How to Build Retaining Walls
Some blocks are specifically designed to build sturdy retaining walls. A locking flange on the bottom of each block clicks into place slightly behind the block above, which helps prevent the wall from being pushed outward by soil pressure.
Stepping back the walls and installing good drainage also help prevent frost heave and structural failure. Sweep each course of blocks clean as you go to ensure proper bonding.
Level the Site
Retaining walls add functionality and curb appeal to the landscape, but they can also cause issues if they’re not properly designed or built. As a result, most local codes require that walls taller than four feet be designed by an engineer and constructed by a professional.
The key to any retaining wall is a solid foundation. This means that the soil behind and in front of the wall must be level.
A flat foundation allows for modular blocks, stone or timbers to contact each other more effectively. In addition, it helps prevent soil erosion above and behind the wall.
A key factor in achieving this is drainage. Backfill the space behind your retaining wall with drainage gravel and insert a perforated drainpipe. This will help reduce hydrostatic pressure in the soil, which can eventually lead to a wall collapse.
Dig the Footings
When retaining walls are built on slopes, they must be designed to push the soil back instead of pushing the earth outward. This design, called "step-back construction," requires a sturdy wall structure and a system of support that pushes the soil from behind. Whether you are using concrete blocks, timbers or natural stone, the foundation for a retaining wall must be strong and properly prepared.
A retaining wall without proper drainage will fail. Groundwater saturates clay-type soils and swells, creating excessive pressure behind the wall that can eventually break it apart. To prevent this, backfill the area behind retaining walls with drain pipe and a gravel base. This will carry water to daylight or a dry well before it builds up too much pressure behind the wall.
Set the Blocks
As you build the wall, take the time to check each row for levelness. It's important to get this right so you don't end up with a wall that looks like a pyramid or a leaning house.
Some block systems have grooves in them that help you line up blocks. If not, use a chisel and hammer to split the block in half where you need to make a cut. This process is dangerous and should be done in a safe area wearing proper protective gear.
When you set the first block, be sure it's perfectly level from side-to-side and front-to-back. Small differences here will have a major impact on how the final wall looks. Also, be sure to call the local number to have utility lines marked before you dig.
Backfill
When building retaining walls, proper backfill is essential. You should backfill with a combination of gravel and native soil. A perforated drainage pipe should be installed at the bottom of this layer to facilitate water flow and prevent pooling behind the wall that can erode blocks, or even break down the whole structure.
Never backfill with topsoil; it will degrade over time. Instead, use a sandy or gravelly material that compacts and tamps easily with a hand tamper, and that also provides excellent drainage.
Shovel in the backfill as you build each tier of the wall, rather than adding it all at once. This will ensure it is thoroughly compacted, and that the lateral pressure is evenly distributed. If you have a taller wall, add geogrid at the base every two courses for added strength.
Add Capstones
After backfilling, install the first course of caps. These large stones create an attractive, finished look and provide extra support. Be sure to leave gaps that are big enough for plant roots to grow into, which reduces erosion behind the wall.
Build each new course in an offset pattern, like very shallow stair steps. This helps the retaining wall resist force from soil pressure, and it also looks better than a solid block wall.
If your retaining wall is over 3 feet tall, consider adding drainage provisions. A perforated pipe or a gravel backfill can help water escape the area behind the wall. This keeps the soil from saturating, which can cause it to heave upward and damage the retaining wall. Also, it gives water a place to go when it rains, rather than flowing down the slope and potentially damaging your yard or other areas of the landscape.
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Some blocks are specifically designed to build sturdy retaining walls. A locking flange on the bottom of each block clicks into place slightly behind the block above, which helps prevent the wall from being pushed outward by soil pressure. Stepping back the walls and installing good drainage also help prevent frost heave and structural failure.…