Performance and Design of Slab-on-Grade and Pier Foundation Systems: Theoretical Considerations and Practical Applications

The design of slab-on-grade and pier foundation systems requires knowledge of the site conditions including soil type, climatic conditions, and consideration of site anomalies. Case studies of poor performance, well-designed (in theory) slab-on-grade foundations indicate that site anomalies including subsurface perched water, pre-existing vegetation, pre-site development practices (including grading, drainage and surficial anomalies) control the designs. In this study, a heuristic approach is revealed for producing guidelines to avoid substantial site anomalies that cause unexpected and unwanted foundation movement and to provide better performing slab-on-grade and pier foundation systems. The heuristic approach will also be shown to comport with simple laws of thermodynamics including the consequence of the 2^nd Law—the Principle of Increasing Entropy. Relative elevation surveys, unsaturated soil mechanics theory, geotechnical testing, and geo-electrical moisture material imaging resistivity surveys were used to evaluate the performance of the slab-on-grade and pier foundation systems. Exploration of the theoretical design of post-tensioned slab-on-grade foundations is provided including an analysis of the types of factors considered by most designers and a parametric analysis of slab performance under the assumed design conditions. Analysis of the design is performed using well-known industry methods including the Post-tensioning Institute's design methodology as well as finite element method analysis (FEM). The FEM program PLAXIS was used to model the soil-structure interaction mechanism. A new guideline that includes additional design considerations for a slab-on-grade and pier foundation system has been proposed based on theory and the case studies reviewed.