How to Build a Dry Stack Stone Retaining Wall

25 Jan How to Build a Dry Stack Stone Retaining Wall

We’re building another retaining wall. A previous blog entry demonstrated our technique of using a blasted rock in building a retaining wall. This customer wanted to have us use a quarried rock, in this case we chose to use Eagle Mountain basalt. It’s a beautifully weathered stone, very angular, great color variations and in some cases covered in lichens and moss. The retail cost of this variety is around $450/ton. I calculate this size rock to cover approximately 12 square feet per ton. Also, for each ton of rock I generally use three yards of crushed and washed structural fill rock. Here in the Northwest, its generally a basalt which is very angular. It makes for excellent structural back fill.

We begin every project with a demolition phase. We generally stockpile the spoils for use in backfill but in this case the job site is on virgin land that has been used as an edibles garden for the known recent history of a few generations and in this case it meant the soil was replete with weed species. Lots of the soil got trucked to the landscape supplier. They can cope with sterilizing the morning glory roots and the blackberry DNA.

Next we level the footing by hand with a shovel and the Spectra laser level. Next we add a 4″ bed of that 5/8″ clean crushed rock and compact it with the Wacker compactor. None of us are psyched to operate that machine. Now I determine the exact position of the wall and we run a masons string to establish a straight line for the the footing. We select the biggest, most rectangular rocks for the footing. We favor placing the rocks solidly in the bed of crushed rocks over trying to level the base rocks. When we get the line of foundation rocks installed we back fill with crushed stone, making sure to work crushed rock into every crevice and space between the stones. This is vital to the strength of the wall. The aggregate rocks act as wedges against the individual rockery stones, providing the friction that keeps the rocks from moving away from the wall.  As we set stones we shim and level the rocks to provide even mating surfaces for the next layer.

I didn’t mention the use of a footing drain in this project. The soil on this project is very free flowing. Due to the smaller size of the foundation rocks in this type of wall, I favor the compacted crushed stone at the base of the wall over a hollow pipe for strength. This type of dry stack wall has lots of permeability and lots of rock-to-rock contact. Water won’t back up behind this wall, it’ll flow through freely.

Building with this nicely rectangular stone gives us an opportunity to use soldier stones, rocks that we orient into the slope to help lock the wall into the hillside. Each layer of rocks gets a few soldier stones, staggered from one lift to the next. As we erect the wall, we slope the wall into the hillside at around 15 degrees of lean. On some projects I make a storey board to indicate the lean, but the crew I have now has done so many of these, they’ve gotten good at eyeballing the angle of lean.

On this project Nancy designed a bench to be incorporated into the rock wall. I selected a good sized slab of Bluestone for the seat. As we continue to build, the bench will have an integrated backrest. I should remind myself to get a picture of the finished project!

As always, thanks for reading.