Block 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 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
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 Interlocking Block
1. Fill your trench with 6 inches of coarse gravel as a
base. “Class 5” gravel will work well for this trench. Pack
this gravel using a tamper. A rented tamper will do a better
job of compacting than a hand tamper.
2. Lay the first row of block into the trench. Use the level
string as a guideline, but also use your level side to side
and front to back on the base to make sure it is level. This
step is critical; and if done right will make the following
rows an easier task. When using blocks that have a flange on
the back, place the first row of block upside down and
backward. This first row will be covered with soil. There
are two different types of block you may use for this first
row. The regular block may be used, but we recommend block
with extensions to hold it in place for this row. It is more
expensive, but the extensions are hollow and allow you to
fill it with crushed rock for a firm base. You may have to
ask for this special block at home centers, as it is not
always in stock; and may only be available at lumber or
cement stores. Check the blocks frequently with a level and
adjust as necessary, by adding or removing gravel below the
3. Lay the second row of blocks according to the
manufacturer’s instructions. Stagger the block so the
vertical seams do not overlap. Make sure flanged blocks are
tight against the lower block.
4. Fill behind the block with crushed rock, making sure the
landscape fabric remains between the rock and the hillside.
If you have extension blocks, fill the extensions with
crushed rock. Pack the rock with a hand tamper.
5. Lay perforated drainpipe along the entire back of the
block. Lay the pipe on top of the crushed rock and at least
6 inches from the block, with the perforations facing down
to drain water. Make sure that at least one end of the pipe
is open to an area that can accommodate drainage of runoff
water. You can now lay more layers of block until the wall
is 18” above ground level.
6. Fill over the perforated drainpipe with more crushed rock
to the top of the 18” wall and pack it down.
7. Lay the remaining rows of block, except for the top cap
row. Back fill with crushed rock and pack it down as you go.
It is important to have at least 12 inches of crushed rock
behind the entire wall for water drainage.
8. You are now ready to lay the cap blocks, but first you
must fold the end of the landscape fabric over the crushed
rock back fill. Add a layer of black dirt over the fabric
and pack it down.
9. Fold any excess landscape fabric back over the crushed
rock. Apply construction adhesive with a glue gun to the top
of the blocks. Lay the cap blocks in place. Fill in behind
the caps with black dirt. You may add sod or plants, as is
10. Go have a drink. You deserve it after this project.
Tools & Material
Tools: Shovel, wheelbarrow, rake, line level, hand tamper,
maul, masonry chisel, work gloves, circular saw with diamond
masonry blade, level, tape measure. Caulking gun and glue
for top layer of stone or block.
Materials: Landscape fabric, gravel for sub-base, crushed
rock for back fill, perforated drain pipe, your choice of
block or stone.
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