Three separate tunnels will be constructed, two metres in diameter, bored at depths of between four and 19 m and each about 400 m long. The first tunnel will run from Washdyke Lagoon to the Alliance Smithfield access road. A second tunnel will run directly underneath Westcott and Richmond Streets, and the third from Ashbury Park to Virtue Avenue. All sections of pipe will then be linked, and joined to pipes already installed across Caroline Bay and around the Washdyke Lagoon.

The successful tenderer for the tunnel drilling is Auckland-based Harker Underground Construction Ltd, while the pipe linking contract went to Works Infrastructure Ltd. The contracted completion date for the project is mid 2009.

Harker said that it is delighted to have been awarded the Timaru contract, which was won by combining a back-hoe TBM erecting ring beam and lagging sections inside an erecting can. A tried and true method of installation in the USA, Harker believes this a first for New Zealand.

The company said that although other methods were considered, it was felt that this was the most cost effective method that guaranteed line installation within the client’s tight construction tolerances and at a grade of only 0.06 per cent. Mixed ground conditions ranging from alluviums (which will suit the TBM) to basalt rock which will be excavated using a drill and blast technique.

Timaru Council District Services Manager Ashley Harper said the tunnelling was a specialist operation with only a few firms in the country capable of undertaking the project. Test bores at the site of the tunnels were conducted this month and were required by the tenderers prior to their formalising the tender process. He said Harker Construction would relocate to Timaru for almost two years while carrying out the work.

“While connections with treatment stations have yet to be completed, large diameter sewer pipes have been laid from Station Street across Caroline Bay and the tunnelling will be completed prior to upgrading work at the northern end of Caroline Bay.

“This is part of our co-ordinated approach to both projects,” Mr Harper said.

The tunnelling program will have a minimal effect on residents, Mr Harper said.

“While on the surface there will be no lasting changes to the landscape, the tunnelling process and the installation of pipe connection stations will see increased activity by workmen and machines with some minor short-term access disruption to a few Richmond Street properties.

“But that’s about all,” he said.

The Timaru District Council is almost half way through its multi- million dollar sewer replacement program. The work is being done in a manner that is timed to avoid unnecessary disruption to the public, and as part of an upgrade that must be done before the town’s right to discharge sewage into the sea expires in 2010.

At the time of going to print, a crew was establishing the first compound area ready for tunnelling early in the new year.