A void is an absence, not an event
It helps to think of a void as the accumulated record of soil that has left, rather than as something that happened. Every tide cycle that moved material through the wall contributed to it.
That is why the timing of discovery is misleading. The sinkhole that opens in the lawn on a Tuesday is not a Tuesday event — it is the moment a process that has been running for months or years finally reached the surface.
- 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
Why the ground holds up for so long
Soil arches. Compacted surface material, bound together by grass roots and tree roots, behaves somewhat like a shallow dome over a cavity. It transfers load sideways to the intact ground around the void rather than down into the gap.
That arch is surprisingly strong, and it gets strengthened by anything that binds the surface — turf, root mats, pavers, slabs. It is also why waterfront properties with mature landscaping often hide voids longer than bare lots do.
What eventually fails it is some combination of the void growing wider than the arch can span, saturation reducing the soil’s cohesion, and a load crossing it — a vehicle, equipment, or simply someone walking in the wrong place.
Where voids concentrate
Voids are not randomly distributed. They form where the pathway is.
Immediately behind panel joints
The most common location, because joints are the most common pathway.
Below and behind the cap
Where surface water has been entering through a separated cap and washing material down the back of the wall.
Deeper, where the toe is scoured
Soil moving under the wall rather than through it produces voids that sit lower and further back than joint-driven ones.
Beneath hardscape
Slabs and pavers bridge cavities particularly well, so voids under them can reach significant size before anything is noticeable.
Near stormwater discharge points
Where concentrated runoff enters the soil near the wall and erodes from above.
Why voids matter structurally
Two distinct risks, and they are often confused.
The first is what everyone pictures: ground failing under foot or vehicle load. That is real and it is the reason to keep traffic off suspect ground.
The second is less obvious and often more expensive. The soil that has left was providing lateral support to the wall and bearing support to anything built above it. A void near a pool deck, a slab or a footing means those structures have lost some of what they were resting on, whether or not anything has visibly moved yet.
Distinguishing these from true sinkholes
In waterfront settings, these cavities are erosion features in the backfill. They are not the karst sinkholes associated with limestone dissolution that parts of Florida are known for.
The distinction matters because the response is different. An erosion void has a traceable water pathway that can be closed; a karst feature is a geological condition. Establishing which you have is part of what an assessment does.
Frequently asked questions
Can I tell how big a void is from the surface?
No, and that is the central difficulty. The visible opening is typically a fraction of the cavity because the surrounding crust is still bridging. Probing is what establishes extent.
Is it safe to walk near a void?
Treat ground you suspect is undermined as unsafe until someone has probed it. Soil that feels firm underfoot is not evidence that there is material beneath it — the arch is doing the work, and arches fail suddenly.
Will a void keep growing?
If the pathway that created it is still open, yes. The mechanism that formed it does not stop on its own.