Christchurch City Council’s largest project ever undertaken is complete. The $87 million Ocean Outfall Project takes the city’s treated wastewater from the oxidation ponds and transports it three kilometres out into Pegasus Bay.
- Location: Christchurch
- Started: Dec 2006
- Cost: $87,000,000
- Product: Rip Rap Boulders
- Completed: Dec 2009
- Contractor: McConnell Dowell
Christchurch Ocean Outfall
Christchurch City Council’s $87 million Ocean Outfall Project takes the city’s treated wastewater from the oxidation ponds and transports it three kilometres out into Pegasus Bay.
Beginning in 2006, the project encompassed a five kilometre underground pipeline to take treated wastewater from the pump station, under the Estuary, South New Brighton Spit and sand dunes and then three kilometres out to sea. McConnell Dowell, the contractor tasked with supplying and installing the pipeline used a $7 million tunnel boring machine built by Herrenknecht in Germany to tunnel between the oxidation ponds to around 250 metres offshore (beyond the surf zone). Nicknamed Dora the Bora, the 13.3 metres long machine was painted in the red-and-black colours of the Canterbury rugby team.
The pipeline itself is made of polyethylene-lined concrete pipe while the offshore section, out to the diffuser, was built with a float and sink technique using high-density polyethylene pipe with concrete weight collars.
Fitting the pipestrings
The first of the project’s seven pipestrings were sunk into place in February 2009. The 360 metre pipestring was towed from Lyttelton Harbour and sunk in a dredged trench excavated from South Brighton Beach and then backfilled. The pipestrings were progressively joined together by a team of divers and then connected to the concrete tunnelled pipeline.
To hold the pipeline stable in the trench, the marine section of the Ocean Outfall has concrete weights, weighing around 16 tonnes each, attached every six metres along the length of the pipe. The final pipestring discharges the treated wastewater through a series of 13 smaller riser pipes (diffusers), ensuring the treated wastewater is quickly and efficiently mixed with the ocean waters.
To support to the diffuser structure, large rip-rap boulders were originally to be placed on the sea floor to form an apron, providing the necessary scour protection from sea currents. But following tests to provide indicative grading of the boulder supply a washed, round, 140mm plus boulder was specified.
Following a rigourous search within the region, Winstone Aggregates’ Yaldhurst Quarry was selected to supply the boulders. Quarry Manager Geoff Cooke says the nature of Christchurch’s alluvial material means that rip-rap boulders are often in short supply, and therefore priced accordingly. “Yaldhurst Quarry was able to produce an extremely good quality product, while at the same time, keeping the cost within budget. Operating our own fleet of truck and trailer units means that we can significantly reduce cost through efficiencies of scale.”
Each of the thirteen diffusers required 400 tonnes of boulders as support and the first deliveries from Winstone Aggregates to McConnell Dowell’s stockpiling area began mid 2009. Over the following eight months, Winstone Aggregates’ South Island Transport division carted from Yaldhurst to Lyttelton (a two hour round trip) around 6000 tonnes of boulders, totalling more than 200 trips.
From the stockpile, a motorised barge carted boulders out to the pipeline. Once the barge was positioned, a 20 tonne digger unloaded the boulders which sunk to the sea floor, forming a 12 metre apron around the pipe structure.
Completion of the three year project was expected mid-2009 but prolonged storm conditions caused open sections of the marine trench to infill, requiring re-dredging. Crane barge and diver crews were unable to work safely and the final six weeks required to fit the remaining 8 of 13 diffuser risers, completing the connection between the tunnel and marine pipe, was delayed by six months.
- The fifth Ocean Outfall to be built in the South Island during the last decade
- The longest in New Zealand
- The largest construction project ever undertaken by the Christchurch City Council
- It took more than 200 trips from Yaldhurst Quarry to Lyttelton Harbour to deliver the 6000 tonnes of boulders
- Up to 350 tonnes of boulders were carried by the barge at any one time
- Project completed in less than four years