One paver rocks when you step on it. A section of path has dropped a centimetre. A crack in a slab that was hairline last year is now something you notice. Each of those on its own reads as ordinary wear, and most of the time that is exactly what it is.
On sandy ground there is another explanation worth ruling out, and it connects to something that seems unrelated: the drain that keeps blocking. Material that moved into a pipe, or that a leak washed along, came from somewhere. In free-draining sand, where it came from ends up as a void, and the void eventually shows at the surface.
Sand carried into a drain through a defect, or moved by a leak, leaves a space behind it. In ground that does not bridge over voids the way clay does, that space migrates upward over time and appears as a rocking paver, a dropped path or a crack that keeps opening. If those signs sit near a drainage line that has a history, the two are worth investigating together.
- Note where the movement is, relative to any drainage run
- Note whether the drain nearby has ever blocked or been repaired
- Photograph it with something for scale, and again in a few months
- A camera on the line answers whether there is a defect feeding it
- Anything structural belongs to a building professional, not a plumber
On this page
1. The material had to come from somewhere
This is a conservation-of-matter argument and it is worth stating that plainly, because it is what makes the connection obvious once somebody points at it.
If a drain has been silting up with sand for years, that sand was previously in the ground beside it. It did not appear. Every bucket of material that came out of a line during a clearance is a bucket-sized space that used to be somewhere else.
The same applies to a leak. Water escaping under pressure moves fine material along with it, and over a long enough period that is a substantial quantity of ground redistributed away from where it was.
Clay bridges over a space and holds. Free-draining sand does not have that structure, so the void works its way up rather than staying put.
2. What it looks like at the surface
- A paver that rocks when nothing about it has changed and its neighbours are solid.
- A section of path that has dropped, often with a clean edge where it meets the part that has not.
- A crack in a slab that keeps opening, rather than one that appeared and stayed the same.
- A depression in a lawn that is not wet, which is the confusing one.
- Edging or a garden bed that has slumped along a line rather than in a spot.
- A driveway with a dip that arrived rather than having always been there.
None of those is proof of anything on its own. What makes them worth investigating is position: whether they sit along or near a drainage run, and whether that run has any history.
3. And the leak that never surfaces
There is a related problem specific to this ground, and it is worth knowing because it removes the sign everybody is watching for.
In permeable sand, a leak almost never surfaces. The water drains vertically instead of pooling, so there is no wet patch, no soft spot and no greener grass, even where quite a lot of water is escaping. The ground takes the evidence away.
So on this ground the absence of a wet patch is not evidence of no leak, and the meter becomes the only reliable test. Everything off, nothing filling, read the meter, wait an hour, read it again. Movement means water is going somewhere, and here it goes down rather than sideways.
4. Connecting the two things nobody connects
The households who ring us about this have usually been living with two separate annoyances for years without joining them up: a drain that blocks periodically, and a bit of paving that has never sat right since.
- Mark on a rough sketch where the surface movement is.
- Mark where the drainage runs, as best you know. A sewer service diagram helps.
- Note the dates of any blockages or drainage repairs.
- If the two overlap, that is worth investigating as one problem rather than two.
- A camera on the line, with a locate, tells you whether there is a defect at that point.
Where they do line up, dealing with the drainage defect is what stops the void growing. Reinstating the paving without doing that is repairing the symptom.
Paving moving near a drain with a history?
We can camera and locate the line, and tell you whether there is a defect there.
5. What we do not assess
This needs saying clearly, because it would be easy for a plumber to overreach here and it would be unhelpful.
We do not assess structures. We will not tell you whether a crack in a slab or a wall is structurally significant, whether a building is moving, or what the ground under a footing is doing. Those are questions for a building professional or an engineer, and the honest answer from a plumber is that we are not qualified to answer them.
What we can do is establish whether there is a drainage defect at a particular location, whether water is being lost, and whether material is entering a line. That is a genuine and useful contribution, and it is frequently the piece of information somebody else needs.
6. The order worth doing things in
- Photograph the surface movement, dated, with something for scale.
- Do the meter test, because on this ground it is the only leak evidence you will get.
- Have the drainage nearby filmed and located.
- Fix any defect found, so the cause stops.
- Then reinstate the paving or the path.
- Photograph again in a few months to confirm it has stopped moving.
Reinstating first is the common order and it is the expensive one, because the new work sits on ground that is still going somewhere. Fixing the cause first is slower to start and considerably cheaper to finish.
- Sand that moved into a drain, or that a leak washed along, came from the ground beside it.
- In free-draining sand that space does not stay put, so a void migrates upward over time.
- It shows as a rocking paver, a dropped path, a growing crack or a dry depression.
- A leak in permeable sand almost never surfaces, so the meter is the only reliable test.
- If surface movement sits near a drain with a history, treat them as one problem.
- Fix the drainage defect before reinstating the paving, or you will pay for the paving twice.
Voids under paving FAQs
Is a rocking paver always a drainage problem?
No, and most of the time it is not. Paving moves for plenty of ordinary reasons. What makes it worth investigating is its position relative to a drainage run, and whether that run has any history of blocking or repair.
There is no wet patch, so surely there is no leak?
On this ground that inference does not hold. Permeable sand drains water vertically instead of pooling it, so a leak can be substantial and leave nothing at the surface. Use the meter instead of your eyes.
Can you tell me if the crack in my slab is serious?
No, and we would not try. Structural assessment belongs to a building professional or an engineer. What we can tell you is whether there is a drainage defect nearby that might be contributing, which is a different and answerable question.
Should I just fill the void?
Not before establishing why it is there. Filling a space that something is still creating means doing it again, and the second time usually costs more because more has moved.
How do I find out where my drainage runs?
A sewer service diagram from Sydney Water shows the layout, and a camera with a locate confirms it on the ground and marks the position and depth. On this ground that locate is worth paying for.
Could a leak really move that much sand?
Over years, yes. It is not a dramatic process, it is a small amount continuously, and the reason it becomes visible is that this ground does not bridge over the space the way heavier soil would.