Liquefaction is the process where, during earthquake shaking, sand and silt grains in wet soil are rearranged and the water in the spaces between the grains is squeezed.
Pressure builds up until the silt and sand grains float in the water, and the soil behaves more like a liquid than a solid. The pressurised water is forced up to the ground surface through the easiest path it can find - often through cracks and crevasses in the ground or concrete.
The water takes silt and sand with it, forming sand boils or volcanos. The ground surface above liquefied soil often tilts and sinks. Buildings, roads, pipes and tanks on or in liquefied soil can be damaged by the tilting or sinking of the ground.
To liquefy, the soil must be:
- loose - compacted soils tend not to liquefy
- sandy or silty - clays and gravels tend not to liquefy
- wet - the soil must be below the water table.
Liquefaction usually only happens in susceptible soils in moderate to strong ground shaking.
During an earthquake, liquefied soils can sometimes move sideways, usually towards streams or rivers, but also if the surrounding land is lower than the liquefied land. This is called lateral spreading. Lateral spreading can cause cracking of the ground surface as the soil underneath it moves sideways.
None of the reports give the likelihood of liquefaction for a particular site or land parcel. This is because soil types can change a lot over very short distances - even from one neighbouring property to another..
Assessing the liquefaction hazard for every single property in Canterbury would cost tens of millions of dollars. Instead, the reports and maps show areas where liquefaction is less likely (so special geotechnical assessment is not needed), and areas where liquefaction is more likely.
In these areas a geotechnical assessment may be needed before building to assess more accurately how susceptible the soil is to liquefaction and what can be done to minimise any future damage from liquefaction. In this way a geotechnical assessment for a particular site is only done where and when it is needed.
In 2012, a joint project to review liquefaction hazard information in light of the Canterbury earthquakes was completed by Environment Canterbury, Christchurch City Council, Selwyn District Council, Waimakariri District Council and the Natural Hazards Research Platform.
The project area extends from Southbridge to Amberley, and is based on the area of the Greater Christchurch Urban Development Strategy. This is the area where development is most likely over the next 35 years.
The project area also incorporates the south-eastern part of Hurunui district, because this area has similar soils to coastal Waimakariri district. Rather than concentrating on zoning land for rebuilding, this project focuses on future development, so it covers only land outside the Ministry of Business, Innovation and Employment Technical Category areas.
The report includes maps of where liquefaction occurred during the September 2010 Darfield (Canterbury) earthquake and the February 2011 Christchurch earthquake.
These maps show general areas of different liquefaction susceptibility, based mainly on underlying geology, but within these areas there can still be some variation in the type of soil. They do not show where liquefaction will or will not happen in a strong earthquake, only areas of more or less susceptibility.
The liquefaction susceptibility at a particular site can only be determined through a geotechnical assessment at that site.
You can see where the liquefaction susceptibility zones are on our interactive map on Canterbury Maps.
Note that the eastern parts of the Waimakariri, Selwyn and Hurunui district maps have been superseded by the 2012 Review of liquefaction hazard information in eastern Canterbury, including Christchurch City, and parts of Selwyn, Waimakariri and Hurunui Districts.