Providing Drainage for Retaining Walls
The life span of a retaining wall depends largely on its drainage. Dry soil places very little stress on a wall, but wet, heavy soil causes the wall to buckle or lean, and means extensive repairs can't be far behind. No drainage system can handle a flood, but a good drainage system will prevent most water damage. The biggest trick-short of the excavation- is finding a place for the water to go. Bring the drainage pipe around the wall and continue running it until it comes to the surface.
DIG A TRENCH
Lay out the wall with batterboards and level lines, and dig into the hill to create a level trench as deep as is required by the wall you're building. Place some of the excavated soil on the slope's low side to help flatten the area. Make sure the trench is a uniform depth (and level) by taking measurements at various points along a level mason's line. If the measurements vary, add dirt (and tamp it) or remove dirt where necessary.
COVER WITH LANDSCAPE FABRIC
Landscape fabric prevents the soil from clogging your drainage system. Unroll enough fabric to run along the length of the trench twice, plus an amount a little more than the depth of the excavation. Pull the fabric across the back and bottom of the trench.
LAY THE FIRST COURSES OF THE WALL AND THEN ADD A GRAVEL BED
Leave space at one end for the drainpipe to exit through the wall. Behind the wall, lay a gravel bed that slopes 1 inch every 4 feet as it travels across the hill. Check the slope with a 4-foot level sitting on a straight 2x4 with a 1 -inch spacer under one end. Add or remove gravel to get a level reading. Lay drainpipe along the back of the wall. It, too, should slope 1 inch every 4 feet. Continue the pipe around a sidewall, if any, without kinking the pipe. Continue to run the pipe downhill until it reaches the surface.
LAY GRAVEL IN THE TRENCH
Cover the drainpipe with gravel as you build the wall. Lay the first few courses and fill in behind them with gravel. Tamp the gravel firmly. Continue to build the wall, filling in with 4-inch layers of gravel and tamping as you go. Lay gravel behind the wall until it is about 6 inches from the top. Pack down the gravel with a tamper, then fold the landscape fabric over the gravel. Fill the remaining space with soil and tamp it in place. Fill any low spots that result, then rake the area level. Sod, seed, or plant with the groundcover of your choice.
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- Perforated plastic drainpipe