One thing is that “underground” is not a homogenous single thing. Sometimes it is loose water logged sand, or clay, or gravel, sometimes it is solid hard rock, sometimes it is large very hard pieces of fractured hard rock forming a loose rubble. Which kind of obstacle are you thinking when you are thinking of your method of digging? Based on your method description it sounds like you are thinking of loose soil with a few big rocks?
> The silicon arms would be full of actuators that measured their resistance in terms of the momentum they want plus the gravity weight of any nodes after them.
That sounds very complicated. Actuators are expensive. Actuators which are strong enough to drill through stone are even more so. Having many of them per arm and many arms per machine sounds very expensive and also a maintenance nightmare. Look at a real world tunnel boring machine: they have a cutting head rotated around by a single electric motor and hydraulic jacks to keep the cutting head pressed against the formation. It is conceptually simple, even though of course real world constraints make it complicated in practice. You are proposing to replace that conceptual simplicity with something much much more complicated. It is not clear what benefit you are hoping to achieve with the complexity you are thinking of.
> tunnels in every city on earth so we didn't have to destroy woodland to build suburban cities at such a gorgeous rate
How would that work in practice? Would people live in tunnels under a pristine forests? I’m not sure i understand your concept.