Pressure-treated wood makes a good, solid retaining wall, which stands up well to shifting soils and should last for decades. Here’s how to build a sturdy timber wall.
For a short 1-2 foot wall, you can use 4x4s, but you’ll want larger dimension lumber for taller walls.
If you’re willing to go for the heavy equipment and have access, it’s easier to excavate the whole area, then backfill. But you can do it by hand, too. Start by excavating the area where the wall will stand. Go deep enough to bury the first timber on 6″ of gravel. About every 8 feet, you’ll also need to excavate about 3 feet back into the hill.
Add gravel to the base trench, then lay and tamp your first course of timber, getting it as level as possible.
You’ll need to drill through the first few courses and drive rebars through the timbers and into the ground. The best bet’s a long auger bit and a powerful drill, but you can manage with a shorter (8″) bit this way. Position two more courses — offsetting the ends of the timber by at least two feet — and mark their positions carefully. Now every 4 to 6 feet, drill straight down until you hit the course below. Remove the top course and find the mark left by the bit. Drill down again, and repeat. Then carefully replace each course in the same position, and ensure the holes are lined up using a rebar. Inject some wood preservative into the hole and drive the rebar into the ground.
By roughly the fourth course — at least two courses below the top — you need to install a deadman. On that note… we’ll have more info, and a diagram, next time.