What is liquefaction?

Liquefaction is the process whereby, during earthquake shaking, sand and silt grains in wet soil try to compact and, as a result, 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 things on top, like 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 crevisses in the ground or concrete. The water takes silt and sand with it, forming sand boils or sand volcanos, or when there is lots of it, filling up large areas with sand and silt.

Liquefaction only happens in loose sandy or silty soils that are 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 and it doesn't take much shaking to for them to liquefy. Others are less susceptible, and the shaking needs to be very strong, or go on for a long time, for them to liquefy. This explains why different areas might liquefy in different earthquakes.

You can find out more about liquefaction from our liquefaction poster.

Liquefaction susceptibility maps

Various liquefaction susceptibility maps are available for different parts of the Canterbury region.

A joint project between Environment Canterbury, Christchurch City Council, Selwyn District Council, Waimakariri District Council and the Natural Hazards Research Platform is currently underway to review liquefaction hazard information for these local authorities in light of the Canterbury earthquakes. There is more information on this project below.

Christchurch City

Several liquefaction susceptibility maps have been produced for Christchurch City over the last 20 years. The most recent maps are those developed between 2001 and 2005 by Beca Ltd. The map is broad scale and based on geological and limited geotechnical data. It indicates general areas of different liquefaction susceptibility, but within these areas there can still be some variation in the type of soil. Because of the variability of soil types within Christchurch city, it is extremely difficult to determine liquefaction susceptibility at a particular site without a site specific investigation.

Liquefaction hazard map - summer groundwater level (2005) (746kB)
Liquefaction hazard map - winter groundwater level (2005) (802kB)
Liquefaction ground damage map - summer groundwater level (2005) (746kB)
Liquefaction ground damage map - winter groundwater level (2005) (744kB)

Solid facts on Christchurch liquefaction poster (2006)

Waimakariri District

A liquefaction study for coastal areas of Waimakariri was undertaken in 2000. This report is available as a hard copy from Environment Canterbury customer services or Waimakariri District Council.

Predicted settlement from liquefaction in coastal Waimakariri district from an earthquake in the Canterbury foothills (2000) (110kB)

Further geotechnical data gathered since 2000 shows that the liquefaction susceptibility in coastal Waimakariri is likely to be even more variable than what is shown on the above map. Because of the variability of soil types within coastal Waimakariri district, it is extremely difficult to determine liquefaction susceptibility at a particular site without a site specific investigation.

A district-scale map of liquefaction susceptibility was produced for Waimakariri District Council as part of an earthquake hazard assessment for engineering lifelines in 2009. The map is broad scale and based mainly on geological information. It indicates general areas of different liquefaction susceptibility, but within these areas there can still be some variation in the type of soil. Liquefaction susceptibility at a particular site can only be determined through an analysis of that particular site.

Waimakariri district liquefaction susceptibility map (2009) (1.2MB)

Selwyn District

A district-scale map of liquefaction susceptibility was produced for Selwyn District Council as part of an earthquake hazard assessment for engineering lifelines in 2006. The map is broad scale and based mainly on geological information. It indicates general areas of different liquefaction susceptibility, but within these areas there can still be some variation in the type of soil. Liquefaction susceptibility at a particular site can only be determined through an analysis of that particular site.

Selwyn district liquefaction susceptibility map (2006) (2.5MB)

Following the September 2010 earthquake, Selwyn District Council commissioned a report to outline areas where liquefaction occurred during the earthquake, and where liquefaction may occur in future. This report, and additional information, can be found on the Selwyn District Council website.

Other districts

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)

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)

Review of liquefaction hazard information for Christchurch City, Selwyn and Waimakariri districts

A joint project between Environment Canterbury, Christchurch City Council, Selwyn District Council, Waimakariri District Council and the Natural Hazards Research Platform is currently underway to review the liquefaction hazard in light of the Canterbury earthquakes.

The Canterbury earthquakes, and the geotechnical data collected since they started, have provided a huge amount of information. We are using this information to update the liquefaction hazard information we hold using a consistent methodology across the three local authority areas.

The study area covers the area from Southbridge to Amberley, and is roughly based on the area of the Greater Christchurch Urban Development Strategy, as this is the area where development is most likely over the next 35 years. It also incorporates the south-eastern portion of Hurunui district, as this area has similar soils to coastal Waimakariri district. Just because an area is within the study area doesn't mean it is susceptible to liquefaction - the study area boundary is based mainly on the Urban Development Strategy area, not on liquefaction susceptibility.

The project builds on the land zoning recently produced for greater Christchurch by Canterbury Earthquake Recovery Authority and the Department of Building and Housing. However, rather than concentrating on zoning land for rebuilding, this project is focusses on future development. The information will also be useful for engineering lifelines asset management and emergency management planning.

The project includes four work streams:

1. Compiling maps of where liquefaction and lateral spreading occurred during the September 2010, February 2011 and June 2011 earthquakes, and collating available information on other historic earthquakes. Comparing these maps to existing liquefaction susceptibility maps.
2. Collating available geotechnical data, including data from council records, and creating a 3D geological/geotechnical model for the project area.
3. Calculating ground shaking hazard from the regional earthquake model.
4. Creating liquefaction hazard maps for the project area for a range of return periods, and maps that the city and district councils can use to guide future development and the level of geotechnical investigation required.

We anticipate that draft results will be available around mid-2012. The information will assist local authorities and lifeline utilities in issuing building consents, managing infrastructure networks and emergency management.