About the project
Background
Christchurch has about 136,000 households connected to the city’s wastewater system collecting 150 million litres each day. The wastewater is pumped to the Christchurch Wastewater Treatment Plant where the purification process begins.
Simply put, the wastewater is treated by separating the organic and inorganic solid wastes, removing odours and killing the bacteria and viruses present. The latter part of the process uses our natural elements such as UV light from the sun, wind-induced aeration and the photosynthetic action of naturally occurring algae for disinfection in the oxidation ponds.
After treatment at the plant and the oxidation ponds, the treated water is returned to the environment, with about 90% of organic material and 99.99% of bacteria removed. At present, the treated water flows to the ocean via the Avon–Heathcote Estuary, or Ihutai.
This process has been the basis for the treatment of the city’s wastewater for the past 40 years, although wastewater treatment has taken place at this site since the 1880s. So why change it all now?
In 2002, the City Council applied for an extension of another 15 years to the existing resource consent based on upgrades to the Christchurch Wastewater Treatment Plant for additional capacity and significantly improved water quality.
In October 2002, Environment Canterbury granted consent for another five years only. They wanted the wastewater out of the Estuary. So the city had to come up with another solution quickly.
The most promising alternative, after many options had been explored, was for an ocean outfall. This was also supported by two independent, expert panels reviewing the public health and ecological impact in the Estuary
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An Ocean Outfall has been chosen because:
- the large and open ocean environment will allow more dilution than in the closed, shallow Estuary.
- dilution means water quality guidelines can be easily met, and public health risk minimised.
- it’s further away from recreationalists.
- The Estuary environment will improve significantly while having very little affect within the ocean.
- it will be better for swimmers at the beach.
The beaches in the Estuary and the Ocean beaches will be improved and graded “good” once the Major Sewer Upgrade is completed and the Ocean Outfall is built.
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What is the Ocean Outfall and how will it be built?
The Ocean Outfall consists of a new pump station being built on the edge of one of the oxidation ponds at Christchurch Wastewater Treatment Plant and a 5km underground pipeline that will take treated wastewater from the pump station - under the Avon-Heathcote Estuary, South New Brighton Spit and sand dunes, and then 3km out to sea.
The pipeline – the landward section
Micro-tunnelling is being used to build the 2.3km land-based section of the pipeline and will use reinforced concrete piping with an internal diameter of 1.8metres.
Micro-tunnelling is carried out by lowering a tunnelling machine into a hole, known as a jacking shaft, several metres into the ground. The machine bores a tunnel the same size as the pipe. A powerful hydraulic jack is used to progressively push each section of pipe forward behind the machine.
The tunnelling machine can operate 24 hours a day, seven days a week. When this happens about 15 metres of pipeline a day is constructed although this does depend on ground conditions.
The tunnelling occurs between 4.4 and 6.8 metres under the Estuary and between 9 and 12 metres under the street surface. Because of the depth of the work the tunnelling machine doesn’t make any audible noise.
This method of construction is less disruptive to the environment and to the community in the vicinity of the pipeline route, than the traditional ‘dig and lay’ method which involves digging trenches and backfilling.
Construction sites
Three areas are required during construction of the landward section of the pipeline where the contractor will work from and where the jacking and receiving shafts are set-up. They are:
- construction area 1 – at the City’s Wastewater Treatment Plant near the site of the new pump station
- construction area 2 - in South New Brighton Park. This means an area of the Park, adjacent to the playing field will be fenced off for safety reasons
- construction area 3 – in the sand dunes, opposite the end of Jellicoe Street, between Marine Parade and the beach. This means an area in the dunes will be fenced off for safety reasons but access to and along the beach will be maintained. The affected area of the dunes will be restored once the outfall is completed.
These areas will also house temporary buildings, stockpiling of sand, construction materials, equipment and vehicles and other materials required for construction activities.
The tunnelling machine
The 55 tonne tunnelling machine was manufactured in germanay specifically for this project. In a long-held tradition of naming construction machinery, the tunnelling machine was christened ‘Dora-the-Bora’ at a ceremony held in April 2007. The name was suggested by local school student Roy Toner.
The tunnels
Tunnel 1:
- from South New Brighton Park under the Estuary to the edge of the oxidation ponds
- about 875 metres long
- the jacking shaft at South New Brighton Park is round with a diameter of 14 meters and is 15 metres deep
- when the tunnelling machine completes the first tunnel it will be retrieved from a receiving shaft (hole) at the oxidation ponds and returned to South New Brighton Park.
Tunnel 2:
- from South New Brighton Park to the sand dunes, opposite the end of Jellicoe Street, between Marine Parade and the beach
- about 600 metres long
- the jacking shaft at the sand dunes is ?m x ?m and xm deep
- when the tunnelling machine completes this tunnel it will continue through the jacking shaft to start Tunnel 3.
Tunnel 3:
- from the sand dunes, opposite the end of Jellicoe Street, between Marine Parade and the beach to just beyond the surf
- about 600 metres long
- the tunnelling machine will be retrieved from the water and the sea-based section of the pipeline will carry on from here.
A DVD, produced by the Herrenknecht (Asis) Ltd the manufacturer of the tunnelling machine, which explains the tunnelling process in more detail is available from Central and New Brighton libraries
The pipeline – the sea-based section
This section of the pipeline will use tough, light plastic pipe with a similar diameter as the concrete piping and a wall thickness of 70mm. At a construction site at Naval Point in Lyttelton, sections of pipe will be joined together, to produce long pipe strings which will be floated across the harbour and sunk temporarily at Diamond Harbour until all the pipe strings have been assembled.
During the 2007/08 summer, individual strings will be floated from Diamond Harbour and towed around to the outfall site before being sunk into a dredged trench in the seabed. The trench will then be backfilled.
The pipeline will be laid 3km out into the ocean from the beach and will be buried along its full length to a depth of 2metres. A series of smaller pipes called ‘risers’ extend up through the seabed along the last section of the pipeline. Diffusers will be attached to the upper section of the risers to ensure the treated water is quickly mixed with ocean waters.
Each pipestring will have a pair of concrete weights, weighing 15.72T, attached every 6m along the pipelength to hold the pipeline in the trench.
The pump station
A new pump station will be located at the oxidation ponds at the Christchurch Wastewater Treatment Plant. Three 350kw pumps will operate the new pump station. These are being manufactured in Sweden.
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How the tunnelling machine works
Lowered into the jacking shaft, the tunnelling machine bores a tunnel the same diameter as the outfall pipe. See Fig 1 below.
Fig 1
As the machine is jacked/pushed forward through the ground, a 3-metre section of reinforced concrete pipe is dropped into the frame behind the machine. This is used to jack the machine forward again. As each section of pipe advances through the ground behind the tunnelling machine, another section is lowered into the shaft so the pipe string gets progressively longer as the machine advances further through the ground.
A container with an operating control panel is located at the top of the jacking shaft. See Fig 2 below.
Fig 2
This computerised system enables the machine operator to continuously monitor and control the progress of the tunnelling machine through a guidance system and the various loads and pressures being recorded by the tunnelling plant.
The tunnelling machine has a circular cutterhead which is specifically designed to suit the anticipated ground conditions. See Fig 3 below.
Fig 3
A crusher area is located within the cutter wheel directly behind the cutting face. The wheel has reinforced spokes which act in the same way as a coffee grinder. Any larger rocks are reduced in size until they pass through the opening for removal from the tunnel. See Fig 4 below.
Fig 4
A type of slippery clay called bentonite is pumped into the pipe tunnel to help the tunnelling machine and pipeline slide through the ground.
A lighter bentonite mixture, or slurry, is used also to carry the material being excavated at the cutterhead to the separation plant at ground level. The separation plant receives the slurry material which has been pumped from the tunnelling machine, and separates the excavated solid material from the liquid. The clean slurry liquid is returned to the cutterhead and the sandy excavated material is trucked to the paddocks next to the oxidation ponds.
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Did you know …
- This is the fifth ocean outfall to be built in the South Island during the last five years
- This is the longest ocean outfall ie the one that goes the furtherest out to sea, to be built in New Zealand
- The maximum flow if treated wastewater will be 6 cubic metres/second
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