Firstly, keep in mind that bedrock isn’t solid rock as most people seem to think, but any form of soil and/or rock that has developed cohesive forces within its structure.
Secondly, you pretty much need tunnels to be going through bedrock otherwise you need to treat the soil as if it was a liquid.
Most of Los Angeles probably has bedrock between a couple of feet below the surface to about 20 feet deep. You are going to have pockets much deeper at valleys, beaches, or any other courseway.
MKDesign 