This is primarily a soil boring machine; some small boulders and rocks which get ground up (hopefully). But the soil down there is quite compressed and hard; I have heard that if you used a pick on it, it would seem like concrete. No problem for the cutters, though.
They thought the steel well casing they went through may have caused problems. But looking at the timeline, WSDOT was concerned by the elevated temperatures from the beginning. So perhaps a design flaw, or some problem at the very start. (Hey - did any body put the lubricant in those things?)
Last year in Seattle a 10-year old highrise had to be demolished because someone didn’t put a dab of silicone on the ends of the stress cables in the concrete and they started rusting.