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Project Hobson

Project: Project Hobson
Client: Watercare
Contractor: Fletcher - MacDow
Location: Orakei - Parnell
Date: Started May 2007 to be completed in 2010
Brief: Live sewage diversion works

The sewage pipe line across Hobson Bay (Orakei Main Sewer – OMS) has been part of the scenery for more than 80 years. It is being replaced by a new 3.7 km tunnel from Orakei to Parnell called project Hobson. Part of the works we have performed on this project was a live diversion.

The scope of the work Lowery undertook included:
• Detailed planning sessions in conjunction with works engineers
• Preparatory drilling and cutting
• Wire sawing openings for butterfly valves
• Wire sawing of OMS in live flow.

The old sewer is more than 2km long and runs on piers above Hobson Bay.  It is in a very frail condition, being held together with stainless bands installed serveral years ago as a stop gap measure. It is rectangular outside, 2m wide x 2.9m high, with chamfered top corners.  Inside it is egg shaped with the point at the bottom so the thickness varies greatly.

During construction of the new sewage tunnel from Orakei to Parnell, diversion works to allow the connection to be built had to be performed. To make the diversion to the live sewer required much planning in concert with the contractor and engineers.
The principal was to isolate a section of the old OMS by inserting 2 butterfly valves and removing two sections, one either side, and running the flow through a diversion channel parallel to the OMS. To prevent inundation of the diversion channel during cutting works, gates were made to seal up the openings as they were cut to minimise flows.
In order to cut 2 x 2m squares from the outside the only practical way to work was using a wire saw to cut pieces above the flow level, then, taking a deep breath, cut out the lower portions lift them out and drop the gate!
Despite set backs due to weather giving high flows and debris in the sewer, the work was done and the diversion completed and the new tunnel should be commissioned early net year. Demolition of the old OMS will return Hobson Bay to its former glory soon after.

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Wairau Valley
0800 456 937
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Unit 5, 26 Goodshed Road
Upper Hutt
0800 456 937
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