What's below Auckland's busiest arterial road?

26 June 2018

WSP: Opened in July 2017, the Waterview tunnel, a twin road tunnel in west Auckland, is New Zealand’s longest road tunnel.

With an extensive planning history, the project has had the greatest impact on how Auckland connects, travels and moves since the opening of the Auckland Harbour Bridge in 1959.

While the NZ
Transport Agency’s project to connect Auckland’s Northwestern and Southwestern Motorways first appeared in transport plans for the city in 1954, complex geotechnical ground conditions and geotechnical constraints underneath the country’s busiest arterial road, Great North Road, meant that it took more than sixty years for it to come to fruition.

Building a tunnel link that could traverse the challenging environment while ensuring minimal disturbances to community activities above was crucial to the Well-Connected Alliance tasked with delivering the project.

Covering a Million Years of Ground

The geology of Auckland is heavily impacted by its volcanic roots. The Waitemata Basin, formed between 24 and 18 million years ago features a thick sequence of weak siltstones and sandstones called the East Coast Bays Formation. Tectonic uplift in the last 18 million years has resulted in faulting, folding and erosion while the eruption of Mt Albert near the tunnel construction site more than 30,000 years ago filled low lying valleys and gullies with basalt.

The Waterview Tunnel alignment itself passed entirely through a unique ground sediment made up of sandstone and siltstone commencing from underneath a basalt lava flow at the southern end of the alignment. Towards the northern end, the tunnel passed at a shallow depth of 8 m below a busy arterial road.

Dr Doug Maconochie, WSP’s Technical Director of Tunnels and Tunnel Design Manager for Waterview says,”Excavation required the use of a Tunnel Boring Machine (TBM) that could operate in all possible ground conditions and resist potential high ground inflows, while limiting possible ground settlement to within consented limits.

“The TBM used was the 10th largest in the world. It was designed specifically for Waterview’s challenging conditions.”

Unique Approach to Excavating the Tunnel

Named Alice, the TBM was 87 m long, weighed 2400 tonnes and featured a 14.4 m diameter rotating cutting head attached to the front of a 12 m long shield. To construct the project’s twin road tunnel, Alice had to be turned around 180 degrees at the end of the first drive in order to drive the second tunnel returning back to the southern portal.

“Turning Alice around to build the second tunnel was one of the project’s most technically challenging tasks owing to its size and weight, combined with restricted access below and above ground, with the construction site bordered by the Great North Road and Waterview Primary School,” explains Dr Maconochie.

“Moving a TBM of Alice’s size is considered rare – the project attracted widespread media interest internationally.

“At times, the clearance was only a few centimetres. To achieve the turn, Alice and her back-up gantries had to be separated into four pieces.

We covered the shaft floor with greased steel plates, jacking each of these sections onto steel cradles. Jacks were then anchored around the shaft perimeter to manoeuvre each piece of Alice into position for the second tunnel.”

The decision to use Alice proved to be the best option, supporting on-time and on-budget delivery of the tunnel design.

“We were able to manage the project’s geotechnical risks while also helping to minimise costs and programme risks,” concludes Dr Maconochie.

 

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