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Liquefaction


March 2011

Fact sheets compiled and distributed by the Institution of Professional Engineers of New Zealand

Download the 3 fact sheets (pdf 1 MB)

The recent sequence of earthquakes and aftershocks in the Christchurch area has highlighted a phenomenon that previously has had a very low public profile. Now, 'liquefaction' is visible. Its effects in Christchurch are extensive and have resulted in significant damage to property, buildings and infrastructure, not to mention creating a widespread mess. Silt, sand and water bubbled up in people's backyards, in streets and parks and even through the concrete floors of buildings. Some refer to the sand and silt as liquefaction, but that is not correct. The soil at the surface is a result of liquefaction.

WHAT IS LIQUEFACTION AND WHY DOES IT OCCUR?

Liquefaction is the process that leads to a soil suddenly losing strength, most commonly as a result of ground shaking during a large earthquake. Not all soils however, will liquefy in an earthquake. The following are particular features of soils that potentially can liquefy:

  • They are sands and silts and quite loose in the ground. Such soils do not stick together the way clay soils do.
  • They are below the watertable, so all the space between the grains of sand and silt are filled with water. Dry soils above the watertable won't liquefy.

When an earthquake occurs the shaking is so rapid and violent that the sand and silt grains try to compress the spaces filled with water, but the water pushes back and pressure builds up until the grains 'float' in the water. Once that happens the soil loses its strength it has liquefied. Soil that was once solid now behaves like a fluid.

WHAT HAPPENS NEXT?

Liquefied soil, like water, cannot support the weight of whatever is lying above it be it the surface layers of dry soil or the concrete floors of buildings. The liquefied soil under that weight is forced into any cracks and crevasses it can find, including those in the dry soil above, or the cracks between concrete slabs. It flows out onto the surface as boils, sand volcanoes and rivers of silt. In some cases the liquefied soil flowing up a crack can erode and widen the crack to a size big enough to accommodate a car.

Some other consequences of the soil liquefying are:

  • Settlement of the ground surface due to the loss of soil from underground.
  • Loss of support to building foundations.
  • Floating of manholes, buried tanks and pipes in the liquefied soil - but only if the tanks and pipes are mostly empty.
  • Near streams and rivers, the dry surface soil layers can slide sideways on the liquefied soil towards the streams. This is called lateral spreading and can severly damage a building. It typically results in long tears and rips in the ground surface that look like a classic fault line.

Not all of a building's foundations might be affected by liquefaction. The affected part may subside (settle) or be pulled sideways by lateral spreading, which can severely damage the building. Buried services such as sewer pipes can be damaged as they are warped by lateral spreading, ground settlement or floatation.

Liquefaction

Prepared with the assistance of Members of the New Zealand Society for Earthquake Engineering - 4 March 2010

For any further information:

Institution of Professional Engineers of New Zealand www.ipenz.org.nz

New Zealand Society for Earthquake Engineering Inc www.nzsee.org.nz

 


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