Freiberg pool. Photography: Mark Mitchell
When the state of Wellington’s earthquake at Freyberg Pool was on a knife edge, an engineer told the city council he would see if they could “rebound” a little more to put the building in a clean spot.
It turns out they can, reports and correspondence issued under the Local Government Meetings and Official Information Act revealed.
Despite an initial assessment finding the building was earthquake-prone, the pool was rated 40 percent of the New Building Standard (NBS) after engineering consultancy Beca was brought in for a second opinion.
A building is considered earthquake-prone if it is rated at less than 34 percent of the NBS.
If a building is considered earthquake-prone, it must be strengthened for safety reasons, which can be a costly, time-consuming and cumbersome process. Therefore, the opinion of engineers can have serious consequences for the owners of the premises.
In this case, the owner of Freyberg Pool is Wellington City Council.
liquefaction concerns
In 2016, detailed seismic assessments were conducted for several council ponds to verify the earthquake status.
Freyberg Pool has an indicative NBS rating of 45 percent, but there has been some uncertainty about liquefaction and what that might mean for the building’s structural performance.
The main point at stake was how the building, located on Wellington’s waterfront, would interact with the land.
Liquefaction is the process that causes soil to behave more like a liquid than a solid, usually during a moderate or strong earthquake.
Therefore, a geotechnical investigation of the land below the pond was carried out, which included taking readings and soil samples through the wells.
These findings were brought together in a draft report by Spencer Holmes, with geotechnical assessment by Tonkin & Taylor, and presented to council officials in June 2021.
The report considered Freyberg Pool to be between 25 to 30 per NBS suppressed, which means it is vulnerable to earthquakes.
Workers clean up black liquefaction sticky substance in Christchurch. Photo/Brett Phipps
It was found at a moderate level of ground shaking under the pond and was expected to leak and the building’s foundations were no longer supported properly.
A summary of the report stated that the connection between the building structure and the piles would likely fail, causing movement at the base of the stands.
“Because the terraces are attached directly to the roof structure, there is a possibility that the roof could be demolished using the terraces.”
One treatment option involved installing new piles around the perimeter of the pool and floor beams to hold the structure together, at a cost of $46.7 million.
The NBS rating was significantly lower because something called a “step change” had to be implemented, effectively halving the rating. It is applied when engineers find that the ground cannot support the building, resulting in catastrophic failure.
Given the significance of these findings, council officials considered it prudent to seek a second opinion.
second opinion
BECA’s chief structural engineer, Rob Gorey, told the Herald that their job was to facilitate discussion between geotechnical and construction experts.
“He was really trying to encourage them to look at different things in terms of what they actually found, but to put the two things together.”
During this process, in November 2021, an engineer from Beca sent an email to the board official informing him of where the audit was headed.
The engineer said the building will likely remain earthquake-prone.
“We are not fully there yet, and we are doing some additional checks to see if we can make it happen [sic] A little more, and get the building up by more than 33 percent. There is still a chance.”
Freyberg Swimming Pool is located in the Oriental Parade. Photography: Mark Mitchell
The jury said he did not consider this email inappropriate and was satisfied with what was said.
“We’re trying to raise these scores and ratings for these buildings to the highest level you can reasonably justify…. You have to go down every road I think because as I say, you should basically give these buildings the best chance without over-inciting.”
Stuart Palmer, technical director of engineering for the Tonkin and Taylor Seismic Foundation, acknowledged that the initial assessment was on the conservative side.
“It’s relatively easy to be conservative about these things, it’s really hard to be realistic.”
“It was nice to have Beca to challenge the simplistic approach we’ve been taking so far and let us think ‘Well, maybe if we look at it this way, or that way, is that going to change our perspective?'” “
In the end, Palmer said, the three advisers were happy with their final opinion.
Further evaluation found that pond piles can actually resist the flow of earth around them, which means they can still support the roof.
This means that there is no longer a justification to apply the geological step change point because in reality there will be no catastrophic failure of the building.
So Freyberg Pool was given an NBS rating of 40 percent, meaning it was not earthquake-prone.
It’s easy for engineers to be too conservative
It’s not unusual for NBS’s rating to change between the initial rating and the final rating, Palmer said. In this case, the difference between applying the step change or not applying it was the difference between considering the building to be earthquake-prone.
“Rob and I have been in a number of these fights. Do we apply gradual change in this situation, or not? And it’s not easy, it’s not easy at all.”
The jury said that just because there were initially different opinions about the pool didn’t mean any of them were completely wrong.
“It just means that you have to take the results objectively and put them together.”
The jury said it was easy for the engineers to be too conservative because they had nothing to gain from being too liberal.
“The reality is we have to be more realistic in the way we rate these buildings. We can’t be conservative, it’s just sending all the wrong mixed messages out there. If the community is really worried about going into a low rating they won’t be able to move.”
The CTV building became a symbol of the February 2011 earthquake – when it collapsed and killed 115 people. Photography: Mark Mitchell
Both the jury and Palmer agreed that there was an overemphasis on NBS ratings.
Palmer said he was concerned that some companies were seeking ratings of 80 percent and 67 percent, well above the legal requirements of 34 percent.
The jury said there was still a lot of uncertainty, and the NBS ratings were absolutely nothing.
Earthquake-prone buildings can still survive earthquakes while others do not. For example, Statistics House, a relatively new building on the waterfront in Wellington, partially collapsed in the 2016 Kaikoura earthquake.
The jury said the focus was on prioritizing buildings and keeping the “bad” out of the way.
“Our evaluation approaches really find the worst buildings, because we want to find the security cameras.”
The CCTV building in Christchurch quickly and almost completely collapsed, killing 115 people, when the February 2011 earthquake struck.
So, is Freyberg Pool safe?
Wellington City Council’s chief customer and community officer, Kim Vail, said he was confident the pool was not earthquake-prone following the final opinion of three technical experts.
“I’m comfortable with that, I don’t have any real concerns about that.”
Phil said there was plenty of life left at Freyberg Pool.
In the meantime, the pool will be undergoing general maintenance until information is available indicating significant population growth in the area, Vail said.
He then said that the council might consider new facilities or expansion.
So, is Freyberg Pool safe? Yes it is, but the jury pointed out that “safe” is not an absolute term.
He also said he wouldn’t have a problem with his grandchildren swimming there.
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Sources
1/ https://Google.com/
2/ https://www.nzherald.co.nz/nz/how-engineers-decided-wellingtons-freyberg-pool-wasnt-actually-earthquake-prone/2GQEWCYGOMSNQ475PLEOB5YV3Y/
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