subsidence

Subsidence, Collapse and Cave Formation

based on thelecture notes of Prof. Stephen A. Nelson, Tulane University

Karst
Caves
Sinkholes
Removal of Salt
Coal
Fluid Withdrawal
Groundwater
Oil and Gas
Sinking Cities


Surface Subsidence and Collapse

Subsidence hazards involve either the sudden collapse of the ground to form a depression or the slow subsidence or compaction of the sediments near the Earth's surface.  Sudden collapse events are rarely major disasters, certainly not anywhere near the scale of the earthquake, volcanic, tsunami, or landslide disasters, but the slow subsidence of areas can cause as much economic damage, although spread out over a longer period of time.

Carbonate Dissolution and Karst Topography

Carbonate rocks such as limestone, composed mostly of the mineral calcite (CaCO3) are very susceptible to dissolution by groundwater during the process of chemical weathering.  Such dissolution can result in systems of caves, sinkholes, and eventually to karst topography.

Dissolution

Water in the atmosphere can dissolve small amounts of carbon dioxide (CO2 ).  This results in rain water having a small amount of carbonic acid (H2CO3) when it falls on the Earth's surface.  As the water infiltrates into the groundwater system and encounters carbonate rocks like limestone, it may start to dissolve the calcite in the limestone by the following chemical reaction:

CaCO3 + H2CO3 = Ca+2 + 2HCO3-2,

which states that calcite reacts with carbonic acid to produce dissolved Calcium ion plus dissolved Bicarbonate ion.

This reaction takes place as the water moves along fractures and other partings or openings in the rock.  This results in dissolution of much of the limestone if the reaction continues to take place over a long period of time.

Caves & Cave Formation CaveFormation.GIF (20321 bytes)

Most caves are thought to form near the water table (the surface below which all open space in rock is filled with water) , and thus the openings are initially filled with water.

Sinkholes

sinkhole2.GIF (14983 bytes)

Karst Topography

In areas where highly water soluble materials lie close the surface, dissolution below the surface can eventually lead to the formation of caverns and sinkholes.  As the sinkholes begin to coalesce, the surface topography will become chaotic, with many enclosed basins, and streams that disappear into sinkholes, run underground and reappear at springs.  Such a chaotic topography is known as karst topography.  Karst topography starts out as an area with many sinkholes, but eventually, as weathering and dissolution of the underlying rock continue, the ground surface may be lowered, and areas that have not undergone extensive dissolution stand up as towering pillars above the surrounded terrain.  The latter type of karst is called tower karst.

Removal of Solids and Mine Related Collapse

Humans can play a large role causing collapse of the surface.  Mining activities that remove material from below the surface can result in collapse if precautions are not taken to ensure that the there is adequate support for the overlying rocks.

Removal of Salt

Coal Mining

Since mining often removes material from below the surface without dissolution, mining can create voids that may become unstable and collapse.

Subsidence Caused by Fluid Withdrawal

We have seen how fluids (particularly water) in the subsurface can dissolve rock to undermine support and cause collapse of the surface.  Here we look at another role that fluids may play in causing subsidence.  Any fluid that exists in the pore spaces or fractures of rock is under pressure due to the weight of the overlying rock.  So long as the pressure of the fluid is enough to support the overlying rock, no subsidence at the surface will occur.  But, if fluids are withdrawn from below the surface, a decrease in fluid pressure may occur resulting in the removal of support and possible collapse.  The two most important fluids that occur beneath the surface are water (in the form of groundwater) and petroleum (in the form of oil and natural gas).   Both of these fluids are often withdrawn for human use, and thus humans are often responsible for fluid withdrawal related subsidence.  But, such withdrawal can also occur by natural processes.

Groundwater

ChWatTab.GIF (13962 bytes)

 

Movement.jpg

Clays.GIF (15481 bytes)

FloridaCompact.GIF (11633 bytes)

Compaction.jpg

After Coch (1995)

  Oil & Gas
Oil and Natural gas are both fluids that can exist in the pore spaces and fractures of rock, just like water.  When oil and natural gas are withdrawn from regions in the Earth near the surface, fluid pressure provided by these fluids is reduced.   With a reduction in fluid pressure, the pore spaces begin to close and the sediment may start to compact resulting in subsidence of the surface.

OilSubs.jpg

This has occurred recently in the oil fields of southern California.   For example, in the Wilmington oil field of Long Beach, California, subsidence was first recognized in 1940 due to withdrawal of oil from the subsurface.  The area affected was about 50 km2.  Near the center of this area, the surface subsided by up to 9 meters .  In 1958 repressurization of the area was attempted by pumping fluids back into the rocks below.   By 1962 further subsidence had been greatly reduced, and the area continuing to subside had been reduced to 8 km2.  Still, up to this point, very little uplift had occurred to restore the area to its original elevation.  This subsidence event has cost over $100 million.


Sinking Cities

Cities built on unconsolidated sediments consisting of clays, silt, peat, and sand are particularly susceptible to subsidence.  Such areas are common in delta areas, where rivers empty into the oceans, along floodplains adjacent to rivers, and in coastal marsh lands.  In such settings, subsidence is a natural process Sediments deposited by the rivers and oceans get buried, and the weight of the overlying, newly deposited sediment, compacts the sediment and the material subsides.  Building cities in such areas aggravates the problem for several reasons.

  1. Construction of buildings and streets adds weight to the region and further compacts the sediment.

  2. Often the areas have to be drained in order to be occupied.  This results in lowering of the water table and leads to hydrocompaction.

  3. Often the groundwater is used as a source of water for both human consumption and industrial use.  This also results in lowering the water table and further hydrocompaction.

  4. Levees and dams are often built to prevent or control flooding.   This shuts off the natural supply of new sediment to the area. In a natural setting sedimentation resulting from floods helps replenish the sediment that subsides and thus builds new material over the subsiding sediment, decreasing the overall  rate of subsidence. When the sediment supply is cut off, the replenishment does not occur and the rate of subsidence in enhanced.

Many subsiding cities are coastal cities like London, Houston, and Venice, or are built on river flood plains and deltas, like New Orleans, Baton Rouge, and the San Joaquin Valley of central California.  Mexico City is somewhat different in that it was built in a former lake.

Predicting and Mitigating Subsidence Hazards

The exact place and time of a disaster related to subsidence cannot usually be predicted with any degree of certainty.  This is true of both slow subsidence related to fluid withdrawal and sudden subsidence related to sinkhole formation or mine collapse.   Mitigation is the best approach to these hazards.  In an ideal world, all areas susceptible to such hazards would be well known and actions would be taken to either avoid causing the problem if it is human related, or avoid inhabitance of such areas if they are prone to natural subsidence. 



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