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Build a great wall
1 Mar, 2007 By: Jamie Gooch Landscape ManagementSegmental retaining wall systems come with blocks that are pre-engineered to set back into the slope. By lining up each course with lips on the blocks or pins included in the system, contractors are assured of the proper setbacks. Segmental retaining wall systems also include guidelines for determining the geogrid needed to tie the wall into the compacted backfill.
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"With boulder materials, those predetermined measurements don't exist," says Poynter. "So if we're building a boulder wall more than three feet high, we bring in an engineer, especially if there's a slope above or below the wall."
Neither Poynter nor Gallegos construct many timber walls, due to a lack of client demand and issues with them rotting over time. However, the same need for a stable base, set back and tying into the backfill is true. Long T-shaped timbers ("deadmen") are often used to tie the timber wall into the backfill, similar to geogrid.
 Rules of thumb |
Water flows downhill
A carefully constructed base, properly set back and leveled wall, and compacted backfill with tiebacks can all be undone by erosion if drainage isn't handled correctly. Water that isn't led away by drainage will soak into the soil making it heavier and adding hydrostatic pressure to the forces a wall has to withstand. The water has to go somewhere. If it isn't controlled it could eventually wash away the base of the wall.
 Typical retaining wall design |
"The biggest mistake I see is contractors not using drain rock behind a wall," says Gallegos. "They're just asking for failure."
Poynter agrees. "A segmental retaining wall is designed to let a certain amount of water through the face of it," he says. "But gravel behind the wall allows you to relieve a lot of hydrostatic pressure. You have to manage watershed areas so that water is not allowed to build up in front or behind the wall."
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In a well-constructed wall, compacted soil just behind the top course of the wall allows much of the rainwater to flow over the top of the wall. What water does seep down behind the wall takes the path of least resistance through the drainage gravel. It can seep through the face of the wall or down to the base where a drainpipe leads it away.
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