The ground works at the Bayfair roundabout are needed to prepare construction of a new underpass and a flyover bridge.

The project can now make progress following uncertainty caused by the behaviour of a layer of pumice. “The design for the ground improvements needed altering after the pumice layer in the area was not performing as we expected,” said Waka Kotahi NZ Transport Agency portfolio delivery manager Jo Wilton. Lattice piling framework will provide further stability and limit liquefaction during an earthquake, she said.

Sandy soils and a high-water table in the area have already led to the installation of about 5,600 stone columns across the site.

Two drilling rigs, weighing approximately 72 tonnes each and standing up to 25 metres high, will now be located on site for about six months. The lattice piles will be installed around each of the five supports for the flyover bridge.

“Installing the lattice framework involves overlapping concrete columns to create a sealed wall,” said Wilton. “It is estimated 9000m3 of concrete will be poured - equalling more than 21,600 tonnes, which will be delivered by around 1,800 truck movements."

The pilint contractors and on-site team will continue to conduct ongoing tests to ensure the new design is constructed and performing as required, she added.