How sinkholes form
Sinkholes form in karst: areas where the bedrock is limestone, dolomite, gypsum or salt that slowly dissolves in slightly acidic groundwater. According to the U.S. Geological Survey, about 20 percent of the United States is susceptible, and the most damage tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania.
The USGS distinguishes two common types. Cover-subsidence sinkholes develop gradually where sandy sediments trickle down into openings in the rock, forming shallow depressions over months or years. Cover-collapse sinkholes form where clay-rich cover bridges a growing cavity until it fails, sometimes within hours. Human activity can speed both up: leaking pipes, concentrated stormwater from downspouts or paved areas, and changes in groundwater levels.
Where our service areas have karst
| Region | What is documented |
|---|---|
| Central Florida and Southwest Florida | Limestone close to the surface; Florida has special sinkhole insurance rules |
| Middle Tennessee | USGS maps the Inner and Outer Nashville Basin as karst regions; sinkhole flooding studied around Murfreesboro |
| East Tennessee and Chattanooga | Valley and Ridge limestone and dolomite |
| Montgomery and Chester County, PA | Carbonate rock in the Chester Valley; Pennsylvania Geological Survey karst inventories |
| Loudoun County, VA | Limestone Overlay District north of Leesburg |
| North Alabama and Birmingham | Karst limestone valleys with caves and sinkholes |
Warning signs around the house
The USGS and state agencies list signs that can point to sinkhole activity:
- Doors and windows that suddenly stop closing properly
- New cracks in walls, floors, driveways or the foundation, especially if they grow quickly
- A circular pattern of cracks in the ground
- Small new ponds where rain did not collect before
- Trees or fence posts that tilt or sink
- Previously buried items, such as foundations or pipes, becoming exposed
- Muddy or cloudy well water
- Vegetation that wilts in a patch as water drains away
None of these proves a sinkhole on its own; ordinary settlement and drainage problems cause many of the same symptoms. Several signs together, or signs that appear suddenly, deserve a professional look.
What to do if you see one
- Keep people and pets away from the depression or collapse.
- If a depression is next to or under the house and doors, walls or floors are changing quickly, leave the house and call your local emergency number or building department.
- Redirect downspouts and stop leaking water lines if you can do so safely, because water speeds up erosion.
- Do not fill a sinkhole with trash or loose soil; that can hide the problem and make it worse.
- Photograph and measure the area, then contact your insurer and a geotechnical engineer or foundation contractor.
How sinkhole-related foundation damage is repaired
Repair starts with investigation. Engineers may use borings, cone penetration tests or ground-penetrating radar to find voids and loose soil. Common repairs include:
- Compaction or chemical grouting to fill voids and densify loose soil below the foundation
- Underpinning with steel piers driven to competent rock, so the house no longer depends on the soil above the void. See pier types compared.
- Drainage changes that keep roof and surface water from concentrating over the problem area
- Structural and cosmetic repairs once the ground is stable
Sinkhole repairs often cost more than typical settlement repairs because of the investigation and grouting. Get the engineer’s report and the repair scope in writing.
Sinkhole or ordinary settlement?
| Clue | Ordinary settlement | Possible sinkhole activity |
|---|---|---|
| Speed | Slow, over seasons or years | Can be sudden, over days or weeks |
| Pattern | Cracks near corners, linked to dry or wet seasons | Circular ground cracks, new depressions, tilting posts |
| Location | Around the perimeter of the house | Often concentrated in one spot, sometimes away from the house |
| Water | Drainage and plumbing leaks involved | New ponds, water disappearing quickly, cloudy well water |
| Region | Anywhere, especially clay soils | Karst areas with limestone or dolomite |
These are clues, not a diagnosis. Only an investigation can confirm what is happening underground.
Buying in a karst area
If you are buying in a known karst region, check state or county sinkhole and karst maps, ask the seller about any depressions, repairs or insurance claims, and have an inspector look closely at cracks and door operation. In Pennsylvania, the Geological Survey maintains karst inventories for counties such as Montgomery and Chester; Loudoun County, Virginia, maps its Limestone Overlay District; in Florida, ask about the property’s sinkhole claim history and coverage options. A geotechnical engineer can advise whether further testing makes sense.
Common myths
- “Sinkholes only happen in Florida.” The USGS lists Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania among the states with the most damage too.
- “Filling the hole fixes it.” Dumping soil into a depression hides the symptom; the void and the water that created it are still there.
- “Every crack in a karst area is a sinkhole.” Most cracks, even there, come from ordinary settlement, shrinkage or drainage.
Insurance
Outside Florida, sinkhole damage usually falls under the earth movement exclusion of standard homeowners policies unless you add an endorsement. Florida law requires insurers to cover catastrophic ground cover collapse, which requires an abrupt collapse, a visible depression, structural damage and the building being condemned, and to offer broader sinkhole coverage for an extra premium. Our guide Does homeowners insurance cover foundation repair? explains the details.
Who to call
For a collapse or a depression next to the house that is changing quickly, start with safety: leave the house if needed and contact your local building department or emergency services. For slower signs, a geotechnical engineer can investigate what is happening underground, and a foundation contractor experienced with grouting and underpinning can price the repair once the cause is known. In Pennsylvania, the Department of Environmental Protection also publishes guidance on how to tell whether a depression is a sinkhole and whom to contact. When you call us, tell the specialist you suspect sinkhole activity so you are matched with a contractor who handles it.
Reducing the risk
You cannot change the rock, but you can control water. Keep gutters clean and discharge downspouts well away from the house, fix leaks in water and sewer lines quickly, avoid directing stormwater into a single spot, and check local karst or sinkhole maps before buying in a known karst area. If you have a well, watch for sudden changes in water clarity.
Sources
- USGS Water Science School: Sinkholes
- USGS: Sinkholes as a natural hazard (share of the US on karst, most-affected states)
- USGS: Depressions, sinking streams and karst areas of Tennessee (geospatial dataset)
- Pennsylvania DCNR: Sinkholes (geologic hazards)
- Pennsylvania DEP: How do I know if it’s a sinkhole?
- Loudoun County: Limestone Overlay District
- Florida Statutes § 627.706: Sinkhole insurance; catastrophic ground cover collapse