Sustainability Case Study #4

Future-Proofing Wastewater Treatment

Case Study 4: Mangere Wastewater Treatment Plant
New-generation, precast concrete panels were critical to the $140 million expansion to Auckland’s wastewater infrastructure, built for a 100-year life.

New Zealand’s largest wastewater treatment expansion in 20 years brought an additional 250,000 Aucklanders into the South Auckland network. Precasting 10,440 tonnes of concrete on site underpinned the project, and was essential to affordability.

Constructed between June 2015 and October 2017, the Watercare project had up to 2,000 workers on site. Commissioned in March 2018 after six months of water flow testing, the McConnell-Dowell-HEB Construction joint venture won sustainability awards for the contract.

INNOVATION WITH CONCRETE
Precasting 174, 60 tonne concrete panels on site, including for two, four-stage, 28,000 m³ bioreactors, shortened construction time by 4 months, also delivering cost savings.

Precasting 174, 60 tonne concrete panels on site, including for two, four-stage, 28,000 m³ bioreactors, shortened construction time by 4 months, also delivering cost savings.

Aside from reduced transport, each completed panel was stored in the correct order, to streamline the work of a 280 tonne crawler crane on site, space being at a premium.

Post-tensioning of concrete panels and floors, using 144 km of steel cable, was the key technology to futureproof the structure. This increased resilience and durability, avoided cracking, and reduced wall and floor thickness, saving on materials. It also reduced the extent of ground improvements, because of the structure’s flexibility and strength.

The design of the concrete mix and of reinforcing met the New Zealand standard for the required 100-year life for the plant expansion. The plant being sited on a peat bog, this achievement is remarkable.

ATTENTION TO DETAIL
Keys risks identified for the bioreactors and the two 52 m diameter clarifier tanks were concrete cracking and leakage, also through the join between vertical panels, and where they meet the floor. Access points below the operational liquid level were built into the structure to allow scheduled maintenance to deal with the issue.

The floor to a new pump station was placed with two pours, 115 m² for a dry-well, and 210 m² for a wet well. Five walls took eight placings of 20-30 m³ each, all of this work carried out in tightly-constrained space. On-site casting reduced the number of construction joints, reducing the risk of groundwater corrosion of reinforcing. A four-coat epoxy lining separates concrete from the area of settled sewerage, to protect against chemical attack.