Comment by duckymcduckface

Geotech is a bitch. Soils aren't homogenous and vary a lot in both composition and characteristics. There are multiple types of TBMs that are designed to operate under very specific conditions, for example a soft rock/soil TBM excavates through different mechanisms than a hard rock TBM. One of the big problems in any excavation is you have no idea what's actually down there until you start working in it. The underlying material can differ a lot over very short distances. You can very easily be minding your own business driving the TBM and run into a transition that will straight up mess you up. Combine that with all the previous mentioned stuff and you get a lot of cost overruns and time overruns. But yea, subsurface is a bitch.