- 1). Dig a trench 2 feet wide and 6 inches deep for the base of the wall. The base should extend from two inches in front of the proposed wall, under the wall and on behind it. If you’re using 8 by 8 inch timbers, this means the trench will extend 14 inches behind the retaining wall. Line this trench with plastic sheeting, wide enough to accommodate the height of the wall plus 3 1/2 feet. Leave a 2-inch lip over the front edge of the trench and lay it up the side of the backfilled earth wall.
- 2). Determine how deep your footer will sit. It should be 10 percent of the height of the wall. For a 3-foot wall, the footer would be set a little less than 4 inches deep. For the 3-foot wall, then, install 2 inches of decomposed granite before installing the bottom timbers. Grade it smooth. Finish filling the trench around the bottom timber until it is full.
- 3). Lay each subsequent course of timber with a 1/2-inch back-step. Hammer galvanized spikes or rebar into partially pre-drilled holes at the front 2-inches of the second course and every subsequent course. Space these spikes 4 to 8 feet apart down the length of the wall and drive them down into the lower course.
- 4). Install a perforated 4-inch PVC pipe behind the wall, centered. This will control water pressure behind the wall. Install a PVC joint at the lowest point of the pipe and join this with piping extended to an outlet for excess water. Cover the perforated pipe with pea gravel.
- 5). Install a deadman for approximately every 2 feet of wall height, every 4 to 7 feet along the the wall. This is a 3 by 3 T of the timber you’re building the wall out of, whose bottom end sits flush with the front of the retaining wall. The top of the T acts as an anchor in the soil behind the retaining wall. Drive a spike down into the base of the deadman T to anchor it in the course below.
- 6). Backfill soil behind the plastic and pea gravel in front of the plastic. Each 3-inch layer of backfill should be compacted before adding the next layer. The top 3 inches behind the retaining wall will be planting soil. The 3 inches below that will be a highly compacted layer of backfill over the pea gravel.
Source...