Tips for Adding Drainage To Your Retaining Wall
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Proper retaining wall drainage can be the difference between a sturdy wall and one that leans. Learn how to prevent and mitigate drainage problems.
Retaining walls add value to your home and property, but only if they’re properly built. Maximizing your wall’s drainage is one of the most important factors — and one of the most often overlooked, too. “Ever see a wall leaning forward, being pushed over from the top? That’s a sign of poor drainage behind the wall,” says Vince Christofora, professional engineer and owner of Woodstock Hardware in Woodstock, NY.
There are ways to help walls showing these signs. And if you’re installing a new drainage wall, it pays to get it right from the start, saving you time, money and headache down the road. “Installation of a retaining wall’s drainage features is not the place to cut corners or scrimp,” Christofora says.
Read on for tips for adding drainage to your retaining wall, new or existing.
Consider Your Conditions
Site conditions are your key determinant. “Dry areas and sandy, well drained, soils need less drainage. Wet areas with poorly drained, and clay type soils need more drainage,” Christofora says. He recommends consulting with a structural engineer, if possible, especially if you’re dealing with poorly drained clay soil. If that’s the case, at the very least, aim toward drainage overkill.
Don’t Skip the Footing Drain
Nearly every retaining wall should have a footing drain at its foundation. These have a pipe leading out, AKA “daylighted,” so water can flow out. “Given the cost of materials and the effort required to build any retaining wall, the increased cost of the footing drain is small compared to assuring the wall will perform over the long run,” Chirstofora says.
Pick the Right Pipe
There are two main choices: perforated pipe and solid pipe. Use the perforated kind under or behind your wall, for maximum subsurface drainage. Use solid to quickly carry water from your wall to the drainage site.
Don’t Skimp on Gravel Amount
Add an angular aggregate — clean, crushed stone or gravel between 1/2- and 3/4-in. — as a base under the retaining wall and as back fill behind it. Convention says to add at least 12 inches of gravel backfill, but Christofora’s motto here: The more the better. “Depending on the size and scope of the project, the minimal increase in cost of installing 24 inches of gravel behind the wall will provide increased drainage and maximize the structural integrity,” he says.
For existing walls, you may want to excavate to see if the gravel is at that 12-inch minimum. If needed, you could dig down far enough to see if there is the necessary footing drain and gravel under the wall as well. Just be careful not to undermine the wall.
Compact the Soil
Although it might seem counterintuitive, use a compactor on the soil behind your completed retaining wall, after adding your 12-plus inches of drainage gravel. Water-saturated soil can cause tremendous hydrostatic pressure to be exerted on your retaining wall. Compacting the soil makes it less permeable, sparing your wall of potential (and potentially damaging) water weight.
If hand-tamping, lay a 1- to 2-in.-thick layer of soil, tamp, then repeat until finished. If using a gas-powered tamper, you can lay up to six inches of soil between tamps.
Use Plenty of Filter Fabric
Filter fabric, AKA geotextile underlayment fabric, is a permeable fabric commonly used as a filter between soil and gravel surfaces. It keeps dirt and debris from clogging the gravel, and therefore preserves retaining wall construction. Christofora recommends using it at every place where soil and gravel meet, including under the wall’s gravel footing, between the footing drain’s gravel and soil and under the gravel added behind the retaining wall.
Install (or Add) Weep Holes
Weep holes are small, evenly-spaced holes along the bottom section of your wall. They protect the structural integrity by allowing underground water to seep through, preventing pressure build-up. Every retaining wall should have them. And yes, you can add them retroactively — drill them into an existing concrete, stone or brick wall using a core drill and drill bit, which you can often rent at your local hardware or DIY home improvement center.
Your wall’s scale will determine weep hole size and spacing. For large walls, around 6 ft. or taller, incorporate 3- to 4-in. dia. weep holes every 3 to 4 feet. If your wall is smaller than that, make your holes more like 1-1/2 in. dia., spaced at 8-ft. intervals. (Though Christofora notes that these figures can vary depending on wall height as well as soil type, topography and other site conditions. “The more holes, the better,” he says.)
One caveat from Christofora: “Adding weep holes may help save a retaining wall, but if the wall was backfilled with soil, and there’s no gravel behind it, you may not get enough drainage through the weep holes.”
Ensure There’s Enough Slope
Even with lots of correctly installed drainage around your retaining wall, if the soil on the other side doesn’t slope away sufficiently, that water won’t go anywhere. Any amount of slope will do, provided it doesn’t reverse a short distance away.
Keep in mind where the water will end up. You don’t want the excess water draining onto someone else’s land or into a pond, lake or river. Other options, if you’re in a pinch: “Build a French drain or a dry well, where water can flow into a gravel trench or pit and then be absorbed into the ground,” Christofora says. “They’re both fairly involved projects, but can be grassed over or turned into attractive landscape features.”