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Because composite timbers are made from recycled materials, such a plastic bags, they won't decompose, making them ideal for projects involving ground contact. Dig the trench from the lowest point of the wall; this gives an idea of how deep the trench needs to be dug in order to keep the area level. When building a retaining wall, it is crucial that the first layer of the foundation be level, since the remaining wall will be built on top of the base course.
Lay the first timber in place and check for level (Image 1). If necessary, use a mallet to "stamp" the timber into the trench, getting rid of any small rocks that might cause the level to be wrong. Repeat the process until the first course of timbers is in place. When the first course is level, a framing square is used to match corner pieces, then timber spikes are used to secure the pieces together (Image 2). An easy trick to remember for hammering timber spikes is to hammer them at an angle, which is easier to do due to the more natural arm motion of swinging up and back, rather than straight up and down (Image 3).
Next, further secure the timbers by drilling three holes, one close to each end of the timbers and one in the middle, and use a sledgehammer to drive 2' sections of rebar through the holes into the ground. Secure all first-course timbers with rebar, checking for level as each piece is secured.
When the first course is secure, begin adding the second course, making sure the joints of the second course don't line up with the first course. Overlapping the joints increases the stability of the structure. For shorter timbers, trim the edges with a saw. Secure each layer with timber spikes.
As the wall grows, dirt taken from the trench is poured in around the outside perimeter and tamped down to fill in air pockets and prevent further shifting of the soil. With the wall in place, a compact utility loader is used to move dirt from higher sections of the yard into place behind the wall. Utility loaders, which also have backhoe, auger and bucket attachments, are available at rental centers and are a great time-saver when moving dirt, rocks, etc.