Water is the main enemy in expansive soils problems. There is either too much water, causing the soil to swell, or not enough, causing the soil to shrink.
Wet clay soil is the most serious threat since the swelling potential of expansive soil is much greater than the shrinkage potential.
Drought conditions cause the supporting soil to pull away from footings and foundation walls. Leaving them with less support then settlement and cracking of walls and footing can occur. Once the soil pulls away from the foundation walls, this leaves an opening wide enough for water to run in all the way to the footing. Now too much water at the footing causes hydrostatic pressure and interior leakage can occur.
Although reinforced concrete footings and foundations are strong, they are not flexible. With the seasonal weather changes and over the course of many years, it is not surprising so many homes have foundation problems.