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 in the water 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 sand volcanos, or when there is a lot of it, filling up large areas with sand and silt. The ground surface above the liquefied soil often tilts and sinks. Buildings, roads, pipes and tanks on or in liquefied soil are often damaged by the tilting or sinking of the ground.
To liquefy, the soil must be:
Liquefaction usually only happens in susceptible soils in moderate to strong ground shaking (when household items start to fall and break).
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.
Environment Canterbury has commissioned several liquefaction reports and maps for Canterbury since 2000.
The reports and maps outline broad zones where liquefaction is more or less likely due to the general nature of the soils and water table in the area. None of the reports or maps give the likelihood of liquefaction for a particular site or land parcel. This is because soil types, and thus the tendency for liquefaction, can change a lot over very short distances - so soils on one property might be prone to liquefaction, while soils on the neighbouring property aren't. To assess the liquefaction hazard for every single property in Canterbury would cost tens of millions of dollars. Instead, these reports and maps show areas where liquefaction is less likely and special geotechnical assessment is not needed, and areas where liquefaction is more likely. In these more likely 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 CERA/MBIE Technical Category areas.
The report includes maps of where liquefaction occurred during the September 2010 Darfield (Canterbury) earthquake and the February 2011 Christchurch earthquake.
GIS maps of the liquefaction zones are available on Canterbury Maps and the GIS data files are also available through Environment Canterbury Customer Services.
Greater Christchurch liquefaction zone map
Greater Christchurch liquefaction hazard information full report (2012) (15.5 MB).
Greater Christchurch liquefaction hazard information report - frequently asked questions
Greater Christchurch liquefaction hazard information - GIS map
The report has also been broken up into four parts for faster download.
Report and Appendices 1-3 (Land classification, Background to the earthquakes, Mapping of liquefaction during the September 2010 and February 2011 earthquakes) (4.8 MB)
Appendix 4 (Review of pre 2010 liquefaction susceptibility maps) (3.1 MB)
Appendix 5 (Geological information relevant to the review) (5.9 MB)
Appendix 6 (Probabilistic liquefaction hazard maps) (1.6 MB)
The report does not define areas that will and will not liquefy in a large earthquake. Soil types vary considerably throughout the greater Christchurch area, and liquefaction may happen on one property but not next door. In the report, the project area has been split into two zones. In the "Damaging liquefaction unlikely" (grey) zone the soils tend not to liquefy. If liquefaction does occur in this zone it would be in very small areas wouldn't cause significant damage. In this zone special liquefaction assessments are not required for building. In the "Liquefaction assessment needed" (yellow) zone the the soils are much more variable - some will liquefy, causing varying amounts of ground damage, and some are unlikely to liquify. Because soils can be very different over short distances in this zone, special geotechnical assessments are needed before building to determine how likely the soil is to liquefy at that particular site, and what foundations or soil treatment is required so that future liquefaction is less likely to damage buildings. Just because a property is within the "Liquefaction assessment needed" zone doesn't mean that the soil will liquefy in an earthquake - it means that further investigations are required to determine whether the soil is likely to liquefy or not.
A liquefaction susceptibility map was developed for Hurunui district in 2011. Liquefaction susceptibility maps had been developed for other districts in Canterbury as part of earthquake hazard assessments for engineering lifelines over the past decade, but Hurunui district, being the first assessment in 2000, did not have a liquefaction map. The accompanying report outlines historical liquefaction occurrences in Hurunui district, and gives an explanation of the four susceptibility zones. The zones are based primarily on geological information and limited borehole data. GIS maps of the liquefaction susceptibility zones are available on Canterbury Maps and the GIS data files are also available through Environment Canterbury Customer Services.
Hurunui district liquefaction susceptibility map (852 kB)
Liquefaction hazard in Hurunui district report (2011) (2.8 MB)
Hurunui district liquefaction susceptibility - GIS map
Note that the southeastern corner of the Hurunui district map has been superseded 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).
The liquefaction susceptibility maps for Timaru district, developed as part of the earthquake hazard assessment for engineering lifelines in Timaru district in 2001, have been updated. The original maps were based primarily on geological information and some limited borehole data. This new study incorporates borehole information collected since 2001, as well as data collected from test pits in Geraldine and Washdyke, and Cone Penetration Tests (CPTs) in three parts of Timaru township. GIS maps of the liquefaction susceptibility zones are available on Canterbury Maps and the GIS data files are also available through Environment Canterbury Customer Services.
Timaru district liquefaction susceptibility map (4.2 MB)
Timaru township liquefaction susceptibility map (3.7 MB)
Liquefaction hazard in Timaru district - full report (2013) (21.1 MB)
Liquefaction hazard in Timaru district report - frequently asked questions
Timaru district liquefaction susceptibility - GIS map
The report has also been broken up into parts for faster download.
Liquefaction hazard in Timaru district report (2013) (114 kB)
Appendix A - Figures (including susceptibility maps) (4 MB)
Appendix B - Methodology (19 kB)
Appendix C - Geraldine Growth Areas (2.2 MB)
Appendix D - Washdyke (2.1 MB)
Appendix E - Timaru City coastal areas (5.7 MB)
Appendix F - Ground Shaking Hazard (66 kB)
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)
Waimakariri district liquefaction susceptibility map (2009) (1.2MB)
Selwyn district liquefaction susceptibility map (2006) (2.5MB)
Ashburton district liquefaction susceptibility map (2002) (1.9MB)
Waimate, Mackenzie and part Waitaki districts liquefaction susceptibility map (2008) (8.3MB)
GIS maps of all these liquefaction susceptibility zones are also available on Canterbury Maps.
Note that the eastern parts of the Waimakariri and Selwyn 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 (report downloadable above).
Liquefaction hazard maps created by Beca for coastal Waimakariri district (2000) and Christchurch City (2005) 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 (report downloadable above). However, these maps and the accompanying GIS data are still available from Environment Canterbury Customer Services.
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