Retaining walls are attractive and practical pieces of landscaping that can fit into many yard designs. They are often great for yards with swimming pools since they can help tame hilly areas and add natural privacy. I’ve built a bunch of retaining walls over the years, especially living in western North Carolina, with a lot of rocky and mountainous land to work with. In this article, I’ll cover what a retaining wall is, how they work, your options for building one, and several other common questions.
What is a Retaining Wall?
A retaining wall is a structural feature that holds back a sloping patch of soil. Instead of long, natural inclines, retaining walls allow flat areas to step up to sloping areas without mixing the two. They are made from various materials and can range in height from a few inches to several feet.
Retaining walls can come in a variety of appearances and designs, as well as heights. They do not have to be straight, and a curved wall can be attractive for some landscapes. Most are under four feet high because walls taller than this typically require designing by an engineer and may require permits.
Sometimes, retaining walls meld into other landscaping features such as ponds or fountains. The same material used to make a retaining wall can be used to build up the outside containment of a water feature or an above-ground planter bed. This type of retaining wall is usually placed during a large-scale landscaping redesign. However, if your yard already has existing landscaping and you do not want to perform a major overhaul, the variety of options available means that there is at least one type of retaining wall that will fit in nicely.
How Does a Retaining Wall Work?
The pressure of soil against a wall is not inconsiderable, so a retaining wall needs to work in a sophisticated way. There are several parts to how a retaining wall works, some of which are pretty technical. Basically, a retaining wall needs to be strong enough to hold back the soil that would not remain sloped if the wall was not there.
This wedge of soil immediately behind the wall would give in to gravity and fall down the slope if the wall were not there. The rest of the soil would generally remain in place as a natural slope. The one exception to this would be a natural disaster, such as an earthquake, when a retaining wall might have to hold up to considerably more pressure.
Retaining walls are usually built at a slight inward lean so that pressure from the soil does not cause them to lean outward. They must be carefully designed to prevent water from backing up behind them, as this will create pressure and may damage the wall. Some designs incorporate more sophisticated drains than others, but all require some form of drainage. Here are some of the most popular designs.
Types of Retaining Walls
There are several different designs you can choose from when building a retaining wall. Here are some of the most popular types for homes.
- Gravity retaining walls. Gravity walls use their sheer weight to hold soil behind them, which means they need to be constructed with heavy materials like stone or concrete.
- Crib retaining walls. These are a type of gravity wall, but use a grid-like structure made with pre-cast concrete and filled with crushed stone. These walls are typically good for supporting planters, but not suitable for holding back a ton of soil.
- Gabion retaining walls. These walls are held together with mesh and filled with large stones. Typically used when erosion is a big concern.
- Cantilever retaining walls. These walls are very strong because of their design to create leverage, having a base slab that slides underneath the soil.
- Counterfort walls. These are pretty similar to Cantilever walls, but require additional support on the backside of the wall.
Signs of a Yard in Need of a Retaining Wall
There are two primary reasons you might need a retaining wall: to create more usable space or to prevent a hillside from losing soil. Flat spaces made by retaining walls can be great places to garden, put in a patio, or just hang out in the yard. If you have a yard with very little usable flat space, a retaining wall might offer you the option of creating and maintaining an area without a slope.
Hillside soil erosion can be a problem in several ways, especially in mountainous territories. It may wash onto the lower-lying areas around your home, covering your yard in dirt. It can also affect the foundation of your home in extreme cases. A properly-built retaining wall can keep hillside soil in place and help prevent these problems. If your yard regularly gets covered with mud and soil during wet weather, a retaining wall will help prevent this.
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Common Retaining Wall Materials
There are several options when it comes to materials for retaining walls. Some of the most common are wood, prefab concrete blocks, and natural stone. Other options include poured concrete, brick, and stone veneer. Wood and prefab concrete block walls are relatively easy to design and install, while natural stone retaining walls take more work and knowledge.
Features that will impact your choice of retaining wall material include cost, strength, and durability:
- Cheapest: Wood
- Most durable: Poured concrete
- Strength: Poured concrete
Wooden retaining walls are made from waterproofed planks or timbers, sometimes using rebar for support. Deadman anchors hold taller walls to the slope and prevent bowing outward. Poured concrete is sometimes used to create a base that rebar is anchored in before attaching the planks or logs.
- Wood is one of the most attractive choices in natural yards and gardens
- Wooden retaining walls are relatively easy to install
- Retaining walls that are properly installed and made from the right wood can last 20 years
- Wooden walls are usually reasonably cost-effective compared to some of the other choices
- Building materials for plank walls weigh less and are easier to transport than concrete or stone
- Does not usually last as long as other types of materials
- Generally cannot be used for retaining walls that are more than 4 feet high
- Susceptible to rotting in areas with a lot of rain or water flow
- The material for timber walls is heavy and can be unwieldy to handle
- Cannot be used to make curved walls
Poured concrete is a rigid wall made around a rebar frame. It forms a T-shape in the ground, with concrete and rebar extending forward and back of the wall underground. It must be poured into a frame made to be the negative of the desired style.
- One of the strongest materials used in retaining wall building
- Can be made into any shape and style that can be molded
- One of the better materials for building taller walls
- Modern, sleek design
- Designing the molds usually requires a landscape architect (that’s us)
- Cracks are difficult to patch and can affect the integrity of the entire wall (our pouring technique minimizes cracks)
- The concrete base can interfere with drainage in the area around the wall (our designs help mitigate this issue)
Natural stone walls are made of rock in its original shape or cut into blocks. Walls made from rocks retaining their original shape are generally broken into two categories by size: rubble walls and boulder walls. These walls typically do not use a base, although they may be laid on a layer of gravel.
- Attractive and natural in appearance
- Some are heavy enough to hold back a significant amount of weight
- Uncut stone is typically a relatively cheap building material
- Can be used to make curved designs
- Certain designs have extremely good natural drainage
- Can be difficult to assemble properly and may require experts to build
- Building materials for stone walls are heavy – even small rocks can weigh a lot, and boulders may weigh hundreds of pounds
- Uncut stone walls are time-consuming projects because each rock must be individually chosen and placed
- Cut stone pieces can be expensive
- May not control water flow or drain well
- Walls over about 3 feet require specialist engineering in most cases
- Walls made of smaller stones cannot usually hold back a lot of weight
The Right Wall Materials for You
Although there are objective facts about retaining wall materials (stone is heavy, and wood can rot), there is also a lot of personal taste involved in choosing one. Some people think that concrete is not attractive, while other people love the sleek, modern lines. Stone may be too heavy for some people but not a problem for others. Logistics are also important because things like how the materials will get to you and how well your property drains will affect the cost and the practicality of various materials.
Natural stone laid in the dry stone manner is another durable option, with examples in Europe that are hundreds of years old. However, it must be done correctly, or water getting into the wall will destroy it. It is also the most expensive. Wood is the least durable, but it is also the cheapest option, and even wooden walls may last twenty years. Poured concrete offers a nice balance of cost, durability, and strength.