Stabilising the tunnel is the hard part; anyone can dig a hole, but keeping that hole stable is what makes it a tunnel. The big innovation from Brunel and the a

Stabilising the tunnel is the hard part; anyone can dig a hole, but keeping that hole stable is what makes it a tunnel. The big innovation from Brunel and the ancestor of most modern tunnels was the tunneling shield [1], which provided an area to safely work ahead of the supporting structure. Modern Tunnel Boring Machines (TBMs) incorporate a tunneling shield, with the addition of a large machine behind the cutting head which builds the tunnel walls as it goes along. The innovation here is to build the stabilisation first and tunnel out the rest; it seems more akin to pipe/box jacking [2] in that sense, where you basically force the outer walls through the material using hydraulics, then excavate the spoil. Their demonstrations so far are at the scale where box jacking would probably make more sense, but if they can deliver on the promise of having effectively an arbitrary long tunnel then that'd make the technique more broadly applicable. [1]: https://en.wikipedia.org/wiki/Tunnelling_shield https://en.wikipedia.org/wiki/Tunnelling_shield [2]: https://en.wikipedia.org/wiki/Pipe_ramming https://en.wikipedia.org/wiki/Pipe_ramming