What are the issues if your house is on top of an active, deep, slow moving landslide? I had a conversation with a geotechnical engineer from Alan Kropp and associates. He said that they often specify 18-30″ deep concrete rafts underneath the entire house. This way the house can ride the slide atop the raft and not get pulled apart differentially. They also recommend flexible utility connections. (If the landslide is shallower, the foundation design usually requires piers down to bedrock, impossible with such a deep slide) Probably only about 10% of the structures in Berkeley built on landslides have this type of foundation underneath. Most have fairly typical foundations. These houses can experience periodic cracking and shifting problems. Conventional construction techniques aren’t really designed for these sorts of movements….but the damage can usually be repaired.
Beyond the massive expense of creating these concrete rafts, there are other complications that arise when an entire hill filled with houses is moving slowly downslope. Here is an article that describes some of the legal mess that can occur. The basic problem is that the property lines are not moving. They are fixed, but everything else is moving. Because of this some buildings have crossed the property lines, and most houses have setbacks that are getting bigger on one side and smaller on the other. Your driveway might eventually end up on your neighbor’s property. What a legal fiasco!