TBM excavation of the Pillans Point stormwater sewer in Tauranga, New Zealand, has been stopped until the contractor sources replacement, larger centrifuges after the original units failed to cope with slurry processing volumes.
With the difficulties having been going for some weeks, local contractor Harker Underground Construction said late September that the new centrifuges were unlikely to arrive until the end of the year, reported the client, Tauranga City Council. After commissioning and testing of the purpose-built centrifuges, tunnel excavation is unlikely to resume before January 2008.
The TBM, which is excavating a 2.5m diameter sewer, had only advanced about 20m into the 400m long drive at Otumoetai, in the Bay of Plenty, when work had to be stopped due to the centrifuge problems. On the project the capacity of the centrifuges, used to separate volcanic ash from water, were found to be too small to cope with the volume of material.
The client had wanted the excavation completed by November. The centrifuge difficulty is the second problem to affect the scheme since tunnelling started in August as excavation was also held up briefly when the slurry shield needed a new O-ring seal.
The Pillans Point stormwater scheme is a design and build contract that includes construction of the main tunnel plus 70m of 1.8m diameter plus 260m of storm water pipelines, and inlet and outlet structures.
