21A in Virginia: Base Under the Drive, Not the Top of It

The most common gravel driveway failure in this state is not really a failure of gravel. Surfacing stone goes down straight onto ground nobody prepared, the clay or the low sandy soil underneath takes on water through a long wet spell, and by the second spring there are ruts where the tires run and a soft patch where the truck turns. 21A is the material people order to stop that happening a second time, and it earns its keep only when it goes in as the layer underneath rather than the layer you stand on.

The drive that was surfaced but never given a base

Most orders for this material trace back to one of four jobs, and all four have the same shape: something above has to stay level while the ground below moves.

• A residential drive or parking pad that has started to rut, where the fix is a compacted course beneath a fresh surface rather than more loose stone on top.
• A shed, garage or hot tub pad that has to stay flat after the ground swells and drains through a Virginia year.
• A patio or walkway base under pavers or flagstone, where any settlement shows in the joints.
• A site entrance or farm lane that has to carry loaded equipment in and out through the wet months.

Size is 3/4 inch minus, graded as a mix of crushed stone, gravel and binding fines. The fines are the point. They close the voids between the larger pieces under compaction, so the layer locks up instead of shifting. That also means it is not a drainage stone, and treating it as one is the second mistake on this list.

Locally, base material is specified by the VDOT Road and Bridge Specifications, where Section 208 covers the dense-graded sizes including 21A and the coarser 21B. That is how aggregate is described on plans and by yards across the state. Treat it as shared vocabulary rather than as a mark this storefront holds.

How 21A sits on Piedmont clay, Tidewater sand and valley rock

Three grounds, three different reasons to compact properly.

• Piedmont clay. Across the Richmond, Charlottesville, Lynchburg and Danville areas the subsoil is heavy, often red, weathered in place over a thick layer of soft decayed rock. It drains slowly, so it gives the base very little help. The compacted course has to spread the load itself, and it needs to be thick enough to do that rather than thick enough to look right.
• Tidewater and Coastal Plain. On the flat country of Hampton Roads, the Eastern Shore, the Northern Neck and the Middle Peninsula the profile is layered sand, silt and clay, and water sits close to the surface. Sandy spots drain quickly; the marshy low ground beside them does not. That difference can occur inside one lot, which is why a base laid to a single depth across a wide pad sometimes settles at one end only.
• West of the Blue Ridge. In the Shenandoah, Roanoke and New River valleys the ground is limestone and dolostone with karst, and rock often sits close to the surface. A base there may be resting almost directly on stone in places and on soft pockets in others. Concentrated runoff is the thing to watch, because in karst ground water that gets channeled goes down into the rock rather than across it.

Freeze-thaw is moderate through most of the state and heavier in the Blue Ridge and the western mountains, which is another argument for tight compaction: a dense layer takes in less water, and water is what moves a base over a winter. Frost depth is set locality by locality here, from around 12 inches in Virginia Beach, Norfolk and Chesapeake to 24 inches in Fairfax and Prince William counties and 18 inches in Henrico, Goochland, Campbell and Madison counties, so check the figure your own locality uses rather than assuming a single statewide number.

Shaping the subgrade, wetting the fines and rolling it hard

Order of operations matters more than tonnage here.

1. Strip the organic layer and any soft material until you are on firm ground. Base laid over topsoil or over a saturated pocket will find that pocket again.
2. Set the fall before any stone arrives. A couple of percent across a pad sheds water; a flat pad holds it, and on slow-draining clay that water stays.
3. Place in lifts rather than in one deep layer, and compact each lift. A plate compactor suits a shed pad or a patio base; a roller suits a lane or a parking area.
4. The fines need some moisture to bind. Bone-dry material will not knit, and saturated material pumps under the roller. Damp is the target.
5. Check the surface against a string line before the top course goes on, because a dip left in the base becomes a puddle later.

Wall backfill and a pond edge want a different stone

Two jobs come up constantly in the same conversation as a base order, and neither one takes this material.

The nearer relative is 21B, the coarser dense-graded size with fewer fines. It drains a little better and packs a little less tightly, which suits some patio and pad work. If what you actually want is the wearing course that goes over a finished base, look at Driveway Gravel instead.

Sizing a 12 by 40 foot construction entrance

Length in feet times width in feet times depth in inches divided by 12 gives cubic feet. Divide by 27 for cubic yards.

• Example, a site entrance 12 ft wide and 40 ft long at 8 inches: 12 x 40 = 480 square feet. 480 x (8 / 12) = 320 cubic feet. 320 / 27 = 11.9 cubic yards.
• Example, a 10 ft by 20 ft shed pad at 4 inches: 200 square feet x 0.333 = 66.7 cubic feet. 66.7 / 27 = 2.5 cubic yards.
• Example, a car drive 10 ft wide and 60 ft long at 6 inches: 600 x 0.5 = 300 cubic feet. 300 / 27 = 11.1 cubic yards.

This site publishes roughly 3,000 lb per cubic yard for road base, about 1.50 tons per yard, and that weight is approximate: the site entrance above is 11.9 x 1.50 = roughly 17.9 tons, with the shed pad about 3.8 tons and the car drive about 16.7 tons.

Those are loose cubic yards before compaction, so round up rather than down. The 21A calculator converts cubic yards into tons for this material and carries the depth guides.

Pick a quantity and a delivery date on this page once the subgrade is cut and the fall is set. Delivery gets arranged at checkout from your ZIP code, so decide in advance where a loaded truck can pull in, tip and back out without crossing soft lawn.