Tuesday, April 18, 2006

Earthquake liquefaction

[One hundred years ago today...]
Earthquake liquefaction, often referred to simply as liquefaction, is the process by which saturated, unconsolidated soil or sand is converted into a suspension during an earthquake. The effect on structures and buildings can be devastating, and is a major contributor to urban seismic risk. Ancient earthquakes have caused liquefaction, leaving a record in the sediments (paleoseismology).

Sand Compaction

Liquefaction essentially means that the soil is turned into a liquid. The key ingredient is a formation of loose, saturated sand. As seen in the figure, uniform sand grains can be packed either in a loose or a compact (dense) formation. Loose sand has usually been deposited gently underwater, either naturally, or sluiced into what is called hydraulic fill. The loose grains can support considerable weight, with the help of the water, which forms a good portion of the mass.

Once strong earthquake shaking begins, the grains are sheared into the more compact arrangement. The water, however, interferes, and the grains float in a liquid slurry. The excess water is squeezed out which causes the quicksand condition at the surface. If there is a soil crust or impermeable cap, then the sand boils out in the form of sand volcanoes (commonly called sandblows).

Soil liquefaction can be dangerous if it leads to landslides or building foundation failures. Mapping the location of old liquefaction zones, called paleoliquefaction allows scientists to determine the strength and location of ancient earthquakes.

In certain areas of the world there is something called quick clay, which is just as dangerous as liquefaction sands. Either should be avoided or stabilized, which typically involves the field of geotechnical engineering.


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