Do Floods Affect Slab-On-Ground Foundations?

The short answer is: no, but the devil is in the details. 

Why floods have little effect on slab-on-ground foundations

Many people are concerned about their slab foundation when their property floods. The reasoning is valid, but the unspoken premise is wrong. The reasoning goes like this: clay soils swell when they get wet, a flood makes the clay wet, so that causes the clay to swell making the slab distort.

Clay soils do swell as they absorb moisture, including flood water, but……  

The problem with this line of reasoning is time; clay absorbs moisture very slowly. So slowly that it is considered impermeable. If you want to build a pond, you have to get down to a clay layer. If you dig down to a sand layer, any water will drain away through the sand.

Clay soils do absorb water, but only very slowly. The most common rate I have seen is 1/8 inch per day.  The clay that can swell and distort the slab is directly underneath the perimeter grade beam. Let’s assume that it is 16 inches below the surface of the soil. Divide 16 inches by 1/8 inch and you get 128 days for the flood water to reach the clay soil underneath the perimeter grade beam. 

An exception 

If the soil is underneath your slab foundation and you are in a floodway, the moving water can wash the sand out as it moves through the sandy soil beneath the foundation. This sand erosion problem is pretty much confined to areas north of I-10.    

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