Bulkhead problem
Sinkholes and Voids Behind a Seawall
Holes opening up, hollow-sounding ground, collapsing areas
The short answer
A void behind a seawall is an empty space left where backfill used to be, created by water carrying soil out through or under the wall. The surface often stays intact over the void until the crust can no longer bridge it, which is why sinkholes appear suddenly even though the cause developed slowly.
What this usually indicates
- An active water pathway through the wall has been removing soil for some time
- The void is likely larger than the surface opening suggests
- Nearby slabs, decks, pools and walkways may have lost support
- The wall itself may have lost lateral support from the soil that is gone
- 1The void — far larger than any opening visible at the surface
- 2Root-matted crust still bridging the cavity, which is why nothing shows yet
- 3Soil exiting through the wall — the mechanism that created the void
What causes it
Soil migration through the wall
The dominant cause. Water moving through joints, cracks or failed filter fabric removes fines first, then progressively larger particles, hollowing out the backfill from the wall face inward.
Soil migration under the wall
Where the toe has been scoured, soil can move beneath the wall instead of through it, producing voids that sit deeper and further back than expected.
Arching in the soil
Compacted or root-bound surface soil can bridge a growing cavity for a long time. This is why the collapse appears abrupt and why probing matters more than visual inspection.
Concentrated stormwater
Downspouts and drains discharging near a wall can excavate backfill from above and create voids independent of what is happening at the waterline.
What an assessment looks at
These are the specific questions an inspection is trying to answer for this symptom.
- Probing and sounding to estimate void extent beyond the visible opening
- Whether the void connects to a discharge point on the wall face
- Proximity to slabs, pool decks, footings and utilities
- Whether the wall shows signs of having lost lateral support
- Where stormwater currently discharges relative to the void
What tends to happen if it is left
Voids tend to grow toward the surface and laterally along the wall. The practical risk is twofold: the ground can fail under foot or vehicle loads, and structures behind the wall can lose bearing support. Both are considerably more expensive to resolve than the void itself.
Frequently asked questions
Is this a real sinkhole?
In waterfront settings these are usually erosion-driven voids in the backfill rather than the karst-related sinkholes associated with limestone dissolution. The distinction matters for how it is addressed, and it is one of the things an assessment establishes.
How are voids behind a seawall filled?
Typical approaches inject a flowable grout or expanding polymer into the void so it fills the space and stabilises the surrounding soil. The important part is that the water pathway that created the void is addressed as well — otherwise the fill can be undermined in turn.
Should I keep people off the area?
Keeping foot traffic, vehicles and equipment off ground you suspect is undermined is a sensible precaution until someone has probed it. Surface soil that feels firm is not evidence that there is material underneath it.
Next
Services that address this
Seawall void filling
Voids behind a seawall are almost always bigger than the hole you can see. The surface is the last thing to go.
Learn moreSoil stabilization
The soil behind a wall is a structural component. When it loses density, both the wall and anything built on top of it lose support.
Learn moreSeawall erosion repair
Erosion behind a wall is a plumbing problem before it is a structural one. Water found a route out, and it has been taking soil with it.
Learn moreOther symptoms
Seeing more than one of these?
Problems rarely arrive alone — several symptoms together usually point at a single mechanism.
Soil washing out behind the wall
Dips, depressions or disappearing yard along the wall
Learn moreWall leaning or bowing
The wall is tipping waterward or bulging outward
Learn moreCracks in concrete
Cracked panels, cap or returns
Learn moreWater coming through the wall
Seepage, jets or streams through joints and cracks
Learn moreNext step
Seeing sinkholes or voids?
Send photographs and a short description. You will get help understanding what the symptoms point to and which repair options are realistic.