#4 Crushed Stone for Virginia Wall Drains and Site Entrances

More of this stone goes behind block and boulder walls than anywhere else in the state, and the reason is the ground it is holding back. Piedmont clay takes water in slowly, so after a summer storm the pressure behind a wall builds where the water cannot leave. A wide zone of coarse open rock gives that water a route down to a drain line instead of a route through the wall. Nothing about a wall failure is sudden: it starts as a bulge in the second winter, and the drainage zone is what prevents it.

The drainage zone behind a wall, and why it wants coarse rock

Size here runs about 3/4 inch to 1-1/2 inches, graded to the #4 designation, angular crushed stone with the fines washed out.

Behind a wall the layer has two jobs at once. It has to let water fall freely to the drain at the base, which needs big void spaces, and it has to stay where it is put under the weight of whatever is above it, which needs angular faces that lock against each other. Coarse washed rock does both. Finer material drains more slowly and can pack into the space the water needed; soil backfill does not drain at all, which is the condition that pushes walls over.

The same reasoning puts it at a construction entrance. A vehicle leaving a wet site carries clay on its tires, and a coarse open layer flexes the mud off as the wheel passes over it rather than holding a smooth sheet of it. It also stops the entrance becoming the softest ground on the plot.

How #4 crushed stone behaves on a red-clay slope and on wet coastal ground

The stone gives different results on the three grounds this state is built on.

• Piedmont clay, including the cleared slopes that shed soil once they are bare. Heavy angular rock resists movement on a grade better than anything rounded, so it holds in a swale or on a bank where runoff would scour bare clay. Keep it off saturated clay without separation, or the fines come up into it.
• Flat, low Coastal Plain ground around Hampton Roads, the Eastern Shore and the river necks. The water table sits close to the surface, so the stone is pressed into soft ground under load unless the subgrade has been firmed and separated. Along a tidal edge, moving water and storm tides work at the toe of anything placed there.
• The limestone valleys west of the Blue Ridge. Bedrock is often shallow, so the excavation is the constraint rather than the stone, and any concentrated discharge into karst ground is worth spreading out.

Frost is a factor in the north and the mountains, and an open layer that drains freely has less water in it to freeze.

Building the wall drain in lifts, and keeping clay out of the voids

Build the wall drain from the bottom up. Set the drain line at the base with its fall established, bring the coarse stone up in lifts rather than tipping the whole zone in at once, and keep the stone back from the soil face with a separation layer so silt cannot wash into the voids. Soil is the enemy of an open drainage layer: once the spaces are full, the zone has become expensive backfill.

At a construction entrance, strip the soft material, firm the subgrade, then lay the stone in a single thick layer wide enough that a vehicle cannot drive around it. It does not need rolling flat, and flattening it would defeat the point.

At a pond or shoreline edge, cut the toe in below the working water level so the bottom course is keyed rather than resting on the face, then work upslope. Any job at a tidal edge or in a stream usually brings the locality into it, so settle that before the stone is ordered.

When #57, ballast or RipRap is the better order

Four products on this site take most of the jobs this one gets ordered for by mistake, and the table sorts them by what is actually going wrong.

Counting a 48 foot wall drain, a pond edge and a site entrance

Length in feet by width in feet, times depth in inches divided by 12, divided by 27 gives cubic yards. This site publishes roughly 2,900 lb per cubic yard for #4 stone, about 1.45 tons per yard, and that approximate figure is used below.

• Example, retaining wall backfill along 48 ft of wall, a 12 inch wide drainage zone 4 ft high: 48 x 1 x 4 = 192 cubic feet. 192 / 27 = 7.1 cubic yards, roughly 10.3 tons.
• Example, a pond or shoreline edge 60 ft long and 5 ft wide at 8 inches: 300 sq ft x 0.667 = 200 cubic feet. 200 / 27 = 7.4 cubic yards, roughly 10.7 tons.
• Example, a construction entrance 14 ft by 55 ft at 6 inches: 770 sq ft x 0.5 = 385 cubic feet. 385 / 27 = 14.3 cubic yards, roughly 20.7 tons.

Those are worked illustrations, not jobs that were built. Open stone settles into the ground it is placed on rather than compacting within itself, so on soft subgrade expect to lose some depth into the surface. Check your own numbers in the stone calculator, then choose a quantity and a date here. Delivery is arranged at checkout by ZIP code, and a loaded truck needs a clear run to somewhere it can tip.