Compaction is the whole job.

What compaction actually does

Freshly placed granular has significant void space — air gaps between the particles. Compaction forces the particles into those voids so they lock together and the layer becomes dense and stable.

An uncompacted base will compact anyway. It just does it slowly, under your vehicles, over the following years — and unevenly, which is what produces dips and rutting rather than uniform settlement.

The difference between compacted and uncompacted is not subtle. It is the difference between a base that is done settling and one that has barely started.

Why lifts, and why four passes

Compaction energy only penetrates so far. A plate compactor effectively densifies roughly 75 to 100 mm depending on the machine and the material. Place 150 mm and compact from the top, and the lower half stays loose.

So the base goes in two lifts of about 75 mm, each compacted before the next is placed. Minimum four passes per lift with a plate or roller, more where the material is dry or the machine is light.

The number is a floor, not a target. Density is what matters; passes are the practical way of specifying it on a residential job where nobody is running nuclear density tests.

Hand tamping is not compaction

A tamper on a stick moves the surface stones around. It does not deliver the energy needed to densify a lift of granular, and it does not do it uniformly across an area.

Where you will legitimately see hand work is at edges and around obstacles where a plate cannot reach. Those spots are also where settlement most often starts, which is why a good crew works them carefully with a smaller machine rather than a hand tool.

If you watch a driveway being built and there is no plate compactor or roller on site, you are watching a driveway settle in slow motion.

Subgrade compaction matters too

Before any granular goes in, the exposed subgrade gets proof-rolled or compacted. If the soil beneath is soft, everything above it inherits that.

This is also when soft spots get found. A section that pumps or deflects under the roller is telling you it needs excavating deeper and replacing, and it is far cheaper to discover that with the excavation open than two winters later.