Subsidence

Subsidence underneath a concrete road has caused damage
Concrete road subsidence showing serious cracks and damage. (Photo by enviromantic / Getty Images)

What Is Subsidence?

Subsidence is the gradual settling or abrupt sinking of earth as a result of natural or man-made causes. Subsidence is capable of damaging roads, buildings, bridges, and sewer systems, as well as altering slopes and streams.

There are two types of subsidence: endogenic subsidence and exogenic subsidence. Endogenic subsidence is caused naturally as a result of tectonic plate movement, folding and faulting, and earthquakes.

Exogenic subsidence is caused by human activities such as underground mining, groundwater withdrawal, oil and natural gas extraction, and changes in soil composition.

Endogenic Subsidence

Endogenic subsidence is the result of the Earth’s natural processes and can be found in phenomenon such as folding, faulting, and earthquakes.

  • Folding and Faulting

    The shifting of tectonic plates causes folding and faulting. Folds are bends in the rock that are the result of compressional forces, and faults are shears in the rock that form as the result of immense stress.

    There are three main types of faults: normal faults, reverse faults, and strike-slip faults. The type of fault determines which direction the earth shifts. For instance, normal faults cause one side of the separated rock to become displaced upwards while the other side of the rock is displaced downwards, so the earth above the fault splits in two different directions.

    The movement of the surface above the folds and faults can occur in any direction. Folds and faults take thousands of years to develop, so this type of endogenic ground subsidence develops slowly.

  • Earthquakes

    The subsidence caused by an earthquake is extremely fast compared to the subsidence caused by folding and faulting. Earthquakes instigate liquefaction, a process in which rapid vibrations cause water-saturated sediments to temporarily liquefy, so the material over the liquefied sediment subsides.

    Earthquakes occur with little or no warning, and are not only capable of altering the land located at the epicenter, but are also capable of affecting land that is further away from the center of the earthquake. For example, an earthquake that struck Nepal in April of 2014 caused damage hundreds of miles away from the earthquake’s epicenter and impacted parts of Northern India, Bangladesh, and China.

  • Caves

    Another type of endogenic subsidence is found in natural caves. Caves form as underground water dissolves limestone. A landscape composed of sinkholes and caves that is formed from dissolved limestone is commonly referred to as karst topography.

Exogenic Subsidence

Exogenic subsidence is the result of man-made activities, and may occur as a result of underground mining, groundwater withdrawal, oil and natural gas extraction, and organic soil drainage.

  • Mining

    Mining is the process of extracting ore from the ground, and is a valuable and popular technique used worldwide. Mines are built to extract minerals, gemstones, and fossil fuels from the earth.  Although mines are essential, they are capable of creating subsidence-prone environments.

    Mine subsidence is the movement of the Earth’s surface that is the result of the collapse or failure of mine environments. The presence of mines creates voids in the earth, and causes the ground above the mines to weaken significantly and collapse inwards. This collapse creates sinkholes and troughs.

    Sinkholes are limited to the boundaries of the mine itself, but are incredibly dangerous to the immediate area in which they are located. Sinkholes are known to occur abruptly and with little warning, and are capable of swallowing both natural and man-made structures.

  • Groundwater Withdrawal

    Groundwater withdrawal is also a process that causes subsidence. Groundwater is a beneficial source of fresh water and is stored in aquifers.  Pumps are utilized to extract water from between the pore spaces in the sand, clay, silt, and gravel.

    However, the pumping of groundwater results in a reduction in water pressure, which causes the soil within the affected region to lose support and weaken. As the soil thins and compacts, the overall land surface is lowered.

    Mexico City, Mexico has sunk more than thirty feet in the last century.  This is largely because water from the aquifer under Mexico City is withdrawn faster than it can recharge. The proposed solution to prevent additional subsidence is to stop pumping water from the aquifer. However, even if pumping stopped and the aquifer were completely recharged, the subsidence that has already occurred is permanent and irreversible.

    Italy is another example of a city that experienced subsidence as a result of groundwater withdrawal. The sinking of Venice is the result of the rapid withdrawal of water from sandy aquifers that are located under the city. Venice relied largely on the aquifers between 1950 and 1969, but switched to withdrawing water from the nearby Sile River after scientists connected the subsidence to the groundwater pumping. In the time since the switch, the aquifer has recharged and the city has stopped sinking. As a result of the sinking, the city is closer to sea level, and periodically suffers from periods of intense flooding from the Adriatic Sea (Dolan and Goodell, 1986).

  • Oil and Natural Gas Withdrawal

    Oil and natural gas withdrawal also causes subsidence. The process is similar to subsidence induced by groundwater withdrawal. Oil and natural gas are imbedded in silts, peats, and clays, so the extraction of oil and natural gas creates soil that is quick to weaken and prone to subsidence.

    The harbor area of Los Angeles-Long Beach in California suffers from subsidence because of oil and natural gas pumping. The Wilmington Oil Field provided millions of barrels of oil and natural gas between 1936 and 1969, causing the land to sink up to 29 feet (9 meters) in the most heavily impacted parts (Dolan and Goodell, 1986).

  • Change in Soil Composition

    Lastly, subsidence may occur as the result of a change in composition within the soil. Organic soil drainage leads to compaction and soil shrinkage, and is common in peat and muck soils.

    The drainage of organic soil is dangerous because it reduces the overall volume of the soil, which leads to subsidence. Subsidence is higher in the first few years after organic soil is drained.

How to Learn More

Subsidence is a persistent and destructive phenomenon and thus it is important that we are careful and aware of its dangerous effects. To learn about the subsidence in your own area, contact your local government office. Additionally, geographic and geologic institutions are equipped to educate people about the indicators of subsidence.

References  

Dolan, R. & Goodell, G. (1986). "Sinking Cities: A combination of human activities and geological processes endangers many of the world’s largest cities." American Scientist, 38-47.

Pennsylvania Department of Environmental Protection. (2015). "Technical Guide to Mine Subsidence." Retrieved from: http://www.dep.state.pa.us/MSIHomeowners/whatisms.html