I'm not a construction engineer by any means but there are a few things I can't warp my head around. Why are they using robots at all? It seems like a pretty big hassle to construct those robots that deposit large amounts of material with high pressure through a hole that needs to be drilled in their own tunnel. Why not excavate the volume between the small boring holes with the same technique they used to create them in the first place? Bore those smaller holes with equal spacing, fill them with concrete, and then repeat for the volume in between. What about steel? Not needed for in tunnels?