Completed in February 2011, the project has mitigated the severe economic and social impacts associated with flooding, and was delivered by the Pipelines Alliance, which is comprised of the following partners:
- Melbourne Water (client)
- GHD (designer)
- Capacity JV, Jaydo and Fulton Hogan (constructor)
- Sandgate Stage 1 “Ó Sub Alliance, Winslow (tunnel constructor)
The Frankston Central Activity District (CAD) contains significant infrastructure of regional importance. However, like other catchments located in older areas of Melbourne, the planning of Frankston gave little consideration to where overland flows would travel once underground drains had reached capacity. Infrastructure and property developments were built in the valleys where overland water flows occur, resulting in extreme flooding to Frankston’s CAD.
Stage 1 of the Sandgate Avenue Drain Flood Mitigation Project involved the construction of a 1,550 m long, 2.5 m internal diameter tunnel, commencing upstream at the Monash University Peninsula Campus and traversing west to the outlet located at Kananook Creek.
The alignment of the tunnel traverses through a well-developed urban environment, with the alignment passing under roadways, parkland, sporting facilities and a railway reserve. Third party assets along the alignment include residential and commercial structures, sporting clubs, car parks, roads, railway, and service pipelines.
Project objective and scope
Melbourne Water used an intolerable risk matrix which rated the Sandgate catchment as an “÷extreme’ flood risk. The objectives were therefore defined by what would reduce the category from “÷extreme’ to “÷high’.
Project objectives were initially developed through a review of Melbourne Water’s Flood Mitigation strategy, which resulted in the project focus shifting from the economic consequence to the recognition of the importance on the social and safety risks caused by flooding.
This change resulted in the development of the Flood Risk Assessment Framework that gives equal consideration to all of these factors.
Assessment found approximately 20,000 properties were affected by “÷intolerable’ or “÷extreme’ flooding, and as a result MWC set a target that this number would be reduced by 10 per cent by 2013 and 30 per cent by 2018.
The Sandgate Avenue Drain Flood Mitigation Works consists of three stages of works, estimated to reduce the flood risk category for approximately 1,400 properties, or approximately seven per cent of properties assessed as subject to intolerable flooding. Stage 1 contributes to over half the total reduction.
Project challenges
The key elements of the project requiring innovative solutions were the tunnel design, the tunnel boring machine (TBM) and slurry treatment plant selection, the tunnel survey and the Nepean Highway crossing.
Tunnel design
Tunnel design was a key challenge for the Pipelines Alliance, due to the size of the tunnel, length of the required tunnel drives, limited cover to the crown of the tunnel and the two 500 m radius curves.
A suitable solution for the design of the tunnel was developed through identifying relevant design criteria and constraints, undertaking a precedence study of relevant tunnel design solutions to understand how similar projects have been designed, and through reviewing relevant design standards and guidelines.
TBM selection
A dual slurry earth pressure balance (EPB) TBM was adopted for the tunnel excavation because it had to cope with all of the expected geotechnical conditions, including clay, sand, potentially high groundwater inflow, the possibility of gravels within the Brighton Group material and the constant variability of the geotechnical conditions.
An MTS Perforator machine was selected because it had the ability to support the face of the excavation by both slurry and EPB mode whilst continually transporting the spoil to
the slurry treatment plant (STP) via a slurry circuit.
The excavated diameter of the TBM was 3,070 mm and the cutterhead design consisted of a mixed ground cutterhead, with a selection of cutting tools including roller cutters and scrapers to ensure that the TBM could cope with all of the expected geotechnical conditions. The main jacking station thrust had a capacity of 1,200 tonnes and the interjack stations had a thrust capacity of 1,000 tonnes.
Slurry treatment plant selection
Because much of the tunnel advance was through fine clays, it was necessary to establish primary, secondary and tertiary separation equipment to remove the solid component from the re-circulated slurry circuit.
The primary separation was achieved through a 400 m3/h desander, which removed solid particles to 50 parts per million. The slurry was then pumped through clarifiers in which chemical polymers and coagulants were added, before being circulated through centrifuges. Once the treatment plant was operating efficiently, tunnel advance rates increased to frequently achieve 30 m in a 24 hour period.
Tunnel survey
Survey for a curved pipe jacking operation is unusual in that it must continuously and accurately maintain the machine’s position whilst the machine, survey equipment and reference prisms are all moving with the pipe string. The long drive length and curved alignment added the complication that a sharp correction or unnecessary variation in excavated alignment could add significantly to the total thrust forces required.
The survey equipment and software utilised was a VMT SLS-RV system which was maintained and operated by a local specialist survey contractor. Across the three tunnel drives an alignment tolerance within 30 mm was maintained.
Nepean Highway crossing
Due to the required drainage outlet level at Kananook Creek it was necessary for the tunnel to progress under the Nepean Highway with less than 1 m of cover. Typically a tunnel of this nature would require a cover of at least two times the pipe’s diameter, or 6 m of
cover to minimise the impact on third party assets.
The Pipelines Alliance cleverly used a weighted system on the highway over the tunnel alignment at the shallow section of tunnel, increasing confining pressure over the TBM whilst tunnelling progressed.
An extensive global review of past tunnelling projects highlighted that this technique had not been previously adopted for this purpose.
Benefit to the community
Project timelines
The occupation of the shaft sites within the community needed to be minimised to reduce negative impacts on stakeholders. The Pipelines Alliance went to considerable lengths to understand the concerns of the community, and these became the dominant influence on project timelines.
With personnel integrity a core value of the Pipelines Alliance, honouring the commitments made to stakeholders became the driver in the project achieving the requested timelines.
Demonstrated commitment to continuous improvement
The Pipelines Alliance demonstrated a commitment to continuous improvement by striving for the best outcomes in all aspects of the project.
They did this by reviewing the ground conditions and engineering controls that had the most impact on production outcomes. Their dedication to a culture of continuous improvement has delivered a drainage solution for current and future generations in a safe, efficient, sustainable and innovative way.
Conclusion
Since completion earlier this year, the Sandgate Avenue Drain Flood Mitigation Project “Ó Stage 1 project has achieved its aims of reducing the likelihood of floods during severe storms to residential, industrial and commercial properties located within the City of Frankston's main catchment area, hence mitigating the severe economic and social impacts associated with flooding. This project has achieved great outcomes for residents and businesses within the area, and is a fitting winner of the ASTT Installation Project of the Year award.