Ask the Builder: Tips for building a small retaining wall
Last week I wrote about my recent trip to Southern California. I was fortunate to spend time with a dear friend and his wife. I don’t know that I’ve ever met a person who’s more in love with a home than he is. Believe it or not, he’s lived in the same home for all of his 72 trips around the sun. It’s a stunning Craftsman home that was built in 1910.
This gorgeous wood-framed house is built on a corner lot in a community that is snuggled in the Crescenta Valley, just below the towering San Gabriel Mountains. His lot has about 8 feet of downward slope across its 150-foot depth. The original builder wanted a fairly level front yard. A 4-foot tall retaining wall was built along the south side of the lot to achieve this goal.
The San Gabriel Mountains contain a wide variety of granite rocks. Flash floods roaring out of the canyons have tumbled these rocks like clothes in a dryer, rounding the sharp edges. There are countless wonderful boulders the size of extra-large watermelons, cantaloupes and grapefruit. You can harvest these stones from nearby streams, and they’re the perfect material for building a durable and attractive retaining wall.
You may not have granite where you live, so make use of any suitable durable stone at your disposal. Coarse fossil-filled limestone was abundant in my hometown, Cincinnati. There are thousands of retaining walls in the greater Cincinnati area built using this dense, strong stone.
My friend’s wall started to slowly succumb to gravity over the decades. Several years ago it was leaning so much that the city condemned the wall. A public sidewalk next to the wall was not safe to walk on, as the wall could collapse without warning.
I was able to view the wall a few years ago before it was condemned. I was amazed that this wall was only one rock-width from top to bottom. This was the primary reason the wall failed. Had the builder constructed the wall so the base was two or three rocks wide tapering to only one rock wide near the top, I’m quite sure I’d not be writing this column.
An elderly local stone mason rebuilt the wall, and it looks as good as new. He salvaged each of the pieces of granite. My friend didn’t take my advice to change the design. The wall was rebuilt exactly how it was done originally. “Tim, the wall stood for over 110 years with it just one stone wide,” he told me. “I won’t be around in 100 years when it fails again.” I didn’t have a plausible objection.
How does this relate to your home? In many respects, my friend’s reasoning is somewhat sound. Earthquakes do happen in Southern California, and his old wall survived quite a few. Frost is not an issue, as it rarely drops below 32 F at my friend’s house. If I were to build his wall at my home in New Hampshire, within a few years the deep frost would start to push over the wall.
Retaining wall design is quite complex. As a wall gets taller, the tipping force increases in a dramatic fashion. Double the height of a wall and the force pushing against it can be three to four times that of the shorter wall. Any retaining wall taller than 3 feet can produce significant tipping forces. This is one reason some retaining walls have a batter, or backwards lean, to offset this force.
Beware of the advice that homeowners and some DIY warriors post on YouTube. I’ve watched many retaining wall videos that are filled with very bad information. I urge you to seek the advice of a structural engineer for any wall taller than 6 feet.
Consider the characteristics of the soil you’re trying to hold back. A well-drained, sandy soil is much easier to deal with than a dense clay soil. Steep hillsides uphill from your wall can be problematic. Soil creep is real. Gravity is working nonstop to pull all the soil uphill from your wall down to the nearest sea or ocean. Never forget that Mother Nature bats last. Your strong wall design will take you into extra innings, but in the end she will win the game.
Leverage physics into your design. An inside curved retaining wall acts much the same as an arch above an opening in a wall. Curves also add eye appeal. Don’t feel everything has to be a straight line.
Provide for excellent drainage behind your retaining wall. Backfill it with rounded gravel the size of walnuts, if possible. Water flows through this rock as it does through a colander in your kitchen sink. Trapped water behind a retaining wall can add thousands of pounds of weight that is working to topple it.
I have several in-depth columns about retaining wall heights, the four primary types of walls, and detailed installation instructions. Just type “retaining walls” into the search box on this page: https://askthebuilder.com/search.
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