Subsidence is the motion of a 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. 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 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. Mining-induced subsidence is nearly
always very localized to the surface above the mined area, plus a margin
around the outside. The vertical magnitude of the subsidence itself
typically does not cause problems, except in the case of 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. This is accomplished through a combination of careful mine
planning, the taking of preventive measures, and the carrying out of
repairs post-mining. Extraction of natural gas
If natural gas is extracted from a natural gas field the initial pressure)
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 exploitation of the Slochteren gas field started in the late 1960s the
ground level over a 250 km² area has dropped by a current maximum of 30 cm.
Earthquake The Geospatial Information Authority of
Japan reported immediate subsidence caused by the 2011 Tōhoku earthquake. In
Northern Japan, subsidence of 0.50 m was observed on the coast of the Pacific
Ocean in Miyako, Tōhoku, while Rikuzentakata, Iwate measured 0.84 m. In
the south at Sōma, Fukushima, 0.29 m was observed. The maximum amount of
subsidence was 1.2 m, coupled with horizontal diastrophism of up to 5.3 m
on the Oshika Peninsula in Miyagi Prefecture.
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. 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
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 a
local area of the crust, the crust subsides to compensate and maintain
isostatic balance. The opposite effect to Isostatic
subsidence is known as isostatic rebound - the action of the crust returning 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 to maintain equilibrium, when
the lake dried up, the crust rebounded. Today at modern Lake Bonneville, the
center of the former lake is about 200 feet higher than the former lake edges.
Seasonal effects 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 25
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
Lateral and subjacent support is a related concept in real property law.
Cave-in References
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