Whether you plan on building a retaining wall for landscaping or drainage, or if your property already has a retaining wall on it, these are engineering marvels that do great things for us. In holding backfill in place and keeping it up off a specific area, retaining walls help us take full advantage of our properties.
An 8’ (2.4-meter) retaining wall should be able to hold 32,000 lbs (14,515 kg), including soil weight and hydrostatic pressure. Larger reinforced walls can hold higher weights assuming they are built sturdy and to code.
There are four main types of retaining walls, and each type can hold different weights, as each is engineered differently and will have various limits. So, let’s discuss the different types of retaining walls, their heights, and how much weight each one can hold.
Just a quick note before we dive in — these are rough napkin figures and I provide no guarantee of any sort. If you’re concerned about whether your retaining wall will hold back the weight you’re trying to give it, consult a professional (like a structural engineer).
Retaining Wall Needed Capacity by Height
Soil weighs approximately 100 lbs. per cubic foot (1,602 kg per cubic meter). Armed with this information, we have most of what we need. We also need to know the height and width of the wall and the size of the sliding wedge to know whether the wall we plan will work.
Let’s assume the wall we want to build is 10’ (3 meters) across, and our backfilled area is 4’ (1.2 meters) deep.
Knowing what we do about calculating volume, we calculate the volume of the sliding wedge using the formula a x b x h x 0.5 = V, where a, b, and h represent the three dimensions of the wedge.
Adding the height of the wall to these numbers gives us the size in cubic feet of the wedge, which we multiply by 100 to get its weight.
So, let’s look at some rough examples:
- For a 2’ (0.6-meter) wall, our formula is 4 x 10 x 2 = 80 cubic feet (2.3 m3) x 100 = 8,000 lbs (3,628 kg) of dirt in the sliding wedge.
- For a 4’ (1.2-meter) wall, our formula is 4 x 10 x 4 = 160 cubic feet (4.5 m3) x 100 = 16,000 lbs (7,257 kg) of dirt in the sliding wedge.
- For a 6’ (1.8-meter) wall, our formula is 4 x 10 x 6 = 80 cubic feet (2.3 m3) x 100 = 24,000 lbs (10,886 kg) of dirt in the sliding wedge.
- For an 8’ (2.4-meter) wall, our formula is 4 x 10 x 8 = 80 cubic feet (2.3 m3) x 100 = 32,000 lbs (14,515 kg) of dirt in the sliding wedge.
Since a properly built 4’ (1.2-meter) wall can support up to 20 tons (18,144 kg) of dirt, we know our walls will offer more than ample support.
Four Types of Retaining Walls
Retaining walls– certainly not a one-size-fits-all creation– vary in size, structure, and purpose.
Knowing the kind you need will help you determine materials, so if you’re building a retaining wall that needs to support more than just the dirt behind it, such as a driveway, your home’s foundation, or a shed, you’ll want a wall that supports more weight. Let’s look at our options.
Sheet Piling Wall
The most simple retaining walls can be pretty resilient if built from the proper materials. When you use weaker materials, however, they can be foolishly flimsy. The concept of a sheet piling wall is that you drive a sheet of material vertically into the ground, and that’s it. That’s the wall.
Now, if you stick a piece of plywood in the ground, it’s not going to hold much weight. Still, using sheet metal piling can create a formidable final product.
Sheet metal piling is usually corrugated to provide additional structural integrity. These sheets get driven into the ground, one by one, each fitting together with the next. Soldier piles use horizontal supports across the front of the wall for added strength.
A sheet piling wall needs to have at least one-third of the building material buried beneath the surface to ensure proper support. If you want a six-foot (1.8-meter) wall, then you’ll need nine feet (2.7 meters) of piling with three feet (0.9 meters) buried.
These walls are simple in design, but they require sturdy, expensive materials to work well. The plywood mentioned above was to illustrate a point. Don’t build a sheet piling wall with plywood. This type of wall is for massive projects and often uses made-to-order, incredibly thick, weather-resistant piles.
Another simple kind of wall, a gravity wall, depends on gravity, and that’s about it. You’ll use a gravity wall for gardening, say for constructing raised flowerbeds or making other landscaping structures.
With this wall, gravity holds the wall together. If you stack heavy enough stones, pavers, or bricks on top of each other, they’ll hold the soil back. You can build a shorter wall just by stacking the wall materials, but anything taller than four feet (1.2 meters) will require additional support, like mortar between the stones and likely a permit from your city.
You don’t have to dig a trench first, but unless you plan on building a wall less than a foot (0.3 meters) tall, a trench will help keep the wall level and less susceptible to leaning.
To lay a gravity wall:
- Dig a trench deep enough for at least one layer of your retaining wall to sit below the ground.
- Compact the ground until it is level.
- Lay down some crushed stone.
- Build your retaining wall.
The trench will stay stable when the ground shifts during freezes and thawing periods, keeping the wall firmly in place.
Cantilevered Retaining Wall
Now we’re talking about a wall built with materials designed just for this purpose, much like the sheet piles discussed above.
Rather than stacking stones to build a wall, cantilevered walls are usually made from concrete and resemble, from the side, an upside-down T. The base of the wall sits on a flat surface under the ground, supporting the vertical piece that constitutes the face of the wall.
When the base is buried and the backfill applied, its weight holds the base in place, and the wall stays upright and keeps the dirt at bay. Some cantilevered walls also feature a counterfort, basically a strut connecting the top of the wall to the edge of the base, which relieves pressure against the wall.
Anchored Retaining Wall
This fourth type of retaining wall isn’t exactly unique. Instead, anchoring can be applied to any of the above walls so that you can build an anchored gravity wall, for instance. Anchoring serves to strengthen the wall to better support the load it bears.
For your two-foot (0.6-meter) garden wall, anchoring it would probably be overkill, but for taller walls that need to hold lots of weight, anchors can drastically increase wall strength.
While several anchoring methods and materials exist, the basic premise is that the wall is built. Then, anchors like the Milspec Heavy Screw Anchor (available on Amazon — affiliate link) are driven through the front of the wall at a downward angle. After that, the ends of those anchors screw into the soil behind the wall and past the depth of the failure plane. We’ll discuss that in a bit.
Determining Your Retaining Wall Needs
As we’ve mentioned, different wall types perform various functions. Depending on the height and depth of the backfill you’re holding up and away (retaining), the failure plane (also known as the plane of sliding) is the point where the soil in your backfill stops being held in place by gravity.
When we build a retaining wall, the wall doesn’t have to keep all the weight of every grain of sand or dirt held up.
Much of the backfill will sit in a pile that won’t move. However, as we move to the edge of a mound of dirt, it slopes away from the bank. The backfill sitting above the angle of that slope is known as the sliding wedge. If it were solid, it would slide, wedge-shaped, down the mound’s slope.
Then, the sliding wedge is the only weight the retaining wall must uphold. This knowledge dramatically reduces the perceived weight the wall must take on.
Using measurements and math allows us to learn the approximate weight of the soil in the sliding wedge with a tool like this one. Still, to know how much our wall will bear, we need another formula, and it’s complicated.
This calculation can tell you exactly what your wall will bear, and if you need a taller wall, you can change your measurements until you reach the desired capacity.
Once you determine your needs, check out our guide on the cheapest kinds of retaining wall you can build. Or, if you have other questions, consult our extensive retaining wall resource.
Calculating the weight of the portion of the backfill that needs support is relatively simple. As the height of the wall increases, more weight will push on it, but for larger projects, using a more sturdy wall is better than using a simple gravity wall, which will more than likely be sufficient for landscaping needs.
Branching out to cantilevered or anchored retaining walls can help strengthen your project’s barrier if your weight calculations require it.
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