A form of the New Austrian Tunnel-ling Method (NATM) developed by Mott MacDonald and Morgan Vinci, known as Lasershell, was used for the approximately 500m of non-bored tunnel on the two rail extension projects – cross passages and connections – speeding construction and improving worker safety.

The tunnel face is inclined to reduce the risk of face collapse. There is no lattice reinforcement system to install, which makes the method faster than conventional NATM, and the lining is installed as a single layer of sprayed concrete. The method gets the full structure in place very rapidly. Laser sighting is used to monitor and control tunnel geometry during excavation and lining.

This technique compares with the traditional method of advancing the tunnel in stages. Conventionally an initial heading of 3m-4m is dug at the top of the tunnel face. This is lined with sprayed concrete. The heading is then deepened and the lining extended downward. Top and bottom headings are then extended before the lower portions of the walls and tunnel floor are excavated and lined.

With the traditional staged approach to excavation, men and machines are required to operate within sections of tunnel that are incomplete, as the tunnel lining only gains integrity when the tunnel profile is fully complete and the concrete lining has cured. With the Lasershell approach, by contrast, excavator and concrete gun both operate from under the protective canopy of a complete portion of tunnel lining.