38 buildings 'in damage zone'

Ground settlement from Auckland's Waterview motorway tunnels is expected to damage 38 buildings. Photo / Supplied

Ground settlement from Auckland's Waterview motorway tunnels is expected to damage 38 buildings, although the Transport Agency expects only three will need extensive repairs.

Engineering consultant Gavin Alexander told a board of inquiry hearing yesterday on a proposed $1.7 billion pair of motorway projects that tunnelling would not necessarily stop if ground settlement threatened buildings with greater damage than expected.

"In some cases it might be better to speed up the work," he said.

"Stopping work and not doing anything could be the worst thing if you get groundwater settlement."

That was because of a need to seal the tunnels with concrete liners to reverse groundwater changes.

Mr Alexander also said in a statement of evidence not read to the hearing but subject to cross-examination that although 38 buildings were likely to be damaged by settlement from a 2.5km pair of tunnels between Owairaka and Waterview, only three would be severely effected.

A further 16 were likely to receive moderate damage, possibly impairing weather-tightness, and the others would need only superficial repairs such as filling cracks.

The Transport Agency would repair non-structural defects once ground settlement was complete, but any structural or weather-tightness repairs would be conducted immediately.

Settlement effects on Oakley Creek and landfills in the Phyllis and Alan Wood reserves were likely to be negligible or minor.

Although owners of multi-storey residential blocks leased to Unitec for student accommodation say they want the agency to buy them before making them potentially uninhabitable, Mr Alexander said they lay outside a zone where "greater than negligible" building damage was predicted.

He acknowledged they had foundations of mixed construction methods, which could increase vulnerability to damage from differential ground movements, but expected that to be "non-structural and repairable."

There would be pre- and post-construction inspections on the buildings, as well as others on the Unitec campus and the Pak'n' Save supermarket in New North Rd.

Geotechnical engineer Peter Millar said that some residents may perceive a hum from night-and-day tunnelling machines, but he told the hearing that the level of vibration from these would be less than 10 per cent of what may cause building damage.

Rock along the tunnels' route was relatively weak, meaning underground blasting was unlikely, although blasting was planned through a basalt layer at their southern portals in Allan Wood Reserve.