How to build natural stone walls:
This post is the first of five on retaining walls and their construction, beginning with the easiest -- gravity-stacked stone walls -- and working through, in ascending order of construction difficulty, pressure-treated timbers, engineered materials, mortared rock, and cultured stone; the last not truly a wall type, but a wall facing. I really like cultured stone and, since this is my posting and I kind of get to do what I want, I've included it. I have not included concrete, brick, or steel walls, as I think they are difficult to incorporate into an informal landscape.
There are several basic types of gravity-stacked natural wall stones: Quarried flagstone, like crab orchard, which is usually a brown or pink color, Tennessee field stone, usually a dark gray to black color, or field stone boulders, also dark gray to black; if you can find either of the latter two types with moss on them, and your wall will be predominantly in the shade, consider getting these guys -- they will look great. Keep in mind that the flatter, or more angular, the rock, the easier to stack the wall -- building with round rock is akin to stacking BB's! Additionally, medium to thick rocks assist in wall stability.
But, to begin at the beginning -- some folks like to pick out their rock first, bring it home, and then start the project; I do it kind of backwards -- I already know the kind of rock I want for the project, so I lay out the project first so I have some idea of the quantity of material required, then I purchase the rock, so I don't have to either go get more or return it. One of the axioms of construction is that you will never, ever, buy the exact amount of material required for the project -- it's always a bit over or a bit short. Save yourself the extra trips to the rock yard.
- Decide the kind of rock you want and how it will fit into your landscape architecture -- formal or informal.
- Retaining walls are best suited to incorporation into a slope -- otherwise you have to bring in a lot of extra dirt for back fill -- not good.
- Calculate the amount of the wall rock as follows -- length x width x height = so many cubic feet of material; multiply that number by 125 pounds per cubic foot (an approximation for most wall rock -- check with your rock guy for the actual weight per cubic foot) -- to give you the pounds of rock; divide that number by 2000 to give you the tons of rock you need; rock yards generally sell by the ton. Have it delivered, or make 128 trips back and forth from the stone yard to your home -- your choice, but I recommend delivery.
Lay out the dimensions of the job -- how long, wide, and tall. Gravity walls are usually shorter than 3'; taller than this height and you have lateral support problems. If the wall is under 3', the width will be approximately 2' wide at the bottom and 1' wide at the top.
- Dig the foundation trench -- for a 2-3' wall, it should be about 6" deep, 2' wide, and as long as you like. It doesn't have to be perfectly flat; you use the varying thicknesses of rock to accommodate the slope, step-downs (where the wall changes height) and corners. You can use gravel as a base, in the trench, if you like, but it's not required for either stability or drainage. You can build the wall to follow the slope or build it with a flat top and step it up and down.
- Now comes the fun part -- look through the rock, try to save some of the flatter stone for cap rocks, and find the biggest guys for the base. Lay them out, side to side and front to rear, for the entire base, butting the joints as close together as possible.
- Continuing laying the wall, side to side and front to rear, increasing the height, and mixing the rocks for appearance and tightness of joints, making sure you "step" the wall back, and back fill and tamp with excavated soil (or extra soil) as you continue vertically. This "leaning back" is called "set back" or "batter" and is essential to ensure the strength of the wall. If you don't step the wall and back fill, it will, as you might imagine, fall over--then your kids will laugh at you and you will have to bribe them to be quiet and not tell their friends.
- Cap the wall with your previously set aside flat rock, tamp the soil, and finish the project by adding a protective mulch, sod, or constructing a flower bed.
- You don't generally have to allow for drainage provisions (gravel or weep holes) for hydrostatic pressure (water pushing against the rear of the wall); the irregularity of the wall rock will allow the water through.
- If there is the possibility that the back-filled soil will stain your wall (e.g. red Georgia clay) or the back-fill will just "leak" through (sand), consider a filter fabric that allows water through.
- Sit back and revel in the admiration of family and friends.
The pictures of gravity walls are to pique your interest and get you started; they are all courtesy of the good folks at Stone Forest Materials, Kennesaw GA.