Understanding Foundation Failure: The Truth About PT Foundations

 

Foundation systems play a crucial role in maintaining the structural integrity of buildings. In our previous articles, we explored the forces that affect conventional foundation systems. Now, let's delve into Post Tensioned (PT) foundation systems and their potential for encountering foundation problems. In this fifth part of our series on diagnosing foundation failure, we will shed light on the Uniform Thickness Post Tensioned (PT) Foundation, its characteristics, and the specific challenges it presents.

 

Post-Tensioned Foundation Systems

 

Over the past three decades, Post-Tensioned (PT) foundations have gained popularity as an alternative to conventional foundation systems. Particularly in the Southwest region, the Uniform Thickness Post Tensioned (PT) Foundation has become widely used. This type of foundation employs high-strength cables, placed in both the north/south and east/west directions, with a spacing of approximately 30 inches along both sides.

 

Understanding the Uniform Thickness PT Foundation

 

The Uniform Thickness Post Tensioned (PT) Foundation represents the most common PT system in the Southwest. Unlike earlier PT systems with deep ribs and cables at the top and bottom, this system features a single cable placed in the middle, known as the neutral zone. The concrete above and below this zone is not supported by the cables, making it more susceptible to tension-induced cracking and slab bending.

 

Edge Protection and Drainage Considerations

 

One notable drawback of the Uniform Thickness PT Foundation is its limited edge protection. With minimal projection, water can easily infiltrate around the edges and penetrate underneath. Consequently, the soil is more vulnerable to the effects of poor drainage. This emphasizes the importance of effective water management to mitigate potential issues associated with this foundation type.

 

Settlement and Heave Concerns

 

The absence of isolated footings in the Uniform Thickness PT Foundation has a notable impact on load distribution. The roof loads exert downward forces that are spread across a larger area, making settlement less likely to occur. However, the monolithic connection between the slab and the foundation increases the risk of heave compared to conventional foundations. Specifically, edge heave is the predominant type of foundation failure observed in this system. The causes of this phenomenon are currently being investigated, including factors such as moisture migration and design considerations.

 

Conclusion

 

In conclusion, Uniform Thickness Post Tensioned (PT) Foundation systems are more susceptible to foundation heave, primarily manifested as edge heave. These foundations' unique characteristics, such as their uniform thickness and limited edge protection, contribute to the increased risk. As ongoing research seeks to understand the underlying causes and refine design practices, it is crucial for homeowners, builders, and engineers to be aware of the potential challenges associated with this type of foundation system. By adopting proactive measures, such as effective drainage and moisture management, it is possible to mitigate the risks and ensure the long-term stability of structures built on PT foundations.