Q. Why do my replacement steps need to be lower than my house foundation? I live near the water, and we dug a hole to put in a new foundation for the front steps that will be bigger than the old ones. We found the groundwater came up at 30 inches, so we stopped digging, but the inspector said our plans showed 36 inches minimum. The house foundation is at 30 inches also, so why are we digging lower? They did it, but I can’t understand.
A. This is a very uplifting topic. The deep question and answer are actually simple and not too deep after all. Science, if followed, can aid in survival against the forces of nature that perpetually exist.
The reason your foundation needs to be lower is that water, when it freezes, expands as ice. So what does that have to do with a big ol’ heavy chunk of concrete, you may ask? Well, the ground absorbs rain droplets, humidity and underground streams, all various forms of water just waiting to expand when the temperature drops below 32 degrees Fahrenheit or 0 degrees Celsius. Frost begins to form, first at the surface and then further and further down below ground level the longer the air temperature remains at or below freezing.
In this part of the world, the average depth of freezing consistently reaches 3 feet, and only slowly thaws when the air temperature rises. The foundation should always have a wide base, or footing, that spreads out at the bottom, and needs to be below the 3-foot depth to resist the phenomenon of heaving. This is because, as the frost builds and expands, the moisture, in all its various forms (I just mentioned), begins to expand and crowd out the foundation of your home, or front steps, or anything else in its way. Think of it like a big zit being forced to the surface from all directions.
When this happens, all the man-made structures in the ground begin to lift, sway, crack and separate in different directions. You have about as much chance of preventing it as you would of standing against a strong tidal wave on the beach, but since the heaving happens in slow motion, it seems to not be well recognized, because this problem comes up all the time on construction sites. Believe it or not, even large buildings, like skyscrapers and shopping malls, are vulnerable to uplift from soil expansion, so it’s not just your front steps or your comparatively smaller building. Workers who don’t understand the science can create some easily avoidable problems.
The reason your house foundation is closer to the surface is probably because it’s tied to deep piers, called piles, that may go down 25 feet below the surface. The adjacent concrete foundation, anchored to the piles, should still have gone down to 36 inches, but at least the existing footings are held to something that may resist the uneven lifting.
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