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Technical Note: Novel sewer pipe installation method in Wellington, NZ

Nick Wharmby, manager engineering, Derek Bilby, operations manager, and Damon Winter, contract supervisor, Brian Perry Civil. This note was first published in GE’s April 2005 issue.

Construction of a 98m long link sewer with an invert level at jibetween 4.5m and 7.5m below ground level within a roadway is not uncommon, but this project in Island Bay, Wellington, New Zealand had some novel features.

Traditionally the 1,050mm diameter pipes would have been laid within a steel sheet piled cofferdam or similar supported excavation and backfilled. Pipe jacking and micro-tunnelling could also have been considered. The site plan and a summary of the ground conditions are given in Figure 1.

The sloping site, in a sensitive urban environment on the edge of Wellington Harbour, has numerous buried services. Ground conditions consist of dune and beach sands, coarse beach gravels, greywacke outcrops and high tidal groundwater. There is a history of pipe installation difficulties in the area. The combination of these issues effectively precluded use of conventional trenching/cofFerdam options, while micro-tunnelling would have required importing a specialist machine for the relatively small project.

Discussions with Bachy Soletanche lead to the investigation and subsequent adoption of its patented Pluss (Pipe Laying Under Slurry Soletanche) method; the project was awarded to Brian Perry Civil on this basis after a competitive tender process. While the method had been used on a number of projects in other parts of the world, it had not been used either in New Zealand or using conventional precast concrete pipes. 

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