Q: Tim, I read a column on your website about rock retaining walls. I’m having a hard time believing you can build a lasting wall and not use mortar between the stones. I’d love to just build a wall and not have to use mortar. What’s the secret? —Karen, Bethesda, Md.
A: I routinely receive inquiries just like Karen’s at the Ask Tim page on my website. You may be a person that marvels at retaining walls that hold back earth without using mortar or poured concrete.
Why the stacked stone works is something you might have learned in your high school physics class. (It’s possible you were sick that week, so I’ll cut you some slack.) There are several formulas that might make your head hurt, but it’s just about mass and friction.
The soil behind the wall is responding to the force of gravity. It wants to be pulled closer to the center of the Earth. Any retaining wall that works is designed to provide a force greater than the pull of gravity, thus holding back, for a time, the hillside no matter how big it is.
Think about how easy it is to push a stone along the ground that’s the size of a baseball. It requires little effort. Try pushing an angular boulder the size of a dishwasher, and you’ll need to enlist the help of a few friends.
Here are a few tips to help you succeed when building a stacked stone retaining wall. The first step is to dig down into the soil and remove the topsoil from where the base of the wall will be anchored. You want the wall to be in the ground at least six inches, though 12 would be better.
If the wall’s going to be taller than about three feet, you should tilt the face of the wall so it leans back into the hillside. The forces that push against the wall increase at an exponential rate as the wall gets taller. As you double the height of a typical wall, the pushing force to tip it over can be three or four times greater.
A stacked stone wall relies on its own weight and the friction between the stones to hold back the soil. Larger stones work best, but be certain the scale of the rock matches the overall look you’re trying to achieve.
I’ve got a collection of retaining wall columns, including superb photos and diagrams, at my askthebuilder.com website should you want to dive deep into the subject.
Tim Carter can call you on the phone free to solve your problem. Go to his website and fill out the form on this page: www.askthebuilder.com/ask-tim/.