Base course
Also called: gravel base, crushed stone base, road base, crusher run, 3/4 minus, aggregate base
The layer of compacted crushed stone under a driveway, slab, patio or paver surface — typically 4 inches under concrete and 6–8 under pavers or gravel that cars park on. Made of 3/4-inch minus stone compacted in 2-inch lifts; it's what keeps the surface from settling and cracking.
Every hard surface outdoors sits on a base course, and nearly every failure — a paver that sinks where the tire stops, a slab corner that drops, a gravel drive that ruts — is a base problem. The surface material gets the attention; the base does the work.
The stone matters. 3/4-inch minus (also sold as crusher run, road base, or by a state spec number) is crushed rock from 3/4 inch down to dust; the fines fill the voids and the angular pieces interlock, so it compacts into something close to a solid. Rounded pea gravel or clean stone without fines never locks and is only used as a top dressing or for drainage. Order by volume: length × width × depth in feet ÷ 27, plus 10% for compaction — about 3.7 cubic yards for 6 inches over a 10×20 strip.
Placing it is the job: over firm subsoil (all topsoil removed) and usually a geotextile fabric, spread in 2-inch lifts, dampened, and run over with a vibrating plate compactor in two directions until it stops moving. Then the next lift. Depth by use: 4 inches under a concrete slab, 6 under vehicular pavers or a gravel drive, 8 or more under an RV pad or on clay. The finished base is flat, at the planned depth everywhere, and pitched the same way the surface will drain.