It has been well established that many billions of dollars in structural damage to homes, buildings, and other manmade structures occur every year because of the effects of expansive soils. These soils may introduce enormous compressive forces of between 10,000-20,000 lbs per sq ft. against foundation systems present in that environment.
What, exactly, does that mean for your home and mine?It's mind boggling to imagine the scale and scope of such big numbers in terms of a threat to your home or other real estate investments. Let's wrap our brains around these big numbers for a moment by looking at two much smaller, contrasting numbers.
Imagine taking a circular saw and cutting a 12" x 12" square out of the typical basement slab floor. Now take that cement section and throw it on a scale. You will find that the slab sample weighs between 50-60 lbs. That doesn't sound like very much weight when compared with the counteracting forces that we know expansive soils can bring to bear. That's not really a fair comparison, though. The slab is not actually a weight bearing structural element of the foundation; it's not the best portion of the foundation to compare against the enormous and contrary forces of expansive soils.
Foundation Footings Added to Expansive Soil Equal Vertical MovementLet's look at the portion of the foundation known as the footer (or footing, depending on your geographical locale). This is a better portion of the foundation to measure since the majority of the weight of the home rests there. Structural engineers tell as that for each linear foot of "footing" that we might randomly sample, we find that between 1500 and 3000 lbs of actual house dead weight are present at the bottom of that footing. Variables that ultimately affect the actual number are one story vs. two, full basement vs. crawlspace, brick exterior vs. siding. However, it's a rare case in residential construction where we would find more that 3000 lbs per linear foot of bearing weight under a footer.
Now contrast that number with the opposing natural forces of expanding clay soils- which we've already identified at 10,000-20,000 lbs per square foot. It's not even a contest. Nature wins hands down, bringing roughly 3-6 times as much force against our foundation as gravity brings to counter it. The potential risk of expansive soils damaging a foundation by acting in a vertical, upward direction to heave it and cause structural cracking are tremendous!
What about lateral (horizontal) forces and the damages they can bring?
Can we quantify the force expanding clay delivers when it pushes sideways on a foundation wall? Let's see. Again, we know that 10,000-20,000 lbs of destructive force per square foot and is what we are dealing with and wish to mitigate. Imagine that we have a typical poured concrete or block basement wall of 8' high and 50' long. Let's further suppose that while the wall measures 8' high from the inside, the soil grade only comes up to a height of 6' on the outside of our basement wall, leaving 2' of exposed foundation above the grade. The 2' of exposed wall is unavailable for the soil to push against. If we take the 6' dimension, which is in direct contact with the soil, and multiply it against our length of 50', 300 square feet of wall area is able to be pushed upon by soil.
Lastly, let us assume a conservative mid-range soil force number of 15,000 lbs per sq foot against each of those 300 square feet of foundation wall, and we quickly see that our humble little basement wall is being faced with a sum total of 450,000 lbs of sideways force against it. Think of it- nearly half a million pounds of force coming at your foundation wall! In perspective, it's a wonder that buildings and homes constructed in expansive soil hold up as well as they do. Nonetheless, structural shifting, settling, bowing, and cracking in the foundation walls, footings, and slabs are a fact of life for many property owners. For those who find themselves in this circumstance, getting professional help from a qualified engineering firm or foundation repair service company is often a good option to explore.
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