Concrete sinks when the soil beneath it erodes, shrinks, or settles—removing the support a slab needs to stay level. This guide explains the specific causes homeowners see most often and what to do so a fix actually lasts.
Key Takeaways (Why Concrete Sinks Q&A)
What is the number one reason concrete sinks?
Concrete sinks because the soil or base material beneath it erodes or settles, creating voids that let the slab drop.
How do plumbing leaks make concrete sink?
Pressurized water supply leaks and sewer discharge leaks saturate and soften soils, wash fines away, and create voids under slabs—especially along low spots where water collects.
Why do new driveways and patios settle more than older ones?
Freshly disturbed soil from construction or utility work can take 5–10 years to compact naturally, so new slabs are more likely to settle at garage entries and along edges.
What soil types cause the most slab movement in Chicagoland?
Expansive clays (often containing montmorillonite) swell when wet and shrink when dry, driving heave in wet periods and drop during droughts.
Why Concrete Sinks (and What Homeowners Can Do About It)
1) Water Erosion & Soil Washout
Water is concrete’s most persistent enemy. When rain, snowmelt, or leaks get under a slab through open cracks, gaps, or unsealed joints, the flow can wash out subbase material and leave behind hollow pockets. Common culprits include downspouts that discharge onto concrete, overflowing gutters, and negative slope toward the home.
Plumbing leaks and failures: Under-slab water supply line leaks (pressurized) can run continuously, softening the supporting soils, auto-compacting loose fill as it wets, and eroding fines that once carried the load. Sanitary sewer discharge leaks also move water through the subgrade; flow typically follows gravity, redistributing soil and collecting in low points beneath slabs. The net effect is threefold: (1) wetting compacts loose backfill, (2) moving water transports soil away and enlarges voids, and (3) saturated soils lose bearing strength—so the slab drops into the space left behind.
2) Disturbed Soil That Hasn’t Finished Settling
Anytime soil is excavated for building, utilities, or concrete replacement, it’s “disturbed.” Even when compacted, disturbed soils continue to consolidate for years. That’s why new flatwork—especially where a driveway meets a garage slab or where patios meet house siding—tends to settle as the ground naturally densifies.
3) Inadequate Compaction Before Pouring
If contractors rush base prep or skip thorough compaction, the slab will ride on a fluffy subbase. After placement, gravity, moisture, and everyday loads squeeze that loose soil down, and the slab follows. Because achieving perfect compaction is difficult, some amount of post-construction settlement is common without proactive stabilization.
4) Shrinking/Swelling (Expansive Clay Soils)
Expansive clays change volume with moisture: they swell during wet periods (heave) and shrink during dry spells. This cycle causes uneven support and differential movement. Repeated swelling/shrinkage gradually degrades bearing capacity, so slabs crack, tilt, and sink—especially when moisture varies across the slab footprint.
5) Freeze–Thaw & Frost Heave
In cold climates, water under or within concrete freezes and expands, lifting slabs. When it thaws, the ground relaxes and the meltwater can carry soil away, leaving voids. Repeated cycles amplify cracking, deterioration, and settlement.
6) Extra Factors That Accelerate Settlement
Burrowing pests (chipmunks, moles, woodchucks) create tunnels that collapse into voids.
Heavy loads (vehicles, equipment) stress weak bases and widen cracks.
Poor base preparation leads to variable slab thickness and trapped water that attacks the subgrade.
Quick On-Site Checks:
Stand back after a rain and note where water sits or flows toward slabs. Look for downspouts aimed at concrete, gaps at control joints, and small soil sinkholes at slab edges. Tap the surface with a mallet—hollow sounds can indicate voids. If interior plumbing lines run beneath the slab, monitor water usage and listen for running water when fixtures are off; anomalies can signal under-slab leaks.
Surfaces Most Affected Around a Home
Driveways, sidewalks, patios, porches, and pool decks: Exterior flatwork is exposed to weather and open edges. When water reaches the base through gaps at joints or along slab edges, it can carry soil away; when clay soils gain or lose moisture, the ground moves. The best defense is to keep water off and out: maintain a gentle, continuous slope away from structures, seal cracks and joints, and direct downspouts well past the slab.
Garage floors: These interior slabs are largely protected from direct weather and surface moisture. When garage floors settle, the primary driver is typically poor initial subgrade construction—disturbed or inadequately compacted fill, uneven base thickness, and soft spots created by construction traffic before the pour. Settlement is often most noticeable at the garage threshold where backfill is hardest to compact and vehicle loads concentrate; incidental moisture at the edges can aggravate weak base conditions, but the root cause is usually the subgrade.
How to Prevent Repeat Settlement
Manage water: Extend downspouts, add splash blocks, and keep roof runoff at least ~10 feet away. Seal cracks and control joints to block direct water paths under slabs.
Fix grading: Target a reliable slope away from slabs to reduce ponding near entries and walkways.
Stabilize weak soils: Where soils are soft or disturbed, strengthening the subgrade and filling hidden voids prevents the slab from dropping again.
Control moisture swings in clay: During droughts, aim for more consistent soil moisture near slabs to reduce shrink/swell extremes.
For a lasting repair, raising and stabilizing sunken slabs with lightweight polyurethane addresses both the symptom (low slab) and the cause (voids/weak subgrade). To see how this works in practice, review our core service for Concrete Raising & Leveling and the proprietary Smart Lift System. After lifting, sealing joints helps keep water out; learn more in Concrete Crack & Joint Sealing After Polyurethane.
Concrete Settlement FAQ's
How do I confirm a hidden plumbing leak under a slab?
Watch your water meter when all fixtures are off; movement suggests a supply leak. Listen for faint hissing, look for warm spots (hot-water lines), and check for persistent damp soil near slab edges. A pro can pressure-test supply lines and camera-inspect sanitary lines to pinpoint leaks that destabilize the subgrade.
Can a sewer line leak really sink my patio or driveway?
Yes. Sewer discharge carries water and fine particles that can erode soils along the pipe path. Leaks often create pathways that channel flow to low spots; over time, the soil redistributes and voids grow, allowing slabs above to settle and crack.
What if I fix drainage but not the voids—will the slab rise on its own?
No. Once voids exist, the slab won’t lift back up. You need a method that both fills voids and raises the slab to plane—polyurethane injection is designed for this and also resists future washout.
Why does the area where my driveway meets the garage settle so often?
That junction is commonly backfilled during construction, so soils there are heavily disturbed and hard to compact perfectly. Vehicle loads and edge gaps amplify the effect, leading to common “bumps” and gaps at garage entries.
Is polyurethane better than mudjacking for preventing resettlement?
Polyurethane is extremely lightweight, water-resistant, and seeks out voids, so it doesn’t add stress to weak soils and resists washout. Mudjacking slurries are heavy and water-based, which can shrink and erode. For a deeper comparison, see Polyurethane Foam Concrete Raising vs Mudjacking.