Tarmac depth surveys: the thickness laid, everywhere

We scan the layer underneath before the tarmac goes down and scan the finished surface afterwards. Both scans are tied to the same site control, so the vertical difference between them at any point is the thickness laid there. It is non-destructive and covers the whole area, not just the core locations.

Only if the layer below was scanned or accurately levelled first. Without a 'before' surface there's nothing to measure the thickness from, and coring is the only option. That's why we recommend booking the sub-base scan as soon as the road is programmed.

That's the adopting authority's call. Many accept full-coverage scan data alongside a reduced number of cores, and it shows them exactly where to core. We provide the data in a form their engineer can check.

Each scan is registered to the same control and we state the registration error on the report. In practice the thickness measurement is good to a few millimetres, which is well inside a ±6 mm binder or surface course tolerance. Where the margin is tight, we say so.

On a single driveway or a short cul-de-sac, a few cores or dips may be enough. Scanning pays off on estate roads, car parks and anywhere a shortfall would be expensive to take up and relay.

The adopting authority's specification sets it, and it varies between councils. As a published example, Lincolnshire County Council allows ±6 mm on binder and surface course thickness and +40/-0 mm on sub-base thickness. Your section 38 agreement or road specification gives the figure for your site. Tell us what applies and the thickness report shows how much of the road falls inside it, and where it doesn't.

Yes, as long as each layer is scanned before the next one covers it. Scanning the sub-base, then the binder, then the surface course gives a separate thickness map for each bituminous layer. Adding the surface course to a binder survey on the same road shares the set-up and travel, so it costs less than surveying it on its own.

Yes, approximately. The thickness map gives the compacted volume laid, which we convert to tonnes at the density you give us. It is an estimate from the scan rather than a weighbridge figure, but it is usually close enough to show whether deliveries match the take-off, or whether a layer was laid thick and the extra tonnage has gone unnoticed until the final account.

It can, but the result is only as good as those levels. If the sub-base was accurately levelled on a grid before it was covered, we can build the 'before' surface from that data, tied to the same control. The thickness between grid points is then interpolated rather than measured. A scan of the sub-base gives a far denser, more reliable starting surface.

They answer different situations. Ground-penetrating radar estimates layer thickness on roads that are already surfaced by reading reflections along survey lines, and is normally calibrated against cores. A before-and-after scan measures the thickness laid directly across the whole surface, but only works if the layer underneath was scanned first. On new estate roads, the scan method is simpler; on existing roads, radar or coring is the realistic option.

Yes, if the layer underneath was surveyed before it was covered. Send the 'before' and 'after' data, scans, drone surveys or levels, tied to the same control, and we map the thickness laid against your specification. Denser data gives a more reliable map; with grid levels the thickness between points is interpolated.