Earthquake tests in Porirua to show how to protect homes on hills

Earthquake tests carried out on a farm in Porirua will help Kiwis understand how hillside homes fare in a big shake, the Earthquake Commission (EQC) says.

EQC and the Building Research Association of New Zealand (Branz) are set to release their research into the issue in June.

The groups worked together with Victoria University to build and test different types of house foundations on sloping farmland, so they could investigate how to reduce earthquake damage to homes on hills.

"Houses built on slopes may have foundations that are significantly taller at one end than the other, or there may be different types of foundation under the same home," said Branz project lead Roger Shelton.

"This can result in a twisting motion, making earthquake damage more likely.

The study follows observations of damage from the Christchurch earthquakes, where the same level of shaking caused more damage to homes in hills than it did to those on flat areas.

"There are several questions that we needed to answer and this is an attempt to start answering those," Shelton said.

Branz identified much of the damage had to do with poorly performing foundations in the homes on the slopes.

New Zealand timber building standards were based on historical practices and testing on level sites with loadings considerably below those on a sloping site.

Shelton said the best way to find out what happens to foundations in an earthquake was to build a field laboratory and "create a shake".

"We built four different types of foundations on a sloping paddock north of Wellington, using pile foundations and combinations of piles and foundation walls," he said.

"The team used details from the building standard NZS 3604:2011, as well as older, more traditional foundation details.

"We added dead weight to the floors of each foundation to replicate the true mass of the house and used a counter-rotating shaker to simulate an earthquake."

Researchers would stop at stages before the foundations were irreparably damaged, and would try out techniques to reduce damage, then start the shaking again.

They found while most foundations performed fairly well under shaking, the weakest link was the connections between the concrete and the timber, which corroded and rotted over time.

Shelton hoped the project would provide updated guidance for house foundations on sloping sites for new homes, as well as existing properties.

"We also hope this project will provide approximate costs for different techniques that homeowners can use to strengthen foundations under their existing houses," he said.

EQC head of resilience strategy and research Dr JO Horrocks said the tests were "super important" for learning the weaknesses Kiwi homes had and figuring out the best ways to fix them.

They found certain homes were more at risk than others, such as split-level houses or ones with more glass at the front compared to at the back.

"A lot of houses in New Zealand towns are built on slopes. It's important we understand what's going to work best on that type of land, not just on flat land, where most of the research has been done until now," she said.

"It's all about making New Zealand homes more resilient to earthquakes."