liquefaction

liquefaction change of a substance from the solid or the gaseous state to the liquid state. Since the different states of matter correspond to different amounts of energy of the molecules making up the substance, energy in the form of heat must either be supplied to a substance or be removed from the substance in order to change its state. Thus, changing a solid to a liquid or a liquid to a gas requires the addition of heat, while changing a gas to a liquid or a liquid to a solid requires the removal of heat. In the liquefaction of gases, extreme cooling is not necessary, for if a gas is held in a confined space and is subjected to high pressure, heat is given off as it undergoes compression and it turns eventually to a liquid. Some cooling is, however, necessary; it was discovered by Thomas Andrews in 1869 that each gas has a definite temperature , called its critical temperature, above which it cannot be liquefied, no matter what pressure is exerted upon it. A gas must, therefore, be cooled below its critical temperature before it can be liquefied. When a gas is compressed its molecules are forced closer together and, their vibratory motion being reduced, heat is given off. As compression proceeds, the speed of the molecules and the distances between them continue to decrease, until eventually the substance undergoes change of state and becomes liquid. Although before the 19th cent. a number of scientists had experimented in liquefying gases, Davy and Faraday are usually credited with being the first to achieve success. The production of liquefied gases in large quantities (and consequently their use in refrigeration) was made possible by the work of Z. F. Wroblewski and K. S. Olszewski, two Polish scientists. The work of Sir James Dewar is also important, especially in the liquefaction of air and its change to a solid. Heike Kamerlingh Onnes first liquefied helium. The critical temperature of helium is -267.9°C, only a few degrees above absolute zero (-273.15°C). The processes for the liquefaction of gases as developed by Linde and others form the basis for those used in modern refrigeration . Liquefied gases are much used in low-temperature research; some, e.g., liquid oxygen, find use as rocket propellants. See liquid air ; low-temperature physics .

liquefaction The process of becoming or making a liquid by heating, cooling, or a change in pressure. In soils, the temporary transformation of material to a fluid state due to a sudden decrease in shearing resistance caused by a collapse of the structure associated with a temporary increase in pore fluid pressure.

liquefaction Liquefaction is a condition in which a soil or sediment behaves as a fluid. This is the result of strong vibration and is normally related to a moderate to large earthquake. The process of liquefaction normally occurs only in sands and silty sands, although recent evidence suggests that gravels may also liquefy. The actual process of liquefaction is complex. Soils are composed of a mineral phase and a fluid phase. During an earthquake the soil mass may begin to compact. When this happens, the water in the pore spaces between grains exerts a higher pressure on the mineral particles. It has also been shown that the passage of seismic waves, which are a transient stress in their own right, also has an effect on these forces, referred to as pore-water pressures. As these pore-water pressures increase, the grains are forced apart, there is less frictional contact between sand grains, and the shear strength of the soil decreases. When the soil loses all its shear strength, liquefaction has occurred. This process is therefore important in soils that gain their strength from internal friction (i.e. grain-to-grain contact). Liquefaction does not occur in clay-rich sediments because the strength of clays generally comes from interparticle bonds.

The susceptibility of soils to liquefaction is controlled by a number of factors. These can be divided into two groups: the ground conditions, and the characteristics of the earthquake ground motions. The ground conditions that give rise to a high liquefaction susceptibility are low density, water saturation, and a silty sand particle-size distribution. The distribution of grain sizes is critical, and it is possible to identify a range of particle sizes at which liquefaction is likely to occur. Dense sands, with low water contents, are less likely to liquefy. The geological history of the soil is important, since river-deposited sands are more likely to liquefy than glacial deposits because of their differences in density.

In general, the larger the earthquake, the greater the possibility that liquefaction will occur. The duration of shaking is, however, a critical factor. A long duration of seismic shaking allows for the gradual increase in pore-water pressures as each stress pulse passes through the soil. The greater the stress pulse (measured as a ground acceleration), the greater increase in pore-water pressure. A long duration of shaking with a high ground acceleration is therefore more likely to cause liquefaction than a short duration of shaking with low ground acceleration. This would normally require an earthquake of magnitude greater than 5.

The engineering effects of liquefaction can be profound. The complete loss of bearing strength had serious consequences during the 1964 Niigata earthquake in Japan. The Kwangishicho apartment block in Niigata itself was tilted through an angle of over 40^o as a result of liquefaction around the foundations, although the buildings themselves did not suffer structural failure. Buried objects, such as septic tanks, have been found to float to the surface in liquefied sediments. Liquefaction can also result in a lateral spread, or flow slide, which can occur on slopes that are normally too low for landslides to occur. This effect was seen during the 1964 Alaska earthquake in which major landslides occurred at Turnagain Heights and Government Hill owing to liquefaction of underlying sand layers.

W. Murphy