The CNC'd shell for the tunnel, they can be in slices, interlocking,

so to be able to pass through the completed shell (in diameter)

 

so say 1/4th the size of the entire diameter, and being completed

little by little. With cement. Or some other construction aggregate.

 

Finish off with hydrophobic coating. But there should be good enough

combination with dirt and cement = so CNC can work with it without cracking.

 

Another way would be... As mentioned some time ago on the construction method,

metal frame first, then earth+construction aggregate shell over it,

to bind with cement. Instead directly from CNC to earthen cubes.

 

The earthen shell wouldn't be CNC'd then. But pressed from pre-existing design.

More expensive this way, but higher structural integrity...