Cantilever Retaining Walls
Cantilever retaining walls are most commonly constructed of reinforced concrete with an L-shaped or inverted T-shaped foundation. The weight of earth behind the wall is transferred onto the foundation, preventing the wall from toppling over from the lateral force of the soil behind it.
Cantilever retaining walls include a reasonably thin vertical stem and a horizontal base slab. The base includes two parts, the heel, which is the part of the base below the backfill, and the toe, which is the remaining portion of the base.
Also, a T-shaped foundation takes advantage of the weight of soil in front of the wall, offering additional stability. Foundations in some cases include a key in their base, which extends downward into the ground to prevent sliding.
A big benefit of cantilever walls compared to other retaining wall types is that they take up little area once they’re built, and are appropriate for heights of as much as 15 feet and more. Construction does require space behind the wall, so they might not be well suited to holding back existing slopes unless temporary support is provided during building and construction.
Cantilever retaining walls:
– Generally use much less concrete than large, heavy gravity walls, but need more design and careful construction.
– Are generally suitable for walls up to 25 ft. in height.
– Can be precast in a factory or built on-site.
How Do Cantilever Retaining Walls Work?
A concrete cantilever retaining wall consists of a reasonably thin vertical stem of steel-reinforced, cast-in-place, concrete or mortared masonry such as CMU blocks. In a cantilever retaining wall style, earth pressure acts horizontally against the side of the wall. The bottom part of the wall counters the earth pressure and pushes back against it.
The bottom element of a cantilever retaining wall is covered by earth with its weight pressing the base slab downward. As a result, a casual observer will not be able to see whether or not a retaining wall was built in a cantilever. However, blueprints or other official documents can reveal whether a retaining wall was installed with a cantilever design or not.
In many public or commercial jobs, and many home projects, a cantilever retaining wall design needs to be presented to local zoning authorities to ensure it is in conformance with regional planning, which may consist of such factors as storm-water overflow strategies and public safety codes. Professional engineers will likewise look at whether the retaining wall has enough strength to manage the lateral earth pressure as explained above.
Another element that engineers assess in retaining wall design is the ability to resist the potential effects of a landslide or avalanche. Retaining walls can be useful in preventing the types of soil movement that are related to such natural disasters. Considering landslide threat, the risk of liquefaction, and the water saturation of earth, is part of examining how a retaining wall will work.
When dealing with historic properties, it may be beneficial to consider transforming an original stone retaining wall into a cantilever style, or similar structure. In a new housing project, a city government board or agency might require a retaining wall to satisfy concerns about the grading or changing topography of the area. Considering various retaining wall choices helps a contractor or homeowner to assess the total costs and advantages of constructing a retaining wall in a location featuring a slope or grade.
Footings must be placed on firm undisturbed soil, or on properly compacted fill material. In areas exposed to freezing temperatures, the base of the footing should be placed below the frost line.
Backfilling against retaining walls must not be allowed until the masonry has actually cured and reached its final strength, or the wall has actually been properly braced. During backfilling, heavy devices should not be used near the top of the wall closer than a distance equal to the height of the wall.
Preferably, backfill must be placed in 12- to 24-inch layers, with each layer being compacted by a hand tamper. Throughout construction, the soil and drainage layer, if provided, likewise needs to be protected from saturation and erosion.
Provisions need to be made to prevent the accumulation of water behind the face of the wall and to reduce the possible unfavorable results of frost. Where heavy prolonged rains are anticipated, a continuous longitudinal drain along the back of the wall may be necessary in addition to through-wall drains.
Climate, soil conditions, exposure and type of construction determine the need for waterproofing the rear face of a retaining wall. Waterproofing should be considered in areas subject to severe frost, in areas of heavy rainfall, and when the back fill material is relatively impermeable.
Applying water repellents is also recommended and the top of masonry retaining walls should be sealed or otherwise protected to prevent water entry.
What is the difference between a cantilever and a gravity retaining wall?
Both types of retaining walls generally hold back soil, although they accomplish this through various methods. A gravity retaining wall utilizes its own weight to hold back the soil. Think of a heavy object with considerable friction along mating faces. It’s very difficult to move such an object. The walls of a gravity retaining wall are thick and generally the exposed face is sloping. As a result, they require more space than cantilever retaining walls.
Cantilever retaining walls use the weight of the soil to hold down a portion of the wall (the foundation or stack) and the stiffness of the connection to accomplish a cantilever effect on the wall. The branch is a cantilever.
Cantilever walls can and typically do have a vertical exposed face. They require large foundations or deep piles. The entire structure is usually built of reinforced concrete or support infilled into a cavity wall. An excellent way to explain a cantilever is to think of a telephone pole buried in the ground. The pole remains upright due to the cantilever action created by the strength of the pole and the weight of the earth around the pole in the buried area.
What most people don’t realize is that the backfill material for a cantilever wall, if engineered properly (preferably compacted and concrete stabilized) can hold its own weight. Ideally there should be a slight slope to the exposed face, which will require some kind of erosion protection. This method can save you a lot of money but can be time consuming to build.
When is a gravity retaining wall preferred over a cantilever retaining wall?
The only reasons to prefer one over the other are cost and the amount of space required. If land is cheap and is available, a gravity retaining wall may be the preferred choice. However, if neither of these factors are true, then the best choice is probably a cantilever retaining wall.
Professional Engineering Retaining Wall Design – Fast and Affordable
Whichever material you choose for your retaining wall, keep in mind that most local building codes require an engineering design for any retaining wall of 4 feet or more in height. And even for shorter walls, a design from a trained structural engineer is highly recommended to insure the security and longevity of your wall.
We can provide a certified engineer design for you within 24 hours (M-F) for just $350. Submit your Design Request here, or contact us if you have any questions or unique requirements.