By definition, a retaining wall is any structure that holds back soil or rock from a building, structure, or area. They have applications in the commercial, industrial, and residential arenas. You might find a retaining wall keeping soil back from sidewalks or roads, along riverbanks, or, more commonly, in your neighbor's garden or landscape. They can be constructed of wood and any number of masonry units, including stone, brick, concrete, and cement-filled cinder blocks.

Functions

Most often retaining walls are used to provide a level surface for gardening or landscaping where the land is naturally sloped. In a more utilitarian sense, they are also used to protect homes from the flow of water, usually when the home site is cut into a hill or mountainside. In a more aesthetic sense, retaining walls provide elevation and accent for landscape design. They can create beautiful stone pathways, support water features, or attractively direct water away from patios or play areas.

Types

Aside from the different types of materials that can be used to build one, there are also four basic types of retaining walls. The difference lies not just in material but in how they are built.

• Gravity walls use their own weight to hold back the soil. Because of that, this type of wall is usually shorter (about three feet tall on average) in order not to overdo things. If the wall was any taller it would be more prone to collapse. Gravity walls are built with strength fully in mind. The base of the wall is at least half as thick as the wall is high. As the wall is built it grows thinner and slants backward, in a process called battering, into the hill or slope that it is trying to keep at bay. This is done to prolong the life and look of the wall. Gravity walls are by far the most common wall used in landscaping.
• Cantilever walls are the same width from top to bottom. However this type of retaining wall uses a footer and reinforced steel bars to strengthen the wall and prevent tipping. Foundation or basement walls are good examples of the cantilever design. The footer will be considerably thicker than the wall standing on it, with steel bars running through the footer and the retaining wall itself--essentially making them one strong unit. Cantilever retaining walls typically hold back a substantial amount of earth and rock and are painstakingly engineered. This is not the type of wall for the do-it-yourself'er.
• Counterfort walls are much the same as cantilever walls with an added piece of support. These walls have a triangular wall that connects the footer to the top of the retaining wall itself. These are common where some seriously heavy-duty support is needed. The triangular portion of the wall system is usually hidden beneath the soil or backfill. Counterfort walls, like their cantilevered cousins, are often made of poured concrete and reinforced with steel bars. Again, these are intricately designed by structural engineers. To build a wall of this type, and have it fail, would likely by very expensive.
• Buttressed retaining walls are the same as counterfort walls except that the supports are on the outside of the wall (i.e. not hidden). These supports offer even more strength because the triangular wall is pushing against the forces that would otherwise topple the retaining wall.

The Pre-Cast Revolution

A sort of revolution has hit residential retaining walls. Pre-cast, interlocking concrete stones have made building walls quick and easy with a wider scope of creative opportunity. Also called segmental walls, this type of retaining wall utilizes natural-looking, concrete blocks that have a lip on their backside. This system of interlocking results in a wall made of components that do not need mortar to bind them. Design possibilities using pre-cast stones include curves, terraces, serpentine walls, steps, and waterfront walls. Because this type of system is similar to gravity walls, and even with the added strength of an interlocking system, segmental walls should not be very tall. Nonetheless, they are ideal for homeowners wishing to do their own work and easily create the lawn and garden of their dreams.