The frost line in Illinois is 40 inches, so the water in the soil has not been known to freeze deeper than that point. In Chicago, building codes require that concrete supports be poured at a depth of at least 3 feet 6 inches.
The frost line marks how deeply below the surface that ground water typically freezes. The thawing and freezing of ground water produces pressure that can cause cracks and structural damage in concrete. In construction, any supporting structures, such as concrete foundations or footings, can be damaged if they are installed above the frost line.
The temperature of the soil below ground depends on soil type, moisture level, snow cover and vegetation. Typically, drier soils cool down and warm up faster than wet soils, and snow and vegetation serve as insulation in temperature extremes.