Liquefaction - Definition and Overview

Liquefaction is the process by which saturated, unconsolidated soil or sand is converted into a suspension. It is commonly observed in quicksand, earthquakes, and wave loading. It can be caused when flowing water reduces the friction between sand particles (as from an underground spring), or when a sudden change in pressure or repeated shock acting on water saturated or supersaturated sediments (as in an earthquake). Although the effects of liquefaction had been observed and understood for years, it was dramatically brought to the attention of engineers and seismologists in 1964 during the Niigata, Japan and Alaska earthquakes. It was a major factor in the destruction in San Francisco's Marina District during the 1989 Loma Prieta earthquake.

Liquefaction in earthquakes

The shock or repeated shock of earthquake waves can cause water-saturated soil to rearrange itself in such a way that it essentially becomes a suspension of solids in the liquid. Heavy structures on such areas can suddenly sink or shift. Buried objects can shift or float to the surface.

"Often during earthquakes, fine-grained water-saturated sediments may lose their former strength and form into a thick mobile mudlike material. The process is called liquefaction. The liquefied sediment not only moves about beneath the surface but may also rise through fissures and “erupt” as mud boils and mud 'volcanoes.'" (Levin 251)

"... the ground shaking reduces the strength of earth material on which heavy structures rest. Parts of many major cities, particularly port cities, have been built on naturally occurring bodies of soft, unconsolidated clay-rich sediment (such as the delta deposits of a river) or on filled areas in which large amounts of loose earth materials have been dumped to build up the land level. These water-saturated deposits often experience a change in property known as liquefaction when shaken by an earthquake. The material loses strength to the degree that it becomes a highly fluid mud, incapable of supporting buildings, which show severe tilting or collapse." (Strahler 202).

This can be demonstrated on a small scale by saturating a bucket of sand with water; place a stone on the currently solid top of the sand, then repeatedly strike the side of the bucket with a hammer and watch the rock sink.

For a video showing liquefaction in action, see this page (http://www.ce.washington.edu/~liquefaction/html/what/what1.html).

Liquefaction in Quicksand

Quicksand forms when water saturates an area of loose sand and the ordinary sand is agitated. When the water trapped in the batch of sand can't escape, it creates liquefied soil that can no longer support weight. Quicksand can be formed by standing or flowing underground water (as from an underground spring), or by earthquakes. In the case of flowing underground water, the force of the water flow opposes the force of gravity, causing the granules of sand to be more buoyant. In the case of earthquakes, the shaking force can increase the pressure of shallow groundwater, liquefying sand and silt deposits. In both cases, the liquefied surface loses strength, causing buildings or other objects on that surface to sink or fall over.

The saturated sediment may appear quite solid until a change in pressure or shock initiates the liquifaction causing the sand to form a suspension with each grain surrounded by a thin film of water. This cushioning gives quicksand, and other liquefied sediments, a spongy, fluidlike texture. Objects in the liquefied sand sink to the level at which the weight of the object is equal to the weight of the displaced sand/water mix and the object floats due to its buoyancy.

Wave-loading

Wave loading is a phenomenon we experience when walking on a beach. As a wave goes back out to sea, the sand assumes a soft, mushy texture, and recedes below one's feet. This happens because at the height of each wave, water is forced into the sand and the sand is saturated. When the wave recedes and a person steps on this saturated sand the increase in pressure causes a small amount of liquification and the foot displaces the liquefied sand.

Liquefaction causes beaches to be sandy. Sand particles rise slightly above the floor as each wave trough approaches, and the current consistently moves toward the beach as liquefaction lifts sand particles above the floor, causing sand to move up the beach, and causing beaches to be sandy (Discover).

Turbidity currents

Submarine landslides are turbidity currents and consist of a flow of water saturated sediments flowing downslope. An example occurred 18 November 1929, when an earthquake struck the continental slope off the coast of Newfoundland. Minutes later, transatlantic phone cables began breaking sequentially, farther and farther downslope, away from the epicenter. Twelve cables were snapped in a total of 28 places. Exact times and locations were recorded for each break. Investigators suggested that a 60-mile-per-hour submarine landslide or turbidity current of water saturated sediments swept 400 miles down the continental slope from the earthquake’s epicenter, snapping the cables as it passed. (Heezen and Ewing 849–873).

See also

• dry quicksand
• flood geology - a creationist view of liquefaction
• liquid limit
• plastic limit (soils)
• thixotropy

References

• Harold L. Levin, Contemporary Physical Geology, 2nd edition (New York: Saunders College Publishing, 1986).
• Arthur N. Strahler, Physical Geology (New York: Harper & Row, Publishers, 1981), p. 202.
• “Breakthroughs in Science, Technology, and Medicine,” Discover, November 1992, p. 14.
• Bruce C. Heezen and Maurice Ewing, “Turbidity Currents and Submarine Slumps, and the 1929 Grand Banks Earthquake,” American Journal of Science, Vol. 250, December 1952, pp. 849–873.
• How quicksand forms (http://ibs.howstuffworks.com/ibs/sac/quicksand1.htm)
• Soil Liquefaction (http://www.ce.washington.edu/~liquefaction/html/main.html)
• detailed dicussion based on research conducted in 1936 (http://www.ce.washington.edu/~liquefaction/html/why/why2.html)
• Niigata earthquake. (http://www.ce.washington.edu/~liquefaction/html/quakes/niigata/niigata.htm)
• Alaska earthquake. (http://www.ce.washington.edu/~liquefaction/html/quakes/alaska/alaska.html)