St. Louis’s location in the Mississippi River valley means homes are often built on alluvial soil deposits – layers of sand, silt, and clay deposited over thousands of years. This soil structure can be unpredictable, with varying densities and moisture-holding capabilities at different depths. During floods or heavy rains, these soils can become saturated and unstable, while during droughts, they may compress significantly. This soil variability often leads to differential settlement, where parts of the foundation sink at different rates.