A fine retaining wall can keep a slope from eroding and can create level land. You can build one of stone, brick, concrete blocks, or poured concrete, but pressure-treated 6 x 6 landdscape timbers are easier to work with ad require no concrete footing. For a wall higher than 4 feet, call a professional.
First, excavate the base of the slope, separating topsoil from subsoil: later, when you backfill, use the subsoil first. Then stretch a string between stakes, and along it dig a level-bottomed trench 12 inches below the finished grade. Put 10 inches of gravel in the bottom. Lay the first course of timbers level end to end; pitch them 1/4 inch from front to back. Cut the timbers with a chain saw if necessary; none should be shorter than 6 feet. To anchor them, drill a vertical hole 1 foot from each end and drive 2-foot¬long steel reinforcing bars or galvanized steel pipes into the ground.
Set the second course and each subsequent course back 1/4 inch. Lay the timbers so that their ends are staggered; secure them by driving 10- inch galvanized spikes into the timbers below. Leave 1 – inch gaps, or weep holes, between ends of timbers in the second course.
After laying the second course, install drainage pipes; excavate behind the base course and lay the pipes atop a bed of gravel. Cover them with gravel, then fill with soil.
Install 4-foot-long anchors atop the ends of the second course and at 14-16 foot intervals between. In subsequent courses, nail timbers atop the anchors, creating a a series of three-sided cribs. Backfill with earth and tamp firmly before adding each course.