Damage and Distortion Criteria for Residential Slab-on-Grade Structures

Various criteria have been suggested throughout the geotechnical and construction community for maximum allowable differential movement for foundation systems. However, the writers have found very few papers pertaining just to slab-on-grade systems. Furthermore, our literature review indicates that most of these criteria have been developed through studies of soil settlement, irrespective of other modes of soil movement, and do not necessarily pertain to residential slab-on-grade structures. There are many different soil/geologic processes that can operate at a given site, each of which can affect the performance of an overlying structure in a different manner. For example, at the same development a house founded on fill of varying depths (differential fill) may settle somewhat uniformly, producing little distortion, whereas a house founded on expansive bedrock may experience “heave” or “edge curl,” resulting in higher levels of foundation distortion. The house settling uniformly may experience an overall differential movement of 2.5 in. with only minor damage, whereas the house affected by expansive soil influence may experience greater levels of damage with an overall differential movement of only 1.50 in. This is because the angular distortion is much greater along the area of influence. Understanding various patterns of movement for different soil conditions is very important and should be considered when designing or reviewing residential slab-on-grade foundations.