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Most retaining walls are used to stabilize a slope. In most cases landscape timbers are used to do this although there are many different materials that can be used for this purpose. A well-built retaining wall can keep a slope from eroding and will create level land. Retaining walls can be made of brick, stone, concrete blocks or poured concrete, but the main reason that pressure treated 6 X 6 landscape timbers are most often used is because they are much easier to work with and require no concrete footing.
To begin building your retaining wall with landscape timbers you will need to excavate the base of the slope. While doing this you should separate the subsoil from the topsoil so that later when you backfill you can use the subsoil first. Make two separate piles of the soil to save confusion when you are ready to backfill. Plant stakes the length of the wall you are building and run a string between the two to insure that your wall will be straight. Dig a level bottom trench that is approximately 12 inches below the finished grade and put 10 inches of gravel in the bottom. Be sure to spread the gravel to level it at 10 inches throughout the trench. Place the first timbers level and end-to-end. Then pitch them at a 1\4 inch grade from the front to the back. It may be necessary to cut the timbers to keep them even. If so use a chain saw and be sure none are shorter than 6 feet long. Next you will need to anchor the timbers. To do this drill a vertical hole one foot from each end and drive in two foot long steel reinforcing bars or galvanized steel pipes. These should extend completely into the ground.
The second course of timbers and each one there after should be set back 1\4 inch. Each timber should be put in place so that their ends are staggered. Secure these timbers by driving 10 inch galvanized spikes into the timber below the one you have just put into place. Be sure to leave one-inch gaps between the timbers of the first and second course. These gaps are called weep holes and allow for water seepage. When you have secured the second course, you will need to install drainage pipes. To do this dig out the soil behind the base timbers and lay the pipes on top of a bed of gravel. Completely cover the pipes with gravel and fill with soil. Any time a steep slope is involved a perforated drainage pipe needs to be placed across the slope and connected to pipes that are running down the slope to an outlet. When building a retaining wall these drainage pipes are very important to draw the water away so water will not stand around the timbers and rot them out. It also protects the soil from being washed out around the wall. Be sure that your drainage pipes are set in such a way as to allow water to move away from the wall. When the second course is set and your drainage pipe is in place you will need to install 4-foot anchors on top the ends. Anchors should run the length of the wall at 14 to 16 foot intervals. Continue adding timbers, nailing them to the anchors to create a series of three sided cribs. Before adding more timbers be sure to backfill with soil and tamp firmly. In a case where your retaining wall will be more than four feet tall it is wise to call a professional.
If you plan to build your retaining wall with bricks, concrete blocks, rocks or other such materials you will need a foundation before you start. A footing is a below ground concrete structure that supports and transmits the weight of an above ground structure or foundation to the soil. The footings size will be determined by the weight of the wall and the load carrying capacity of the soil. The most common type of footing for a retaining wall is a flat top. It is keyed for anchoring concrete or masonry walls. When the retaining wall your are building is made of concrete blocks or poured concrete, your footings must have steel rods or keyed channel connectors. In most cases you will need to check with your building code inspector to find out the required depths of your footings. Then dig your trench to the necessary depth making it 16 to 20 inches wide. If the trench is deep enough that you have to get down inside it to dig you will want to make it wider. If you plan to pour directly into the trench you will need to level the bottom so that the weight of your foundation will be evenly distributed on the soil. Since all footings require rebars, which are steel reinforcing rods, you will need to set these next. Rebar is usually available in 20-foot lengths and in some cases you might need to splice them together with wire. Set the rebars on 2 3\4 inch thick bricks prior to pouring the concrete. When the concrete is dry you will need to dig a small trench to the back of your base for a drainage pipe. Line the bottom of this trench with gravel before placing the pipe on top. As the wall goes up on top of your foundation you will need to brace it from the back. This should be done according to the materials you have chosen you use. When using bricks, they should be cemented three or four deep starting at the back of the wall. In this case the bricks can be laid around your four-foot timber braces. If the retaining wall you have chosen is a solid one-piece concrete wall, concrete wings can be added to the sides in a crib like fashion to hold the soil behind it. No matter what you have decided to make your retaining wall from, always fill in the soil layer by layer behind the wall as you work. When tamping the soil down, be sure it is firm throughout since air pockets can cause settling in time.