The Broken Concrete Guy Leaning Badly
Driving through Everett, Wash., the other day, I heard a shout for help. Startled, I pulled over, got out and searched for the source. Oddly, there was no one around. “Please, please help me,” someone shouted again.
“I’d help… if I knew where you were,” I anxiously replied to a weed-infested vacant lot.
“Over here, I’m the concrete guy; the broken one leaning badly.”
“Ahh, I see you now,” I said, zeroing in on the victim.
It was an ugly sight, one I won’t soon forget — a retaining wall, or what was left of one, broken and indeed leaning badly, ruined and naked to the world. It was so pathetic that I was compelled to take a picture, knowing full well my disclosure might cause revulsion and reverberation across the globe. The horror… the horror.
“How did this… this shipwreck happen?” I asked the wall, anger and emotion tugging at me.
“I’ve got no rebar,” came his sad reply.
And it was true. Nary a stick did I see poking or protruding anywhere. “You weren’t built to code were you?”
“No… not even close. And just a few sticks, even number 3 bars, would have held me together. As you can see, I’m not that tall, the loads just aren’t that great.”
“Whoa now. Easy there big fella,” I said, a little concerned whether or not he truly understood retaining wall theory. “You’re a cantilever wall, eight feet tall if you’re an inch. And there are cars surcharging your top.”
“Just as I suspected,” he moaned. It’s worse than I thought. I’m terminal… I’m terminal…”
His voice trailed away to gibberish after that, making further conversation useless. Now I was left wondering, who could have done this… built this retaining wall so woefully weak? Clearly it was someone with no concept of how retaining walls receive and resist loads.
Retaining walls are one of the most misunderstood structures. They are complex animals that require more space to explain than I’ve got in this column. But, I will cover a few basics now, and perhaps continue the lesson in a future column (if you can’t wait, see my book, "Basic Structural Concepts for the Non-Engineer," at www.constructioncalc.com).
There are four types of retaining walls: cantilever, braced (or propped), gravity and tied-back. Our unfortunate friend above was a concrete cantilever, so we’ll start with them. Knowing which type of wall you have makes all the difference in how they’re designed. For example, a braced wall needs its rebar in different locations and amounts than a cantilever wall.
Cantilever walls are fixed at their base, not connected to a floor or other laterally-resisting element at their top. In other words, the wall cantilevers up from the footing. Here is a sketch of a cantilever wall and the forces acting on it.
Note that there can be four lateral (sideways) forces acting on a cantilever wall. However, many times there is only one: the force due to the retained soil. If your wall won’t have surcharge loading from vehicles or other nearby adjacent structures; and the retained soil isn’t sloping upward away from the wall; and you have a good behind-wall drainage system to convey groundwater away, those forces go away. Now your wall needs less rebar and concrete, thus becoming less expensive.
For cantilever walls to work, the connection between footing and wall must be very strong (moment-resisting), with vertical rebar of sufficient quantity located in precisely the correct place. In the next sketch, you can see as the soil pushes sideways on the wall and it tends to deflect away, there is tension on the dirt side of the wall and compression on the air side. Concrete is very good at resisting compression, but terrible at resisting tension. Rebar is great at resisting tension, thus must be placed where tension forces occur — toward the dirt side. Important note: braced walls have tension and compression in different locations than cantilever walls and, thus, their rebar must be placed differently.
Also, in the footing, tension and compression occur (see above sketch). Transverse rebar in the right locations resists the tension forces. Longitudinal footing rebar is important, also, to distribute gravity (downward) loads evenly throughout the footing (in my sketches, the only gravity loads are from the weight of the wall and footing themselves, but there could have been posts or other structural members sitting on the wall bringing gravity loads too.)
Once we’re sure the wall and footing are structurally sound, we have to check the wall’s overall stability (see next sketch). First we check the soil under the footing to ensure we don’t exceed its bearing capacity. As lateral pressures try to rotate the wall, the toe of the footing (especially) presses downward. The soil under the toe must have adequate bearing capacity or the entire wall can rotate. Using a long footing heel helps spread these forces over the footing’s width and reduce overturning.
Lastly, we check resistance to sliding. Soil placed against the front (toe) of the footing, plus friction between footing bottom and soil beneath get this done. Sometimes a key — a downward chunk of footing projecting into the soil below — is needed if not enough soil can be placed against the front of the footing.
In summary, retaining wall design is complicated; one size does not fit all. Failure to recognize this results in calamities like the broken fellow I met in Everett. The next step for him, tragically, is the wrecking ball.
Tim Garrison of ConstructionCalc.com, is a professional engineer, author and software producer for the building industry. Send e-mail to email@example.com. Tim reads every one.
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The views expressed in this article represent the personal views, statements and opinions of the author and do not necessarily represent the views, statements, opinions or policies of the National Association of Home Builders. NAHB does not necessarily endorse any of the views expressed by the author and NAHB is not responsible for any direct or indirect consequences arising out of the views expressed in this article.
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