To build a timber retaining wall, begin by digging a trench along the line of where your wall will be. The trench should be approximately the depth and width of the timbers you will be using to build the wall. If you need space to work on the back side of the wall, dig that space out before you begin building the wall. Use a line level to level the ground where the timbers will lay.
Place the first row of timbers flat in the trench. After your first row of timbers is laid along the trench begin stacking your second row of timbers. Stagger the ends of the timbers to ensure a strong wall. Attach each layer of timbers to the layer below it with spikes (8 inch long 60D nails). Timber retaining walls are built straight up – not slanted like stone walls – so keep a level or plumb handy as you stack them.
Timber Tie-Backs
If your wall will be higher than about 18 inches use tie-back timbers every eight or ten feet on various levels to hold your wall upright and make sure it will not fall forward due to the constant pressure exerted upon it from behind (top failure). To add a tie-back timber, simply lay one timber perpendicular to the other timbers but with its length extending into the area that will be back filled. When the area is back filled this timber will act as an anchor to hold the wall in place and ensure your timber retaining wall will last.
Timbers United into One Structure
One aspect of my retaining wall design which is a little different from others you may see is that I prefer to unite the entire timber retaining wall structure with re-bar driven vertically through all the timbers and into the ground via a hole that is drilled through all the retaining wall timbers after they are completely stacked. The re-bar should fit tightly into the drilled hole.
This step might be an overkill but I like strong stuff that lasts a long time. An alternative but similar method is to drive re-bar through the bottom two or three layers when the wall is about half-built and then connect the bottom timbers to the top layers once the top layers are added (see pictures).
Use Properly Treated Quality Timbers
Some books and sites will recommend that you use “garden timbers” (those cheap ones with two round sides and two flat edges) to build a retaining wall but I strongly advise against that practice because “garden timbers” are typically made from the cheapest pieces of wood leftover from the production of other lumber or plywood and contain mostly heartwood which does not accept pressure treatments. They will probably be heavily rotted within a few years and will quickly fail.
Building a timber retaining wall is hard work so use timbers that will last. You might even consider using timbers with a vinyl or polymer coating. American Pole and Timber is a reputable company that ships quality timbers nationwide and offers a few types of vinyl coatings that can make wood last virtually forever.