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Land Subsidence
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Subsidence Overview
Land subsidence can result from fluid (e.g. groundwater, petroleum) withdrawal
in weakly consolidated materials. The loss of fluid causes consolidation of the empty pore spaces, which
means that any voids in the soil previously filled with fluid are compressed by the mass of the overlying
materials, effectively decreasing the soil volume and resulting in land subsidence. Examples of places
experiencing land subsidence due to fluid withdrawel and subsequent soil consolidation include: the San
Joaquin Valley, California; Houston, Texas; Phoenix, Arizona; and Venice, Italy.
Geologists and engineers work together to develop computer models of areas undergoing subsidence
and monitor fluid removal and subsidence rates in those areas. To avoid damage to new development,
geologists identify and study areas containing materials undergoing, or susceptible to, subsidence, then
provide recommendations for minimizing or preventing future subsidence.
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Image from USGS, 1977 .
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The photograph to the left illustrates subsidence in the San Joaquin Valley, California. In the
photo, USGS scientist, Joe Poland shows subsidence between 1925 and 1977 due to fluid withdrawel and soil
consolidation. |
Land subsidence also occurs in areas of underground mining where removal of
material causes overlying surface rock to sink or collapse. Although the locations of underground mines are
often denoted on topographic and geologic maps, the potential subsidence hazard from a mine requires
evaluation by a geologist.
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Image courtesy of the Pennsylvania Enivironmental Protection Agency
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The schematic to the left illustrates subsidence due to mining. Subsidence results with the room
and piller system of mining collapses, creating a depression on the land surface. |
Similar to land subsidence due to underground mining, subsidence can also result
from chemical weathering of soluble rocks (e.g. limestone, dolomite, gypsum). As groundwater flows through
limestone, a chemical reaction causes the rock to dissolve, eventually forming cavities within the bedrock.
As a cavity near the surface increases in size, the overlying materials can sink or collapse and create a
depression on the land surface. Areas in the United States known to experience subsidence due to soluble
rock include Florida, Kentucky, Texas, and Missouri. Geologists identify rock types prone to dissolution and
having the potential to form cavities and sinkholes. Additionally, they can explore the locations and extent
of cavities using various hydrologic and geophysical techniques.
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Image from Jennings, 1985 in Environmental Hazards in
Karst Landscapes
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The schematic to the left illustrates subsidence due to dissolution of underlying rock
materials. Subsidence results when the room and piller system of mining collapses, creating a depression on
the land surface. |
Image courtesy of the USGS
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Photograph of the Winter Park, Florida sinkhole that opened in May 1981. For more details visit
Historic Examples of Land Subsidence Events. |
Historic Examples of Land Subsidence Events Back to top
Some notable recent examples of subsidence include:
- New Orleans, Lousiana: Land subsidence in New Orleans has reduced the
land surface elevation at least 0.55 meters. While withdrawel of groundwater is a significant contributer to
subsidence in New Orleans, other natural causes associated with local geolog conditions may also be
responsible. Such natural contributers include rock strata dipping toward the ocean (Rahn, 1996), settling
of coastal sediments, and movement of the Michoud fault. Additional man-made influences include drainage of
wetlands and diversion of sediment-bearing floodwaters from the Mississippi (NASA). Subsidence
exacerbates New Orleans' susceptibility to flooding that can occur during large store events and hurricanes
(Rahn, 1996). For more information on Louisiana land subsidence visit USGS.
- Appalachian Subsidence from Mining: Underground coal mining in
Pennsylvania has resulted in significant subsidence problems in Pittsburgh, Scranton, and Wilkes-Barre. The
room-and-pillar method of mining was employed at those locations and requires removing large sections of
coal and rock and leaving pillars of rock in place to support the roof. Over time these supports have
weakened and collapsed. In 1982, a concrete slab over an 88 meter deep coal mine collapsed in Scranton,
engulfing a parking lot and crane. Fairmont, West Virginia has also experienced its share of mine
subsidence. Built over a maze of coal mines, subsidence in Fairmont resulted in jammed doors, failed gas
lines, snapped electric lines, sinkholes, and condemned properties. Finally, in 1983, 1,000 tons of grout
were injected into the mine voids daily in hopes of preventing the downtown from collapsing (Rahn, 1996) For
more information on mine subsidence visit Pennsylvania DEP.
- Winter Park, Florida, May 1981: This sinkhole formed over a few hours
in a single day and caused at least $2 million in damages. During the day the sinkhole grew to 300 feet by
320 feet and 90 feet deep, swallowing a house, several cars, many trees, and a local swimming pool. An
analysis of the sinkhole revealed that a cavity in the limestone migrated toward the surface and caused the
overlying clayey silt cap to collapse. The sinkhole has since be converted into a lake (Rahn, 1996). For
more information on subsidence due to soluble rock visit USGS.
Links to More Information Back to top
References Cited This Page Back to top
Jennings, J.N., 1985, Karst geomorphology: Oxford, Basil Blackwell Ltd., p. 293.
Rahn, Perry H., 1996. Engineering Geology, An Environmental Approach: Prentice Hall, New Jersey,
2nd Ed., p. 202.
U.S. Geological Survey, Professional Paper 1401-A, "Ground water in the Central Valley, California- A
summary report" Photo by Dick Ireland, 1977
For more in-depth information about land subsidence, check out AEG's Technical References page.
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