Earthquake liquefaction
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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).
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Liquefaction occurs when a saturated sand formation 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, as they are in contact with each other in a statically stable formation.
Once strong earthquake shaking begins, the grains move and are no longer resting on each other. Eventually, the grains will settle into a more compact arrangement. However, this transition is not immediate, and requires excess water to leave the formation. For a short period of time, depending how long it takes for the water to drain from the formation, 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 dry soil crust or impermeable cap, the sand slurry will sometimes come to the surface through cracks in the confining layer, bringing much of the excess water with it, creating sand boils, colloquially called "sand volcanos".
Soil liquefaction can cause damage to structures in several ways. Buildings whose foundations bear directly on sand which liquefies will experience a sudden loss of support, which will result in drastic and irregular settlement of the building. Liquefaction causes irregular settlements in the area liquefied, which can damage buildings and break underground utility lines where the differential settlements are large. Sand boils can erupt into buildings through utility openings, and may allow water to damage the structure or electrical systems. Soil liquefaction can also cause slope failures. Areas of land reclamation are often prone to liquifaction because many are reclaimed with hydraulic fill, and are often underlain by soft soils which can amplify earthquake shaking.
Study of past liquefaction events in geologic formations can provide information about the strength of prehistoric earthquakes, a vital component of paleoseismology
Liquefaction primarily occurs in sands, but there are formations of quick clay, which can cause many of the same problems as liquefiable sands.
Mitigating potential damage from liquefaction is part of the field of geotechnical engineering.
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