Why is there a sinkhole behind my seawall?
A sinkhole behind the cap means fill is leaving. It goes out through an open joint, a crack, torn filter fabric or under the toe. Find the path before anyone adds fill or foam.
The common Key Colony Beach wall drains through weep holes and French drains. KCB Seawall Owner’s Manual When those clog, or the filters behind them fail, water and fill move where they should not.
When should I call about a sinkhole behind my seawall?
- A soft spot that grows after a king tide or heavy rain.
- A sand fan or stain on the water side at low tide.
- Pavers, a pool deck or a slab sinking near the cap.
- A hole beside a drain pipe or downspout line.
- A hole that returns after you filled it.
Where does fill escape from behind a seawall?
| Path | What you might see | Usual fix |
|---|---|---|
Open joint between panels | A sand stain or fan at one joint at low tide | Reseal the joint and restore the filter behind it |
Crack at the waterline | A crack line with sand below it | Crack or panel repair |
Torn or missing filter fabric | Holes that return in the same spot | Dig out and replace the fabric |
Under the toe | Holes along a long stretch, bottom scoured at the base | Toe repair or riprap |
Around a pipe through the wall | A hole beside a drain line | Seal the opening and reroute the water |
Weep holes with no filter | Sand at the weep openings | Add filters and keep the openings clear |
Key Colony Beach’s manual lists joint separation and sinkholes, toe and berm failure, and waterline cracking among the common failures of its walls. KCB Seawall Owner’s Manual
Why is filling the hole not the fix?
Fill poured into a sinkhole follows the same path out. The hole closes for a while, then opens again, often bigger.
Drains and filters do two jobs together: they let water out so pressure does not build, and they hold soil in. Coastal engineering guidance Plugging every opening can trap water behind the wall instead.
Why do king tides and storms make seawall sinkholes worse?
Fill moves when water moves. Monroe County expects seasonal king tides from September through December. Monroe County king tides Each high tide pushes water behind the wall, and each drop pulls some back through any gap.
Storm surge does the same, harder. On Summerland Key after Irma, USGS photos show surge topping seawalls and moving sand landward and seaward. USGS Summerland Key photos At Key West, NWS minor flooding begins about 1.1 feet above the average daily high tide. NOAA Key West flood levels
- Photograph soft spots at the peak of a king tide and again two days later.
- Email flood photos with your address, date and time to greenkeys@monroecounty-fl.gov. Monroe County king tides
- Note heavy rain dates beside each photo.
What do Keys rules say about grade and drainage behind a seawall?
| Where | Rule | Source |
|---|---|---|
Unincorporated Monroe County | Existing grade behind a bulkhead at least 6 inches below its top | |
Marathon | A retention swale, berm or curb at least 6 inches, directly landward of any seawall, bulkhead or riprap | |
Marathon | No reclaiming eroded land by bulkheading and backfilling | |
State exemption | Minimal leveling backfill only, not land reclamation | |
Key West | Shoreline plans must show how water quality is protected |
The swale exists because yard runoff ends up in the canal. Monroe County started its canal program in 2014 after a study found 311 of 502 Keys canals failed state water-quality criteria. Monroe County resiliency
What fixes seawall soil loss for good?
- Find the path: probe, dig a test pit and check the water face at low tide.
- Close it: seal joints and cracks, replace filter fabric and seal pipe openings.
- Fix the toe if fill is leaving underneath. See riprap and toe protection.
- Restore drainage: clear weep holes, add filters and send downspouts away from the wall.
- Fill the void: compacted fill, or foam for a confirmed void behind a sound wall.
- Grade and swale: keep the ground below the cap, and add the swale where the code asks for it.
Skipping the first two steps is why holes come back.
Do I need a permit to fix soil loss behind a seawall?
| Where your lot is | Local permit office | What matters for this job |
|---|---|---|
Unincorporated Monroe County | Stock Island (305) 295-3990, Marathon (305) 289-2501, Big Pine Key (305) 289-2877 Monroe County Building, Stock Island Monroe County Building, Marathon Monroe County Building, Big Pine Key | Wall repair is structural and needs a permit; keep grade 6 inches below the top of the wall. Monroe exemption notice Monroe County code 118-12(k) |
City of Key West | (305) 809-3956 Key West Building Department | Permit for work over $1,000, at $36 per $1,000 of contract price, $100 minimum. Key West permits page Key West Code 14-2 |
City of Marathon | (305) 289-5052 Marathon Building Department | The 6-inch swale applies, and backfilling to reclaim land is not allowed. Marathon code 106.36 |
Key Colony Beach | (305) 289-1212, ext. 3 Key Colony Beach Building Department | Foam or soil-injection firms must be licensed and registered with the city. KCB warning |
City of Layton | City Hall (305) 664-4667 Layton City Hall | Start with City Hall; a contracted administrator reviews building work. Layton city staff |
Local detail is in the Marathon and Key Colony Beach guides and the Lower Keys overview.
What does a lasting soil loss fix cost?
The fix that lasts costs more than a load of fill, because it closes the leak and restores drainage. Price is driven by how much wall must be opened and whether the toe needs work.
- Test pits and probing to find the path.
- Length of joint or fabric repair.
- Toe repair, often the largest item. See the public footer prices on the riprap page.
- Restoring pavers, a pool deck or landscaping.
- Permit fees: Monroe County charges a flat $236 for residential work under $5,000. Res. 277-2025
Compare quotes on the same scope with the cost guide.
What to ask before you sign for soil loss repair
- How did you find where the fill is escaping?
- Which openings are leaks, and which are drains that must stay open?
- Will the filter fabric be replaced, and how much?
- Does the price include the toe, if it is part of the problem?
- How will you compact the new fill, and grade it below the cap?
- What happens if the hole comes back within the warranty?
Seawall sinkhole questions
Is a sinkhole behind my seawall dangerous?
It can be. The void under the surface is often bigger than the hole you see. Keep people, cars and trailers off it until it is checked.
Can I just fill the sinkhole with sand?
Only as a short-term safety step. Sand follows the same path out unless the leak is closed. Drains and filters have to keep water moving while holding soil back. Coastal engineering guidance
What is the swale behind a Marathon seawall?
Marathon requires a retention swale, berm or curb of at least 6 inches directly landward of any seawall, bulkhead or riprap it permits. Marathon code 106.36 It holds yard runoff back from the canal.
Show sourcesHide sources
- City of Key Colony Beach Seawall Owner's ManualSource reviewed 2026-10-28
- Coastal Erosion Control DesignSource reviewed 2026-10-27
- King Tides & High TidesSource reviewed 2026-10-28
- Hurricane Irma pre- and post-storm photo comparisons: Summerland KeySource reviewed 2026-10-28
- CO-OPS Metadata API: Key West station flood levelsSource reviewed 2026-10-28
- Monroe County Land Development Code, Chapter 118 - Environmental ProtectionSource reviewed 2026-10-28
- LDR Sec. 106.36 Criteria For Approval (Hardened Shorelines)Source reviewed 2026-10-28
- Rule 62-330.051, F.A.C. (Exempt Activities)Source reviewed 2026-10-28
- Florida ERP Applicant Handbook Volume I, June 28 2024Source reviewed 2026-10-27
- Code of Ordinances, Subpart B Land Development Regulations, Ch. 110 Resource Protection, Art. IV Coastal ResourcesSource reviewed 2026-10-28
- ResiliencySource reviewed 2026-10-28
- Building and PermittingSource reviewed 2026-10-28
- Building Department - About UsSource reviewed 2026-10-28
- PermitsSource reviewed 2026-10-28
- Building Fees (Code Sec. 14-1 and 14-2)Source reviewed 2026-10-28
- BuildingSource reviewed 2026-10-28
- Building Department and Code EnforcementSource reviewed 2026-10-28
- Warning: Soil Amendment ServicesSource reviewed 2026-10-28
- Contact UsSource reviewed 2026-10-28
- City StaffSource reviewed 2026-10-28
- Resolution No. 277-2025 (Building Department permitting fee schedule)Source reviewed 2026-10-28