Competent soil can be a long way down. Chicago sits on deep glacial deposits, and the upper layers are frequently soft clay, silt and organic material with limited bearing capacity. Load bearing strata can be relatively shallow in one neighborhood and considerably deeper in another. Footings poured at standard depth sit on whatever happens to be there, and that is not always adequate over a century.

Much of the city is built on made ground. Large areas of Chicago were filled, regraded or raised during the nineteenth century, and the city famously lifted entire blocks of buildings in the 1850s and 60s to install sewers. Land near the river, near the lake and across former wetland and marsh was made usable with fill. Fill soil compresses unevenly and continues settling for a very long time.

Clay shrinkage removes support seasonally. Glacial clay contracts as it dries. Through a hot Chicago August the soil under and around a footing shrinks and withdraws support, then swells again in spring. Thirty or forty cycles of that leaves a footing sitting differently than it started.

Mature trees accelerate it locally. A single large tree close to a house pulls a significant volume of water out of the clay each summer. That localized drying causes localized shrinkage, which is why settlement so often affects the side of the house nearest a mature tree rather than the building evenly.

Older footings were shallower. Much of Chicago’s housing stock predates 1940, when footings were commonly narrower and shallower than current standards require, and often sat above rather than below the frost line.

Water makes all of it worse. A downspout discharging against the foundation, a failed drain tile or a negative grade saturates the soil in one spot, and saturated clay loses bearing capacity. A lot of the settlement we see started as a drainage problem nobody addressed.