Q: We would like to build a summer residence on some land we have that is about 5 miles from the nearest community. Many areas around there have swelling soils, but we don't know whether our soil is a swelling type because nobody has built nearby. Are there easy ways to check it?
If we find that the soil swells, how can we choose a safe way to build on it?
A.: To tell whether the soil swells, there are a few signs to look for:
- open cracks in the surface soil when it's dry
- soft, popcorn-like appearance of the dry surface soil
- lack of vegetation because of heavy clay
- unusual stickiness of the surface soil when it's wet
Any one of these would be good reason to suspect that you have a swelling soil. Nevertheless, it would be best to consult a geologist or professional soils engineer to determine whether the soil would present hazards to your foundation. There are several successful methods of building safely on swelling soils, including:
- building on a post-tensioned slab on grade
- building on concrete grade beams supported on concrete caissons located at about 10-foot intervals, and with an isolated, floating floor slab
If the swelling action is low or moderate, a pad and grade beam foundation, continuous spread footings, or extended bearing walls can be used. You should get a professional engineer's advice about what solution would be safe.