The soil is one of the most critical considerations before building a foundation. If the area has expansive soils, then typically a foundation repairperson will use a pier and beam or a slab foundation method. In the southern states you do not use a basement because the expansive soils of the south tend to push in on the sides of the basement and the sides are not supposed to move. With the sufficient force on these walls, the soil can actually collapse the basement walls. Clay is the cause of expansive walls. Another problem with expansive soils is that water can actually percolate through. "Water is a major problem for foundations," says Fred Marshall, owner of Advanced Foundation Repair, who has been working on foundation repairs for 13 years.
When building a foundation, soil height is one of the critical issues. There are other important issues as well, such as the height of the building and how much weight the house has to carry. Each type of foundation is different, Marshall says. The crew and homeowner should choose between three types of foundations to determine which would be most suitable to the soil content.
If a crawl space is used as the foundation, then the site digs a set of trenches and holes. Concrete is poured into these trenches and holes and will act as the interface between the foundation wall and the ground. Once it hardens, it forms a massive concrete beam that the house rests on. "The width of the concrete beam is controlled by the compressibility of the soil," says Marshall. If the soil is lighter then the beam will be wider to try to spread out the load, and in heavy clay content soil it can be narrower. The concrete then takes four to six weeks to cure to full strength.
A slab on grade foundation might also be used. The slab on grade is the least costly form of foundation out of the three. If a pier and beam foundation is used, which is the next one up in costs, then the soil content is a factor as well. Pier and beam foundations isolate some portion of the foundation from the ground. The pier and beam only has contact with the ground along with the perimeter wall and along isolated points in the interior. In northern states where the soil is not expansive, a basement is typically used.
