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Build a great wall
1 Mar, 2007 By: Jamie Gooch Landscape ManagementImproperly constructed retaining walls are not only unsightly when they fail, they can be dangerous. Gravity and water will combine to bring down a poorly constructed wall. To make sure that doesn't happen, contractors need to know the physics involved in retaining walls, whether they're constructed of segmental blocks, bricks, poured concrete, boulders or timbers.
An object in motion ...
Retaining walls are used to hold back sloping earth. While the ground just behind that slope isn't moving very fast, it is in motion. That motion will continue unless a retaining wall is capable of resisting the force that gravity places on the sloping ground. Soil can weigh more than 100 lbs. per cubic foot. That's a lot of weight to hold back. To counteract it, retaining walls need to be built the right way, and that starts with a good base.
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The first course of a retaining wall can't just be set on top of the ground. It needs to be buried. What's more, it needs to be placed on a well-compacted base.
"We always try to use at least six inches of road mix compacted to 95% to get a building pad to set the base course on," says Curt Gallegos, landscape division manager at Leatham Landscaping in Boise, ID. "Depending on what you're trying to retain, sometimes you have to bury the whole block or more."
Site conditions and wall materials will dictate the depth of the base material, but in general it should be buried 1/10th of the wall height. A good base will prevent the forces pushing against the wall to kick out the base of the wall over time.
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"Soil types can have a big impact on wall construction," says Richard Poynter, president and CEO of Poynter Landscape and Construction in Wildwood, MO. The clay soils in his area will support walls up to six feet high without calling in an engineer. But in some parts of the country, walls more than two feet high require an engineer, as per local regulations.
"In most cases, the soil type doesn't affect the base construction unless you run into plastic soils," Poynter says. Plastic soils are those that can be molded or permanently deformed by moderate pressure. "On plastic soils, we end up removing a percentage of it below the footing. We excavate 12 to 18 in. and replace it with compacted rock."
 A well-compacted base and backfill are a must for a properly installed retaining wall. |
The site also impacts the base course.
"If you have a slope in front of the wall as well, then you'll want to excavate the trench for your base down farther so you have two feet from the base material to daylight," suggests Gallegos.
 Segmental retaining wall systems come with blocks pre-engineered to set back into the slope. |
Once a good base has been established, the wall setback and level must be maintained during construction. Setting back the wall, basically leaning it back into the slope, helps it to retain the slope behind it.
 Some manufacturers' blocks have hollow cores, making them lighter to work with during construction. The voids are filled with crushed stone as the wall is built. |
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