Foundation Void Box - Used in foundation construction in areas with expansive clay soil. A box made of a slowly dissolving, water soluble material. Void boxes are placed on the ground prior to pouring of the concrete slab. This method creates an air space or void between the underside of the foundation floor and the expansive soil. A void box may be used in conjunction with poured piers to create the 21st century concrete version of a pier and beam house.