Mark J. Farrow, P.E.1 & Doyle L. Smith, Jr., P.E.2

The continued rise in litigations and claims involving residential foundations on expansive soils has promoted several changes in the state of the art of their design. Investigations of distressed foundations usually indicate that actual differential foundation movements and edge moisture variation distances exceed design values by a large margin. Excluding instances when foundation distress is caused by settlement associated with poor compaction, soil desiccation, or tree root absorption residential distress is usually related to the availability of free-water sources beneath or in close proximity to the structure. A free moisture source causes excessive differential upward foundation movements due to swelling of expansive clay soils. This paper presents the influencing factors affecting the state of the art of slab-on-grade foundation design and the shortcomings of the current design methodology. This paper also presents case histories that suggest that factors critical to performance of slab-on-grade foundations do not receive the proper emphasis during design and construction based on current practices.