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.
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.
Image from USGS, 1977 .
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.
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.
Image courtesy of the Pennsylvania Enivironmental Protection Agency
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.
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 from Jennings, 1985 in Environmental Hazards in Karst Landscapes
Photograph of the Winter Park, Florida sinkhole that opened in May 1981. For more details visit Historic Examples of Land Subsidence Events.
Image courtesy of the USGS
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.