Dirt that is deep underground is like a sponge that is soaked through with water which keeps it stable, and if the water is missing then it leaves behind air pockets which cannot support the weight of the dirt and buildings above it. Additionally, standing water from rain adds to the surface weight. In other words, when one is dealing with dirt that lacks groundwater then one should tip-toe as if walking on egg shells.
Just as ground water saturates dirt and supports the weight above it, so does oil underground, which is one reason why fracking for oil underneath people's houses is a really bad idea.
Homeowners in northern and central California can ask Pacific Gas and Electric ("PG&E") what kind of ground their home was built on top of. The field technicians who dig in the ground know what's underneath every neighborhood. For example, in San Francisco, when they are sent to dig on Potrero Hill they know that they'll be dealing with solid bedrock, when they go to the Outer Richmond and Outer Sunset district they'll be dealing with sand, and in the Marina it's landfill. Every neighborhood is different.