suggest waiting through one spring thaw.