Customer: Waka Kotahi NZ Transport Agency
Location: Auckland, New Zealand
- The new bridge will be over 260m long and eight metres wide
- Main span is on a horizontal curve with a 136m radius
- The bridge’s most stunning feature is a 60m steel tied-arch
- Two southern approach spans are 12m wide creating dedicated fishing bays
- The northern and southern abutments consist of 1.2m wide reinforced concrete blade walls
- The new bridge provides up to eight meters clearance for vessels during low tide, to increase access to the upper harbour
- The replacement bridge will be a shared public space with a gentle slope to provide easy access for many users
McConnell Dowell was awarded the contract for the Waka Kotahi NZ Transport Agency project to deconstruct the 105-year-old bridge (the Old Māngere Bridge) and construct a new and improved replacement bridge to connect the communities of Onehunga and Māngere Bridge.
The demolition of the old bridge and work to construct the new bridge will occur simultaneously from both the North and South sides of the harbour. The project is scheduled to be completed in 2022.
The Old Māngere Bridge Replacement project is a high profile and working under high tension powerlines and in a sensitive marine area means that the construction and deconstruction operations are extremely complex. The challenges of the project are focused on two main areas, working at heights and over water. The high visibility and interest of the public in the project and especially the environmental and health and safety outcomes makes these the two key areas of focus.
One of the technical challenges is the deconstruction of the Old Māngere Bridge, which was closed for public safety reasons in 2018. The demolition work will be completed using a ‘cut and crane’ method to remove the spans and supporting piers from a temporary work platform built on the western side of the old bridge. This method is almost the reverse of how it was constructed over 100 years ago.
Construction of the new 260m long bridge, with its show-stopping 60m central arch in structural steel will be carried out from a similar temporary work platform on the eastern side of the old bridge while deconstruction is underway. These platforms will provide access for the deconstruction works and the teams to construct the piles, cofferdams and other structures required for the new bridge
Careful stewardship of the Manukau Harbour is critical to the success of the project and McConnell Dowell together with Waka Kotahi NZ Transport Agency, is committed to delivering better than expected environmental outcomes.
The team are also working closely with Mana Whenua, who along with receiving regular updates on progress are also invited to visit site to conduct cultural monitoring and advise the team on appropriate safety and environmental protection measures.
Three examples of the controls in place to protect the Manukau Harbour as well as local flora and fauna include a large ‘net’ under the old bridge to prevent debris from falling into the water below, underwater noise monitoring to manage the impact of driving steel piles into the seabed and the use of floating pontoons during deconstruction to catch concrete slurry during the cutting process.
Wherever possible we are reusing and recycling to minimise our environmental footprint. Parts of the old bridge will be salvaged whole, so they can be used in the landscaping around the new bridge as a link to the past for future generations. The rest of the concrete is crushed for reuse.
For our temporary works and staging, we are re-using steel from previous projects in the South Island. The production of steel is a carbon rich process so recycling and reusing as much as possible is an effective way to minimise the project’s carbon footprint.
The Key to Success:
Waka Kotahi has maintained a strong community focus from the early engagement and consultation through to the construction phase.
Maintaining the connectivity and accessibility around the old bridge while work is underway is important as there are two popular community assets on either side of the harbour next to the construction sites. There is a walking and cycling path on the northern side, and the causeway to the boat ramp and Waterfront Reserve on the southern side.
We have developed the traffic management plan in consultation with local board members, cycle advocacy group Bike Auckland and the harbourmaster to ensure that safe access is always maintained around the construction sites. Space is tight so we minimised risk by creating separate areas for construction traffic and the public.