Wood Retaining Wall
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 dont 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
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
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
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
Crushed rock , Class 5 gravel for base
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