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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.
..... Click the link for more information. ). 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.
..... Click the link for more information. . 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.
..... Click the link for more information. ) 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.
..... Click the link for more information. 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.
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.
REFERENCESDenisov, 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