Giant borer makes light work of sewer pipeline

By Bernard Orsman

It looks sci fi, but it has a very basic purpose. For six months now, a boring machine has been drilling a 3km-long tunnel beneath Orakei and Hobson Bay to replace the ageing sewer that crosses the bay.

The $118.6 million Project Hobson is a complex engineering feat that will eventually see the 90-year-old Hobson Bay sewer removed in 2010.

More important, the new underground pipeline will create environmental benefits.

On an average day the underground pipe, 3.75m in diameter, will carry about 3200cu m of sewage an hour. But during high rainfall, it will be capable of carrying 13,000cu m of sewage and water every hour.

Project manager Mike Sheffield says the tunnel does two jobs.

"It's not just about replacing the old pipeline, which is at the end of its economic life. It's also providing significant storage capacity for high-flow periods, such as during a storm, which will virtually eliminate overflows into Hobson Bay."

Another plus will be increased recreational opportunities. Groups such as the Auckland Rowing Association see opportunities for rowing, kayaking, dragon boats, waka, small yachts, outriggers, Sea Scouts and swimming.

Right now the tunnel is halfway across Hobson Bay at a depth of about 25m.

The boring machine - named Te Kaha, meaning "the strength", by the local iwi Ngati Whatua o Orakei - is on track to reach the Parnell end of Hobson Bay at Logan Tce in February.

Tunnelling began under the Orakei Ridge at Orakei Domain.

The 270-tonne machine does all the tunnelling work in one pass; drilling through rock at one end while lining the tunnel with precast concrete segments at the tail end.

Debris from the excavation is being removed to the Orakei Domain construction site using a small train system. A temporary access causeway has also been built from Shore Rd and along the existing sewer pipe to the bottom of Logan Tce and Victoria Ave. Shafts are being built at the bottom of both of these residential streets. The causeway will be removed and the Shore Rd site will be restored and returned to the Shore Rd reserve at the end of the project.

After tunnelling is complete, a large noise enclosure will be removed from the construction site and a temporary shaft, 35m deep and 10m wide, will be filled. This is scheduled to take place before next August.

Next year will also see the electrification and mechanical fit-out of a new pump station at Orakei Domain to get the tunnel operating by December.

Final work, including demolishing the Hobson Bay pipeline, pumphouse and landscaping, is expected to take place in the first six months of 2010.

Project Hobson is a Watercare Services enterprise. Watercare commissioned a joint venture of Fletcher Construction and McConnell Dowell to execute the project, which is proceeding on time and on budget.

- NZ Herald

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