Expansive soils contain clay minerals — particularly montmorillonite — that swell when wet and shrink when dry, generating volume change forces that can crack and move foundations. Montmorillonite-rich soils are common throughout the Wasatch Front valley floor in prehistoric Lake Bonneville lake bed deposits. Sites with plasticity index (PI) over 15 in the active soil zone (typically the top 6–8 feet) need engineering intervention: deeper footings to stable bearing soil below the active clay layer, non-expansive engineered fill under slabs to replace problem material, or pier-and-grade-beam systems that span the active zone. The cost to address expansive soils at construction is modest. The cost to repair differential settlement damage to a structure built over unengineered expansive soils is not. A soil report is the only way to identify and quantify the risk on a specific parcel.