Subsidence
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Subsidence
Subsided house, called The Crooked House, the result of 19th-century mining subsidence.
Mam Tor road destroyed by subsidence and shear, near Castleton, Derbyshire.

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. Ground subsidence is of concern to geologists, geotechnical engineers and surveyors.

Dissolution of limestone

Subsidence frequently causes major problems 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.

Mining

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 uses "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[1]). Mining-induced subsidence is nearly always very localized to the surface above the mined area, plus a margin around the outside.[2] 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.[3]

Where mining activity is planned, mining-induced subsidence can be successfully managed if there is co-operation from all of the stakeholders. This is accomplished through a combination of careful mine planning, the taking of preventive measures, and the carrying out of repairs post-mining.

Types of ground subsidence

Extraction of natural gas

If natural gas is extracted from a natural gas field the initial pressure (up to 60 MPa (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 overburden pressure increases and this may lead to subsidence at the ground level.

Since exploitation of the Slochteren (Netherlands) gas field started in the late 1960s the ground level over a 250 km² area has dropped by a current maximum of 30 cm.[4]

Earthquake

The Geospatial Information Authority of Japan reported immediate subsidence caused by the 2011 T?hoku earthquake.[5] In Northern Japan, subsidence of 0.50 m (1.64 ft) was observed on the coast of the Pacific Ocean in Miyako, T?hoku, while Rikuzentakata, Iwate measured 0.84 m (2.75 ft). In the south at S?ma, Fukushima, 0.29 m (0.95 ft) was observed. The maximum amount of subsidence was 1.2 m (3.93 ft), coupled with horizontal diastrophism of up to 5.3 m (17.3 ft) on the Oshika Peninsula in Miyagi Prefecture.[6]

Groundwater-related subsidence

San Joaquin Valley 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.[7][8] 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.

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 hanging wall of normal faults. In reverse, or thrust, faults, relative subsidence may be measured in the footwall.

Isostatic subsidence

The crust floats buoyantly in the 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 a local area of the crust (e.g., through deposition), the crust subsides to compensate and maintain isostatic balance.

The opposite of isostatic subsidence is known as isostatic rebound--the action of the crust returning (sometimes over periods of thousands of years) to a state of isostacy, such as after the melting of large ice sheets or the drying-up of large lakes after the last ice age. Lake Bonneville is a famous example of isostatic rebound. Due to the weight of the water once held in the lake, the earth's crust subsided nearly 200 feet (61 m) to maintain equilibrium. When the lake dried up, the crust rebounded. Today at Lake Bonneville, the center of the former lake is about 200 feet (61 m) higher than the former lake edges.

Seasonal effects

Many soils contain significant proportions of clay. Because of the very small particle size, they are affected by changes in soil moisture content. Seasonal drying of the soil results in a lowering of both the volume and the surface of the soil. If building foundations are above the level reached by seasonal drying, they move, possibly resulting 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. That 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, which can last up to 25 years, the surface level around the tree will rise and expand laterally. That often damages buildings unless the foundations have been strengthened or designed to cope with the effect.

See also

References

  1. ^ Herrera, G.; Tomás, R.; López-Sánchez, J.M.; Delgado, J.; Mallorquí, J.; Duque, S.; Mulas, J. Advanced DInSAR analysis on mining areas: La Union case study (Murcia, SE Spain). Engineering Geology, 90, 148-159, 2007.
  2. ^ "Graduated Guidelines for Residential Construction (New South Wales) Volume 1" (PDF). Retrieved . 
  3. ^ G. Herrera, M.I. Álvarez Fernández, R. Tomás, C. González-Nicieza, J. M. Lopez-Sanchez, A.E. Álvarez Vigil. Forensic analysis of buildings affected by mining subsidence based on Differential Interferometry (Part III). Engineering Failure Analysis 24, 67-76, 2012.
  4. ^ Subsidence lecture
  5. ^ 23?(2011?)? [Land subsidence caused by 2011 T?hoku earthquake and tsunami] (in Japanese). Geospatial Information Authority of Japan. 2011-04-14. Retrieved . 
  6. ^ Report date on 19 March 2011, [1] Diastrophism in Oshika Peninsula on 2011 T?hoku earthquake and tsunami, Diastrophism in vertical 2011-03-11 M9.0, Diastrophism in horizontal 2011-03-11 M9.0 Geospatial Information Authority of Japan
  7. ^ Tomás, R.; Márquez, Y.; Lopez-Sanchez, J.M.; Delgado, J.; Blanco, P.; Mallorquí, J.J.; Martínez, M.; Herrera, M.; Mulas, J. Mapping ground subsidence induced by aquifer overexploitation using advanced Differential SAR interferometry: Vega Media of the Segura river (SE Spain) case study. Remote Sensing of Environment, 98, 269-283, 2005
  8. ^ R. Tomás, G. Herrera, J.M. Lopez-Sanchez, F. Vicente, A. Cuenca, J.J. Mallorquí. Study of the land subsidence in the Orihuela city (SE Spain) using PSI data: distribution, evolution and correlation with conditioning and triggering factors. Engineering Geology, 115, 105-121, 2010.

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