The London Underground, Paris Metro, New York Subway ... in the not-too-distant future, Auckland will be able to add itself to the list of cities offering underground rail to tens of thousands of passengers each day.

By 2024, all going well, the 3.5km City Rail Link from Britomart to a new above-ground station at Mt Eden will be carrying up to 54,000 passengers an hour to and from the central city from most ends of Auckland. Later, rail could run to the North Shore on a new harbour crossing.

The CRL will save 17 minutes by train from Henderson to Aotea, a new underground station built between Victoria and Wellesley Sts in the central city. A second underground station will be built at Karangahape Rd with two entrances.

Only in Auckland

Herein lies one of those "only in Auckland" stories.

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In 2015, cost pressures meant the board of Auckland Transport decided to build a single entrance for the Karangahape Station at Mercury Lane while "future proofing" a second, deeper and more convenient entrance at Beresford St to shave $30m off the then $2.5 billion cost.

Last year, City Rail Link Ltd (CRLL), the Government/council joint-venture company managing the project, recommended overturning that decision when it became apparent the original projections for use appeared inadequate.

City Rail Link tunnel under construction. Photo / Jason Oxenham
City Rail Link tunnel under construction. Photo / Jason Oxenham

CRLL discovered the underground rail lines were going the same way as the Auckland Harbour Bridge, which was built with four lanes and soon required the "clip-ons" to take it to eight lanes.

In the case of the CRL, it was being planned for a capacity of 36,000 passengers per hour by 2045, but rapid growth in rail meant the date was rushing forward to 2035.

On the recommendation of CRLL, the Government and Auckland Council decided to lengthen the platforms to take longer trains and for a second entry at Karangahape Station. The new capacity is 54,000 passengers per hour.

As a result, the CRL is going to be bigger and better and cost more. Council sources have told the Herald revised estimates for the $3.4b project have increased by more than $500m and "there are fears it is going to be over $1b" by the time the main contracts are awarded this year.

Where is the project up to?

Work continues on the City Rail Link as the tunnels start to take shape under Albert St. Photo / Greg Bowker
Work continues on the City Rail Link as the tunnels start to take shape under Albert St. Photo / Greg Bowker

Anyone who has walked along lower Albert St, or worse, tried driving, will be familiar with the deep trench, braced with three-tonne steel capping beams to prevent it falling in on itself.

This has been followed by hundreds of trucks pouring concrete for the floor slab, then building a shell and twin rail tunnels for the trains to run along and finally backfilling and re-establishing the street above. The job moves along in sections and is more than 80 per cent complete.

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At Britomart, the former Central Post Office Building, built of Oamaru stone and Coromandel granite, is undergoing a second structural overhaul after being refurbished 17 years ago to become the city's main railway station.

"This is really challenging work," CRL construction manager Scott Elwarth says.

"It's hard to do in greenfield sites, but this is the next level".

The station is sitting on massive steel beams and columns. Piles in the path of the tunnels running north and south have been cut out and left hanging in thin air while, deep below a vast underground hole, sections of the reinforced concrete floor are being laid.

In the basement of the towering Commercial Bay project, two 100m-long train tunnels were completed last month, curving a path from Britomart and under Customs St to Albert St.

CRLL chief executive Dr Sean Sweeney says work on the two contracts between Britomart and Albert as far as Wyndham St is tracking well and due to be completed at the end of the year.

Tough times

City Rail Link boss Sean Sweeney at the tunnels under Albert St. Photo / Greg Bowker
City Rail Link boss Sean Sweeney at the tunnels under Albert St. Photo / Greg Bowker

Sweeney said 2018 was a hard year, not on the construction front, but because of losing Fletchers as one of the two preferred bidders and having to find another company to enter the bid process. There was also the "fire storm" when the company awarded the systems contract, RCR Infrastructure (NZ), went into voluntary administration.

He believes those problems have now been resolved by two consortiums bidding and rolling the systems contract into the main contract.

The main contract is in the final stages of a tender process with two shortlisted tenderers. One is the Link Joint Venture (Downer, Vinci Grands Projets, Soletanche Bachy, AECOM, Tonkin and Taylor). The other is WSP Opus and a joint venture between CPB Contractors, UGL, Beca, McMillen Jacobs Associates and Jacobs.

The next steps

Work continues on the City Rail Link under Albert St. Photo / Greg Bowker
Work continues on the City Rail Link under Albert St. Photo / Greg Bowker

How and when the main tunnels get built is a matter for the successful joint venture, but the arrival of the boring machines will mark a key moment in New Zealand's largest infrastructure project.

The CRL eclipses New Zealand's major dam-building projects, such as the Clyde Dam, says Sweeney, an engineer who has worked on Te Papa and major public infrastructure projects in Melbourne and Sydney.

"It has still to be settled whether the boring will start at Mt Eden but I have heard it said the logical preference is to have the tunnel-boring machines going downhill," he said.

The tunnels themselves were relatively easy to build and not as complex and time critical as the two new underground stations. The more difficult and high-risk job was installing the rail systems for controls and signal, safety and fire, Sweeney said.

"The system is bespoke because it is connecting into a live operating network and no-one can foretell or foreshadow the complexity of doing that. It is vexing our minds," he said.

Whatever methods of construction are used, "there will be a lot of disruption", Sweeney said. Huge volumes of dirt have to be removed by trucks and building materials brought in.

Image / NZ Herald
Image / NZ Herald

In Mt Eden, bridges will be relocated and roads realigned. Aotea station will be built the same "cut and cover" way as the tunnels at the city end, excavating down to build the station.

The new Karangahape station is on two restricted building sites 35m below ground. The tunnel-boring machines will bore two holes to be mined out for a big cavity to form a station.

Sweeney said CRLL was tracking for completion of the tunnels and stations by late 2023 before the systems are installed and verified.

"We are looking at the moment at late 2024 [for completion].

"The benefits are going to be massive for Auckland ... it is really going to enable Auckland to grow and develop as an international city," he said.

The series

• Today: City Rail Link
• Tuesday: International Convention Centre
• Wednesday: Newmarket shopping centre
• Thursday: Auckland International Airport
• Friday: Commercial Bay
• Saturday: America's Cup Village Development