It does "sound" expensive, but so does separating out all the steel beams from the concrete of a tunnel. That kind of thing is pretty labor intensive and people

It does "sound" expensive, but so does separating out all the steel beams from the concrete of a tunnel. That kind of thing is pretty labor intensive and people cost a lot of money. If you have a big enough grinding machine, it may well be cheaper overall if it only needs a single operator.