Once you have selected your site, you need to choose your stones. Look for rocks with angular faces—they stack better and a give a firmer repose. Rounded rocks are almost impossible to build into a wall without using copious amounts of mortar. An ideal rock has six parallel faces (like a brick). Regretfully, there aren’t many ideal rocks available, so look for angular rocks with the most flat faces.
To figure out how much stone you’ll need, multiply your wall’s height times the depth times the length. If your wall is 2 feet high, 1-1/2 feet wide, and 20 feet long, you’ll need roughly 60 cubic feet of stone. Most stoneyards will deliver the stones for a slight charge; have them placed as close to the site of your retaining wall as possible.
As for tools, you’ll need a shovel for digging your trench and backfilling, a mattock for attacking the grade, and a small sledgehammer for tamping soil. For marking your site and leveling rocks, you’ll need a line level, a few tall stakes, string, some flour, and a 4- or 8-foot level.
So you’re all set with your tools, some drinking water, and maybe some tunes to work by. The first thing to do is to determine the front face of the wall. If it is straight, a board or a string stretched between stakes can be used to mark the line. Otherwise, use a garden hose and mark the edge with flour.
Now you can start digging. The easiest approach is to cut and fill—that is, dig into the slope where the wall will go and spread the earth below you to create a level terrace. When you cut and fill, the wall is backed by undisturbed soil, which is more stable than fill. For design reasons, however, you might choose to build a freestanding wall and fill behind it with soil from another site. Or you might do a partial cut and fill, which is somewhere between the two.
Walls are built in courses. The base course is structurally the most important, while the final course, the capstone, is the most challenging. For stability, walls should be at least 20 inches wide at the base. They can taper slightly toward the top, but you’ll want a wall that is at least two rocks wide in most places. This can be accomplished by mixing stones of different sizes or by backfilling with a combination of two-thirds rubble to one-third soil.