The ins and outs of installing retaining walls

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Updated Jun 13, 2024
Photo: Redi-RockPhoto: Redi-Rock

We all have some customers that rely on us for every task around the landscape, while others like to be do it yourselfers and attempt a few projects solo.

While there’s no harm in them venturing out on their own to complete a few minor projects, there are a few that absolutely require professional help. One project in particular that needs professional attention is the installation of a retaining wall.

Convincing a DIY customer that you need to be the one to undertake a project like a retaining wall can be difficult, but in the end it is crucial that you help them understand the dangers they could incur if they were to attempt this project on their own.

Retaining walls are used to help landowners overcome sloped areas and often have visual interest as well as a functional purpose. Retaining walls can be used to increase the amount of usable land in a yard, and they can even provide environmental benefits such as protecting areas from saturation and soil erosion reduction.

Building specifics

Techniques for steeper slopes include the use of interlocking concrete blocks, wood retaining walls, riprap (loose rock) areas, terracing and rock retaining walls. If you choose wood, be sure to treat the wood with a preservative to prevent rotting.

If the wall will be higher than 3-4 feet, an engineer will be required to assist with the project due to zoning codes and regulations.

The wall should lean into the hill at a minimum of 1 inch for every 12 inches of height in order to maintain a safe load on the wall. This can also help with the drainage when the soil becomes saturated. It is also a possibility to redesign the grade of the slope before installing the wall; this can help direct water away from the wall and could also decrease the amount of slope needed.

Placing drainage tiles behind the wall can help keep water build-up at bay in soils that drain poorly. Be sure to construct these tiles with a 12-inch-deep layer of backfill that is freely drained, such as gravel.

The cost, function and height of the wall are all dependent on the materials your customers choose for the project. If the wall will be used strictly as a decorative piece in the yard, almost any kind of material would do the job. For the walls that need to serve the purpose of supporting large loads, you will need to use long-lasting, durable materials suited for the conditions of the site.

Wood and solid concrete walls are recommended only where the wall’s height is less than 4 feet tall. Wood walls do deteriorate faster, and concrete walls have drainage issues that can cause water saturated conditions above the wall.

What to look for  

While maintaining your customer’s landscape, be on the lookout for areas that could be in need of a retaining wall. A few things to keep in mind are:

  • What is the slope? If the slope is greater than a 3:1, consult with an engineer. If the slope is over 2:1, it will require structures or special stabilization techniques.
  • How much freezing or frost will the wall and the soil it is holding come in contact with?
  • Check the drainage and see what it is like. If it looks like water will flow heavily on the soil and wall, drainage may need to be added.
  • What kind of soil does your customer have? Soil that has heavy clay content won’t drain well, but it is less erosion-prone. Sandy soil will have the opposite characteristics.
  • Check for other structures near the site where a retaining wall may be needed. If there are existing structures around, consider how they will be affected if/when a retaining wall is installed.
  • Are you and your customers located in an area that is prone to earthquakes? If so, consider consulting an earthquake engineer to analyze the wall and make it more earthquake-resistant.

Four types of retaining walls

Once you’ve determined that your customer’s yard does need a retaining wall and have discussed the importance of having a professional install said wall, the time comes to determine which type of wall your customer needs.

Gravity walls: These hold the earth by the weight of the wall’s material. They can be formal pavers or even a stack of large rocks, but they can fall easily and should be used for short slopes of 3 feet or less.

Anchored walls: These are the strongest type and can be combined with other techniques. An anchor is wrapped around the wall, and a base is placed deeper into the hill, which provides the stabilization.

Piling walls: These use long piles, or poles, that go deep into the soil and above it. Pilings can be made of metal or treated lumber, and they have a good capacity to hold the soil back.

Cantilever walls: These are similar to piling walls however, they get added strength from a sort of “arm” that extends back into the hill. This can increase its capacity to stabilize pressure.

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