Wellington's Freyberg Pool potentially earthquake-prone

Freyberg Pool. Photo / Mark Mitchell

Freyberg Pool, where generations of Wellingtonians have learned to swim, is potentially earthquake-prone.

New information following the 2016 Kaikōura earthquake has raised questions about how the building would cope with something called the "basin-edge effect".

Recently Wellington City Council, the owner of the building on Oriental Parade, commissioned engineering consultants as part of an upcoming five-yearly maintenance closure of the pool next year.

These consultants have raised questions about the relationship between the building and the ground conditions it sits on, city council chief customer and community officer James Roberts told the Herald.

But the information provided to the council is only preliminary.

"The work done by the consultants is complex and the methodology for assessment is relatively new", Roberts said.

"The next step the council agreed, with support of engineering experts, that an independent review of the draft report be undertaken".

The pool currently has a New Building Standard (NBS) rating of between 45 and 50 per cent based on a July 2016 assessment. An earthquake-prone building is anything less than 34 per cent.

The new information provided to the council did not come to a conclusion about what any revised NBS rating would be, Roberts said.

The building remains open and there is also no legal requirement for it to close it if it is deemed earthquake-prone.

The council is scheduled to receive a progress update by the end of November, with the report on the review due by the end of March.

Wellington City Council chief resilience officer Mike Mendonça. Photo / Supplied

Basin-edge effects resulted in concentrated areas of damage during the 1994 Northridge and 1995 Kobe earthquakes in California and Japan.

City council chief resilience officer Mike Mendonça said it was also experienced in Wellington following the Kaikōura earthquake.

Central Wellington is situated on the edge of a geological basin. When energy from an earthquake runs along the seafloor and hits the angle of the basin edge, it can amplify ground shaking.

"The damage from the Kaikōura earthquake is all on the basin edge in spots. The big spot was where BNZ and Statistics House was. Thorndon was also a real hotspot for us", Mendonça said.

"So we've found this in a couple of spots and this is the latest where it's an issue for us - Freyberg Pool is right on the basin edge."

A subsequent investigation into why Statistics House partially collapsed in the Kaikōura earthquake identified the basin-edge effect as one of the reasons.

But at that time, it was noted there wasn't sufficient international or national research to say conclusively where a basin-edge effect could be predicted.

Mendonça said identifying any potential seismic issues with buildings was part of the long-term journey to make Wellington more resilient

"We're being honest enough here to have look at the state of our land, the state of our buildings, and we're fixing them."

In the case of Freyberg pool, the council also acts as the regulator and has confirmed it considers the building to be potentially earthquake-prone.

Owners of buildings thought to be potentially earthquake-prone have one year to provide information to the council to prove otherwise.

The regulatory arm of the council will then decide whether it is in fact earthquake-prone.

Freyberg Pool has been one of the capital's landmark buildings since being completed in 1963.

It's a category 1 heritage-listed building.

The Heritage New Zealand listing said the building is an example of modernist architecture as well as having considerable social significance as a venue where generations of people have learnt to swim.

The history of swimming pools in Wellington and the Clyde Quay area dates to the 1860s.

But it was not until the early 1960s that the city council acted on considerable public pressure for an indoor pool.