Liquefaction

Liquefaction occurs when loose silts and sands below the water table become suspended in the groundwater as the ground shakes during earthquake shaking.

The sediment grains are then no longer supporting the ground above, but the weight of that ground falls back upon a layer that has now basically become a liquid. With continued ground shaking pressure builds up and forces the water to the surface through cracks and crevices in the ground. The water carries with it the suspended sediment and ejects it at the ground surface, and can lead to subsidence as this material settles, and lateral spreading, where stretch or translation of land occurs adjacent to waterways.

Liquefaction and its effects. IPENZ (now Engineering New Zealand) (2011).

What we know

Actual and likely effects of liquefaction

Recent earthquakes have demonstrated vulnerability of the land in the regeneration strategy project area to liquefaction and its effects.

Observed effects of liquefaction and subsidence or uplift following the 2010 and 2011 Canterbury earthquakes included damage to property, stormwater and wastewater infrastructure, roads, changes in river and coastal processes and ecosystems because of uplift and subsidence, and changes in overland drainage patterns, as outlined in figure 19.


Extent of hazard

This map categories the liquefaction and lateral spreading observations following the February 2011 earthquake, and outlines the differences in liquefaction vulnerability across the area.

The worst affected areas of observed liquefaction were generally red zoned after the earthquakes. However, most of the land along the estuary edge in Southshore, and either side of Bridge Street has also experienced ejected material during the earthquake and is more vulnerable to future events then the rest of the Regeneration Strategy project area.


Why this is important

The widespread liquefaction in the Canterbury earthquakes has resulted in an extensive amount of research and guidance, developed locally and nationally, on past occurrence of, and future vulnerability to liquefaction.

There is also a broad understanding of the effects, and how to mitigate these. As a result of this awareness, building constructed after the earthquakes are likely to be built to the latest standards for liquefaction-prone land and more resilient than older building stock.


What we don’t know

While the observed liquefaction is currently the best available information on future susceptibility of land, Christchurch City Council has commissioned more detailed investigations on liquefaction risk which will provide further clarity which will be used to inform this project when available at the end of 2018.

Liquefaction occurs when loose silts and sands below the water table become suspended in the groundwater as the ground shakes during earthquake shaking.

The sediment grains are then no longer supporting the ground above, but the weight of that ground falls back upon a layer that has now basically become a liquid. With continued ground shaking pressure builds up and forces the water to the surface through cracks and crevices in the ground. The water carries with it the suspended sediment and ejects it at the ground surface, and can lead to subsidence as this material settles, and lateral spreading, where stretch or translation of land occurs adjacent to waterways.

Liquefaction and its effects. IPENZ (now Engineering New Zealand) (2011).

What we know

Actual and likely effects of liquefaction

Recent earthquakes have demonstrated vulnerability of the land in the regeneration strategy project area to liquefaction and its effects.

Observed effects of liquefaction and subsidence or uplift following the 2010 and 2011 Canterbury earthquakes included damage to property, stormwater and wastewater infrastructure, roads, changes in river and coastal processes and ecosystems because of uplift and subsidence, and changes in overland drainage patterns, as outlined in figure 19.


Extent of hazard

This map categories the liquefaction and lateral spreading observations following the February 2011 earthquake, and outlines the differences in liquefaction vulnerability across the area.

The worst affected areas of observed liquefaction were generally red zoned after the earthquakes. However, most of the land along the estuary edge in Southshore, and either side of Bridge Street has also experienced ejected material during the earthquake and is more vulnerable to future events then the rest of the Regeneration Strategy project area.


Why this is important

The widespread liquefaction in the Canterbury earthquakes has resulted in an extensive amount of research and guidance, developed locally and nationally, on past occurrence of, and future vulnerability to liquefaction.

There is also a broad understanding of the effects, and how to mitigate these. As a result of this awareness, building constructed after the earthquakes are likely to be built to the latest standards for liquefaction-prone land and more resilient than older building stock.


What we don’t know

While the observed liquefaction is currently the best available information on future susceptibility of land, Christchurch City Council has commissioned more detailed investigations on liquefaction risk which will provide further clarity which will be used to inform this project when available at the end of 2018.