The 3.45km twin-tunnel underground rail link involves tunnels up to 42 metres below city centre streets.
It will unlock our public transport network and transform the way we travel around the city.
The alignment of the tunnels generally follows the roads to minimise the amount of private property affected by the project.
Due to the gradient of the landscape between Britomart and the western line at Mt Eden and the limiting climbing capacity of the electric trains, the twin tunnels will vary in depth.
At each end of the tunnels (Albert Street and Mt Eden) it will be shallow enough to construct the tunnels using the ‘cut and cover’ method (building from the surface).
Cut and cover tunnels are built by installing retaining walls to retain the soil and stop water from entering the site. Tunnels are constructed, then the concrete floor, walls and roof cast and the completed structures backfilled. When the tunnels reach Mt Eden, they will appear at the surface as ‘dive’ structures (like New Lynn Station).
Where the tunnels are deeper, they are constructed using a 7m diameter tunnel boring machine (TBM).
TBM projects require large backup areas to get the excavated material out of the tunnel and the supplies in, including precast lining sections. The backup area will be located at the Mt Eden end rather than Albert Street. For that reason, both tunnels will likely be excavated from the southern to the northern end.
Road header machines are used when the tunnels reach the station box or when the tunnels diverge.
Tunnels under Commercial Bay
An agreement was reached with Precinct Properties for the CRL tunnels to be built below and through their Commercial Bay development, adjacent to the Britomart Transport Centre in Lower Queen Street.
The land was already designated for rail purposes and owned by KiwiRail.
Precinct’s new business, entertainment and retail destination known as Commercial Bay is on the site of the former Downtown Shopping Centre, which opened in 1975 and was demolished in 2016.
The twin-tunnels emerge from Britomart Transport Centre (the former Chief Post Office), under Lower Queen Street and through the Commercial Bay development, avoiding the existing Zurich building, before proceeding up Albert Street.
The Precinct development is by Fletcher Construction and is geographically separate from the City Rail Link contracted works. The two shallow curved tunnels are underneath the 39-storey commercial office tower which is a major part of the Commercial Bay development.