Around Arizona, there’s a tremendous amount of variability when it comes to soil. You can find more information about your soil type by visiting one of these Web sites:
National Resources Conservation Service
Arizona Geological Survey
Here are some general observations:
- Benson: Parts of Benson have highly expansive soils and parts have collapsible soils. Much of Benson has highly collapsible soil. It gets wet, it settles. There’s a Safeway store in Benson that settled 15 inches over a few years. The foundation cracked and it had to be underpinned.
- The foothills of the Valley: In some areas around Phoenix, the soil is expansive, and in other places it’s collapsible.
1. You’ll find clay in the lower portions of the Valley. The clay washes down the mountain with the running water. So in Gilbert and other parts of the lower Valley, you’ll have more clay… But half a mile away, you’ll find soil that’s not expansive.
2. Along the foothills, like in northeast Mesa and Scottsdale, you’ll find collapsible soils made from silt and sand, which is loose and collapsible.
3. In Gilbert, Phoenix and Mesa, the homes are built over a former farm field. The farm field does a lot to help the soil because while it was a farm, there was constant wetting and drying for crops. That activity removed the expansion and contraction over this native soil, so you don’t have much of an issue. Still, if you rework that soil to a higher density, you reintroduce the expansion potential.
- The rim: Flagstaff, Show Low and higher country have clay soil that is a remnant of the bedrock, which has been broken down over time and turned into clay. Some of this is highly expansive and can cause five or six inches of heave.
- Prescott Valley: You’ll find a lot of expansive soil.
- Bullhead City: Close to the river, you’ll find everything from sand-and-gravel soil, which is a good, solid-bearing material. You’ll also find dense clay that can lift a house seven or eight inches.
- Luke Air Force Base, out on the west side, has very expansive soils.
- Tucson: Like in Phoenix, in the lower areas down in the valley, you’ll find expansive soils. But in the Catalina Foothills, the edges of Mount Lemmon, there are silty, sandy deposits that can be collapsible.