What are Retaining Walls?
A retaining wall is a structure that’s used to keep soil on one side from spilling over to the other. They can be easy to miss, but they’re actually very common, and their uses range from industrial to residential. They can keep landslides from blocking mountain roads, or beautify a backyard garden.
Every retaining wall has the same fundamental purpose. Its job is to hold back dirt. There are a few different ways of doing it, but almost all retaining walls will share a few major qualities.
First, almost all retaining walls should have about a tenth of the height of the wall buried underground. That isn’t enough to make for a sturdy wall on its own, but it is an excellent place to start.
Second, retaining walls must have some way of draining off water. Wet soil can be about 133% as heavy as dry soil, and water can weaken certain types of retaining walls. Better to let it get out of the way before it causes any trouble!
Third, retaining walls should never be angled away from the dirt that they’re holding in place. That is to say, the top part of the wall should never be further forward than the bottom. If that happens, gravity is working against the wall. Many retaining walls are angled in the opposite direction instead.
Beyond those similarities, retaining walls can be very different. Here are a few different types of retaining walls, and some information on when to use them.
First things first, there aren’t so much types of retaining walls as there are strategies that go into building them. One can refer to the types of retaining walls because most of the time, only one of the strategies is focused on, but in situations where a retaining wall needs to be especially strong, two or three different techniques might be used to meet the project’s needs. The following strategies aren’t mutually exclusive, but each will have an advantage in certain situations.
Gravity Retaining Walls
This technique is most commonly used in gardens and landscaping, where there isn’t very much dirt to hold back, and where aesthetics are important. A gravity retaining wall works by virtue of its weight. It just needs to be heavy enough to keep the soil that it’s retaining from going anywhere. As such, this strategy usually uses heavy materials like stones, bricks, or cinder blocks. Often, these walls won’t be made out of solid pieces of concrete or metal like some others, because building them out of more portable materials is easier for homeowners and gardeners.
The best small, stone-built gravity retaining wall, will have three specific features.
First, it will be placed on a flat, sturdy footing that isn’t going anywhere. This might be a concrete or wooden base, but it could also simply be compacted dirt, pressed together for extra support.
Second, it won’t be built perfectly perpendicular to the ground. Instead, it will be stepped backwards from the base to the top, so that gravity works in its favor and presses the bricks down onto the dirt.
Third, the top of the wall will be at the same height as the top of the dirt that it is keeping in place. If it’s higher than that, then water could collect behind the top of the wall and work its way between the bricks to weaken the wall. If it’s lower than that, then dirt could spill over the top.
Sheet Piling Retaining Walls
Sheet piling walls are also a common choice for smaller walls. They’re thinner and lighter than gravity retaining walls and made of single sheets of material rather than stacks of bricks. At a residential scale, they are generally cheaper to build than a gravity retaining wall, which can make them the preferred option over gravity retaining walls in some cases. On the other hand, they don’t usually look as pleasant or natural as gravity retaining walls using stone or cinder blocks.
Sheet piling retaining walls are made by taking a flat sheet of a material like wood or metal and driving it deep into the ground so that at least one-third of the wall is below ground level. It’s important to use a durable material in this kind of wall because the most common way for sheet piling retaining walls to fail is by breaking at ground level where the pressure of the soil against them is highest. This method can be paired with the next for extra sturdiness.
Anchored Retaining Walls
This is one of the more straightforward types of retaining walls. It’s similar to a sheet piling wall, although the material used is often thicker than sheet metal or wooden planks. It involves a simple wall to keep the dirt in place, anchored into the same earth that it’s retaining by a series of cables with anchors at their ends. These anchors are driven into something like a rock face or densely packed dirt, then are expanded, often with an injection of pressurized concrete. This way, the soil is trapped between a wall and a hard place, so to speak. This method won’t work unless there’s something sturdy behind the soil that the anchors can be pounded into. If there isn’t, then there’s no force holding the wall in place.
Cantilevered Retaining Walls
Cantilevered retaining walls are usually made out of one solid piece of sturdy material like concrete. They’re built in an L shape so that the dirt that they are retaining is pushing down on the bottom of the L and against the top. This way, the weight of the dirt itself is what keeps it from moving.
This is already one of the most durable types of retaining walls, but it can be made even stronger by including buttresses behind the wall that provides extra support. Because this kind of wall is usually made of reinforced concrete, it isn’t very pretty to look at, but a stone or brick facade could be applied to remedy that.
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