Retaining walls are used in landscaping to reduce soil erosion, create terraces on steep slopes, and many other reasons. Typically, these walls are used to make hilly terrains a usable space by supporting tons of soil. Soil becomes very heavy with rain from a storm, and the retaining wall is built to support the weight of saturated soil. It is important to hire a professional engineer with carefully calculated construction plans to ensure a safe retaining wall is built. Miscalculated construction plans can lead to a wall that is susceptible to failure. Some typical failure risks include bulging walls or collapsing walls. Additionally, do-it-yourself (DIY) retaining walls can be built for areas with minimal soil or to create an aesthetic landscaping wall. This article will go over how to build a retaining wall and various types of retaining walls.
Retaining Wall Design
Like many other construction activities, retaining walls may require permitting from the city or county it is being built in. The main reason is that retaining walls can obstruct the natural drainage path, resulting in flooding a neighbor’s home. Therefore, a permit shall be processed before construction can take place. After the construction of the wall is done, an inspection to ensure the wall is built to standard or the certified design will occur. An engineer can do the design if the project is extensive or you can adopt a DIY method. The information listed below can assist in determining the right design for your retaining wall:
- Retaining walls have a variety of building materials that can be used. Professionals do not do DIY projects, so there are manufactured blocks that can be purchased with design specifications for retaining walls. Manufactured blocks can be purchased at any local home improvement store, and there are various colors and textures to choose from.
- Concrete retaining walls – Poured concrete retaining walls are customizable and have optimal strength if designed precisely. Also, a concrete block retaining wall can be done with pre-cast concrete blocks. These walls will require an engineer to design the layout of the wall and the amount of rebar used for reinforcement. After the concrete footings and wall is poured, the concrete must cure. Once wall forms are removed, waterproofing and the drainage system shall be installed. See the Drainage category below for additional guidance on installing a drainage system.
- Timber retaining walls – Composite timber is a good material choice for a retaining wall. Timber retaining walls offer a sustainable option for a straight retaining wall. A guiding line should be used to lay out a timber wall before digging the trench. The base of the wall should equal out to half the height of the proposed wall; this portion will be beneath the level ground. The first row of installed timber will need added pilot holes at the end of each row of timber. The holes are for adding rebar to reinforce the foundation of the wall. Adding rows to create the wall takes place after this. The retaining wall also needs a conveyance for the drainage, so a perforated pipe behind the wall in a gravel base will be a great option for this. If no drainage pipe is added, the area should be a sloped terrain that allows for drainage runoff. The backfill above the gravel will be dirt, which was added for drainage. After all of the timber is placed and backfilled, then a top cap will need to be placed over the wall. An overhang can be added at the top row of the wall for a design feature. Timber walls can be stained or painted to match the landscape aesthetic.
- Brick retaining walls – Brick retaining walls have two specific design methods. One wall is called a cavity wall, which includes a steel piece running from the foundation to the top along with the spaces in the brick. The other type is a block wall, where the brick is laid outside the structural core. This means bricks will be laid on the exposed portions of the wall. That block wall method is cheaper than the cavity wall method. The brick material used can be bought as a whole brick, half brick, or a thin brick veneer. The first two are self-explanatory, but the thin brick veneer is a thin application of the veneer, which is very similar to a tile. A skilled mason may be needed for laying the brick, so the thin brick veneer would be a good option for a DIYer.
Steps to Building a Retaining Wall-
- The foundation of a retaining wall is the starting point for ensuring a good design. A trench should be added and filled with crushed rock to support the structure and minimize the shifting or settling of the wall. The depth of the wall is calculated based on the height of the overall retaining wall. See Figure 2 for an example of the retaining wall depth to height ratio. A rule-of-thumb is to dig the trench for the foundation an eighth of the overall wall height and add three inches (3”) to that depth.
- Survey the area where the retaining wall is to be built. The survey should include the existing slope of the land, soil properties, and the amount of water in the soil at present or during rain or other worst conditions.
- The wall should not be constructed on an uneven surface. This is because each added row will be uneven if the first layer is. A level gauge should be used at each addition of rows to eliminate unnecessary voids.
- Anticipate building the wall as a slope. Since the wall is retaining soil, each layer should be built at a slope. Use a minimum rate of one inch (1”) for every one-foot (1’) of rising. The locking flange at the bottom edge of a block wall will assist in guiding the next block’s position when adding the slope. Figure 2, above, shows an offset of 1” per block as the wall rises 1’ in height.
- Finalize the material of the retaining wall based on your needs and budget. Refer to the Retaining Wall Design section above to see the different building material options.
- Drainage is a huge part of the design to avoid over-saturation of the soil. Some ways to mitigate drainage will include backfilling the space behind the block or material with crushed rock and installing a perforated pipe. The drain pipe should run horizontally to the ends of the wall to convey the drainage away from the wall. Once the pipe is installed, make sure the backfill is added with a material that can filter water.
- The drainage system should consist of weep holes and drain lines. Weep holes are located in retaining walls to drain the area behind the wall. There should be sufficient spacing between the added weep holes to allow for uniform draining behind the wall. It is recommended that weep holes have filtered material, like a mesh, added between the wall and the backfill. This is so that fine soils do not migrate, clogging does not take place, and the loss of backfill from the retaining wall does not happen.
- Tiering (or terracing) the wall is a better option than creating one tall retaining wall. This adds a landscaping feature and will break up the amount of soil that needs to be supported by each wall section. The distance between terraced walls should be twice the height of the lowest wall. This allows for less soil pressure to be placed on the lowest wall.
- Beautification of the Wall – If the area where the wall is being built is in a garden or near your house, you would want to hide the bare wall and beautify it with some small plants, some cobblestones, or other items.
Hiring for Building a Retaining Wall
As mentioned before, an engineer will need to be onboarded to provide structural calculations and drawings for building a retaining wall. Also, the permitting process will require a certified design before moving forward with construction activities. Design Everest has a team that can assist with this process. Contact us to get a free quote and consultation today.