Liquefaction is the process whereby, during earthquake shaking, sand and silt grains in wet soil compact and the water in the spaces between the grains is squeezed. The water 'pushes back' and pressure builds up in the water until the silt and sand grains 'float' in the water. The soil particles are then not able to support the weight of buildings and roads and they can tilt or sink. 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 sand volcanos, or when there is a lot of it, filling up large areas with sand and silt.
Liquefaction only happens in loose sandy or silty soils below the water table. It doesn't happen in clay soils, which tend to stick together, and it is very unusual in gravelly soils. Some soils are very susceptible to liquefaction. Others are less susceptible.
In 2012, a joint project to review the liquefaction hazard 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 report has been produced in close collaboration with the Canterbury Earthquake Recovery Authority (CERA) and the Ministry of Business, Innovation and Employment (MBIE).
Download the full report here (15.5 MB). The report has also been broken up into four parts for faster download.
Liquefaction Report includes Appendices 1-3 (4.8 MB)
Appendix 4 (3.1 MB)
Appendix 5 (5.9 MB)
Appendix 6 (1.6 MB)
The Canterbury earthquakes, and the geotechnical data collected since they started, have provided a large amount of information. We have used this information to update liquefaction hazard information using a consistent methodology across the three local authority areas.
The study 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 study area also incorporates the south-eastern part of Hurunui district, because this area has similar soils to coastal Waimakariri district.
The project* builds on the land zoning recently produced for greater Christchurch by CERA and MBIE. However, rather than concentrating on zoning land for rebuilding, this project focuses on future development, so it covers only land outside the CERA/MBIE Technical Category areas.
The information will help local authorities and lifeline utilities in issuing building consents, and managing infrastructure networks and emergencies.
The study area has been separated into two "assessment areas". The section of the map (right) in grey, covering the western part of the study area and most of the Port Hills, is labelled "Damaging liquefaction unlikely". The yellow section covering the eastern parts of the study area and some low-lying, coastal parts of Banks Peninsula, is labelled "Liquefaction assessment needed". The green area is covered by the CERA/MBIE Technical Categories, and is not included in the study.
Click here for larger image
This map does not define areas that will and will not liquefy if a large earthquake happens. Soil types vary considerably throughout the greater Christchurch area, and liquefaction may happen on one property but not next door. However, the soils that make up the "Damaging liquefaction unlikely" (grey) assessment area tend not to liquefy, apart from small pockets. In the "Liquefaction assessment needed" (yellow) assessment area, the soils are much more variable, and some sites will have soils that can liquefy, while others have only soils that are unlikely to do so. Because the study was carried out at a regional scale, it cannot be used to say anything about the soil types on an individual property.
This map will be used by councils to help them decide whether a separate geotechnical assessment relating to liquefaction will be needed for development, and for subdivision and building permits to be granted.
The MBIE guidelines cover the whole of the greater Christchurch area. The guidelines outline the amount of geotechnical assessment needed so that buildings can be built safely, taking into account the type of ground they are built on. The guidelines will continue to set out the types of investigation that must to be done in the "Liquefaction assessment needed" (yellow) assessment area, but they will no longer require a specific liquefaction-related assessment in the "Damaging liquefaction unlikely" (grey) area. This means many property owners will now need to do less geotechnical assessment than before, but proper safety standards will be maintained during development.
Geotechnical investigations into other natural hazards, such as subsidence, land instability and flooding, will still need to be done in both assessment areas. The geotechnical professional in charge of these investigations will also be required to see whether the soils are of the type that may liquefy, even if the property is in the "Damaging liquefaction unlikely" (grey) assessment area. This is because there will be pockets of soils that can liquefy within the grey area.
*The project included four workstreams:
District-scale maps of liquefaction susceptibility have been produced for each district in the region as part of earthquake hazard assessments for engineering lifelines over the past decade.
These maps are broad scale and are based mainly on geological information. They indicate general areas of different liquefaction susceptibility, but within these areas there can still be some variation in the type of soil. This means that there is no guarantee that liquefaction will or will not occur in a particular zone during a strong earthquake - it indicates general areas of more or less susceptibility. Liquefaction susceptibility at a particular site can only be determined through an analysis of that particular site. The likelihood of liquefaction in an area also depends on the likelihood of strong earthquake shaking to generate liquefaction, and this varies across the region.
Kaikoura district liquefaction susceptibility map (2009) (834kB)
Hurunui district liquefaction susceptibility map (2011) (852kB)
Waimakariri district liquefaction susceptibility map (2009) (1.2MB)
Selwyn district liquefaction susceptibility map (2006) (2.5MB)
Ashburton district liquefaction susceptibility map (2002) (1.9MB)
Timaru district liquefaction susceptibility map (2001) (1.6MB)
Waimate, Mackenzie and part Waitaki districts liquefaction susceptibility map (2008) (8.3MB)
Note that the eastern parts of the Waimakariri and Selwyn district maps and the southeastern corner of the Hurunui district map have been superceded by the 2012 Review of liquefaction hazard information in eastern Canterbury, including Christchurch City, and parts of Selwyn, Waimakariri and Hurunui Districts (report downloadable above).
Liquefaction hazard maps created by Beca for coastal Waimakariri district (2000) and Christchurch City (2005) have also been superceded by the 2012 Review of liquefaction hazard information in eastern Canterbury, including Christchurch City, and parts of Selwyn, Waimakariri and Hurunui Districts (report downloadable above). However, these maps are still available from Environment Canterbury Customer Services.
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