KEY POINTS:
Four motorway and rail tunnels are being recommended as Auckland's main future harbour crossing for up to $4.1 billion.
Officials yesterday confirmed their preferred option for four deep "driven" tunnels between Takapuna's Esmonde Rd motorway interchange and the Auckland isthmus, running beneath the western side of Wynyard Wharf, as disclosed by the Herald last week.
They are recommending two three-lane vehicle tunnels running 3.9km under the harbour and Victoria Park before emerging to join the Southern and Northwestern Motorways at spaghetti junction, and a pair of single-track rail tubes in each direction for electric passenger trains.
Passengers could catch trains from an underground station beneath a redeveloped Wynyard Quarter, and the rail link would point towards the Britomart transport centre after running parallel to the motorway tunnels beneath the harbour.
The question of whether the railway lines would connect directly with Britomart or join a proposed underground loop for trains running beneath Albert St to Mt Eden has been left open.
Buses would keep using the existing harbour bridge, as would general traffic heading to and from central Auckland.
The bridge would also be available for pedestrians and cyclists, who are pushing Transit NZ to allow them early access, given that it may be 15 years before the tunnels are built.
Traffic using the bridge would arrive at Cook St via a separate "cut and cover" tunnel through Victoria Park, allowing the existing motorway viaduct over the park to be demolished.
The tunnels would become part of State Highway One, leaving traffic bound for the bridge in either direction to leave the motorway network via large off-ramps.
Consultant Richard Hancy - who headed a $1.3 million study for Transit NZ in equal partnership with Auckland Regional Council, its transport authority subsidiary, and Auckland and North Shore cities - confirmed his team's preference for bored tunnels rather than more disruptive "cut and cover" structures.
That would not only protect building foundations but avoid having to dig a trench across the harbour for "immersed-tube" tunnels. Mr Hancy, a former Transit regional manager, said the bored tunnels would be 40m below the harbour bed at their deepest point.
The motorway tunnels would each be 15m in diameter, but the rail tubes needed to be only 6m wide to fit electric trains requiring less ventilation than for diesel or petrol fumes.
A cost estimate ranging from $3.7 billion to $4.1 billion for all four tunnels is about $1 billion higher than assumed for a crossing in previous studies, which examined 159 options.
But it would be $1 billion less than a longer set of tunnels from Esmonde Rd to Grafton, an option which the latest study concluded would provide greater transport "resilience' by taking long-distance traffic further away from central Auckland, although less flexibility between the new link and existing bridge. Mr Hancy said the more direct route would, as well as being cheaper, allow a better distribution of traffic between the two.
Southbound traffic heading past central Auckland would be unlikely to use the bridge, as that would mean wending through city streets to rejoin the motorway.
Although a new bridge would be about $600 million cheaper than tunnels, the study team has ruled that out because of its visual impact on the harbour and surrounds such as St Mary's Bay and Westhaven.
Mr Hancy acknowledged a far cheaper option would be a rail-only tunnel crossing for $1 billion to $1.2 billion, leaving the motorway traffic on the bridge.
The study has indicated a possibility of building the motorway and rail tunnels at different times, although at a greater overall cost than digging them together.
North Shore Mayor Andrew Williams believes the rail tunnels should be dug first, to see whether enough commuters would flock to high-frequency electric trains to avoid a need for motorway tunnels.
"If you have electric trains leaving North Shore every five minutes, each taking 300 to 400 people, you are going to have a pretty dramatic impact on the bridge," he said.
Regional council chairman Mike Lee last night welcomed the study's conclusions, expressing relief that the preferred option would avoid disruption to the Wynyard redevelopment while allowing for an underground station to serve the precinct's growing residential and commercial population.
Auckland City transport committee chairman Ken Baguley said he remained attracted to the longer link to Grafton, but accepted that might prove unaffordable.
