The new pipeline and its associated infrastructure and systems will carry the desalinated water from Kurnell, across Botany Bay, to the city’s main water supply, the City Water Tunnel at Erskineville.
Three tunnel boring machines (TBM) will be employed to minimise the disturbance to residents and also to protect unique tracts of seagrass along the Botany Bay floor.
TBM: managing the environment
The southern shore of Botany Bay contains extensive seagrass beds, which are a valued and protected part of the estuarine environment. Three species of seagrass are present off Silver Beach at Kurnell: zostera capricorni or eelgrass; posidonia australis or strapweed; and halophila ovalis or paddleweed. Posidonia requires the greatest consideration due to its slow reproduction and poor propagation by seed.
Stretching about 6,500 metres in a westerly arc from Silver Beach at Kurnell to Lady Robinson’s Beach at Kyeemagh, the twin and single steel pipelines will impact approximately one per cent of the overall area of Botany Bay. Along the whole route, however, less than half of one per cent of existing seagrass along the southern shore (0.45 per cent) and Botany Bay (0.42 per cent) will be removed as a result of pipeline construction.
Trenching through these seagrass beds would have required a seagrass management plan to be implemented during and after construction, and a compensatory seagrass package involving steps like transplantation. Instead, Sydney Water has chosen to microtunnel the pipeline from its Silver Beach construction area under Botany Bay for a distance of about 800 metres in order to protect the seagrass.
The Water Delivery Alliance will join the single 1,800 mm diameter pipeline from Silver Beach to the twin 1,400 mm diameter pipeline about 800 metres from the shoreline, and (as always) protection of the environment will be a key consideration. Pit construction is nearing completion at the Silver Beach site. This pit is supported by secant piling, has internal jet grouting and is around 10 metres deep. Land-based sections of the pipeline will be constructed first. Material that has been dug up from the pit is being used on site, where possible, in order to minimise truck movements. Continual water quality monitoring is carried out around the Silver Beach construction area. Recent monitoring of the site has indicated good water quality conditions, with similar results both inside and outside the silt curtain.
TBM onshore
A TBM is also an essential tool to minimise disruption onshore. The bore passes beneath Tasman and Dampier Streets, Kurnell for the water delivery pipeline. The model shown is a Herrenknecht earth pressure balance and AVN machine, weighing approximately 100 tonnes.
The first microtunnelling drive through a residential area is now complete. The TBM tunnelled 640 metres from the launch pit in Cook Park, under General Holmes Drive and under Tancred Avenue to the receival pit at Muddy Creek. The TBM used was chosen specifically for the conditions at Cook Park. The pipe liner and services will be installed on this section of pipe in the coming months.
The TBM has also finished tunnelling 645 metres from Canal Road to Botany Freight Rail Line, which was the first tunnel complete on the project. Microtunnelling from Marsh Street in Arncliffe, under the Cooks River to Tempe Recreation Reserve is set to begin in March.
Choosing the route
The pipeline route across Botany Bay was chosen because it avoids known areas of contamination. Every practical effort is being made to protect the Bay environment during construction. Water quality monitoring is ongoing during construction activities, in accordance with the Construction Water Quality Management Plan. The results of this monitoring will help the project team manage their work.
Project scope
The main project works are:
- A drinking water pumping station on the site of the desalination plant in Kurnell.
- Infrastructure from the pumping station to the existing water supply system in Erskineville, via Silver Beach and Kyeemagh, including associated connections and flow and pressure controls.
- Marine works consisting of twin 7.5 km long, 1,400 mm diameter steel pipelines across Botany Bay;
- Approximately 6.4 km of 1,800 mm diameter mild steel cement-lined onshore pipe, slipped inside 2,100 mm diameter concrete pipes and installed by trenchless microtunnelling; and
- Approximately 3 km of 1,800 mm diameter MSCL onshore pipeline, installed by dig and lay conventional trenching methods, with sheetpile and trench box shoring as required.
Overcoming challenges
Given the size of the project, and the locations it must traverse, considerable pre-planning and consultation has taken place with various regulatory authorities and the many wider stakeholders, including the community that will be affected by construction operations along the pipeline route. The Water Delivery Alliance has in place a structured team of proven community and environmental personnel, providing support to the wider team and ensuring that all approvals have been obtained and that stakeholders are well informed of both the program and methods of the activities that will take place in their vicinity. Community feedback has been encouraged and adjustments to the method or timing of activities have been made to accommodate the community’s concerns, wherever possible.
This project is breaking new ground to achieve the distances set for the tunnel drives, while the sizing of the laybarge operations require laying twin 1,400 mm diameter pipe in a pre-dredged trench across the vast Botany Bay waters, which is likely to be challenging. Some of the tunnels will be the longest undertaken by pipe jack method in Australia, and potentially within the Southern Hemisphere. No twin pipeline of this diameter, laid simultaneously from a barge for 8 km, has been executed previously in Australia.
Notwithstanding the considerable challenges that exist, the Water Delivery Alliance team are well skilled and committed to overcoming them, utilising innovation and a team culture that is striving for the completion of this project on time, in a safe manner and surmounting technical challenges to break new ground.