The five causes, in Florida order
1. Storm washout — the Florida special
Heavy tropical rain finds the path of least resistance: the joints and edges of your driveway. Over successive storms, water carries the sandy base out from under the slab, leaving voids. The slab looks fine until one day it drops. If your settlement appeared or worsened after a big storm, washout is suspect #1. This is the cause that changes the method choice: washout voids favor polyurethane foam (washout-resistant) over mudjacking (slurry can wash out again).
2. Poor compaction at construction
If the soil under the driveway wasn't properly compacted before the pour, it compresses gradually under the slab's own weight and traffic. This shows up as slow, uniform settlement over years — the whole driveway eases downward together. Common in newer construction built fast. The tap test usually sounds solid (no void, just compressed soil), which makes this a good mudjacking candidate.
3. Sandy soil consolidation
Treasure Coast soils are sandy, and sand consolidates — especially with a high water table and vibration from traffic. Unlike washout, there's no void; the ground itself densifies and the slab follows it down. Settlement here tends to be gradual and widespread rather than sudden and localized.
4. Tree roots
Roots don't just push slabs up — they also displace and loosen the base, and when roots die and decay they leave voids the slab drops into. Root damage has a signature: cracking that radiates from a point, heaving near a tree, or a slab that's both lifted and cracked. Lifting doesn't fix roots. If roots are the cause, the honest fix involves the tree (removal or root barrier) plus repair or replacement.
5. Water from your own property
Downspouts that dump at the driveway edge, irrigation heads spraying the slab base, a leaking hose bib or water line — all of these erode the base slowly and invisibly. This is the cause homeowners can actually fix themselves, and it's the first thing to rule out: walk the property during a rain or run each irrigation zone and watch where water goes.
Read the symptoms
| What you see | Likely cause | Urgency |
|---|---|---|
| Slab dropped at one corner, hollow sound underneath | Washout void | Act soon — voids grow; the slab can crack suddenly under load |
| Gradual, even settlement across the whole drive | Compaction / soil consolidation | Monitor — cosmetic until trip hazards or drainage problems appear |
| Water ponding against the garage or foundation | Settlement reversed the drainage slope | Urgent — water against the structure damages more than the driveway |
| Slab rocking when driven over | Void underneath, slab bridging it | Act soon — a bridging slab eventually cracks through |
| Cracks radiating from one point near a tree | Root damage | Assess — lifting won't fix it; needs root work + repair |
| Hairline cracks, no settlement | Shrinkage / normal aging | Monitor — seal and watch |
Three homeowner tests (no contractor needed)
- The straightedge test. Lay a 6-foot level or straight board across slab joints. Measure the gap underneath with a tape. Under 1" is cosmetic; 1–3" is the lifting zone; over 3" warrants a professional look at the subgrade.
- The tap test. Strike the settled area firmly with a hammer handle. A solid thunk means supported concrete (compaction). A hollow drum means a void (washout). Mark the hollow areas with chalk — that map is genuinely useful to show a contractor.
- The water test. During the next rain (or with a hose), watch where water flows across and under the driveway edges. Water disappearing under a slab edge is actively eroding your base. Fix the water path before paying for any lift.
Bring your findings to the quote. "Two slabs dropped 2 inches, hollow sound over about 4 square feet, downspout dumps at the north edge" gets you a dramatically better diagnosis — and price — than "my driveway is sinking." Contractors respect informed customers, and informed customers get honest answers.
When sinking is urgent vs. cosmetic
Treat it as urgent when: water now drains toward your house or garage; there's a trip hazard at a public walkway (liability); a slab is rocking or visibly bridging a void; or settlement is visibly progressing month to month. Treat it as monitor-and-plan when: settlement is under an inch, stable over time, drains away from structures, and causes no trip hazard. Cosmetic settlement doesn't get cheaper to fix later — but it doesn't get dangerous either, so you can plan the repair on your schedule and get proper quotes.
Next step: run your situation through the repair-vs-replace decision framework →