Engineering a Retaining Wall

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Sure, retaining walls look like simple stacked stone, block, or timber. But in fact, they're carefully engineered systems that wage an ongoing battle with gravity. They restrain tons of saturated soil that would otherwise slump and slide away from a foundation or damage the surrounding landscape. These handsome barriers also make inviting spots to sit, and can increase usable yard space by terracing sloped properties, something that is increasingly important as flat home sites become ever more scarce in many regions.

Along with sloped landscapes where water runoff causes hillside erosion, ideal locations for a retaining wall include spots downhill from soil fault lines and where the downhill side of a foundation is losing supporting soil or its uphill side is under pressure from sliding soil.

If your property needs a retaining wall, or if the one you have is failing, review these descriptions of the four most common types: timber; interlocking blocks; stacked stone, brick or block; and concrete.

Common Problems
Although retaining walls are simple structures, a casual check around your neighborhood will reveal lots of existing walls that are bulging, cracked, or leaning. That's because most residential retaining walls have poor drainage, and many aren't built to handle the hillside they're supposed to hold back.

Even small retaining walls have to contain enormous loads. A 4-foot-high, 15-foot-long wall could be holding back as much as 20 tons of saturated soil. Double the wall height to 8 feet, and you would need a wall that's eight times stronger to do the same job. With forces like these in play, you should limit your retaining wall efforts to walls under 4 feet tall (3 feet for mortarless stone). If you need a taller wall, consider step-terracing the lot with two walls half as big, or call in a landscape architect or structural engineer for the design work (have the architect or engineer inspect the site thoroughly) and experienced builders for the installation.

If you have your retaining wall built, figure about $15 per square face foot for a timber wall, $20 for an interlocking-block system or poured concrete, and $25 for a natural-stone wall. Preparing a troublesome site—one that includes clay soil or a natural spring, for example—can raise costs substantially. Add 10 percent or so if you hire a landscape architect or engineer. But shop around; some landscape firms do the design work for free if they do the installation.

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