Innovating with Segmental Caisson Construction to Build a New Pump Station in Wellington's CBD
Annual Conference
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Growth is vital to keeping our cities vibrant and attractive places to live. The limited capacity of the wastewater network in the Wellington CBD has restricted the ability for the city to grow and develop.
This paper outlines the investigations, tender, construction and contract supervision for a new wastewater pump station (PS), and associated pipework undertaken on behalf of Wellington Water Limited (Client) for Wellington City Council (Asset Owner).
This was the first PS to be constructed in the CBD in over 40 years. The CBD setting presented significant challenges in terms of feasible site locations; managing existing services, providing operations and maintenance access, and accommodating third party stakeholders and local business needs.
The paper discusses the difficulties of constructing a combined drywell / wet well water retaining structure, in a difficult location, with critical Health and Safety considerations and why these complexities ultimately, drove the project towards the adopted solution.
The client accepted GHD’s proposal to open the tender to alternative construction methodologies that might offer better solutions. The nominated Contractor, Brian Perry Civil (BPC), proposed an innovative proprietary precast caisson segmental system for the PS structure. Caisson construction generally, offers cost effective, efficient and safe method for constructing below ground chambers; the segmental system added speed of construction with significantly improved Health and Safety advantages.
The paper details the close collaboration between GHD and BPC to ensure the precast system met geotechnical, seismic and structural performance requirements and the additional quality assurance (QA) requirements for elements prefabricated overseas.
The paper also describes the challenges that were faced and successfully overcome during construction. Focusing on, first attempt using this construction system for the contractor, managing contaminated ground and associated contaminated groundwater, and managing groundwater drawdown and associated building settlement risks associated with constructing the temporary concrete base plug.
GHD and BPC do not believe that anyone else in New Zealand has inserted internal walls within a pre-cast segmental structure like this, with the intention of using it as a combined operational building and water retaining structure in a seismically challenging environment.