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Underground Rail Doubles Capacity in Auckland

By Catherine A. Cardno, Ph.D.

Preparatory work is under way in Auckland, New Zealand, on an underground line that will significantly expand the city's rail system.

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The City Rail Link will be built beneath Auckland, New Zealand, and will include two new stations. The Aotea station will be at a depth of 11 to 13 m and will be constructed along Albert Street in the center of the city through a cut-and-cover operation. Auckland Transport

September 9, 2014—In New Zealand, property owners own the ground beneath their homes—all the way to the center of the earth. As a result, any tunneling that takes place must be preceded by the purchase of the land that the route will take. This is precisely the formidable task that now faces Auckland Transport, the agency responsible for all of the region's transport services, as it conduct preliminary work for a new, underground rail line in the city that will be built at depths reaching 33 m.

Known as the City Rail Link, the electrified route will include two 3.4 km long tunnels that will extend beneath the center of Auckland, New Zealand's largest city and the nation's main transportation hub. Once complete, the City Rail Link will double the capacity of the city's existing, ground-level rail system, taking it from 15,000 riders an hour to 30,000 riders an hour, according to Auckland Transport's website. The implications of this growth are significant for residents of the metropolitan area. Most riders will be able to commute to the city center in just 30 minutes, a capability that is expected to improve economic opportunities for residents and employers alike, according to Auckland Transport.

"The City Rail Link is New Zealand's largest capital project and its most complex infrastructure project," said Carol Greensmith, a spokesperson for Auckland Transport, who wrote in response to written questions posed by Civil Engineering online. "It is Auckland's number-one and largest transport and economic development project, [and] it will be New Zealand's first underground passenger rail."

The planned 3.4 km line will connect the Britomart Transport Centre, the terminus of the current rail system, with an existing station at Mount Eden. Two new stations will be added along the new line: the Aotea station, which will be at a depth of 11 to 13 m and will be constructed along Albert Street in the center of the city through a cut-and-cover operation, and the Karangahape Road (also referred to as K' Road) station, which will be located at a depth of 33 m. The station at Mount Eden will be redeveloped to connect to a new platform that will be constructed at a depth of 7 m through a cut-and-cover operation for those using the City Rail Link, according to Auckland Transport. 

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The underground line will include two 3.4 km tunnels that will extend beneath the center of Auckland from the Britomart Transport Centre to an existing station at Mount Eden, which will be expanded. Auckland Transport

Constructing a rail line beneath a highly developed urban area is always a complicated task and can be made more challenging by unfavorable ground conditions. "One of the significant challenges for the designers is the inconsistent ground conditions for tunneling," Greensmith noted. She expects that the construction team will encounter reclaimed ground with a high water table at one end of the project and basalt at the other. "This requires the adoption of cut-and-cover, top-down, mined, and tunnel-boring machine (TBM) tunnels to address the ground conditions, all within the densely populated city center," she explained.

The TBM that will be used for the City Rail Link has a diameter of 7.5 m, which is about half the size of the one used for Auckland's Waterview Connection, according to Auckland Transport. 

The changes in depth of the tracks also present a challenge; the tracks "need to climb over 70 m in 3.4 km of track length to connect the existing terminus station to the North Auckland line, leading to track gradients that are towards the maximum levels achievable with the chosen rolling stock," Greensmith explained. 

Auckland today has a population of about 1.5 million, and that number is expected to increase by more than 700,000 over the next 30 years, according to Greensmith. This growth raises the importance of rail expansion. "The current train network will be at capacity in less than a decade, and bus arterials will be at gridlock, with city-center speeds down to a walking pace if the City Rail Link isn't built," Greensmith said. "The City Rail Link needs to be built before that happens."

Removing what Greensmith calls the "dead end" at Britomart will enable trains to travel in two directions as well as around Auckland, and that, together with the new stations, will double the number of people who can reach the city center within 30 minutes, she said. "Auckland is built on an isthmus, and road-based traffic ultimately needs to converge, causing severe congestion on key city access points," Greensmith said. "Making the train system more accessible, functional, and reliable has been identified as a key element toward managing Auckland's future transport demands."

The expansion will also facilitate additional growth of the rail system in the future. "Without the City Rail Link, there is no ability to add other rail lines to other parts of the city," Greensmith noted. With the construction of the two tunnels and new stations, the groundwork will be laid for additional lines should they become necessary.

The project is anticipated to cost NZ$2.86 billion (U.S.$2.38 billion) in inflated 2021 dollars, which includes the cost of the land, the construction, and the rolling stock, according to David Warburton, the chief executive officer of Auckland Transport, in an interview about the project that is available on its website.

The rail expansion is currently in the early design phase, and detailed design and construction are expected to begin in 2015, according to Greensmith. Funds for purchasing the land for the route have been provided by the Auckland Council, according to Warburton, whereas funding for construction is expected to come from a variety of sources.

More than half of the surface parcels of land needed for the route have already been purchased. An underground survey has been undertaken, and it is expected that the purchase of the subterranean parcels will be completed next year.

The target completion date of the expansion is 2021, according to Auckland Transport.


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