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Leaks in a retaining wall could actually be a good thing

A reader is concerned about leaks in a retaining wall, but landscaping experts said there’s little cause for concern in this case. (Reader photo)

Q: My beautiful stone retaining wall has sprung leaks during these recent rainstorms. How can I fix the problem?

Rockville, Md.

A: Your wall is probably just fine. If it were a leakproof dam without another route for the water to escape, you would have a bigger problem.

Retaining walls like yours need to hold back soil as well as the pressure that develops within it as the ground becomes saturated after heavy storms. The water dribbling through is relieving pressure that might otherwise cause the wall to buckle out and collapse and the soil behind the wall to slump down.

A couple of landscaping professionals who build retaining walls looked at the picture you sent. They disagreed on how they think the wall is built, but both said it appears you don’t need to worry as long as sections of the wall beyond the picture aren’t bulging out.

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“The wall shown in the picture is a dry stacked stone wall,” Krisjan Berzins, president and chief executive of Kingstowne Lawn & Landscape in Alexandria, Va. (703-921-9200; kingstownelawn.com), wrote in an email. This means that below the top course, where the picture clearly shows mortar, the stones are just stacked on top of each other in a way that uses their weight and gravity to keep them in place. “It is the natural irregularities in the stone that leave small gaps between the stones, allowing water to trickle through during periods of heavy rainfall or soil saturation,” Berzins said. “Seeing water trickle through a dry stacked stone wall, such as the one pictured, is a good thing. This means that the wall system is functioning properly.” If the water couldn’t dribble out, hydrostatic pressure would build up behind the wall. “Hydrostatic pressure buildup behind a retaining wall is precisely what causes wall failure or collapse.”

A representative from Lifetime Stones (571-436-0370; lifetimestones.com), a company in Silver Spring, Md., that offers a full suite of landscaping services, including construction and repair of retaining walls, looked at the picture you sent and concluded the stones appear to be mortared together all the way down, not just in the top layer. But mortar degrades over time, he said, and it becomes more like sand, allowing water to seep through. As long as the wall itself isn’t bulging out, this shouldn’t be a problem.

A mortared stone retaining wall, as well as those made of concrete or mortared blocks, needs an extensive drainage system behind the wall to keep hydrostatic pressure from causing the wall to collapse. If your wall is mortared, it probably does have a drainage system or it wouldn’t have survived earlier storms. The system might have become overwhelmed by recent storms, or the mortar might now be degraded enough so water flows more easily though the wall than through the initial drainage system.

If the wall is leaning, however, that would be cause for concern. In that case, you should call a landscaping professional who installs retaining walls and ask for an evaluation. If your wall is leaning, Lifetime Stones might charge $3,000 to $5,000 to repair it, depending on the length of the wall.

The risk of collapse from the pressure of waterlogged soil is high enough that heavy-duty retaining walls need to be designed by professionals. Montgomery County requires a permit and professional design for any retaining wall more than four feet high. And if the soil being retained has something on it, such as a car or shed, the limit without professional design and a permit is two feet.

If you were building the wall from scratch, you could build in a way to direct the water down behind the wall and into a pipe that would carry it to a lower spot in your yard. You would need perforated pipe at the base of the wall, with the holes in the pipe angled down to keep it from clogging with dirt. This pipe would need to slope toward one or both sides of the wall and might need to connect to solid pipe to get the water to a place where it could empty out without causing a problem.

Above the perforated pipe, you would add gravel to fill the gap between the wall and the soil. Spaces between gravel pieces would give the water a place to flow. Four inches or so near the top of the wall, where you want to transition from gravel to soil so you can grow lawn or other plants, you would cover the gravel with geotextile fabric (also known as landscaping fabric) and tuck it behind the gravel fill. This would help keep soil from running into the gravel layer and clogging the spaces.

When retaining walls are built of solid materials, such as concrete or mortared stones or blocks, there is usually also a waterproofing membrane against the back of the wall and short pieces of pipe slanted downhill through the front of the wall to act as weep holes where any water that collects in the soil can make its way out without pushing the wall over.

Your wall already has weep holes — the spaces between the stones.

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