I typed up a rant about boring vs drilling and the correct terminology but it sounded super nerdy after reading it. I'm not really qualified to answer your que

I typed up a rant about boring vs drilling and the correct terminology but it sounded super nerdy after reading it. I'm not really qualified to answer your question as I'm not as intimate with the machines as I am with the ground/structure side. Most likely the seal is maintained by the machine being too big for the hole being cut or by inflatable packers. The walls of the bore can range from extremely smooth to not smooth at all depending on ground conditions. Sorry.