McConnell Dowell were awarded the contract to install pipelines to take untreated wastewater from Governors Bay and Diamond Harbour to Lyttelton before being pumped through the Lyttelton Tunnel to the Bromley Wastewater plant. As part of this project, HGM were engaged to carry out the following works:
- Excavation of test pits in the harbour.
- Installation of 1.5-ton anchor on Govenors Bay foreshore, excavation of trench on foreshore at Govenors Bay and winching of pipeline.
- Excavation and breaking out of trench at Diamond Harbour. Drilling and installation of hold-down anchors. Winching of pipe into place.
- Pumping of 24m3 of concrete below water level to secure the pipeline at Diamond Harbour. Backfill of trench in foreshore and reinstation rip rap rock.
- Supply and installation of rip rap rock at Navel point.
Keeping environmental impact to a minimum was one of the key challenges along with timing, having to program work around changing tides. Environmental considerations included minimising the disruption to the foreshore and the preservation of wildlife as well as managing the risks associated with completing an underwater concrete pour. Other challenges included managing the timing of concrete trucks arriving on site at Diamond Harbour and ensuring consistency in both supply and quality of concrete being pumped. Underwater divers were engaged directly by McConnell Dowell to assist with the concrete placing and HGM worked closely with the divers to ensure the pour went to plan with minimal environmental disruption.
Another challenge was getting the pipeline in to place and HGM’s custom built cable drum roller provided a safe and efficient solution by winching the pipeline in to place in a slow and controlled manor.