Retaining Walls are everywhere, but you probably don’t notice them, or may not even realise what they are. In this article, we’ll go through the various types of retaining walls such as Cantilever, Gravity, Nailing, and Piles, how they work, and how the reasons why some of them fail.
In March of 2021, a long-running construction project on a new jersey highway interchange ground to a halt when one of the retaining walls along the roadway collapsed. A project already 4 years behind schedule, the cause of the collapse is still under investigation, but the event brought into the spotlight a seemingly innocuous part of the construction environment – restraining walls and why they fail. As both a civil engineering and structural engineering organisation, we thought we’d shed some light on the different types of retaining walls, how they work, and what can cause them to fail.
Collapsed Retaining Wall
So why do we build walls to hold back soil? What are the different ways to do it? And why do they sometimes fall?
The natural landscape is never ideally suited to construction as it stands, the earth is just too uneven. Before things get built, we almost always have to raise or lower areas of the ground first. We flatten building sights, smooth paths for roads and railways, and build ramps up to bridges. You may notice that these cuts and fills usually connect to the existing ground on a slope.
Loose soil won’t stand on its own vertically. That’s just the nature of granular materials. The stability of a slope can vary significantly based on the type of soil, and the loading it needs to withstand. You can get many types of soil to hold a verticle slope temporarily. However, over time, the internal stresses will cause them to slump and settle into a more stable configuration. For long-term stability, engineers rarely trust anything steeper than 25 degrees. That means anytime you want to raise or lower the earth, you need a slope twice as wide as it is tall, which can be quite a problem.
25 Degree Slope Used by Civil Engineers for Retaining Walls
In dense urban areas real estate comes at a premium, so it doesn’t make sense to waste valuable land on slopes. Where space is limited it often makes sense to avoid this disadvantage by using a retaining wall to support the earth vertically. When you see a retaining wall in the wild, the job of holding back soil looks effortless. but that’s usually only true because much of the wall’s structure is out of view. A retaining wall is essentially a dam, but instead of water, it holds back earth.
Retaining Walls & Surcharged Loads
Soil doesn’t flow as easily as water, but it is twice as heavy. The force exerted on a retaining wall from that soil, called a lateral earth pressure, can be enormous, but that’s just from the weight of the soil itself. Include the fact that we usually apply additional forces; buildings, vehicles, or other structures – on top of the backfill behind the wall. We call these surcharged walls, and they can increase the forces on a retaining wall even further. Finally, water can flow through or even freeze in the soil behind a retaining wall applying even more pressure to its face.
Retaining Wall infographic with lateral earth pressure and surcharge loads
Estimating all these loads and designing a wall to maintain them can be a real challenge for a Civil Engineer. Unlike most structures where loads are verticle from gravity, most forces on a retaining wall are horizontal (like in the above image). There are a lot of different types of walls that have been developed to withstand these staggering sideways forces, more on that to follow.
Cantilever Retaining Walls
Imagine these dowels are soil particles, so we can easily see how different types of retaining walls are able to withstand such tremendous stress, and what happens when they can’t. The most basic retaining walls rely on gravity for their stability, often employing a footing along the base.
Retaining Wall Footing Demonstration
The footing is a horizontal member that serves as a base to distribute the forces of the wall into the ground. Your first idea might be to extend the footing onto the outside of the wall to extend the lever arm, however, it’s actually more beneficial for the footing to extend inwards, into the retained soil. This allows for the earth behind the wall to sit on top of the footing, which acts as a lever to keep the wall upright against lateral forces. Retaining walls that rely only on their own weight and the weight of the soil above them to remain stable, are called gravity walls. And the ones that use footing like the image above, are called cantilever retaining walls.
Gravity Walls and Mechanically Stabilised Earth
One common type of retaining wall involves tying a mass of soil together to act as its own wall, retaining the unreinforced soil beyond. It’s accomplished during the fill operation by including reinforcement elements between layers of soil, a technique called mechanically stabilised earth. The reinforcing elements can be steel strips or fabrics made from plastic fibers called geotextile or geo-grid.
Mechanically Stabilised Earth Example
Gravity walls and mechanically stabilised earth are effective retaining walls when you’re building up or out. In other words, they’re constructed from the ground up. Though excavated slopes often need to be retained as well, consider you’re cutting out a path for a roadway through a hillside or constructing a building in a dense urban area starting at the basement level, in these cases, you need to install a retaining wall before or during excavation from the top down. There are several ways to go about it, just like reinforcements hold a soil mass together in a mechanically stabilised earth, you can also stitch together earth from the outside called soil nailing.
Soil Nailing Process
First, an angled hole is drilled into the face of the unstable slope
Then a steel bar is inserted into the hole, usually with plastic devices called spiders to keep it centered
Cement grout is then added to the hole to bond the soil nail to the surrounding earth
Both mechanically stabilised earth and soil nails are commonly used on roadway projects so it’s easy to spot them if you’re a regular commuter.
Infographic of how soil nailing works to secure retaining walls
Retaining Wall Erosion Prevention
Often times you will see these types of retaining walls with an outer concrete layer, though this outer shell does little in the way of retaining the earth behind, it actually helps to prevent the soil from erosion which would cause further issues down the lines. You may also notice that engineers often use interlocking concrete panels with a decorative pattern, these panels that only look more aesthetically pleasing, but they also allow for movement over time as well as allowing for water to pass through the joints rather than holding behind the wall and building up pressure.
Retaining Wall Facade
Soil Nail Disadvantages
The one disadvantage of soil nails is that the soil needs to settle a little bit before the strength of each one kicks in. The nails also have to be spaced closely together, requiring a lot of drilling (you can imagine the amount needed for a large project). In some cases it makes more cases to use an active solution, usually called anchors or tie backs. Just like soil nails, anchors are installed and drilled holes at regular spacing, but you usually need a lot fewer of them.
Tie Back Retaining Walls
Also unlike soil nails, they are not grouted along the entire length. Instead, part of the anchor is installed inside a sleeve filled with grease, so you end up with a bonded length and an unbonded length. This is because once the grout cures, a hydraulic jack is used to tension each one, the unbonded length of the anchor acts like a rubber band to store that tension force. Once the anchor is locked off, usually using a nut combined with a wedge-shaped washer, the tension in the unbonded length applies a force to the face of the wall, holding the soil back. Anchored walls often have plates, bearing blocks or beams called whalers to distribute the tension force across the length of the wall.
Infographic of how anchor retained walls work
Tangent & Secant Piles for Retaining Walls
One final type of retaining wall is called ‘Piles’, these are verticle members driven or drilled into the ground. Concrete shafts are installed into a massive drill rig like huge fence posts. When they’re placed in a row touching each other, they’re called ‘tangent piles’, sometimes they’re overlapped called ‘secant piles’ to make them more watertight. In this case, the primary piles are installed without steel reinforcement, and before they cure too hard, secondary piles are drilled partially through the primary ones, the secondary piles have reinforcing steel to provide most of the resistance to earth pressure. Alternatively, you can use interlocking steel shapes, called sheet piling. These are driven into the earth using industrial hammers or vibratory rigs.
Difference between tangent piles and secant piles
Pile walls depend on the resistance from the soil below to cantilever up vertically and resist the lateral earth pressure, the deeper you go, the more resistance you can achieve. Pile walls are often used for temporary excavations during construction projects because the wall can be installed first before digging begins. Ensuring the excavated faces have support for the entirety of construction.
Why Retaining Walls Fail
All these types of retaining walls perform perfectly if designed correctly, but retaining walls do fail and there are a few reasons why.
Under-designing for lateral earth pressure. It’s not intuitive how much force earth can apply to a wall, especially because the slope is often holding itself up during construction. Earth pressure behind a wall can build gradually such that failure doesn’t even start until many years later. Lots of retaining walls are built without any involvement from an engineer and its easy to underestimate the loads if you’re not familiar with soil mechanics. Most cities require that anything taller than around 4 feet or 1.5 meters be designed by a professional engineer.
Surcharge Loads: as mentioned, soil loads aren’t the only forces applying to walls. Some fail when there are unanticipated surcharge loads are introduced, such as; large buildings, or heavy vehicles driving too close to the edge. If you’re ever putting something heavy near a retaining wall, whether it’s building a new swimming pool or operating a crane, its usually best to have a civil engineer review beforehand.
Water can also cause incredible issues with retaining walls, as in some climates this water can freeze, and when freezing the water expands causing a near impossible force to restrain, and you don’t want that happening to the face of a wall. Most large walls are built with a drainage system to prevent water from building up. You’ll often see this with holes in the face on walls that allow water out, called weep holes, or pipes that collect and carry the water away.
Texture of old stone block retaining wall with weep hole for draining
Finally, soil can shear behind the wall, even completely bypassing the wall altogether. For tall retaining walls with poor soils, multiple tiers, or lots of ground water, civil engineers perform a global stability analysis as part of a design, this involves using computer software that can compare the loads and strengths along with a huge number of potential shearing planes to make sure that a wall won’t collapse.
Infographic of how soil slippage can affect a retaining wall, resulting in it faulting.
Now you should know everything there is about retaining walls, and you’ll find yourself taking notice to them when you’re out and about, and understand the important role they play to ensure a safe construction environment both before, during, and after construction takes place, as well as the engineering that goes behind each one.
If you’re in need of a retaining wall for your project and now understand the complex details and safety measures that go into each one, you may now realise that this is a job for a highly qualified civil engineer. Speak to one of our team today to get your project underway, and safely. Contact us by email or by phone.