Destructive testing on the decommissioned Whirokino Trestle has given modern engineers some fascinating insights into the expertise and workmanship of their peers a century ago.
The 90-year-old bridge on State Highway 1 south of Foxton (along with the existing Manawatu River Bridge) was recently replaced by Waka Kotahi NZ Transport Agency with a wider $70m structure over the Manawatū River and Moutoa floodplain, and the deconstruction of the old Trestle has given Dr Lucas Hogan from the University of Auckland an unprecedented opportunity for real life earthquake testing.
“It was a pleasant surprise how well these old foundations held their strength under huge deformations,” says Hogan.
“How the Ministry of Works built their bridges was well ahead of their time and world leading, as far as building bridges to withstand earthquakes.”
The past week, Hogan’s team have been pulling the exposed foundation piles back and forth to simulate earthquake movements, before taking other parts of the bridge back to Auckland for additional testing.
The research, funded by the Earthquake Commission (EQC) and QuakeCoRE, is being carried out in cooperation with Waka Kotahi, as the owner of the bridge, and demolition sub-contractors Jurgens Demolition working alongside the lead contractor, Fletcher Construction, who have fine-tuned the deconstruction programme to fit with the scientific needs of the University of Auckland team.
Hogan says the results will be significant to anyone involved in infrastructure design, planning and maintenance.
“The Ministry of Works built a lot of bridges like this, so knowing how this bridge behaves is very helpful. By doing this destructive testing, like what an earthquake would do, we can find out how other bridges in the country would behave,” says Hogan, who adds that the 2km length of the Whirokino Trestle creates additional questions and opportunities.
“These big bridges behave differently from short bridges. With long bridges, one end will start shaking before the other end, which creates a whipsaw effect that really amplifies the motions.”
Hogan says this is effect is not yet well understood in the world because there have not been many long bridges researched with this level of instrumentation.
The Whirokino Trestle is similar to many bridges over the braided rivers in the South Island that may be vulnerable to an Alpine Fault event.
“So knowing how this one behaves, will help us predict how the others may behave.”
Dr Jo Horrocks, Head of Strategic Research and Resilience at EQC, explains that the Commission invests over $17m each year in research to create stronger homes and infrastructure, to reduce the impact of natural hazards and is proud that EQC-funding can support the Whirokino project.
“We’re really pleased to be part of this project. That we have so many organisations involved shows the importance of ensuring New Zealand has resilient infrastructure,” says Dr Horrocks.
Waka Kotahi Senior Manager, Project Delivery, Andrew Thackwray says with more than 4,500 bridges on New Zealand’s roading networks, strong, safe bridges are vital to keep the country moving.
“It’s a testament to New Zealand’s engineers that we have a robust network of bridges that have proven to be very resilient, despite all the natural hazards we are exposed to in this country. We’re looking forward to seeing the results of this research and expect that it will enable us to build even more resilient bridges,” he says.
Hogan says that this unique project will also draw huge interest from overseas colleagues.
“Pile testing like this is rare, and people are rarely allowed to pull them over to the extent we have done here.”
Once the field work is completed, the scientists will create a computer model to assess the vulnerability of other bridges around the country and help prepare for future earthquakes.
“We will be able to predict damage, but also assess what it would mean for transport, communication, or sewerage, that may be running along those bridges. Knowing where the vulnerabilities are, will really pay off when one of our communities is unfortunate enough to face any of these events because it allows emergency management organisations to prepare ahead of the earthquake.”