Take a look at the video below to see a saturated soil in a liquid state. This phenomenon is liquefaction. When certain soils (granulated/sandy) become saturated with water, it behaves like a liquid (see infographic). Imagine putting a structure on this, and then imagine an earthquake. It doesn’t bode well for the structure.
Geotechnical analysis is needed to identify the soils on site and remediation/mitigation of the soils by removal and replacement, or changing the foundation depth to a better soil strata, or drilling down to bedrock are best to support structures with sites with soils susceptible to liquefaction. Controlling runoff and water flow to these areas is essential as well.
A lot of southern California soils are susceptible. A geotechnical report by a certified Geotechnical Engineer or Geologist is always recommended for development.