#4 Crushed Stone for the Layer Under Soft Ground

There is a window every spring between the frost leaving and the ground firming up, and it is the worst possible time to fix anything. A lane that holds water in April is telling you what it needs, and April is the month you cannot give it.

Note the spot. Come back in June when the ground has drained, and dig it out properly.

#4 crushed stone is what goes in the bottom of that hole. Angular crushed rock, roughly 3/4 inch to 1-1/2 inches, with wide gaps between the pieces and no fines to plug them.

The camp road stretch #4 crushed stone is bought to fix

The classic New Hampshire version: forty or fifty feet of lane that pumps and ruts no matter how much material has gone into it over the years.

What is happening underneath is that fine-graded gravel is being pushed down into saturated soil while the soil comes up through it. Both materials end up ruined. Another load of the same thing repeats the cycle.

A coarse stone breaks it. The pieces are too big to be pushed into soft ground easily, they bridge across the weak spots, and the space between them gives water a route sideways instead of holding it under the wheel track.

The same reasoning puts it in ditch bottoms, in gabion baskets, and as a construction entrance where machinery has to cross wet ground.

Till, hardpan and the water that perches on frozen ground

Dense glacial till does not drain on its own. Under it there is often a hardpan layer, and water that arrives in spring has nowhere down to go.

In mud season that water perches. The top few inches thaw, the ground below stays frozen, and everything held above it turns soft until the whole profile opens up. A road built on that needs its own drainage path, not a thicker layer of the same fine material.

Ledge changes the shape of the fix rather than the fix itself. Where rock is close to grade you cannot dig the section, so the layer goes above the rock and the edges do the work. Grade hard to the side and give the water a ditch to reach.

The sand plains around Ossipee and the outwash in the river valleys need none of this. That ground drains itself.

Lift thickness, separation fabric and what goes over the top

Fabric first, always, where this stone meets soil. Without a geotextile between them, the fines work up into the voids and the layer you paid for becomes the ground you were trying to replace.

Then six to eight inches of stone, placed in one lift because it will not compact in the usual sense anyway. A few passes with a loaded bucket or a roller seat the pieces against each other. That is all the compaction it takes.

Then it needs a cover. A 3/4 to 1-1/2 inch stone is not a surface to drive or walk on. Over the top goes a compacted layer of crushed gravel, then whatever wearing course the job calls for.

Keep the fabric edges lapped well past the sides of the trench, and fold them over the top of the stone before the base goes on.

Where #57 stone stops and railroad ballast starts

Three coarse stones that look similar in a pile.

The step up from a house-drain stone to this one is about load. The step from this one to ballast is about how soft the ground is and how heavy the traffic.

Drainage lifts under a flagstone walk, four raised beds and a play area

Three hypothetical jobs where a wet seam runs through the work. This size runs about 2,900 lb per cubic yard on this site's published figures, near enough to 1.45 tons per yard.

Flagstone walk, 45 ft by 3.5 ft, 4 inch drainage lift below the base

• 45 x 3.5 = 157.5 sq ft
• 157.5 x 4 / 12 = 52.5 cu ft
• 52.5 / 27 = 1.94 cu yd
• 1.94 x 1.45 = 2.8 tons, order 3

The flagstone still beds on stone dust over a compacted base. This layer is only there to keep the seam from wicking up.

Pad for four raised beds, 16 ft by 22 ft, 4 inches

• 16 x 22 = 352 sq ft
• 352 x 4 / 12 = 117.3 cu ft
• 117.3 / 27 = 4.35 cu yd
• 4.35 x 1.45 = 6.3 tons

The beds themselves take garden soil. The stone keeps the aisles between them out of the mud.

Play area, 26 ft by 34 ft, 4 inch drainage blanket

• 26 x 34 = 884 sq ft
• 884 x 4 / 12 = 294.7 cu ft
• 294.7 / 27 = 10.91 cu yd
• 10.91 x 1.45 = 15.8 tons, order 16

Other shapes go through the stone calculator.

What a tandem needs at the end of a camp road

Set the quantity and the delivery date on this page. Delivery is arranged at checkout by ZIP code.

Coarse stone is heavy work to move twice, so put the pile as close to the trench as the truck can safely get. Firm ground, room to back, clear height to raise the body, and branches cut back over the approach.

If the excavation is already open, keep the drop a full truck width clear of the edge. Stone this size takes real effort to get back out of a trench by hand.

If you are not sure whether the job wants this or a smaller drain stone, send the dimensions to New Hampshire Rock Supply from the contact page before you order.