The company's back-bolted caisson shaft segment rings were utilised to build a 6.5m diameter deep waste water pumping station in the heart of Wellington's Central Business District (CBD)
The new sewer infrastructure project will also relieve pressure on the existing pumping stations, thereby reducing the possibility of overflow into the harbour.
The 6.50m internal diameter shaft was sunk from ground level using the caisson method. Sinking was partially aided by four hydraulic jacks with FP McCann's back-bolted standard precast concrete rings forming each complete section of the shaft wall. The concrete cutting shoe was initially installed and subsequent rings placed on top, thereby allowing the shaft to sink under its own weight, helped with the jacks and some partial undermining of the hard gravel layer beneath the cutter shoe. Once the shaft build was complete, mass concrete was poured into the base to form a 1.5m deep plug, on top of which the structural floor is currently being constructed. Back grouting of the shaft walls will complete the solid, water tight chamber.
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The contract to build the shaft and other associated pipeline upgrade engineering works was awarded to Brian Perry Civil (PBC), part of the NZX/ASX listed Fletcher Building group.
Commenting on the shaft build, Daniel Doyle, Project Engineer of BPC said: "The precast segmental caisson provided the most efficient solution to deal with the limited working space available and to handle the water table that sat at 4.5 metres below ground level. The much smaller footprint over other options considered, reduced the volume of material to be removed and also allowed us to retain existing street light poles and a protected tree, that otherwise would have had to be removed. From a safety aspect, workers didn't need to enter the shaft until the base plug had been poured."
In leading discussions with Wellington Water and FP McCann, Andrew Burgess, BPC Wellington Regional Manager said: "The key was to ensure that the caisson could withstand the seismic loads that the structure could be subject to. Wellington has some of the highest earthquake loads in the country, so ensuring that the pump station could withstand the design seismic loads was critical. Wellington Water and consultant GHD were keen to progress our solution and we offered to undertake further design modelling to demonstrate compliance. FP McCann were incredibly helpful in providing the design information that they had, which we could then model to suit NZ conditions and building code requirements."
Stuart Carson, National Tunnels & Shafts Manager for FP McCann added: "The construction methodology adopting the precast segmental caisson shaft sinking process is the first of its kind in New Zealand and we're very proud to have played a big part in this project. Moving forward, we see a strong partnership developing between ourselves, BPC and their client base."
(CM/MH)
