Customer: Waka Kotahi NZ Transport Agency
Location: Auckland, New Zealand
McConnell Dowell was awarded the contract for the Waka Kotahi NZ Transport Agency project to deconstruct the 105-year-old Māngere Bridge and construct a new and improved replacement in late 2019. The new connection linking the communities of Onehunga and Māngere Bridge was completed on time in August 2022.
Local mana whenua gifted the bridge its new name Ngā Hau Māngere, which means ‘gentle lazy winds’ before it was officially opened to the public on 27 August 2022.
The Ngā Hau Māngere Project was a high-profile job for the client and the local community were very engaged in the process. A complex sequence of deconstruction and construction was carried out under high tension powerlines and in a sensitive marine environment which both added to the difficulty of the undertaking. The challenges of the project were 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 also made these the two key areas of focus.
One of the technical challenges was the deconstruction of the Old Māngere Bridge, which was closed for public safety reasons in 2018. The demolition work was 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 was carried out from a similar temporary work platform on the eastern side of the old bridge at the same time deconstruction was underway to reduce the length of the project. The temporary platforms provided access for the deconstruction work, the construction of the piles, cofferdams and other structures required for the new bridge.
A 330-tonne pre-cast concrete deck section and the show-stopping 60 m structural steel central arch were installed in six complex lift operations using two 250-tonne cranes, to complete the structure.
Careful stewardship of the Manukau Harbour was critical to the success of the project and McConnell Dowell and Waka Kotahi NZ Transport Agency were committed to delivering better than expected environmental outcomes.
The team also worked closely with local mana whenua representatives, who along with receiving regular updates on progress were also invited to visit site to conduct cultural monitoring, advise the team on appropriate safety and environmental protection measures and celebrate key milestones.
Three examples of the controls in place to protect local flora and fauna and the Manukau Harbour itself included:
- a large ‘net’ under the deconstruction work to prevent debris from falling into the water below,
- the use of floating pontoons during deconstruction to catch concrete slurry during the cutting process,
- and underwater noise monitoring to manage the impact of driving steel piles into the seabed on marine mammals such as seals, penguins and dolphins which call the harbour home.
Wherever possible we reused and recycled materials to minimise our environmental footprint. Parts of the old bridge were salvaged whole and reused as landscaping features, a link to the past for future generations to enjoy. The rest of the concrete from the old bridge was crushed for reuse as fill onother projects.
We also reused steel from a bridge project in the South Island in the temporary work platforms. The production of steel is a carbon-rich process so recycling and reusing as much as possible was an effective way to minimise the project’s carbon footprint.
The Key to Success:
Waka Kotahi maintained a strong community focus from the early engagement and consultation through to the construction phase and McConnell Dowell supported this engagement through construction.
Maintaining connectivity and accessibility around the old bridge while work was underway was important as the construction sites were next to popular community assets on both sides of the harbour. Space was tight so we minimised risk by creating separate areas for construction traffic and the public. The walking and cycling path on the northern side was diverted to ensure safe travel around the construction site, and access to the causeway boat ramp and Waterfront Reserve on the southern side was maintained.
We developed the traffic management plan and the walking and cycling diversion route in consultation with local board members, the cycle advocacy group Bike Auckland and the harbourmaster to ensure that safe access was maintained.
To keep the community updated about progress at major milestones we produced regular newsletters and videos so people could see some of the amazing engineering taking place. You can watch videos of the demolition, construction, timelapse and more here.