Thirty-plus years ago a guy developed an electrical sensor that could measure the differences in soil types and used parts on the TBM bit. Basically a fancy resistance meter. It could tell if they were in soft or hard soils, and even where a boulder’s general location was on the bit. It would also be a very good method to see if they were grinding against metal I would imagine.
I would be surprised if they didn’t have something like that. But I haven’t heard anything like that in the news.
Makes sense to me...Could also measure the thrust load on the bearings...After thinking about it, they likely have some mechanism to measure the rpm of the cutting head...Seems that the cutters are in groups and they rotate within the cutter plate which itself rotates...I'll have to look one up on the internet...