“It’s a very short journey – three millimetres at most – but it’s one of the most demanding engineering jobs undertaken in New Zealand and one rarely done overseas,” said Scott Elwarth, head of delivery for City Rail Link - New Zealand's largest transport project.
Elwarth said that those three millimetres are the most the 107-year-old 14,000t building is allowed to move as its weight settles on new foundations.
“Underpinning a building the size and weight of the CPO is an extremely challenging task – something only done when other methods are not available and, then, done very slowly,” he said.
Buildings such as the old Birdcage Tavern near Auckland’s Victoria Park road tunnel have been physically moved out of the way before, but Elwarth said that there was no room in central Auckland to do that for a building as large and as heavy as the CPO.
“The CPO is one of the most historically important buildings in the country – a building with a top heritage rating,” he said. “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 weight transfer will be gradual, over several weeks. It includes removing some 350 tonnes of steel used for the underpinning structures that provided temporary support for the building during tunnel construction.
“It’s a delicate, careful and well-planned operation,” Elwarth said. “People will not notice any change to the CPO.”
The CPO’s latest ‘journey’ reverses one completed last year when its weight was transferred to the temporary supports and 15 original concrete foundation columns were demolished to clear the way for the tunnels.
The aim is for the return to be completed at the end of October, when the building will be supported on new foundations that include diaphragm walls sunk 20m below ground, new foundation columns, cross beams and the tunnel boxes themselves.
After the weight transfer, work will start on restoring the CPO’s interior. Restoration work is due for completion in late 2020.