3/4" Crushed Stone in Maryland: Not #57, and Not CR-6 Either

Most people reading about 3/4" crushed stone have three lines of an order form open and are trying to work out why they all describe roughly the same rock. The answer is in what has been taken out. This one is angular stone at a nominal 3/4 inch. #57 is washed to a 3/16 to 1 inch band and sold for maximum flow. CR-6 is a graded blend that runs from a similar top size right down to dust, and the dust is what lets it bind into a hard surface. On a Maryland lot those three end up in three different parts of the same project.

Size: approximately 3/4 inch. Type: angular crushed stone. Approximate weight, from this site's own published figures: about 2,800 lb per cubic yard, or 1.40 tons per cubic yard.

Backfilling a wall and basing a drive: where most of these loads end up

Two jobs account for the bulk of it. The first is the drainage zone behind a retaining wall, where the stone has to carry the weight of the soil above without closing up, and give the water collecting behind the wall a route out instead of a place to push. The second is a supporting layer under something else: a drive or road subbase, a pad, a footing area, the layer under a patio build-up. Angular faces key together under a plate or a roller and stay keyed, which is the property both jobs depend on.

It also does trench and stormwater work, where consistent size keeps the flow predictable, and it backfills block, timber and stone walls alike. What it does not do is finish a surface. There is nothing fine in the gradation to fill the gaps at the top, so left exposed it stays loose underfoot and under a tire.

What Piedmont clay and a Bay-side water table each do to a 3/4 inch layer

Central Maryland clay and silt loam hold water and turn soft when they are saturated, which is the single most common reason a base layer fails here. Stone tipped onto wet Piedmont subgrade presses into it, the two mix, and a layer that measured 4 inches on the day loses half its thickness to the ground beneath. The fix is separation and timing rather than more stone.

Near the Bay and the tidal rivers the problem is height rather than softness. Flat Coastal Plain ground sheds water slowly and the water table can sit close to the surface, so a layer built at grade is a layer sitting in water for part of the year. Raising the build-up is usually a better answer than deepening it.

Freeze and thaw then works on whatever holds moisture. Frost runs to about 30 inches across most counties, 24 inches in Charles and several Eastern Shore counties, and 36 inches in Garrett and Allegany. Open stone holds very little water and heaves comparatively little, and that is part of why it goes behind walls and under slabs rather than fine material.

One planning note for waterfront work. Within 1,000 feet of tidal water, Critical Area rules limit lot coverage, and gravel and stone surfaces count toward the figure.

Lifts, plate passes and the fall you set before the stone lands

• Get the subgrade shaped and the fall set before anything is delivered. Every layer above copies the shape of the one below, and a compacted stone layer will not correct a slope that runs the wrong way.
• Build in lifts rather than one deep dump. Four inches at a time is about as much as a plate compactor will work through properly.
• Compact each lift before the next one goes on, and run the plate in overlapping passes in two directions.
• Behind a wall, keep the drainage zone clear of the soil backfill and bring both up together in step, so neither side is left unsupported.
• Under a patio or a pad, check level after each lift rather than at the end, when the only remedy is taking stone back off.
• The truck needs clear access to the dump spot, and a load of stone is far easier to place twice by machine than once by wheelbarrow, so choose the pile position before delivery day.

3/4" crushed stone next to #57 and CR-6

The short version. If the layer has to carry weight and pass water, this one. If the layer is purely about water moving, buy washed. If the surface has to firm up under a tire, buy the graded blend. Maryland Rock Supply lists all three separately for that reason, and Crusher Run sits alongside CR-6 as the other binding option.

Tons behind a 42-foot wall, along a pond shelf and across a site entrance

Square feet times depth in feet, divided by 27, gives cubic yards, and cubic yards times 1.40 gives tons. These are worked examples, not real projects.

• Example, the drainage zone behind a 42 foot retaining wall, 12 inches wide and 4 feet high: 42 x 1 x 4 = 168 cubic feet, divided by 27 = 6.22 cubic yards, about 8.7 tons. Wall height rarely stays constant, so average the height along the run rather than using the tallest point.
• Example, a pond shelf 60 feet long and 3 feet wide, or 180 square feet, at 6 inches: 180 x 0.5 = 90 cubic feet, divided by 27 = 3.33 cubic yards, about 4.7 tons. That figure holds for an ornamental pond edge. A tidal shoreline taking wave action will move stone this size, and that job wants RipRap.
• Example, a construction entrance 50 feet by 14 feet, or 700 square feet, at 6 inches: 700 x 0.5 = 350 cubic feet, divided by 27 = 12.96 cubic yards, about 18.1 tons. For light traffic over firm ground with a separation fabric underneath, this size holds. Where loaded trucks are running over soft Coastal Plain ground, go up to #4 Crushed Stone, which is large enough not to be pressed straight into the mud.

Work your own dimensions, then check the conversion in the stone calculator and set a quantity and a delivery date here. Where a wall and a drive are going in on the same job, send both sets of measurements through contact and have the split between materials worked out before you order.