Crushed stone is almost always a base material — the layer that carries the load under something else. Because it is buried, it is the layer people cut short to save money, and it is the layer that causes the failure when they do.
Base depth by what sits on top
Under a paver patio that only takes foot traffic, four to six inches of compacted base is the usual specification.
Under a driveway that takes vehicles, eight inches or more, and more again in cold climates where frost heave is a factor.
Under a concrete slab, four inches of compacted stone gives drainage and a level bearing surface.
It has to be compacted in lifts
Dumping eight inches of stone and running a plate compactor over the top does not compact the bottom of it. Base goes down in lifts of three to four inches, each one compacted before the next goes on.
Expect the material to lose 10 to 20 percent of its loose volume when compacted. Order for the compacted depth you need, not the loose depth you pour.
Pick stone with fines in it
For a base you want crusher run, road base, or the product your local yard calls three-quarter minus — angular stone with the fine dust left in. The fines fill the voids and lock the layer solid.
Clean washed stone with no fines will not compact into a stable base. It is the right material for drainage, where you want water to pass through, and the wrong one for bearing load.
What the numbers mean
Crushed stone is sold by grade numbers and the system is not intuitive. Number 57 is a clean stone around three quarters of an inch with no fines, which drains freely but does not lock together.
Crusher run, sometimes called 21A or dense grade, contains everything from stone down to dust. It compacts into a hard, stable layer and does not drain well.
Which one to use follows from the job: base under pavers or a slab wants a dense grade that compacts; a French drain or a drainage layer wants clean stone that water moves through.
Geotextile under the base
Over soft or clay subgrade, a woven geotextile between the soil and the stone stops the two mixing. Without it, stone works down into the soil and soil pumps up into the stone under traffic.
That mixing is invisible and it is what causes a driveway base to lose thickness over a few years without anybody adding or removing anything.
The fabric is inexpensive relative to the stone it protects, and on any base over poor soil it is the cheapest insurance in the whole assembly.