It may not look like a bowl, but most basements are constructed in a below-grade depression and then surrounded by backfill which is far more absorbant than surrounding soils.
The clay bowl effect refers to the natural consequence of excavating and backfilling when building a foundation. The backfill consists of soil that's been removed when a house's foundation is dug, creating an empty shape (hence the "bowl"). This looser soil is returned after the foundation is built and is more porous and more absorbent than the hard-packed undisturbed soil around it.
So it's no surprise that, when it rains or storms, the ground closer to the foundation is more heavily saturated. This problem is made worse if proper gutters and downspouts are not installed or if the water table is very high.
Additionally, over time, the soil around the house settles, altering the original slope of the house and affecting drainage when it rains or storms. In some instances, a foundation is built near springs or other underground water sources, adding to the risk of water seepage into the basement.
Why worry about the Clay Bowl Effect? Water in the soil close to the foundation will and can build up and find its way in, seeping through porous basement walls or through gaps and basement wall cracks. In fact, cracks often appear early in the life of a basement, formed during construction from the backfill striking the walls, sometimes before the concrete has completely cured.
Where does the term "Clay Bowl" come from? This backfill effect is especially pronounced in regions that have high-clay soils, such as Kansas, hence the name "Clay Bowl Effect." But the principle holds true wherever looser soil is used to fill in around a foundation.
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