How To Build A Landscape Timber Retaining Wall Properly
Building a retaining wall of timbers that will last is not difficult provided you know a few key tips!
Choosing the right timbers for the application is important. A small wall holding back the dirt in planting beds up to 2 1/2 feet high could be constructed with 3 x 5 inch timbers. I would go with 6 x 6 inch timbers for anything taller.
You'll need timbers, spikes (large nails to hold the wall together) and 2-3 foot rebar sections (to hold the first course in the ground). While most articles include gravel and perforated schedule 40 pipe for drainage, I've built many walls that lasted for years with the only drainage being holes drilled through the wall near the ground. Tools required include a small sledgehammer, saw, and drill.
Now the most important tip: To prevent collapse of the wall you must plan for timbers every few feet and staggered, that are turned perpendicular to the wall so that only the small 6 x 6" end shows while the bulk of the timber shoves back into the hillside (you'll need to dig out for them). It's the leverage of the dirt pressing on these "deadmen" timbers that keep the wall standing rather than falling over eventually. They may not need to be full length timbers, I generally go with slightly over half length. If you look at a well built wall and study the pattern, you'll notice these little squares- the tips of the deadmen- every few feet.
The first step is to dig out the ground and install the first course. By setting it beneath the surface it prevents the wall from kicking out. If you want it to sit on gravel, now is the time to spread the gravel, digging even deeper to accommodate it. After setting the timber, drill it in the center every few feet and pound rebar in the holes with the sledgehammer until flush. The rebar will stick in the ground, more insurance against kickout. From there it's a simple matter of stacking the timbers, staggering them as you go so the seams don't line up. Typically this is done by cutting a timber in half to begin every other course. So start course 2 with a cut piece, course 3 with a full piece. Connect each course to the course below with the spikes. Be sure to nail in the deadmen pieces as well or they won't be tied in to the wall. When you get to course 2 to 4 you can run your schedule 40 drainage pipe behind the wall. It can exit around the end or you can cut a hole for it. If you're skipping the gravel & pipe, now is a good time to drill some holes near the ground for drainage.
Basically that's all there is to it, it's a wall made of timbers tied together with spikes that includes rebar on the bottom course and deadmen ties to prevent kickout/fallover. If you like, chamfer the end pieces of the top course slightly for a more finished look.