Liquefaction is the process which causes soil to behave more like a liquid than a solid during an earthquake.
The shaking rearranges sand and silt grains in wet soil underground and the water between the grains is squeezed. Pressure builds up until the grains float and water is forced up to the surface through the easiest path it can find, often through cracks in the ground or concrete. The rising water takes silt and sand with it, forming sand boils or volcanos. The ground surface above liquefied soil often tilts and sinks, damaging buildings, roads, pipes and tanks.
To liquefy, the soil must be loose, sandy or silty (clay and gravel tend not to liquefy) and wet (below the water table).
Liquefaction usually only happens in susceptible soils in moderate to strong ground shaking (when it is difficult to stand up, things are being moved around, and buildings and infrastructure are being damaged).
Liquefied soils can move sideways, usually towards streams or rivers, or downhill towards lower ground. This is called lateral spreading. The ground's surface can crack when the soil underneath it moves.
For other areas in Canterbury, see our liquefaction storymap
Liquefaction susceptibility has been mapped in most areas of Canterbury to show whether liquefaction is possible, or unlikely, during strong earthquake shaking. The maps are mainly based on the underlying geology (for example, rock, old river gravels, relatively young estuary silts) and on borehole and groundwater information (depth to the water table).
Within these areas soils can change a lot over very short distances, even from one neighbouring property to another, because of the way sediments were laid down by different river, estuary or beach processes. The only way to determine the actual liquefaction susceptibility at a particular site is to undertake a geotechnical assessment involving testing of the soil on that site.
Our mapping means that site-specific geotechnical assessments to determine liquefaction susceptibility generally only need to be done in areas where liquefaction is considered possible, rather than requiring assessments for every property.
Most areas in Canterbury, outside urban Christchurch City, have very low to no liquefaction susceptibility and, in general, a site-specific geotechnical investigation is not required before building.
In areas where the underlying geology and groundwater information indicate that damaging liquefaction is possible, your city or district council may require you to get a site-specific geotechnical investigation as part of a subdivision or building consent. This will more accurately assess how susceptible the soil is to liquefaction and what can be done to minimise damage from liquefaction - for example, building stronger foundations.