The surface is not what carries the load
Look at a failed driveway — cracked, rutted, sunk into hollows where the car wheels sit — and the instinct is to blame the asphalt. Almost always the asphalt is innocent. It failed because whatever it was laid on moved, and a flexible surface can only be as stable as the thing beneath it.
A paved surface is a system of layers, and each one does a different job.
- The subgrade: the natural ground you dig down to. You do not choose this, you inherit it. Its strength is what everything above has to work with.
- The sub-base: a thick layer of well-graded crushed stone, compacted. This is the structural heart of the construction and usually the deepest layer.
- The base course: the lower layer of asphalt, using a coarser mix, laid where the construction needs more strength than the sub-base alone provides.
- The surface course: the top layer, using a finer mix. It provides the running surface, the texture and the waterproofing, and it is the thinnest layer of the lot.
Load spreads as it travels down. A wheel puts a concentrated pressure on the surface, and each layer distributes that pressure over a wider area of the layer below, so by the time it reaches the subgrade the pressure is low enough for the soil to carry without deforming. That is the whole engineering idea, and it explains everything else here. If a layer is too thin or too loose to spread the load properly, the pressure arriving at the layer below is higher than it was designed for, and the failure works its way up to the surface where you can see it.
What makes a good sub-base
- Graded, not uniform: the stone should contain a range of particle sizes, so smaller particles fill the gaps between larger ones and the whole mass locks together. Single-sized stone looks tidy and stays loose, because there is nothing filling the voids.
- Angular, not rounded: crushed rock has faces that interlock. Rounded gravel behaves more like ball bearings and can always find a way to move.
- Compacted in layers: a deep sub-base must be built up and compacted in successive layers, not dumped to full depth and rolled once. A roller only densifies so far down, and material below that depth is simply loose stone with a firm crust over it.
- Deep enough for the ground it sits on: this is the part that varies most. Firm, well-drained gravel subgrade needs relatively little; soft clay that holds water needs considerably more, and may need the ground improved or a geotextile separator before any stone goes down at all. This is a question for someone who has looked at your actual ground, not something to take from a table on a website.
Drainage is half the job
Water is what destroys pavements, and it does it in several ways at once. Saturated subgrade loses strength dramatically — a clay that would carry the load when damp will deform under it when wet. Water trapped in the sub-base cannot drain away, so every wheel load pumps it around, washing the fine material out and leaving voids. And where it freezes, it expands and lifts the surface, then leaves a gap behind when it thaws.
So a properly built pavement handles water deliberately rather than hoping it goes away.
- Falls: the finished surface should shed water to a defined place. A driveway with no fall holds puddles, and puddles find every weakness in the surface.
- A drainage path: water that gets into the sub-base needs somewhere to go, whether that is a permeable edge, a drain, or free-draining ground below.
- Edge restraint: an unrestrained asphalt edge crumbles under load and lets water in from the side. A kerb, an edging or a haunching detail keeps the edge supported.
- Never pave over the problem: laying asphalt on ground that stays wet buys you a season or two. The water is still there.
The separation layer
On soft or fine-grained subgrade, a geotextile between the soil and the stone is cheap and does something the stone cannot do for itself. Without it, the fine soil below gradually works up into the voids in the stone, and the stone works down into the soil, until the sub-base is contaminated and behaves like the soil rather than like stone. That process is slow, invisible, and irreversible.
What this means for your estimate
The asphalt tonnage is only part of the material for a driveway, and often the smaller part by volume. The sub-base stone is usually the larger excavation and the larger delivery, and the excavated spoil has to go somewhere too, which is a cost people routinely forget.
Work out each layer separately, at its own thickness and its own density. Then check the arithmetic that catches people out — the depth you excavate is the total of every layer, not the depth of the asphalt. A 100 mm asphalt construction over 150 mm of sub-base means digging out 250 mm plus whatever it takes to reach ground you can actually build on.
If the budget will not stretch to both a proper base and a thick surface, spend it on the base. A thin surface over a sound base can be resurfaced in ten years. A thick surface over a bad base is a demolition job.