St Marys Bay to Masefield Beach Water Quality Improvement
challenging urban tunnelling project
The NZ$37.8m project aims to clean up the Waitematā Harbour.
The new pipeline will reduce wastewater overflows to St Marys Bay and Masefield Beach by 95%. The high flows after rain events will be stored in the new larger-capacity pipeline and be pumped back into the sewer network when there is capacity.
As well as reducing overflows, once complete the new marine outfall will discharge to an outfall away from public swimming beaches.
This was a traditional construct-only contract. While Aurecon was lead designer, McConnell Dowell was responsible for the design of the outfall ballast blocks and the marine piling operation.
Using a slurry TBM and locating the separation plant in Point Erin Park minimised potential disturbance to nearby stakeholders, with the TM advancing to smaller shaft sites located in an urban neighbourhood.
how did we optimise, earlier?
We were commissioned by McConnell Dowell to support the project team with excavation designs for the three main tunnelling shafts. We produced final design solutions for the main launch shaft which used unreinforced secant piles with 50% less cement and 90% less reinforcement than conventional designs.
At the London Street retrieval shaft, careful logging of the rock face during excavation allowed the 6m high and 7m diameter cavern ‘belled’ cavity in the rock to be constructed completely unlined, resulting in a reduction of over 70% in the face support. Such design optimisations resulted in substantial time and cost savings for the contractor and a significant reduction in disturbance for stakeholders.
Our optimised solution was better for the client, the contractor, the community, and the environment. It was a win all around – with less cost, carbon, time, and less wastewater in the harbour. Who wouldn’t be happy?
Andy O Sullivan had a great understanding of the contractors focus to provide a safe and efficient solution for the shaft works on the Saint Marys Bay Project. Their willingness to investigate change and be flexible during the construction phase allowed the project to realise opportunities and achieve a successful outcome