Driveway sub-base & foundations

Everything visible on a driveway is a wearing surface. The structure is the compacted stone underneath it, and its job is to take a concentrated wheel load and spread it out until the pressure reaching the soil is something that soil can carry without deforming. Get that right and almost any surface performs. Get it wrong and the best blocks, the best resin and the thickest tarmac in the world will still rut, crack and sink.

The depth is set by two things: what the ground is, and what will drive on it. A firm, well-drained subgrade under a normal car might want 150mm of compacted sub-base; soft clay, a slope, or somewhere a motorhome or a delivery lorry parks needs considerably more. In practice a domestic drive usually finishes 200–250mm above formation once the surface and laying course are added — but that is a typical figure, not a specification, and the right one comes from looking at the ground.

MOT Type 1 is the usual material and it is worth knowing what it is. It is a graded crushed stone to the Specification for Highway Works, with a controlled range of particle sizes from about 40mm down to dust — and the dust matters, because those fines are what let it lock together into a dense, stable layer when it is compacted. Clean single-sized stone, by contrast, does not lock; it is used deliberately in permeable construction where drainage is the point, and it is the wrong material under a conventional drive.

Compaction is where corners get cut invisibly. Stone has to be laid and whacked in layers of roughly 100–150mm, each one compacted before the next goes on. A 250mm layer tipped in and rolled once is compacted at the top and loose underneath, and it consolidates under traffic over the following two years — which is exactly what a drive that has developed wheel-track ruts is doing. There is no way to tell from the finished surface, which is why it belongs in writing.

The geotextile membrane is cheap and frequently omitted, and its job is separation rather than strength. Without it, on soft ground, wheel loads pump the fine subgrade up into the stone and press the stone down into the subgrade, and within a few years there is no clean structural layer left. On firm, granular ground it may genuinely not be needed; on clay it is close to essential. Where the subgrade is very soft, a geogrid — which interlocks with the stone and spreads load far wider — does more than simply adding depth.

One last point about how to read a quote. A driveway quote that gives a rate per square metre and says nothing about excavation depth, sub-base thickness, membrane or compaction is not telling you what you are buying, and the difference between two such quotes tells you nothing useful. Asking for those numbers in writing costs nothing, is entirely reasonable, and is the single most effective way to compare one contractor with another.