>>7749860
>how do they overcome the extreme ground pressure at depth
The temperatures at the depths they are forging these tunnels are not so high that the rock's plasticity is high enough for the rock to behave like a hydraulic fluid. It's solid enough that you can punch a hole through it without the overburden flattening the tunnel out right away–but there is some downward and inward pressure, and that's part of why the machine exerts immense outward pressure as it goes.
>cooling issues
The support systems following the TBM include everything required for bringing the work site and completed tunnel linings to the necessary temperatures. As you might imagine, just like with a nuclear reactor for any other purpose, cooling is one of the most important and costly concerns. They exchange heat with the air outside the system using a liquid ammonia refrigeration piping system being pumped in and out at very high pressure. The entire TBM operation is incredibly fucking dangerous, should anything go wrong.
>encountering oil or gas
Many strata have oil and\or gas trapped in them, and this isn't a huge problem for the TBM, as it can be managed with proper venting to the outside of the tunnel system. Hitting a larger pocket or a natural cavern or other void with the TBM would be a titanic disaster for the project and indicative of a massive engineering and planning failure that would likely cost the lives of everyone operating inside the system at the time of the breech. Basically, losing pressure due to the TBM breaking into a void would destroy the TBM, and if the void contained a massive quantity of oil, gas, or another fluid, that fluid would enter the TBM tunnel and that would be a catastrophe like you cannot imagine.