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Wood Retaining Wall

Retaining Walls
Retaining walls are a bit more complicated. It is important to remember, retaining walls are subject to thousands of pounds of pressure from the weight of the soil and water. Even a three foot wall can collapse if not installed correctly. If you have a tall hill, it will be easier to terrace the hill with several short walls. This is a very big job, and you may want to hire professionals who are knowledgeable in this area, but if you want to tackle it yourself, there are three major steps:

1. Preparing the foundation: This is the most important step, as it will be the base for the wall. It is important to add drainage to prevent erosion and make sure that you have an adequate base. If this step is skipped, and water saturates the soil behind the wall, damage can occur.

2. Installing the first row: When you set your first row, you are establishing the shape and scope of the wall. It is very important to get the first row level, or your wall will be leaning when it is finished.

3. Installing the remaining rows: This is usually the easiest part of the job. If you have installed your base correctly, this step should go smoothly.
Preparing the Site
1. Dig out the hill, creating a level base for the block or timbers. Make sure that as you dig, you don’t disturb the soil you are going to leave in place. For stone or interlocking block, allow at least a 12” space for the crushed rock back fill between the block and the back of the hill. For timber walls, allow at least 3 feet of space..

2. Use stakes and a string to mark where the first row of block or timber will go. Level the string and use it for a base. You will be digging a trench to lay the first block or timber, and this will help you measure the depth of the trench.

3. Dig a trench 6” deeper than the thickness of one row of block or timber. If your block is 8” high, your trench should be 14” deep. Do not cut corners on this task, as it will keep the wall from leaning in the future.

4. Line the entire excavation area with heavy duty landscape fabric. If you must overlap fabric, make sure there is at least 6 inches of overlap. Cut the fabric on top so that it is 3 feet longer than the height of the intended wall, overlapping onto the hillside for now.

Building a Retaining Wall with Wood
Pressure treated timber walls will usually last about 15 to 20 years if installed correctly. Do not use 4x4 timbers, as these are not strong enough for a retaining wall. Your timbers must be at least 5x6”. Do not use old railroad ties, as these are soaked in creosote that can contaminate your soil and kill your plants.

1. Prepare the site as directed above. (Preparing the Site)

2. Spread a 6” layer of Class 5 dirt in the trench and tamp down. Lay your first row of timbers, making sure they are level side to side and front to back. Each row of timbers should be set with a ½” batter. This is accomplished by setting each row of timbers ½” behind the preceding row. This will cause the wall to tilt slightly toward the hillside. Use 12” galvanized spikes or rebar to anchor the ends of each timber to the underlying timbers. Stagger the ends of the timbers at the corners to form stronger corner joints. Drive additional spikes to the underlying timbers at 2’ intervals along the length of the timbers. Lay 3 more rows of timbers, making sure to stagger the joints.

3. Add 6 inches of Class 5 gravel or crushed rock behind the timber wall at a width of 3 feet. Lay perforated drainpipe on top of this layer across the entire length of the wall. Place the pipe with perforations down to allow water drainage. Make sure at least one end of the drainpipe is open to an area that can accommodate drainage of runoff water.

4. Install deadmen, spaced 4 feet apart, midway up the wall. Deadmen a are timber extensions that extend into the hillside from the back of the wall. Deadmen will prevent the wall from sagging. Build the deadmen by joining two 3’ lengths of timber into a T with 12” spikes. The cross of the T will lay into the hill and the slash of the T will insert into the wall where 2 timbers meet. Anchor the deadmen to the wall with spikes. You will have to cut a hole in the landscape fabric and insert the slash of the T of the deadman through the hole before you anchor it to the wall.

5. Fill over the drainpipe and deadmen with crushed rock to the top of the wall. Remember the depth of the crushed rock should be 3 feet for timbers.

6. Add remaining timbers and back fill with crushed rock as you go.

7. Install vertical anchor posts to the front of the wall as reinforcement. Space the posts 3 feet apart, and dig them so that the buried depth of each post is at least half the height of the exposed wall.

8. Drill weep holes through the second row of exposed timbers. Space the holes 4 feet apart and angle them upward to encourage drainage.

9. Fold over the excess landscape fabric over the top of the crushed rock and fill in with black dirt. Place sod or plants to finish your project.
Tools & Material
Chain saw or reciprocating saw.
Gloves, Safety Goggles
Metal rebar for reinforcement.
12” galvanized spikes.
Hammer or mallet
Perforated drain pipe
Landscape fabric
Crushed rock , Class 5 gravel for base


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Retaining Walls

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