New Zealand’s ambitious tunnelling project

Auckland Council’s Watercare is undertaking one of New Zealand’s most ambitious tunnelling projects by constructing a 13 km wastewater tunnel at depths of up to 100 m underneath the city.

An additional 4.4 km of link sewers will also be constructed as part of the project, with the trenchless method of microtunnelling likely to be used.

After completing the detailed design, Auckland’s water utility Watercare is in procurement phase for one of the
New Zealand’s largest tunnelling projects, the Central Interceptor. The project will see the construction of a 13 km long wastewater tunnel, with an internal diameter of 4.5 m, as well as 4.4 km of link sewers that range between 2.1 and 2.4 m in diameter and a terminal pump station with 6 m3/s capacity.

Project

The tunnel will run between Western Springs and the Mangere Wastewater Treatment Plant and will be constructed at a depth of between 22 and 110 m. It will cross Manukau Harbour approximately 15 m below the seabed and will connect to the existing wastewater networks along the route, which will divert flows and overflows into the tunnel.

The project is needed to ensure there is sufficient capacity in Watercare’s network to meet planned population growth and developments in Auckland. Without this, more overflows into the waterways and harbours will occur.

In heavy rain, the wastewater network can overflow to the Waitemata Harbour at more than 100 points and to the northeastern part of the Manukau Harbour at 14 points. The Central Interceptor is expected to reduce the annual average overflow volume by 80 per cent.

One part of the existing network, the lower section of the Hillsborough Tunnel and Manukau Siphon, is estimated to have a residual life of between 15 and 25 more years. If this fails, it could mean continuous untreated wastewater discharge from more than 200,000 customers into the Manukau Harbour for an unknown period of time, including industrial flows presently treated at the Mangere Wastewater Treatment Plant.

Detailed design

The detailed design for the project has now been completed and, on 20 October 2017, an expression of interest was issued that will result in a shortlist of contractors who can tender for the works during 2018. Physical works will commence in 2019 and the project is expected to be completed in 2025.

Watercare’s Stephen Grace has acted as the client-side Design Manager on the project for the last three years, working with the design consortium of Jacobs, AECOM and McMillen Jacobs. Mr Grace’s role has involved reviewing all of the deliverables that comprise the detailed design, which included more than 1,800 drawings, 200 technical memos and specifications, and the contract documents.

“The project will enhance network capacity to allow for growth in Auckland City and increase the resilience of the network by duplicating sections of it,” says Mr Grace.

“It will also reduce wet weather overflows from the combined wastewater network in the older parts of Auckland.”

Tunnelling and microtunnelling

He says that as a result of the size and depth of the tunnel, it has been determined that a tunnel boring machine (TBM) is the best method for the construction of the Central Interceptor. It is expected that the link tunnels will use a microtunnel boring machine (MTBM).

“The size of the wastewater tunnel and depth determined that TBM tunnelling was a good option,” says Mr Grace.

“The shafts are up to 110 m deep and are spaced 1 km to 4 km apart, so they were beyond the capacity of a microtunnelling pipejack. The tunnel is deep to avoid the near-surface basalt rock that characterises the Auckland volcanic field.

“Auckland is sometimes referred to as the ‘City of Volcanoes’, with over 50 scoria cones.

“We expect the 4.4 km of link sewers will be constructed by MBTM using pipejacking techniques. Additionally, there is over 1 km of smaller pipes, under 1 m in diameter, that connect the tunnel and link sewers to the existing wastewater network.

“Some of these connection pipelines are well-suited to trenchless methods.”

The TBM specified for the project will be earth pressure balanced (EPB) machine that can tunnel through the typically weak sandstones that lie under the volcanic flows that stretch across central Auckland. Watercare has undertaken extensive geotechnical investigations, which include more than 200 boreholes.

The EPB machine will need to handle the groundwater pressures (particularly under the Hillsborough Ridge, where the deepest section is 110 m below the surface) and will minimise the risk of settlement where the tunnel passes under buildings, highways and railway lines. Mr Grace says the project won’t be without its challenges.

“The initial launch of the TBM presents some challenges as the mixed ground conditions at the southern-most point of the tunnel are in alluvial and marine sediments. This could require ground improvement techniques to launch the TBM,” he says.

“Under the Manukau Harbour, the tunnel passes under the existing Western Interceptor pipe that may be fragile. And in the link sewer pipejacking, we have one drive length that is almost 1 km long and around 66 m depth to invert.

“The tunnel is very deep through most of its length, so the impact on properties above will be negligible. The main tunnel and link sewers will be driven under 1,085 properties, so Watercare has undertaken extensive consultation and prepared materials to explain why the project is needed and how it will operate.”

Watercare is also taking extra steps as the tunnel crosses under Manukau Harbour.

“We will monitor the health of the Manukau Siphon before, during and after the harbour section is constructed,” says Mr Grace.

“We also have to avoid power pylons on the northern foreshore, so the tunnel alignment was moved to avoid the footings in the mud flats. The tunnel will be 15 m under the floor of the harbour, which was determined by a bathymetry survey, and 23 boreholes along the corridor.

“The segmental lining has been designed to withstand the ground and hydrostatic loads, with conventional reinforcement south of the harbour and fibre reinforcement through the subsea section and north of the harbour.”

The Central Interceptor is one of several projects Watercare is undertaking using Trenchless Technology and tunnelling methods.

“We are currently constructing a number of trenchless projects of size, including the Army Bay outfall and Wairau rising main. Army Bay is using Herrenknecht’s Direct Pipe, while Wairau is using a Herrenknecht AVN MTBM,” says
Mr Grace.

“The Glen Eden rising main was also installed recently with a Vermeer Axis machine. A lot of smaller water and wastewater pipelines across our network are regularly installed using horizontal directional drilling methods.”

This article was featured in the March edition of Trenchless Australasia. To view the magazine on your PC, Mac, tablet, or mobile device, click here.

For more information visit the Watercare website.

If you have a project you would like featured in Trenchless Australasia contact Journalist Chloe Jenkins at cjenkins@gs-press.com.au

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