What causes sinkholes or depressions behind a seawall, and are they dangerous?
Most holes and dips behind a seawall form because backfill is escaping, usually through open joints, failed filter fabric, unfiltered weep holes or under a scoured toe. That leaves a hidden void that eventually collapses at the surface. Because the void is often larger than the hole you see, treat any new depression as a hazard, keep people and loads off it, and have the wall checked.
Warning signs to look for
- A dip, soft spot or open hole in the lawn, pavers or slab within a few feet of the wall.
- Pavers, a sidewalk or a pool deck near the wall settling, tilting or cracking.
- A depression that comes back after it has been filled.
- Ground near the wall that feels spongy, or stays wet after high tide when it has not rained.
- Soil-colored water, sand fans or backfill at the base of the wall at low tide.
- Open joints, gaps or cracks in the wall face roughly in line with the depression.
- Fence posts, light poles or irrigation heads near the wall leaning or dropping.
How urgent is it? A new or growing hole near a seawall can sit over a larger hidden void, so treat it as a fall and collapse hazard and arrange an evaluation promptly. If a depression is large, near a building or opening quickly, Florida's Geological Survey advises calling local law enforcement when there is a risk of physical harm.
What a Seawall Sinkhole Is, and What It Is Not
A depression behind a seawall is usually the surface sign of soil that has already left. Fine backfill is carried out through an opening in or under the wall, a void grows below the surface, and the ground above eventually sags or caves in. Key Colony Beach's seawall owner's manual pairs this with slab joint separation, noting that backfill leaking through the wall creates a sinkhole on the land side.
That is a different process from a geological sinkhole. The Florida Geological Survey explains that true sinkholes form when overlying sediments collapse into cavities in limestone that has been dissolved by slightly acidic water. It also lists events that can mimic sinkholes, including collapsed or broken sewer and drain pipes, improperly compacted soil after excavation and buried debris. Next to a seawall, the wall is usually the first suspect, but a cracked storm pipe running through it, or loose fill over an old trench, can look the same at the surface.
How Voids Form Behind Florida Seawalls
Soil needs two things to leave: an opening and moving water. Florida waterfronts supply the water twice a day with the tide, and again with every heavy rain and boat wake. The openings are where the wall's soil-tight details have failed.
- Open or failed joints. Key Colony Beach's manual describes interlocked concrete panels as not designed to be watertight. When panels separate, tide water moves in and out through the gap and carries backfill with it.
- Torn or missing filter fabric. The U.S. Army Corps of Engineers' low-cost shore protection guide recommends filter fabric behind bulkheads to prevent loss of soil through cracks, and warns that unstabilized fabric degrades quickly in sunlight.
- Weep holes without filters. Weep holes relieve water pressure, but the same guide says drain holes must be backed with filter cloth or graded stone. Fort Lauderdale's seawall master plan describes retrofit tidal valves that let water out while preventing the loss of fines.
- Undermining at the toe. Fort Lauderdale's plan explains that when water removes the channel bottom at the toe, fill can escape from underneath the wall and the ground behind it settles.
- Pipes and drains through the wall. Storm outfalls, pool drains and their connections can crack and draw in soil.
- Runoff and overtopping. Water pouring over the cap or running across the yard toward the wall can wash fill down into open joints and weep holes.
How Professionals Evaluate a Depression
- Map the Soft GroundThe area around the depression is probed to find how far the soft or hollow ground extends. The void below is often wider than the hole at the surface.
- Trace the PathwayThe wall face is inspected at low tide for open joints, cracks, staining and failed weep holes in line with the depression. A diver may be recommended when the opening is likely below the waterline.
- Check the ToeWater depth and bottom condition at the base of the wall show whether fill could be escaping underneath rather than through the face.
- Rule Out Other CausesDrain lines, irrigation and utilities near the hole are located. If the pattern does not fit the wall, a geotechnical engineer or licensed geologist may be the right next call.
- Judge the Wall's ConditionLarge soil loss can leave slabs, the cap or the anchors with less support. The evaluation should say whether the wall itself needs structural attention, not just the yard.
Digging Is a Contractor's Job
Exposing a void means excavating next to a wall that may already be weakened, often near buried lines. Section 556.105, Florida Statutes, requires excavators to give notice through the statewide one-call notification system at least 2 full business days before excavating anywhere that is not beneath state waters. Leave the shovel work to the people doing the repair.
Typical Repair Approaches
A repair that lasts closes the exit first and refills second. Filling the hole alone usually fails because the pathway is still open, and the next tides start a new void under the fresh fill.
| Where soil is escaping | Typical repair approach |
|---|---|
| Open panel joints | Sealing the joints and restoring filter fabric behind them; Key Colony Beach's manual describes sealing vertical slab joints with filter fabric |
| Unfiltered or failed weep holes | Cleaning and adding weep filters, or retrofitting tidal valves that pass water but hold back fines |
| Undermined toe | Toe protection such as added berm, riprap, sand-cement bags, concrete-filled mats or extended sheet piling, options Fort Lauderdale's plan lists for undermined walls |
| Broken pipe or outfall | Repairing or replacing the pipe and sealing its connection through the wall |
| The void itself | Removing loose material and backfilling with suitable, compacted fill once the exit is closed |
Where sinkholes keep coming back, Key Colony Beach's manual suggests a French drain behind the wall if one is not already installed, along with replenishing the rock in an existing drain. If the investigation shows the wall has lost support or moved, the repair grows to include the structure. See Seawall Repair and Erosion Control.
What Happens If It Is Ignored
A void that stays connected to the water tends to keep growing, because each tide cycle can carry out more fill. Voids can spread along the wall and back under patios, pool decks and sidewalks before the surface gives way, and the collapse can be sudden under a foot, a mower or a vehicle. Fort Lauderdale's plan notes that settlement behind a wall adds further distress to the wall's overall stability. Not every depression behaves the same way, but few stop on their own while the opening remains.
Prevention and Routine Checks
Keeping Soil Behind the Wall
- Walk the strip behind the wall every month and after storms, looking for new dips, cracks or soft ground.
- Keep weep holes clear. Fort Lauderdale's plan notes they clog with rocks and oyster shells, and clogged weeps trap water behind the wall.
- Adjust sprinklers and redirect roof and yard drainage so water does not pond or run directly behind the wall, as Key Colony Beach's manual recommends.
- Have joints, weep holes and filter fabric looked at during routine seawall inspections, not only after a hole appears.
- Watch the toe at low tide and replace lost berm or riprap early.
- Report small depressions while they are small; the repair is usually simpler.
Getting a Depression Checked in Northeast Florida and Elsewhere
Owners in Duval, St. Johns, Nassau, Clay, Flagler and Putnam counties can send requests straight to Barefoot Marine Construction, the owner and operator of Florida Bulk Head. After you Request a Quote, the Site Assessment focuses on finding the exit point, then the work moves through Plan & Permit, Build or Repair, and Protect Your Property. Photos of the hole with a scale reference, plus low-tide photos of the wall face in line with it, make the first conversation far more useful.
For a depression elsewhere in Florida, you can submit an inquiry for the partner network Florida Bulk Head is assembling. None of its partner contractors are confirmed yet, and any match depends on contractor availability, so keep the area roped off in the meantime. The Seawall Health Check can help you sort symptoms first, and Find a Contractor explains how your area is handled.
What you can safely do now
- Rope off or cone off the depression well back from its edges, and keep children, pets, mowers and vehicles away.
- Photograph it with a tape measure or a familiar object for scale, then repeat from the same spot every few days to see whether it grows.
- Do not fill it before it has been looked at. Filling hides the evidence, and the void below can keep growing.
- At low tide, look from a safe spot on land or a neighboring dock for soil staining or open joints on the wall face in line with the hole, and photograph what you see.
- Turn off irrigation zones that spray near the wall and point downspouts away from the area.
- Note whether the hole changes after high tides or heavy rain, with dates.
Leave structural work, excavation behind the wall and anything in the water to licensed professionals.
Frequently asked questions
Is the hole behind my seawall a real sinkhole?
Probably not in the geological sense. Florida's Geological Survey describes true sinkholes as collapses into dissolved limestone, and lists broken pipes, poorly compacted soil and buried debris as things that can mimic them. A hole within a few feet of a seawall is far more often soil escaping through or under the wall, but an evaluation should confirm the cause.
Can I just fill the hole with sand or dirt?
Filling it may make the yard look normal for a while, but if the opening in the wall is still there, the new fill usually washes out the same way. It can also hide how big the void below has become. Rope it off and have the pathway found and closed before anything is refilled.
Why does the hole keep coming back after it is filled?
Because the exit point is still open. Each tide moves water through the gap, joint or weep hole and carries some of the new fill with it. Key Colony Beach's manual suggests a French drain for walls with repeated sinkholes, alongside sealing joints and restoring the berm.
Will insurance cover a sinkhole behind my seawall?
That depends on your policy and on the cause, and this page cannot answer it. Ask your agent how your policy treats soil loss behind a seawall compared with a geological sinkhole. The Florida Geological Survey's sinkhole page directs sinkhole insurance questions to the Department of Financial Services' Division of Consumer Services.
Who should I call first, a geologist or a seawall contractor?
For a depression right behind a seawall, a marine contractor who can inspect the wall face and toe is usually the practical first call. If the hole is far from the wall, near the house foundation, or the investigation rules out the wall, a geotechnical engineer or licensed geologist is the better fit. The Florida Department of Environmental Protection notes that it does not inspect possible sinkholes.
How big can a void get before it shows at the surface?
There is no reliable rule. Turf roots, pavers and slabs can bridge a void and hide it until it is much wider than the surface dip. That is why probing around the depression is part of any evaluation, and why the area should be kept clear until then.
Sources and references
- Seawall Owner's Manual — City of Key Colony Beach Building Department. Accessed November 2, 2026. Slab joint separation and sinkholes: causes, symptoms and remedies, including French drains and weep hole filters.
- Sinkholes — Florida Department of Environmental Protection, Florida Geological Survey. Accessed November 2, 2026. Karst sinkhole formation, events that mimic sinkholes, safety and insurance contacts.
- City of Fort Lauderdale Seawall Master Plan: Summary Report — City of Fort Lauderdale (prepared by BCC Engineering, 2018), hosted by the Southeast Florida Regional Climate Change Compact. Accessed November 2, 2026. Sections 4.6 to 4.8: undermining and scour protection, weep holes and tidal valves, joint sealing.
- Low Cost Shore Protection: A Guide for Engineers and Contractors (1981) — U.S. Army Corps of Engineers, via the U.S. Government Publishing Office. Accessed November 2, 2026. Filter fabric behind bulkheads, filtered drain holes and ultraviolet degradation of fabric. Older guidance, cited for general mechanisms.
- Section 556.105, Florida Statutes: Procedures — The Florida Legislature (Online Sunshine). Accessed November 2, 2026. Notice to the one-call system before excavation.
Regulations, programs and conditions change. Always confirm requirements with the agency that has jurisdiction over your property.