Digging Deep: Adventures in Foundation Follies (Part 1)

Well, here we are, embarking on what could be a riveting exploration into the world of construction or, alternatively, a therapeutic exercise to maintain my sanity. You see, my wife has understandably grown weary of my ceaseless ramblings about the intricacies of construction—especially foundation walls. Any type of wall, really. Give me the slightest opening, and I could expound on walls for eternity.

I can't blame her reluctance. Imagine if I could travel back in time to inform my teenage self that my future passion would revolve around the marvels of foundation walls and cantilevered retaining walls. Younger me would likely concoct a plan akin to a rather lackluster version of Oedipus, desperately trying to alter this peculiar fate. At the very least, the storyline would be far more digestible—for which we can all be thankful. Yet, fate has brought us to today’s topic, and here I stand comfortably on my metaphorical soapbox, ready to declare to the world, “You should care!”

But why should you care? Frankly, because very few people do—even those within the industry—and that's alarming. It's the relentless mundanity of this subject that makes it so elusive: people have lives to lead, and learning about walls usually doesn’t fit into that itinerary. Yet, therein lies the rub: if your builder or contractor isn’t savvy (ahem... boring), they could unknowingly pass on a colossal problem to you—and I mean colossal. Now that I have your attention…

The calamity at hand, hinted at earlier, concerns tall crawlspaces. Seemingly innocuous, yet I've personally witnessed some of the largest, most costly errors related to tall crawlspaces. Think this doesn’t concern you? Well, if you’re considering buying new construction, chances are you’ll inherit a tall crawlspace more often than not. This is because we reside in the Piedmont. Look up the definition of piedmont and you'll find words like ‘slope’ and ‘flat land’. The latter has rapidly been built upon because it’s typically cheaper. This leaves us with those pesky hills. To accommodate slopes, one must construct taller walls and piers. And to prevent your yard from resembling Mount Kilimanjaro, we must bring in fill to level the lot. This fill comes in the form of dirt, introducing hydrostatic and soil pressure against the walls—and a lot of it. So what are the failure modes? For part one of this two-part article, we’ll narrow our focus to footings.

Footing Failure Modes

Sliding

It turns out that when you stack something heavy against an object—especially when that something can exert hydraulic force—that object tends to move. In the case of footings, this means the wall and the footing can literally slide. Yes, indeed. If the wall is securely attached to the footing, and the footing surpasses the frictional forces and resistance of the surrounding soil, the entire assembly will slide. Ingeniously, engineers have coined this failure mode as “sliding.”

Now, how does one prevent this? By fortifying the structure with rebar and reconsidering the footing's geometry. A popular method involves shaping the footing like a “T,” offering more surface area to distribute the sliding forces. This clever addition to the traditional rectangular shape is known as a ‘shear key.’ It's an elegant solution, akin to adding a keel to a ship for stability—except here, we’re ensuring your house doesn’t decide to take a stroll.

The black circles are rebar that run through the length of the footing. The green is often called ‘transverse’ rebar and run through the width of the footing. The red is bent rebar that extend into the shear key. In higher loading conditions, additional rebar is often placed at the top of the footing to address areas of tremendous tension.

Insufficient Bearing Capacity

Wouldn’t it be remarkable if builders and contractors actually read the building plans? I mean truly read them. You know, the way you pick up something and scan the words to grasp what’s written? Engineers refer to these items as details and notes. One thing people should know about engineers is they’re not typically known for their vibrant personalities—not exactly the life of the party. They're not big talkers. These generalities are all in jest, of course; however, when engineers do decide to communicate with the world in the form of the written word, they expect those words to be read. One of the critical pieces of information on every structural footing design typically states, “This design is contingent upon a soil bearing capacity of 2,500 psf.” This bearing capacity can change, and if the on-site soil falls short, so too will your foundation. It’s crucial that a geotechnical engineer tests the soil for all footings, but especially for tall crawlspaces and retaining walls. The geotechnical engineer's findings are crucial.

Why are they so important? Suppose their tests reveal the soil bears only 2,000 psf. No problem! The engineer can likely enlarge the footing to accommodate the softer soil. Perhaps the soil bears 3,500 psf. Excellent news! We may reduce the footing size and save money on materials and labor. This harmony hinges on communication across multiple parties, which, in a male-dominated industry like construction, is considered a man’s greatest strength. I’ll see myself out…

Overturning Failure

I have a penchant for immediately contradicting myself. Remember that part about ensuring your soil is sufficient? Well, bearing failure can still happen. From my own experience, I’ve seen failures due to inadequate water management, insufficient backfill material use, or last-minute changes that lead to taller walls than the engineer intended. Essentially, these issues create pressure against the wall that can exceed the soil's capacity. When this happens, one side of the footing becomes overloaded. The rotational force on the wall increases loads over a smaller footing area, specifically the ‘toe’ of the footing. This effect can worsen if the ‘heel’ of the footing lifts upward. The ‘toe’ effectively acts as a fulcrum, leading to the unfortunate outcome of the wall toppling over.

Note the ‘heel’ and ‘toe’ of the footing. They both resist the wall overturning. The heel has dirt piled on top, using the total weight of the dirt to anchor it in place. The toe extends past the wall far enough to distribute the remaining overturning forces over the bearing soil. If the toe is too short, the wall can rotate.

Summary

So, here we are, venturing deep into the labyrinthine world of construction, where foundation walls and footings reign supreme. In this first installment, we've glimpsed the quirks of sliding footings, the dramas of insufficient bearing capacities, and even the occasional somersaulting wall—each a testament to the unpredictable dance between construction plans and Mother Nature's whims.

Stay tuned for part two, where we'll uncover more tales of heroic wall design, dashing rebar reinforcements, and the never-ending quest to keep houses firmly rooted and upright in the chaos of new construction.