Ask the Builder: How a retaining wall works

Humans have been building retaining walls for thousands of years to create flat, or gently sloping, terraces on hillsides. A lot of my readers have them on their own property – or need to build one – and they come to me for advice.

Retaining walls are far more complex than you might imagine. If you see a tall retaining wall that hasn’t tumbled over or doesn’t lean, it might have been a stroke of luck on the builder’s part, but most likely its construction involved some serious engineering.

Let’s discuss a simple gravity retaining wall for now. This wall is the easiest to build, materials are plentiful and often free, and you can achieve success in almost all cases. A gravity retaining wall is made using any solid material that you can stack. This wall relies on its own mass to hold up the earth that’s pushing against it. It’s a simple battle of gravity, and you can win.

There’s an old saying: You don’t know what you don’t know. I can attest to this. Looking back at my building career, I started out so ambitious, and I had no fear. I jumped into projects like a small child into a blow-up swimming pool on a hot August day. I had lots of early success, and I also had a few dust-ups. Dust-ups are great learning experiences.

I was quite wet behind the ears, just about 26 years old, when my lovely wife asked me to build a retaining wall for her at our second home. The wall would serve two purposes. First, it would allow us to put a 6-foot-tall fence up in the dirt behind the wall that would give us lots of privacy from the apartment building parking lot that abutted our backyard. The 3-foot-wide strip of fill dirt behind the wall would also give her a wonderful opportunity to place shade plants and flowers.

This wall was about 50 feet long and started out up near the detached garage. There it extended up just 1 foot tall. By the time it got to the far property line, it was 42 inches high. I thought all along this 8-inch-thick red-brick wall would do really well. I was wrong.

My first mistake was not battering the wall. Battering means making it lean backward out of plumb, into the earth behind the wall. Think about how you lean into something when you push it. Retaining walls, especially simple stacked gravity walls, look better with a batter to them. The batter sends a subliminal message of strength. Usually a batter of about 5 degrees out of plumb is plenty.

Within five years, I noticed the tall part of my wall started to lean. I had good drainage behind it, but the wall was lacking in mass. The base of the wall was as thick as the top. The base of the wall at that end should have been at least 20 inches wide and tapered to 8 inches as it got to the top.

Keep in mind that retaining walls are simple vertical levers. The weight of the ground above the wall and frost in cold climates pushes against the top of the wall trying to tip it over. You probably remember from your high school physics class that Archimedes said, “Give me a lever long enough, and I can lift the Earth!”

He was right. The taller the wall, the longer the lever. This is why you need to make your gravity retaining wall look much like a pyramid that’s cut in half. Imagine how stable a pyramid is with its wide base!

The second retaining wall I built for my wife fared much better. I learned from my mistakes. It was a red-brick serpentine wall that she requested. It was stunning once complete, and it’s still in perfect shape 35 years later.

A curved retaining wall works well for the same reason arches worked for the Romans to support their aqueducts. Imagine an arch laid on its side and built into the ground. It has enormous strength.

Most gravity retaining walls work well about 3 to 4 feet tall. It’s best to make the width of the base about half the height of the wall. You should also bury the first row of stones, blocks or timbers about 6 to 8 inches into the soil. Don’t just lay the first row on the ground, and definitely do not lay it on grass or other organic matter that will decay and become a slippery mass of goo.

The soil type at your home will make a difference. I’ve found that sandy soils are the ones that seem to play the best with gravity retaining walls. Dense clay soils that are slippery might put far more pressure against the top of your wall. The amount of hillside above your wall also comes into play. A slope that extends hundreds of feet uphill from your wall will place much more pressure against the wall than a much shorter slope.

Do some due diligence. Walk, bike or drive around your neighborhood. Pay attention to other gravity retaining walls that you like and might have been in place for decades. Don’t be afraid of asking the homeowner or business owner about the wall. They may have built it and might be able to share how it was done.

I have five gravity retaining walls on my property here in New Hampshire built with giant granite boulders. My neighbor has one that’s 10 feet tall. All of them have been in place for more than 20 years and look as good as the day they were installed. Believe me, your new gravity wall is going to look fantastic. Send me a photo of it using the form on my “Ask Tim” page at askthebuilder.com.

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