There are two main problems with increasing depth: Rocks and water. Rocks are the biggest. The ratio of rock to finer sediment increases with depth until bedr

There are two main problems with increasing depth: Rocks and water. Rocks are the biggest. The ratio of rock to finer sediment increases with depth until bedrock is reached. This might be anywhere from 1 to 1000 meters depth but in most cities it's probably <30 meters average (less in hilly parts of town, more in river valleys etc). Bedrock can be bored through and is quite strong, but it's slow and may be more expensive (although it could cut down on regulatory and post-boring structural reinforcement costs quite a bit). Mixed soft sediment and large boulders are often trickier to deal with--the boring rate can vary wildly, and it's difficult ta maintain the proper rate balance between cutting and sediment removal. Remove too slow and things clog, remove too fast and the pressure becomes too low and voids can develop, which are then filled in by overlying sediment as the ground above collapses a bit. Shallow seismic techniques could certainly be used for imaging/boulder detection some number of meters in front of the borer if it was built in right. But I don't know how useful it would be. Can they steer or reroute? Probably not much... Water is a lesser concern but is still important. The water table is pretty shallow (<10 m almost everywhere) and tunnels need to be sealed. This is pretty straightforward, but I really wonder about LA during an earthquake with a deep transit tunnel (especially in winter when the water table is higher). What if a long tunnel's walls are seriously breached? Could it flood and drown passengers who may be blocked by debris etc?