As structural and civil engineers, we encounter multiple site conditions that require retaining walls. At times, it's replacing a wall that has failed or is failing. Some new developments need retaining walls to shape the land. We specialize in designing retaining walls of various materials, shapes, and sizes.
Our retaining wall design services include:
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Structural Engineering Design of Concrete Retaining Walls
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Structural Engineering Design of Block Retaining Walls
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Structural Inspections of Retaining Walls
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Construction Inspections of Retaining Walls
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Soil slope stability analysis (avoids the use of retaining walls for up to 4 feet)
Retaining Walls Explained
Basics of Concrete and Block (stone) Retaining Walls
In this article, we'll try to cover the most common questions and basic topics about concrete retaining walls as well as block retaining walls(more commonly known as modular or segmented block walls). We'll also give you one good, inexpensive solution for homeowners who'd like to save a buck or two.
What is a Retaining Wall?
A retaining wall, as the name implies, is a wall meant to retain something. Engineering-wise, a retaining wall resists lateral forces imposed on it, such as wind, earth, fluids, etc. However, in the construction world, everyone understands it as being a wall built to retain soil or earth. In fact, the term “retaining wall” usually refers to a concrete retaining wall.
So, in terms of performing a function, a retaining wall is just a retaining wall. The only difference between the types of retaining walls lies in how the “retaining” is done. There are many different types of retaining walls, but in this article we'll deal with the most common ones (which happen to be also the cheapest and most economical): concrete (cantilever) retaining walls and (modular or segmented) block or stone retaining walls.
The term "concrete retaining wall" usually refers to a cantilever retaining wall. The reason it's cantilevered is that the wall is supported by the footing below the wall, and there's nothing supporting the top of the wall. As the soil pushes against the wall, the wall will want to slide away and also topple (i.e., overturn). To resist these lateral forces, the concrete wall retains the soil by being heavy enough to prevent sliding and wide enough to prevent overturning. The footing is typically long enough that dirt will also be over it, increasing the weight and thus helping prevent the sliding and overturning of the concrete retaining wall.
Factors affecting the size of a retaining wall.
An important factor in any retaining wall is considering the possibility of water accumulation in the dirt, because water makes the soil heavier. So you need to carefully consider how you plan to provide drainage behind the wall to relieve the water pushing against the wall (called hydrostatic forces). Drainage is either done by means of weep holes (holes in the bottom of the wall) or by means of a pipe drain running along the back of the wall.
Using the correct type of soil is key to a successful design and an economical retaining wall because different types of soils act differently when water is added. For example, gravel provides good drainage while clay doesn't; therefore, gravel will not be affected too much by water, but clay will (however, gravel is expensive, but clay is dirt cheap).
Other important factors affecting the forces in the retaining wall include: the slope of the retained soil at the top of the wall, the slope of the soil at the toe of the wall, parking lots/houses/buildings nearby, vehicle barriers, fences, etc.
How to estimate the size (area) of a retaining wall for pricing purposes.
A very good rule of thumb to start preliminary engineering design of a concrete retaining wall (as well as a good estimate of the cost) is that the wall height is about the same as the footing width (Example: if from top of footing to top of wall you have 10ft, then you should expect about 10ft of width of footing). Another good starting point is to assign one inch of thickness to the wall for every foot of height of wall (Example: if your wall is 10ft tall, then you should expect about a 10in thick wall). However, the wall should never be less than 6in thick. For the thickness of the footing, a good guess is about twice the thickness of the wall. In San Antonio, Texas, a good way to have a general estimate of the cost is to figure about $700 per cubic yard (divide cubic feet by 27 to get cubic yards), or about $50 per square foot of face, assuming the site is accessible. These rough values include labor and materials for the concrete work, as well as the required earthwork and backfill. Once you've hired a Structural Engineer to design your retaining wall, you will be able to get a set of plans that will help you get a very good pricing estimate.
Differences between a stone retaining wall and a concrete retaining wall.
Modular block or stone walls are also retaining walls, but the way they retain soil is completely different from that of your typical cantilever concrete retaining walls. It is very difficult to explain, but the main component of a stone/block modular or segmented retaining wall is the stuff you can't see, which is buried in the dirt behind the wall: the geotextile fabric, also known as geogrid. As the soil pushes the wall away, it also tends to slice (so there are two modes of failure, just like a typical concrete retaining wall). Layers of a kind of geogrid are placed at different intervals across the height of the wall and attached to the block. As the soil pushes against the wall, it is restrained by this geogrid, which extends past the slice point, thus putting the grid in tension.
The cost to build a concrete retaining wall is about the same as, or about twice, the cost of a modular block wall. It all depends on 1) what you're doing, 2) where you're doing it, 3) who's doing it, and 4) what structural engineer designed the retaining wall. For instance, are you putting some really nice stone blocks? Are you building the wall in a space where a lot of earthwork will be required? Are you building the wall in San Antonio, Texas, or San Francisco, California? In California, labor and materials will be substantially more expensive than in Texas, and because of earthquakes, you'll need more materials and engineering.
Advantages and disadvantages between types of retaining walls.
Each type of retaining wall will have advantages and disadvantages. One advantage of the tall concrete retaining wall is that you won't lose much land to build it compared to a block wall, because the block wall will not be completely straight and will need to be built at an angle. Another advantage of the concrete retaining wall is that it is fast and easy to build and doesn't require all the trigger work a modular block wall does. An advantage of a block wall is that it can be substantially cheaper and look much more aesthetically pleasing than a concrete retaining wall. Again, you want to make sure that you have a Structural Engineer design your wall.
An affordable retaining wall.
Residential customers have an advantage. If you have a problem in your house or property where you need a short wall (Example: 2ft high), then you may want to consider a very economical solution: sandbag the soil, and step it in layers so that no soil can push or slide away. You would need to protect each bag in plastic to prevent water infiltration, but besides that, it's all earthwork: no concrete, no geogrids, and, best of all, it could save you tons of money & and it may be years before you need to do any type of remedial work. Get a structural engineer and start saving money.