Preserved wood foundations have become popular in some areas over the last few years. Wood in a below-grade, damp soil environment has historically not had a long life, particularly as a structural member. As a result, there are several design challenges with respect to wood foundations.
They are more likely to be successful in dry soils than in wet soils. For the most part, their modes of failure will be similar to what we will look at on most other foundation systems, with a couple of exceptions. Since wood is less brittle or more flexible than concrete, for example, cracking is likely to be less common and bowing may be prevalent. Rot and insect damage or obviously possibilities with wood foundations, while these are not issue with most other foundation and footing materials.
In most cases, the interiors of preserved wood foundations are finished as living space, and it may be difficult to identify the foundation system, let alone inspect it.
Some areas have expansive soils that make it risky to use conventional footings and foundations. A special reinforcement technique for concrete grade beams and floor slabs is sometimes used to resist the forces of the soil and to prevent differential movement of the structure.
Post-tensioned slabs and grade beams use steel cables or tendons that are laid in place before the concrete is poured. The cables are most often surrounded by a plastic sheathing. After the concrete is poured, jacks are used to pull the cables tight, strengthening the assembly. These post-tensioned cables sometimes snap, and in some cases, they shoot out from the foundation or come up through the floor slabs. Fortunately, this problem is rare, at least so far.