Liquefaction

Soil liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. Liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world.

Liquefaction occurs in saturated soils, that is, soils in which the space between individual particles is completely filled with water. Prior to an earthquake, the water pressure is relatively low--the weight of the buried soil rests on the framework of grain contacts that comprise it. However, earthquake shaking can disrupt the structure, the soil particles no longer support all the weight, and the groundwater pressure begins to rise. The soil particles can move farther, and become entrained in the water--the soil flows. Liquefied soil will force open ground cracks in order to escape to the surface. The ejected material often results in flooding and may leave cavities in the soil.

Whether and where liquefaction will take place depends on many factors. These include the degree of saturation, the grain size distribution and consistency at a site, the strength, duration, and frequency content of the shaking and even the grain shape and depth of soil. There is much active research into the mechanisms of liquefaction, because its effects can be so severe yet its process remains imperfectly understood.

The following movies are of flooding and sandblows from the February 2011 Christchurch NZ, 6.3 earthquake:

The consequences to structures and utilities of earthquake-induced liquefaction include:

1) Non-uniform and differential settlement of structures often resulting in cracking.

2) Loss of bearing support

3) Flotation of buried structures such as sewer lines, tanks, and pipes.

4) Strong lateral forces against retaining structures such as seawalls.

5) Lateral spreading (limited lateral movement)

6) Lateral flows (extensive lateral movement)

Areas with a high to moderate susceptibility to liquefaction are often mapped in urban areas.  See the page Liquefaction Hazard Maps for more information.