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Research On Wellington Reclaimed Land Provides Seismic Treasure Trove For Engineers

New research into the 2016 Kaikōura earthquake’s impact on reclaimed land in Wellington’s waterfront is giving engineers invaluable information for improving infrastructure design.

University of Canterbury researcher Ribu Dhakal successfully defended his PhD recently, based on four years researching liquefaction and the behaviour of reclaimed soils along Wellington harbour.

“The ground motion from the Kaikōura earthquake was relatively minor in Wellington, compared to what would occur in an earthquake on the Wellington fault, but the damage to the ground and consequent effect on buried structures and building foundations was still quite significant and more severe than we would have historically expected,” says Dhakal, whose research identified how the different areas around the harbour were affected in 2016.

The research, which was funded by Toka Tū Ake EQC, has already provided great insight for New Zealand geotechnical engineers for designing buildings that will cope with the seismic hazards on parts of the reclaimed land.

Toka Tū Ake EQC Chief Resilience and Research Officer Dr Jo Horrocks says that Dhakal’s research will help towards the organisation’s goal to improve Aotearoa New Zealand’s resilience to natural hazards by understanding what is happening under the surface of the Earth.

”Toka Tū Ake EQC invests in research that helps build resilient communities and Ribu’s PhD research will lead to useful information for building engineers, designers and planners,” she says.

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Dhakal says that the research has attracted interest from colleagues and engineers around the world grappling with similar issues.

"The port of Wellington has an extremely well-documented case history including recorded ground motions for several earthquakes within the port with detailed liquefaction observations, making it appropriate for an in-depth study of liquefaction problems."

“Our project is unique in that it is possibly one of the most well-documented liquefaction case history of reclaimed soils anywhere in the world.”

He explains how the team conducted over 100 cone penetration tests in a small 0.5 km2 area at the port of Wellington, as well as drilling dozens of soil samples which were tested at the University of Canterbury geomechanics lab.

The team discovered that the liquefaction in the southern part of the harbour had been severe, including gravelly soils where experts historically may not have expected it to occur. As a result of the liquefaction, some of the land area settled by half a metre and experienced more than a metre of lateral movement.

“All this data has provided us with invaluable information to truly quantify the seismic hazard of different types of reclaimed land. Engineers working in this area will now have a much better understanding about where they can expect liquefaction and how much ground movement may occur,” says Dhakal.

While some buildings on Wellington’s reclaimed land suffered catastrophic damage, Dhakal says that the research also showed that existing liquefaction assessment methods for most areas in the Wellington port had performed reasonably well.

“But even within those existing methods, we identified areas where they could be improved by providing better estimates of how certain soils will behave,” says Dhakal, who has started on the next phase of the project in his post-doctoral work.

“The next part includes more lab testing and numerical simulations that will give engineers even better modelling for future seismic events.”

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