What were engineers expecting to bore through? I’d say it’s something the cutters weren’t designed for. If engineers were expecting to bore through sedimentary rock but now find themselves confronted by a dome of igneous rock, that might explain it.
They are drilling in glacial till. Sands and gravel and clay that have been compacted by the ice to form something very similar to concrete. Although every so often the glacier will drop a huge boulder that it carried down from Canada - so not a dome of igneous rock, but it probably is a huge boulder of igneous (and very hard) rock.
Pure diamond is about the only thing I can think of that could stop Bertha. It'd have to be the biggest damn chunk of diamond the world has ever seen.
That thing should do igneous, albeit slowly. It may be that it has the wrong teeth mounted on it for lots of igneous though (that was run into at a tunnel project nearby, and the machines needed to have something like 1500 teeth each switched out at ~$750 apiece).
If it ran into something like a big ole nickel iron meteorite, now THAT would be cool... Or a monster hunk of native copper, the maleability of which is a b***h on boring and cutting tools I hear.