New Zealand will be mapped for liquefaction-prone soils raising foundation costs for new homes

Foundations will become more expensive for houses built on liquefaction-prone soil across New Zealand, not just in Canterbury.

Rules about specially designed foundations for new homes on liquefaction-prone sites are being extended to the rest of the country after applying in Canterbury following the earthquakes which started in September 2010.

The Government has given territorial authorities - city and district councils - a two-year transition to map their areas for liquefaction. The changes have been brought in with the six-monthly update to the Building Code.

Ministry of Business Innovation and Employment (MBIE) says that buildings on liquefaction-prone ground will require specifically-designed foundations suitable for the site soil conditions. The changes will promote more robust, safer and more resilient buildings.

Groups of volunteers are clearing sand and silt from liquefaction in the east Christchurch suburb of Parklands after a swarm of earthquakes at Christmas 2011.
KIRK HARGREAVES/STUFF
Groups of volunteers are clearing sand and silt from liquefaction in the east Christchurch suburb of Parklands after a swarm of earthquakes at Christmas 2011.

"When we consulted on the proposed changes in August 2019, we received strong feedback that the changes are necessary and the building sector is supportive," Dave Robson, manager of building performance and engineering at MBIE said.


A "waffle" foundation design, also known as a "raft" foundation has been used widely in the Christchurch rebuild. The foundation contains polystyrene blocks, reinforcing steel and concrete.
SUPPLIED
A "waffle" foundation design, also known as a "raft" foundation has been used widely in the Christchurch rebuild. The foundation contains polystyrene blocks, reinforcing steel and concrete.

"The changes are already in place in the Canterbury region, so this provides much needed clarity to both councils and engineers. It ensures new homes across New Zealand are being built safely and strongly enough to withstand liquefaction risks."

Robson said there were a range of foundation solutions that had been designed by the engineering community, often referred to as 'raft foundations', for use in Canterbury.

​Some councils had already completed the liquefaction mapping, "but for the ones who have not, there will be a slight increase in work until this is done," he said.

MBIE, alongside other Government agencies, would also be developing an education campaign to make sure the sector fully understood the new rules and how to incorporate them into the work they did.


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