Granular A, and why the number is 150 mm.

What Granular A actually is

A specified blend of crushed stone with a controlled range of particle sizes, from roughly 19 mm down to fines. That gradation is the point: the small particles fill the gaps between the large ones, so when it is compacted it locks together into a dense, stable layer.

It is not the same as clear stone, which is washed and single-sized and used where drainage matters more than compaction. It is not the same as pit-run or crusher-run either, though those get substituted on cheap jobs.

The gradation is what lets it be compacted to high density. Substituting uncontrolled material means it never reaches that density regardless of how it is rolled.

Why 150 mm for a residential driveway

The base spreads the point load of a wheel out over a wider area of subgrade. Thicker base means the load reaching the soil is lower and spread wider.

On the clay-heavy soil common across the GTA, 150 mm compacted is the working minimum for vehicle traffic. Below that, load concentrates and clay deforms — which shows up as rutting in the wheel paths and settlement over a few seasons.

Pedestrian areas like patios and walkways are fine at around 100 mm because the loads are a fraction. Commercial drive aisles need 200 mm or more.

Two lifts, not one

A plate compactor or roller only densifies material to a limited depth. Dump 150 mm in and compact from the top, and the bottom half is barely touched.

So it goes in two lifts of roughly 75 mm, each compacted separately with a minimum of four passes, before the next goes on. It takes longer and it is the only way the full depth actually gets compacted.

This is invisible in the finished job and impossible to verify afterwards, which is exactly why it should be written on the quote and photographed during construction.