Christchurch City Council Ocean Outfall

Project: Christchurch City Council Ocean Outfall
Location: Christchurch
Client: Christchurch City Council (Subcontractors to McConnell Dowell)

The backhoe dredge (BHD) Machiavelli and the tug Kurutai were used to excavate and backfill a 2.5 kilometre long trench for the new 1.8 metre diameter sewer outfall pipeline being built by McConnell Dowell for Christchurch City Council. The dredging tolerance for the trench was + 150mm from design; this tolerance was achieved over the 2500 metre long trench in depths of water of over 20 metres.

To eliminate the need for hopper barges WH761 & WH762 on this project Heron utilised one of the cutter suction dredge booster pumps and 540m of floating pipeline to dispose of the dredge material. The dredged material was placed directly into a feed hopper by the BHD Machiavelli, at the bottom of the feed hopper was 2 adjustable speed screw augers, this allowed the flow of sand to the dredge pump to be controlled. The hopper assembly, water intake and piping was designed and constructed in Heron’s Papakura workshop before being disassembled and trucked to Christchurch. This innovation allowed the dredging of a trench and the backfilling of a pipe-string to happen concurrently, it also minimised the floating plant required on the site and increased the operational hours of the dredging spread.