How HS2 build tunnels
A green tunnel – or cut-and-cover tunnel – is where a trench is excavated and roofed over, then the land on top of the tunnel is restored so it blends into the landscape. This technique will be used by HS2 for the following tunnels; Wendover in Buckinghamshire, Burton Green in the West Midlands, and Chipping Warden and Greatworth, both in Northamptonshire. The tunnel at Long Itchington wood, in Warwickshire, will be built using both green tunnel and bored tunnel techniques.
A twin-bore tunnel is where two parallel tunnels, each containing a single rail track, are constructed using tunnel boring machines. The rotating cutter-head at the front of the TBM bores the tunnel. The TBM also installs the round concrete segments that form the tunnel walls.
HS2 tunnels have large internal diameters. The trains will be travelling at high speed through the tunnels and larger tunnels help lower the pressure variations inside making it more comfortable for passengers.