Projects

Harris Street

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Previously in storm events, surface stormwater would pond in Wellington’s lower CBD area resulting in the flooding of roads, footpaths and business premises.

The ponding and subsequent flooding would occur due to the capacity of the existing stormwater system being exceeded and the overland flowpath (from the CBD to the harbour outlet) obstructed by the road level of Jervois Quay being at a higher level than the lower CBD area.

This project was for the upgrade of the existing stormwater system by providing additional conveyance capacity to eliminate the stormwater flooding problems in Wellington’s lower CBD area.

The key objectives of the strategic works included:

  • Tripling the capacity of the stormwater network around Willis, Bond, Victoria and Harris Streets
  • Providing a bypass pipeline for the upper catchment flow independent of the CBD network
  • Providing 50 year protection without any overland flow.

Two options, open cut and microtunnelling, were investigated and the option to microtunnel was selected. Microtunnelling was the more beneficial option particularly as the works were located within the road carriageway of the lower CBD area and traffic flows had to be maintained; there was a requirement for installing the pipeline at depths greater than 6.5 metres to avoid working within the reclaimed land; potential dewatering issues due to a high watertable combined with weak ground; and the discovery of a number of existing unmarked underground services when excavations were undertaken for shaft construction.

For this particular project, Harker Underground Construction with TunnelTEQ designed and manufactured the TT1500H slurry TBM and separation plant.

Stage 1 of the project involved a manual pipejack of 90 metres long along Bond Street, at a depth ranging from 4 metres to 6.5 metres. Ground conditions encountered included weak greywacke and groundwater two metres below the surface.

Stage 2 was a 188 metre machine drive along Victoria Street at a depth of approximately 6.5 metres. Ground conditions encountered included weak greywacke, marine sediments, reclamation materials, timber and steel obstructions (such as steel pile point protectors), and groundwater one metre below the surface.

 

Stage 3 was undertaken as two drives. The first drive was a 155 metre machine drive along Harris Street, starting from a downstream depth of 16 metres and finishing upstream at a depth of 6.5 metres. Ground conditions encountered included weak greywacke, marine sediments and groundwater one metre below the surface.

The second drive commenced from the lower end of Harris Street (by the Public Library), crossed under Jervois Quay and terminated within Frank Kitts Park on the harbour side. This 150 metre drive was originally agreed to be a machine drive however the results from the magnetic and ground radar surveys identified six anomalies within the line of the drive. The anomalies detected turned out to be steel obstructions. Hence the final drive was a manual pipejack commencing at a depth of 16 metres and terminating 14 metres below the ground surface at the outfall.

Ground conditions encountered in this final drive included marine sediments, reclamation materials and excessive groundwater flows (due to the tidal influence).

This project won Harker Underground Construction the New Zealand Contractors Federation 2005 Hirepool Construction Awards Category Four.

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