Building a Strong Retaining Wall

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Strong Retaining Walls:

There are many common blunders that people make in their retaining wall design. On the outside, a retaining wall appears to be strong and capable of holding back tons of dirt.

However, simply laying out bricks haphazardly can result in eventual failure of the structure. The longest lasting retaining walls do not hold back mounds of soil but only a small wedge of soil behind it. Compacted soil behind the retaining wall supports itself and will not press on the wall.

The compacted soil is angled at a plane which allows it to hold its own without failing. Filler dirt is placed between the wall and compacted soil, which is the only force that the wall will be resisting.

The slope of the compacted soil directs most of the filler weight down to the base of the wall, creating a lot of pressure. Therefore, strong and heavy wall materials are needed. Materials like brick, concrete, or even wood can be used. When installed in the right way, the heavy materials will stay intact for many years.

Basic Steps:

A channel is dug into the ground along where the wall will be placed. The depth of this channel should be around 10 percent of the total wall height. The purpose of the channel is to lock the base into position, preventing the wall from sliding forward under the force.

A foundation, typically compacted soil, is placed at the base of the channel which the bricks will be set on. The bricks are set on top of each other with the seams offset for strength.

Bricks are stacked with each row overlapping in a stepped pattern. When built at an angle, the wall will have greater resistance. Walls that are built straight up are likely to start tipping under the pressure.

Many retaining wall bricks are designed with a lip on the bottom which grips the brick below it and creates the perfect incline. Gravel, fabric, drainage tubes and other materials are then placed behind the wall.

Combating Water Weakness:

A common problem involves water leaking into the base of the wall and washing out material. Water from rain, sprinklers and irrigation can flow under the bricks and wash out the foundation. An even greater threat is when the water builds up behind the wall to create huge pressure levels. The best prevention for this is to create a wall that allows the water to flow out quickly. Keep the following in mind when brainstorming retaining wall ideas:

  • Impervious soil layer:    Create a layer of impervious soil just behind the top brick to prevent most water from seeping behind the wall.
  • Gravel:    Put a layer of gravel directly behind the bricks to allow water to drain quickly.
  • Landscape Fabric:    Place a fabric liner between the compacted soil and the gravel to prevent the soil from entering the gravel.
  • Ditch Depth:    If the ditch for the wall is dug too deep, water can flow underneath.
  • Drain Tube:    Ultimate safety can be guaranteed by installing a drain tube at the base of the wall just behind it.
  • Backfill:    Use only good draining backfill behind the wall.
  • Wall Height:    The top of the wall should be flush with the ground, allowing water to flow over the wall if need be, not get trapped behind it.

Compaction and how to do it right

The filler soil behind the wall must be compacted as much as possible to create greater friction and direct as much of the force downward as possible. This reduces stress on the wall, especially if the filler wedge is particularly large. This cannot be done with anything but a compaction tool. While loud, these vibrating tools are the best resource for the job. Add a few inches of soil, compact it, and repeat the process until you reach the top. If your wall is tall or made of timber, you will need a reinforcing grid behind the wall. Brick suppliers should be able to tell you what grid placement is best. Topsoil is worthless for use as filler, as it settles over time. Sand or gravel is much better. Don’t compact the ground so much that you push the wall outward.

Wooden Walls:

Timber alone is not strong enough to serve as a reliable retaining wall. The pieces are simply not heavy enough. You can still create a wooden wall by reinforcing the wood with deadman anchors. These anchors travel from the wall to the soil behind to lock the wall in place and prevent it from moving forward. They are installed as the wall is erected. As with brick walls, timber ones should be stepped and have good internal drainage.

Tall Walls:

Even if you exceed 4 feet in height, the same principals still apply. However, the material behind the wall may be far too heavy for the bricks to support. In this case, builders can use a combination of pins through the bricks and a reinforcing grid. A permit may be required to build a wall taller than 4 feet. Check your local city ordnance.

Another method is to create a series of levels instead of a single large wall. In essence, you would create several retaining walls on top of each other This tiered method is useful in gardens for planting. Set each wall back 2x the height of the wall below to keep pressure off of the lower wall.

Tool List: (see more general garden tools here)

  • Vibration plate OR soil tamper (for compaction)
  • Measuring tape
  • Level
  • Gravel
  • Interlocking stones
  • Wheelbarrow
  • Stakes and string (To check for levelness)
  • Safety Tools (Gloves, glasses)

Cutting Stones:

There are times when you need to break a stone in half to use as an end-stone. Using a chisel and hammer, create a score line around the whole brick. Place the chisel on one score line and hit it hard. You may need to give it a couple tries, but eventually, you will obtain a smooth, professional half-brick that you didn’t have to pay extra for.

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