A Deep Dive into Foundation Types in Arizona: Are You Built on a Floating Slab or Piers and Beams?
A Deep Dive into Foundation Types in Arizona: Are You Built on a Floating Slab or Piers and Beams?
Understanding the different types of foundations is crucial when diagnosing potential issues related to heave and settlement. By identifying the type of foundation present in a structure, one can gain valuable insights into the possible causes and effects of foundation problems. In this article, we will explore the three most common types of foundations: floating floor slabs, post-tensioned foundations, and pier & beam foundations with a crawl space. What type of Foundation do you have? The type of foundation plays a significant role in helping determine heave vs. settlement. There are three common types of foundations:
- Floating Floor Slab
- Post-tensioned Foundation
- Pier & Beam Foundation (with a crawl space)
More detail on these foundation types may be found in my book (pages 5-7).
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Floating Floor Slab: A floating floor slab is characterized by a large surface area with minimal weight distribution. The majority of downward forces in this type of foundation are concentrated on the footings. Due to the relatively small size of the footings compared to the floor slab, they can more easily exert pressure on the soil, leading to settlement. Although the floor slab itself is less prone to settling, it may follow suit if the footings settle first.
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Post-Tensioned Foundation: Post-tensioned foundations utilize steel tendons or cables that are tensioned after the concrete has been poured. This construction method imparts additional strength to the foundation. When diagnosing foundation problems in post-tensioned foundations, it is important to consider both heave and settlement factor. The forces pushing up on the floor slab have a more significant impact than those acting on the footings. The expansive surface area of the slab, coupled with minimal weight, makes it susceptible to soil heaving. Conversely, the footings, with their smaller area and lower position, are less prone to heave and more likely to experience settling.
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Pier & Beam Foundation (with a crawl space): A pier and beam foundation is supported by a series of vertical piers and horizontal beams, with a crawl space beneath the structure. In this type of foundation, both heave and settlement can occur. The footings, which provide support for the piers, are susceptible to settling. If a footing settles and creates a void, the floor slab may also settle, but typically only if the footing leads the way. Meanwhile, the floor slab itself, being a large surface area with minimal weight, is more prone to heaving caused by upward forces acting on it.
It is important to note that while these rules of thumb generally apply, there are exceptions to consider. In rare cases, a slab may settle independently when deep soil problems cause the soil to settle under its own weight, dragging the slab along. Additionally, specific circumstances such as high stem walls (greater than 24") and poor backfilling inside the stem wall can lead to floor slab settlement without accompanying footing settlement, especially when suboptimal fill materials like cinder rock are used (as seen in Northern Arizona).
Understanding the characteristics of different foundation types and their tendencies toward heave and settlement can provide valuable insights when diagnosing foundation problems. However, it is essential to remember that each situation is unique, and professional assessment by a qualified foundation expert is always recommended for accurate diagnosis and appropriate remediation.