How They Work: Retaining Walls

Retaining walls are structures built with the intention of holding back earth from moving. They are commonly used for stabilization of slopes and to make useful areas out of different elevations. For example, terraces for buildings, roads, railways, and agriculture.

The walls are designed to resist any lateral pressure from soil when there is a change in the elevation of the ground that exceeds the angle or repose of soil. Every wall is built to support a specific wedge of soil. A basement wall is one example of a retaining wall. Typically, this term is used in reference to a cantilever wall that is freestanding and has no lateral support from the top.

There are four main types: gravity, piling, cantilever, and anchored. Gravity models are the standard type. It holds the earth through its weight, and may topple over easily because the internal leverage of pressure from the earth is high. Piling models, as their name suggests, use long piles. The wall is fixed, with soil on both sides of its lower end. If the piles can withstand the bending forces, the wall can handle high loads.

A cantilever model uses the pressure of the earth trying to topple it in order to stabilize itself through a second lever arm. Finally, there are anchored models. These units have cables connected to them that keep them from toppling over. The cables are driven into rock or soil and fixed by expanding anchors.

There are other methods that can be used for retaining purposes. Soil slopes, retaining walls, or excavations are reinforced in a process called soil nailing. This technique involves the insertion of slender elements, usually steel-enforcing bars. These units are installed, untensioned and at a downward incline, within a drilled hole and grouted or drilled into place. A facing or isolated soil nail heads can be used on the surface.

Retaining systems that are not solely a wall are considered soil-strengthened systems. They may also include features that reduce the earth pressure that acts on the wall. Most of these systems are used in combination with a wall. Gabion meshes are a type of soil strengthening commonly used with an outside wall. These devices contain wire mesh boxes where roughly cut materials, such as stone, is filled. The cages reduce internal movement and erosion forces.

MSE, or mechanically stabilized earth, is soil that is constructed with artificial reinforcements through layered mats called geosynthetics attached to their ends. The mats offer added internal resistance. Steel traps, which are also with layers, do this as well. This type of technique requires outer facing walls that the layers can be affixed to. The wall face is generally concrete structures that can handle differential movement.

Retaining walls are constructed in places where it is necessary to hold the earth back. They are built to resist pressure from the soil and earth. There are four primary types, each one having a different level of tolerance and resistance ability. Sometimes these structures must be used with other systems in order to be effective.

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