Thousands of homes and businesses every year experience problems stemming from the movement of the floors that support their structures, walls, equipment, and exterior flatwork - differential vertical slab-on-grade movement.  This is a very common insurance claim and most owners do not understand the causes.  Since the soils in Arizona are commonly expansive, but remain relatively dry for the majority of the year, the slightest bit of water infiltrating the soils results in soil expansion with vertical and horizontal forces.  Expansive soils can be defined as any soil or rock material that has a potential for swelling due to a change in water content.  In some areas of the country, homeowners are advised to “water the foundation” to keep a constant moisture amount in the soil.  In Arizona, the soils are dry, and the discussion below indicates the need to keep the soil dry.