Auckland’s 14,000-tonne, heritage-listed Chief Post Office will not be allowed to move more than 3 millimetres while it is transferred from temporary support onto new permanent foundations as part of the City Rail Link project.
The historic CPO was transferred last year from its 15 original concrete foundations onto temporary supports, so work crews could destroy those original foundations and build two CRL train tunnels through the CPO’s old basement.
With that work now progressed, it’s time for crews to move the building onto the new permanent supports built during the tunnelling process.
But CRL’s head of delivery Scott Elwarth says the 107-year-old building will only be allowed to move 3 millimetres during that weight transfer.
“Underpinning a building the size and weight of the CPO is an extremely challenging task,” he said, “something only done when other methods are not available and, then, done very slowly.”
The weight transfer will take place over several weeks, during which time 350 tonnes of steel underpinning the structures providing temporary support will gradually be removed, allowing the building to rest on its new foundations.
“It’s a delicate, careful and well-planned operation,” Elwarth said
“The CPO is one of the most historically important buildings in the country, a building with a top heritage rating. All our planning, design and construction of the tunnels has been dominated by the need to protect the CPO from any damage. Add in the tight working conditions for our teams under all that masonry and concrete and the ‘live’ Britomart station on the other side of the wall, then you’re dealing with a challenging engineering operation.”
The CPO’s tiny but lengthy return journey will be finished by the end of October.
Once complete, the building will be supported securely on new foundations that include diaphragm walls sunk 20 metres below ground, new foundation columns, cross beams and the tunnel boxes themselves.
After all that, work will begin to restore the CPO’s interior, with targeted completion in late 2020.
The CRL project will deliver twin 3.45-kilometre rail tunnels between existing stations at Britomart and Mt Eden. The link will complete a ‘loop’ in Auckland’s centre, while adding another two stations, at Aotea and Karangahape, and rebuilding the station at Mt Eden.