Elon thinking about something like this:
Elon Musk on Twitter
Makes sense as always. Weld from the inside. Welder would rotate rather than the tube. Increases speed at the from end but also simplifies logistics - no concrete to get to the front alongside the spoil going out.
The question regarding spiral welding Super Heavy was raised by KarenRei at the beginning of the year.
Twitter
He mentions in that tweet that the hyperloop is spiral welded. That one was made in free space which is easier
For a positive pressure tunnel, they will still need to pump slurry to the front to seal and fill the tunnel to excavation gap. Pumping is easier than solid segments though.
Elon mentions reinforcement which may relate to the rebar in the segments versus the tunnel itself.
If he is talking rebar, that is also interesting. Feeding and bending rebar is much simpler that sheet stock. Concrete pipe uses a similar technique with rebar on the long axis and rewire spiral welded along the length.
In this setup, they could feed the rebar, bend into shape, then cast the whole thing in place. It might make the most sense to do the inner tunnel seperate from the slurry fill due to material requirements and access. I'm not sure if the cure tine and process control lines up with tunneling speed goals. However, by gripping more cured segments further down the tunnel, they can trade mechanics for cure time.
Continuous spiral welding of a tube in place is possible with the difficulty dependent on thickness and width. (Bend radius/ feed angle/ uncoiling). Curves would requires shaving or corrugating if doing edge welding. If doing overlap (telescoping) then that is easier. The shield can provide a gap for access to the back side of the weld. SpaceX uses friction stir welding which might be an option and works well with overlap.
Possible hybrid approach is steel sections. The sections could be one piece of tunnel diameter size sent down the tunnel with the ends overlapping (so as to fit). The piece would then be threaded around the TBM push rods which engage with features on the previous segment. Once in place, the piece is expanded and welded to itself and the previous section. With these, they can pre cut the segments with a taper. That taper allows the tunnel to curve based on rotational orientation similar to HVAC ducting.
They could feed and cut these sections in place. That eliminate the issue of rotating the feed material around the circumference since they can pull one or two wraps off at a time with a free end, versus a captive end and moving the coil.
Steel tunnels of various designs are used in immersesed environments:
https://www.sciencedirect.com/topics/engineering/concrete-tunnel
Check out the Bai Yun paper.
The outer concrete slurry would provide protection, and they can add a protective coating while the steel is still inside the shield.
(Grain of salt, not a tunnel engineer)
