Just 12 months after the arrival of 510 metres of large diameter HDPE pipework from Thailand, the Daldy Street Outfall has been handed over to Auckland Council’s Healthy Waters. Wynyard Edge Alliance (WEA) consisting of McConnell Dowell, Downer, Beca, Tonkin & Taylor and Auckland Council carried out the work.
Relocation of the existing outfall within Wynyard Basin was required to improve water quality within the basin after the breakwaters had been installed that provided tranquility for the forthcoming 36th Americas Cup.
The new extension runs along the entire length of Wynyard Point, an area formed nearly 100 years ago by reclamation from the Waitemata Harbour. A basalt breakwater was formed around its perimeter and infilled with hydraulic fill and excavated arisings from the CBD development. Historically, the area had been used for heavy industry – gas works, fuel storage and was heavily contaminated.
The design of the new pipeline determined the pipe to be 3 metres in diameter for its upstream half, expanding to 3.5 metres diameter for the downstream half. HDPE pipe, in 15 metre lengths was ordered from Uponor in Thailand and delivered onto Wynyard Wharf last September.
A 5 metre deep excavation was required for the new pipeline which was tidally influenced. Key to the successful outcome of the project was the ground stabilisation required to support the western face of the excavation, the eastern face being the existing 100 year old basalt breakwater. The design finalised on the “Cutter Soil Mix” methodology (CSM) which injects grout through rotary cutter heads driven into the ground, forming panels of stabilised ground to a depth of up to 8 metres, locking in place the contaminated material typical of the site.
Excavation for the pipeline was carried out between the CSM stabilised wall and the basalt breakwater, typically 2-3 metres below tide level. A sophisticated array of temporary works schemes were designed to hang concrete cradles at each pipe joint position, lower the pipes into place, flood the annulus of the HDPE pipe to sink the buoyant pipes, pour flowable fill under the pipe to provide support, then adding cement to the excavated material to produce mudcrete and backfill the pipe. This installation cycle was repeated typically every 4 days.
A new outfall end structure was constructed at the northern end of the pipeline, on the tip of Wynyard Point. Pre-cast panels with a featured pattern and black oxide coluring help blend the new structure in with the existing basalt breakwater were used for this structure. A steel frame was set 1 metre below lowest tide, with the aid of divers, to support the precast outfall units. A flap valve was added across the outlet to prevent unauthorised entry up the pipeline.