A structural foundation assembly for use with expansive and other soils. The foundation includes slab and wall assembly in which foundation walls are poured on an upper surface of a previously poured slab such that the slab supports the walls. The slab is poured in a slab form supported upon a center, elongate support beam that is supported by piers. The height of the piers and beam provide a void between the slab and soil to allow the soil to expand and contract. The wall is attached to the slab with angled connector members in the slab. The foundation includes a dam that has planar members positioned about the periphery of the slab contacting the soil and the slab to block soil from entering the void space. A sump pit is provided exterior to the slab and wall assembly with access provided by a window well in the foundation.