Where this starts
A pavement is a stack: the ground, a compacted sub-base spreading the load, a thin laying course seating the units, and the units themselves. Every settlement failure is one of those layers, and the pattern on the surface says which.
MAP THE DEPRESSION FIRST, with a straightedge or a string line rather than by eye. A BROAD DISH over a metre or more, with the units still sitting flat relative to each other, means the whole stack has gone down together — the sub-base has consolidated or the ground beneath it has. A SINGLE unit below its neighbours, with the surrounding field level, is the laying course under that one unit. A RUT in the wheel tracks is the base failing under repeated load.
WHY THE LOCATION IS INFORMATIVE. Settlement over a trench line is the backfill consolidating and is extremely common — a service trench cut across a drive and backfilled with the spoil that came out of it will settle for years. Settlement at the edges is usually a missing or failed edge restraint letting the field creep sideways. Settlement at a gully, a manhole or a drain is often the drain itself, and that is the one to take seriously.
WATER IS THE MECHANISM BEHIND MOST OF IT. Water entering the pavement and not leaving washes fines out of the laying course and the sub-base, which is why sinking so often follows a wet winter, why it clusters where a downpipe discharges, and why a blocked or broken drain beneath a pavement produces a hole that keeps coming back however many times the units are relaid.
ONE THING WORTH RULING OUT EARLY, because it changes everything: a depression that is still growing, or one that appeared suddenly rather than gradually, over or near a drain run. Material washing into a broken drain produces a void that continues to enlarge under the surface, and the visible dip is smaller than the cavity below it.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Sub-base too thin or not compacted
Most commonA broad dish across several units with the units still level relative to each other, often worst where vehicles turn or park, and appearing within the first year or two.
How to check: How wide the depression is. Wider than a unit or two means the layer beneath them went down, not the bedding.
Backfill over a service trench consolidating
Most commonA depression in a line across the pavement rather than a patch, often matching a known service run, and continuing to settle over years.
How to check: Whether the dip is linear. Trench backfill compacted in one lift settles for a very long time.
Fines washed out of the laying course
CommonIndividual units below their neighbours, open joints, sand visible at the bottom of a slope or at a gully, and worsening after heavy rain.
How to check: Whether the joints are empty and where the sand has gone. A laying course losing material leaves the units unsupported one at a time.
No edge restraint, so the field has crept
CommonMovement concentrated at the perimeter, joints opening, units rotating at the edge and the whole field slightly wider than it was.
How to check: Whether there is a restrained edge. Block paving relies on interlock and interlock relies on the edge holding.
A broken or blocked drain beneath
CommonA depression near a gully, a manhole or a drain run, appearing suddenly or continuing to grow, sometimes with a hollow sound or a visible void at a joint.
How to check: Whether it is near a drain and whether it is still growing. Material washing into a broken drain leaves a cavity larger than the dip.
Ground movement — clay shrinkage, tree roots, made-up ground
Less commonSeasonal movement that rises and falls, or settlement near a tree or over an area known to have been filled, affecting the pavement and often a nearby wall too.
How to check: Whether anything else nearby has moved, and whether it changes with the seasons. Ground moving takes everything on it.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
A broad dish, a single low unit and a wheel rut are three different layers failing, and eye judgement is unreliable over a shallow depression.
- 2
A line is almost always a service trench; a rut is the base under traffic; a patch is bedding or a void.
- 3
Most sinking has a local cause within a couple of metres, and repairing the surface without it is a repeat visit.
- 4
Empty joints mean the jointing material is being washed out, which both removes the interlock and shows where water is getting in.
- 5
A depression that appeared suddenly or is still growing near a drain is a void forming underground, which is a different order of problem from consolidation.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where a depression is near a drain run and is still growing, since material may be washing into a broken drain and the cavity below is larger than the dip.
- Where a vehicle uses the area and the base is clearly inadequate, because relaying the surface over the same base repeats the failure.
- Where settlement affects a nearby wall or structure as well as the paving, which is a ground question rather than a pavement one.
- Where the pavement is a shared driveway or adjoins a highway, since the crossing and the edge details are usually subject to the authority's requirements.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.