What is liquefaction?
Liquefaction occurs when sandy or silt soils are saturated and then exposed to ground shaking during an earthquake. The soil particles are shaken so that they settle and water that was between the soil particles rises to the top, meaning that the ground behaves more like a liquid than a solid. The result is silt and fine sand boiling up and covering streets and gardens. It’s a bit like when you stand at the water’s edge and stand in the wet sand. You will probably sink a bit after a while, as the sand around your feet becomes sloppy.
Liquefaction is most likely to happen at the coastal edge, in flood plains, along the edges of waterways like rivers and lakes, in estuaries and swamps and where fill has not been compacted properly or land has been reclaimed.
What are the consequences of liquefaction?
The ground level can rise due to sand and silt coming to the surface and creating a mess, with dust and contaminated soil being a health hazard for people. Houses become structurally unsafe and uninhabitable, sinking as the ground underneath can no longer support them. There can be damage to other property e.g. cars can become stuck in the silt/sand. People’s health and wellbeing can also be affected. Many people had to move out of their homes permanently as a result of liquefaction after the Christchurch earthquake.
Liquefaction can produce a lot of material that needs to be disposed of – 500,000 tonnes of material had to be stockpiled after the Christchurch earthworks and it is still sitting there. Waterways can also change shape and course as a result of liquefaction.
Infrastructure can be affected too. After the Christchurch earthquake, power lines dropped into the ground so that they were at an unsafe height. Wastewater tanks and pipes shifted in the ground so that there was no longer enough slope to carry the wastewater through the lines. In the Kaikoura earthquake, the main fibre optic cable for the South Island was damaged.
How do I know if my property is affected by liquefaction?
A geotechnical engineer will be able to provide you with the best information about whether your property is at risk. They will determine the level of risk based on what you want to do and local soil, groundwater conditions and seismic hazards. However, the understanding of liquefaction is not perfect, nor the actual risk of an earthquake happening, so there will always be some uncertainty.
At the moment, there is a lot of inconsistency around hazard mapping across New Zealand. There is a project underway to help councils make their hazard maps consistent across the nation. There is also a new guidance document to help professionals analyse the risk of liquefaction i.e. whether it is unlikely or possible and what the potential damage is. If you are interested in this topic, you can read the document here Planning and engineering guidance for potentially liquefaction-prone land.
How does this affect planning?
Some District and Regional Plans do not have any references to liquefaction and where references are included, they are inconsistent across the country. There are no specific references to liquefaction in the Auckland Unitary Plan. However, resource consent applications should address the risks of natural hazards where appropriate and this is a requirement of the RMA. In the future, expect to see more references to liquefaction in District and Regional Plans.
What can we do to minimise the risk of liquefaction when building?
At the moment, land defined as ‘good ground’ under New Zealand Standard 3604 (which is a standard for timber structures) may still be prone to liquefaction. If you are uncertain, ask more questions of your building professionals!
You should seek guidance from a professional engineer about the best foundation options for your build, like pile foundations or a stiff raft slab. Some bigger builds also use techniques to improve the ground conditions before construction, like compacting the ground or putting in earthquake drains.
Need some help?
Please contact us if you need resource consent for land development. We can prepare your resource consent application and we have a network of contacts to advise on specific liquefaction or other geotechnical matters. You can contact us on hello@planningplus.co.nz or 09 427 9966.
Disclaimer As with all our blogs, the information detailed here is general in nature and meant as a preliminary guide only. This should not be substituted for your own investigations or use of your own professional’s. Planning Plus is not liable for any errors or omissions.
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