subsidence


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subsidence,

lowering of a portion of the earth's crust. The subsidence of land areas over time has resulted in submergence by shallow seas (see oceansocean,
interconnected mass of saltwater covering 70.78% of the surface of the earth, often called the world ocean. It is subdivided into four (or five) major units that are separated from each other in most cases by the continental masses. See also oceanography.
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). Land subsidence can occur naturally or through human activity. Natural subsidence may occur when limestone, which is easily carved by underground water, collapses, leaving sink holes on the surface, such as in Florida. Earthquakes can also cause subsidence of the land because of the movement of faultsfault,
in geology, fracture in the earth's crust in which the rock on one side of the fracture has measurable movement in relation to the rock on the other side. Faults on other planets and satellites of the solar system also have been recognized.
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. Permafrost, or the permanently frozen ground in tundra regions, can subside during local warming trends, a phenomena called thermokarst. Oceanic crust produced at spreading ridges (see seafloor spreadingseafloor spreading,
theory of lithospheric evolution that holds that the ocean floors are spreading outward from vast underwater ridges. First proposed in the early 1960s by the American geologist Harry H.
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) subsides after cooling, as do calderas, the craterlike features at a volcano's peak. An atoll is a coralcoral,
small, sedentary marine animal, related to the sea anemone but characterized by a skeleton of horny or calcareous material. The skeleton itself is also called coral.
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 reef that forms a ring with no apparent central peak and may form when volcanic islands subside—an explanation first proposed by Charles Darwin. Human activity has contributed greatly to subsidence over the last few centuries. For example, withdrawal of oil from the field at Long Beach, California, beginning in 1936 resulted in subsidence at rates ranging from 0.5 to 2.0 ft (0.15–0.61 m) per yr in the center of the field. By 1962 the center of the oil field had subsided slightly over 27 ft (8.5 m), caused by the removal of fluid from the pore spaces in the underground rock, allowing the grains to compact. Similarly, withdrawal of groundwater through well pumping has resulted in subsidence in such cities as Mexico City, Houston, Tex., and Venice, Italy. Subsidence is also caused by the collapse of underground salt, ore, and coal mines.

Subsidence

 

the compaction of soil owing to external stress or the soil’s own weight. It occurs in loess and loesslike deposits after artificial wetting and may result from thawing (thermal subsidence in frozen soils) and such dynamic actions as vibration subsidence. The soil’s surface may subside from a fraction of a centimeter to 2 meters. Subsidence may cause cracks on the surface and in the soil mass. If percolation of moisture in soils compacted by wetting occurs after subsidence, deformation of the soil is possible owing to leaching from the soil of water-soluble compounds.

The cause of subsidence in loess and loesslike deposits is insufficient compaction of the soil and accompanying loss of stability in particle bonding as a result of wetting. With soil of a given moisture content, a specific porosity corresponding to each degree of pressure decreases with increasing pressure. The bonds between particles in soil may keep it compact despite an increase in pressure caused by the weight of new deposits or of structures. This gives rise to a disparity between porosity and pressure, or a state of insufficient compaction. Subsidence also results from lowered stability of particle bonding in soil when loess is wetted by leakage from water pipes or when the groundwater level near a reservoir rises. Insufficient compaction of loess and loesslike deposits is characteristic of such arid semi-desert or steppe regions as those in Middle Asia, the Ukraine, the Northern Caucasus, China, Southern Central Europe, and the Mississippi basin. Thermal subsidence may occur in the permafrost zone.

The subsidence properties of loess and loesslike soils are studied in compression testing devices and by wetting trenches. The extent of the compaction of soil when wetted in relation to the original elevation of a soil sample is called the relative degree of subsidence and may vary from 0 to 0.1 or more. Subsidence may arise when the moisture content of soil increases to a certain level (initial moisture content of subsidence) or when pressure exceeds a certain level (initial pressure of subsidence). Conditions for construction on loess and loesslike soils are of two types: surface subsidence of less than 5 cm resulting from the weight of the wet ground itself, and surface subsidence of more than 5 cm. Different conditions require different construction measures.

Steps taken to control subsidence during construction include wetting of the ground, silicification, compaction, and firing. Structural measures are also taken, and the danger of water harming the foundations of structures is eliminated.

REFERENCES

Denisov, N. Ia. Stroitel’nye svoistva lessa i lessovidnykh suglinkov, 2nd ed. Moscow, 1953.
Abelev, Iu. M., and M. Iu. Abelev. Osnovy proektirovaniia i stroitel’stva na prosadochnykh makroporistykh gruntakh, 2nd ed. Moscow, 1968.
Gil’man, Ia. D., and V. P. Anan’ev. Stroitel’nye svoistva lessovykh gruntov i proektirovanie osnovanii i fundamentov. Rostov-on-Don, 1971.

N. I. KRIGER

subsidence

[səb′sīd·əns]
(meteorology)
A descending motion of air in the atmosphere, usually with the implication that the condition extends over a rather broad area.
(mining engineering)
A sinking down of a part of the earth's crust due to underground excavations.

subsidence

A sinking of an entire area, in contrast to the settlement of an individual structure.

subsidence

A descending motion of air in the atmosphere over a broad area, as in polar regions.
References in periodicals archive ?
We've installed over 300 ground and property monitoring points across the estate, which we've been monitoring for over 14 months and in this time the subsidence zone has not increased in area, so we're confident to say that it is confined to this one part of the estate.
This is the first time that groundwater and subsidence data have been put together to illustrate the story of what is happening to our land and water resources in the region, said Sachin Shah, USGS chief of Gulf Coast hydrologic studies and research.
Three more villages in Bojnurd County are also prone to land subsidence and may need to be relocated, he said.
Monitoring of the subsidence using geodetic methods has a long tradition in this region (Neset, 1984).
Relatively easy solutions can often be found for subsidence, such as fixing a defective drain or water pipe.
Cracks that are wider at the top than the bottom, and that run diagonally, can be a sign of subsidence, too, as can a crack that's on both the inside and outside of a wall.
Research showed that in the past, BC geologists conducted a technical study in the immediate area after the 1990 earthquake and concluded the area was considered unstable due to the presence of fault zone and reports of localized subsidence.
Apart from the increased risk of floods and associated diseases, experts consulted for this article say subsidence threatens health in other ways.
In other words, the regional subsidence of earth's surface includes the sinking of the lower layer of the earth's surface which can also has a little horizontal shift.
Neighboring Fort Bend County, on the other hand, which still relies on the Gulf Coast Aquifer for 60 percent of its water, is farther inland, and the effects of subsidence can be less tangible.
When we told our insurance firm they informed us of the high excess for subsidence claims.
That's bad news for the local government, which had already put a stop to groundwater pumping in an effort to curb the city's subsidence.