The CRL will use twin 3.4km long tunnels up to 42 metres below city centre streets.
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 will be built by installing retaining walls to retain the soil and stop water from entering the site. Tunnels will be constructed, then the concrete floor, walls and roof will be 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 will be 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 will be used when the tunnels reach the station box or when the tunnels diverge.
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