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 oceans). 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 faults. 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 spreading) subsides after cooling, as do calderas, the craterlike features at a volcano's peak. An atoll is a coral 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.

In geology, engineering, and surveying, subsidence is the motion of a surface (usually, the Earth's surface) as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation. There are several types of subsidence, listed below in order of increasing scale:

Dissolution of limestone

Subsidence frequently occurs in karst terrains, where dissolution of limestone by fluid flow in the subsurface causes the creation of voids (i.e. caves). If the roof of these voids becomes too weak, it can collapse and the overlying rock and earth will fall into the space, causing subsidence at the surface. This type of subsidence can result in sinkholes which can be many hundreds of meters deep and can provide areas of ecological isolation which see the evolution of new branches of animal and plant life.

Mining-induced

Several types of sub-surface mining, and specifically methods which intentionally cause the extracted void to collapse (such as pillar extraction, longwall mining and any metalliferous mining method which utilises "caving" such as "block caving" or "sub-level caving") will result in surface subsidence. Mining induced subsidence is relatively predictable in its magnitude, manifestation and extent, except where a sudden pillar or near-surface underground tunnel collapse occurs (usually very old workings). Mining induced subsidence is nearly always very localised to the surface above the mined area, plus a margin around the outside [1]. The vertical magnitude of the subsidence itself typically does not cause problems, except in the case of drainage (including natural drainage) - rather it is the associated surface compressive and tensile strains, curvature, tilts and horizontal displacement that are the cause of the worst damage to the natural environment, buildings and infrastructure. Where mining activity is planned, mining-induced subsidence can be successfully managed if there is co-operation from all of the stakeholders [2]. This is accomplished through a combination of careful mine planning, the taking of preventative measures, and the carrying out of repairs post-mining.

Faulting induced

When differential stresses exist in the Earth, these can be accommodated either by geological faulting in the brittle crust, or by ductile flow in the hotter and more fluid mantle. Where faults occur, absolute subsidence may occur in the footwall of normal faults. In reverse, or thrust, faults, relative subsidence may be measured in the hangingwall.

Isostatic rebound

The crust floats buoyantly in the plastic asthenosphere, with a ratio of mass below the "surface" in proportion to its own density and the density of the asthenosphere. If mass is added to the crust (e.g. through deposition), the crust is thought to subside minisculely to compensate and maintain isostatic balance.

Extraction of natural gas

If natural gas is extracted from a natural gas field the initial pressure (up to 600 bar) in the field will drop over the years. The gas pressure also supports the soil layers above the field. If the pressure drops, the soil pressure increases and this leads to subsidence at the ground level. Since exploration of the Slochteren (Netherlands) gas field started in the late 1960s the ground level over a 250 km² area has dropped with a current maximum of 30 cm [3]. See also this subsidence lecture.

This type of subsidence can similarly be caused by extraction of other resources, e.g. ground water, petroleum or rock salt.

Groundwater-related

Main article: Groundwater-related subsidence

The habitation of lowlands, such as coastal or delta plains, requires drainage. The resulting aeration of the soil leads to the oxidation of its organic components, such as peat, and this decomposition process may cause significant land subsidence. This applies especially when ground water levels are periodically adapted to subsidence, in order to maintain desired unsaturated zone depths, exposing more and more peat to oxygen. In addition to this, drained soils consolidate as a result of increased effective stress. In this way, land subsidence has the potential of becoming self-perpetuating, having rates up to 5 cm/yr. Water management used to be tuned primarily to factors such as crop optimisation but, to varying extents, avoiding subsidence has come to be taken into account as well.

Seasonal effects

See also: expansive clay

Many soils contain significant proportions of clay which because of the very small particle size are affected by changes in soil moisture content. Seasonal drying of the soil results in a reduction in soil volume and a lowering of the soil surface. If building foundations are above the level to which the seasonal drying reaches they will move and this can result in damage to the building in the form of tapering cracks. Trees and other vegetation can have a significant local effect on seasonal drying of soils. Over a number of years a cumulative drying occurs as the tree grows, this can lead to the opposite of subsidence, known as heave or swelling of the soil, when the tree declines or is felled. As the cumulative moisture deficit is reversed, over a period which can last as many as 20 years, the surface level around the tree will rise and expand laterally. This is often more damaging to buildings unless the foundations have been strengthened or designed to cope with the effect.

See also

• Soil consolidation

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