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
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.
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
Caves are large underground open spaces. If there are many
interconnected chambers in a cave system, it is called a cavern.
Most caves are formed by the chemical dissolution process described above,
as a result of circulating groundwater. The dissolution begins along
fracture systems in the rock, widening the fractures and connecting them
to other fractures, until a cave is formed.
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.
After the water table is lowered due to changing geologic conditions,
further seepage of water into the now open cave system results in the
deposition of stalactites (icicle like stones) where the water drips into
the cave. If water is absent from the floor of the cave stalagmites
form where the water drips on the floor of the cave. Both
stalactites and stalagmites are composed of newly precipitated calcite,
initially dissolved from the limestone above, carried in the groundwater,
and re-precipitated when the water reaches a low pressure area like a
The rate at which caves form depends on such factors as the acidity of the
water and the velocity at which the water moves through the rock.
Highly acidic water and high flow velocity increase the rate of
dissolution, and thus the rate at which a cave forms. Based on
observations of the rate of dissolution from currently forming caves, it
appears that the cave formation process can take anywhere between 10,000
years and 1 million years.
A sinkhole is a large dissolution cavity that is open to the Earth's
surface. Some sinkholes form when the roofs of caves collapse,
others can form at the surface by dissolving the rock downward.
Because we are here concerned with subsidence disasters and hazards we
will concentrate on the formation of sinkholes by collapse.
Sinkholes are common in areas underlain by limestone. Central
Florida is such an area, and in one small area of about 25 km2, over 1000
sinkholes have formed by collapse in recent years.
Sinkholes may form as a result of lowering the water table by excessive
pumping for human use of the water. This appears to be responsible
for sinkhole formation in Florida.
Caverns that were forming just below the water table were filled with
water. The water table was lowered over the years resulting in the
level of groundwater in the caverns to become lower. While the water
table was high, the water in the cavern helped to support the roof of the
cavern. This support is removed when the water table is lowered, and
thus the unsupported roof eventually becomes unstable and collapses to
form a sinkhole.
Sinkholes may also form by slow enlargement of caverns by continued
dissolution of the limestone. This may no matter what the level of
the water table.
When sinkholes collapse to expose the water table at the surface, the
sinkhole will be filled with water forming small circular lakes.
Although common in areas underlain by limestone, sinkholes can form in any
area where highly water soluble rocks occur close to the surface.
Such rocks include rock salt made of the mineral halite, and gypsum
deposits, both of which easily dissolve in groundwater.
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
Salt occurs beneath the surface in areas that were once below sea level in
restricted basins where extensive evaporation caused the concentration of
salt in seawater to become so high that the salt was precipitated on the
bottom. This occurred, for example during the Jurassic Period (about
150 million years ago) in the area of the Gulf of Mexico. Later
deposition of other sediments on top of this salt, resulted in low density
salt underlying higher density sediments. Since salt is rather
ductile, it began to flow upward toward the surface, and in many cases
became detached from the original layer of salt at depth to form what is
called a salt dome. Since the salt now occurs close to the surface,
it can dissolve and collapse to form sinkholes.
The salt can also be mined to produce salt for human usage. One
mining technique involves injecting fluids into the salt to dissolve
it. The fluids are then recovered and the salt re-precipitated from
the solutions. Such mining, because it dissolves large cavities in
the salt can lead to instability and collapse. Such solution mining
resulted in a sinkhole 300 m in diameter in Hutchinson Kansas .
Another salt mine related collapse occurred as the result of oil
exploration near a salt dome at Jefferson Island in southwestern
Louisiana. Jefferson Island is not an island in the strict sense,
but is an area of high topography that formed as the result of salt moving
upward through the sediments and uplifting the surface. The salt
dome contained active salt mines, where mining was accomplished by digging
tunnels in the salt. An oil company attempted to drill a well from a
lake adjacent to Jefferson Island, hoping to skirt the edge of the salt
dome and find oil. Instead, the oil well penetrated the salt and
entered one of the mining tunnels. This cause the lake to
drain into the salt mine, which then caused some of the salt to
dissolve. Eventually this undermined support above the mines and led
to collapse of the surface. The lake drained into the salt mine, ten
barges used for oil drilling were sucked into the hole, and a home and
entire botanical garden collapsed into the depression. Fortunately
no miners or other inhabitants of the area were killed .
Since mining often removes material from below the surface without
dissolution, mining can create voids that may become unstable and
Coal occurs beneath the surface as extensive layers called coal seams.
These seams were once swampy areas on the surface where much vegetation
flourished, died, and became buried before it could decay. Processes
acting over long periods of geologic time have turned dead vegetation into
coal. Other useful substances are mined by digging tunnels in rock,
but in most mining techniques the useful substance occurs along narrow
zones only these enriched zones need to be removed. In mining coal,
however, all of the material is useful, so large masses of material are
removed. The technique used in coal mining is referred to as
"room-and-pillar" mining. The rooms are where the coal has been
removed, and the pillars are left to support the overlying rock.
Sometimes, too few pillars are left, and the overlying rock collapses into
the mine. This is not only dangerous to the miners, but can also
cause hazards to areas on the surface where the collapse occurs
Underground fires in coal mines can also lead to collapse hazards.
Fires can start by spontaneous ignition of coal dust or methane gas
released from the coal. Such fires are difficult to extinguish, and
often are left to burn for years. In Pennsylvania, for example, coal
mine fires have burned for more than 25 years. Burning of the coal
results in removal of the coal, and thus may undermine support for the
roof of the mine resulting in collapse.
Subsidence Caused by:
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
Groundwater occurs nearly everywhere below the surface of the Earth,
where, as we have said before, it fills the pore spaces and fractures in
rock at levels below the water table. The zone beneath the
water table is called the saturated zone. Groundwater flows into the
saturated by percolation downward from rainfall on the surface.
Surface bodies of water, like streams, lakes, and swamps, are areas where
the water table is exposed at the surface. Springs are also areas where
the water table is exposed at the surface. If one digs or drills a well to
intersect the water table, water will flow into the well and fill it to
the level of the water table. The level of the water table
can change as a result of changing amounts of input in the groundwater
system (called recharge) and output from the groundwater system (called
discharge). Recharge takes place by water infiltrating down from the
surface. Discharge occurs as a result of outflow through surface
bodies of water, springs, and wells. During the wet season the water
table is generally higher because recharge exceeds discharge. During
dry seasons the water table is depressed because discharge exceeds
recharge. Likewise, during periods of drought the water will be
Groundwater moves through the saturated zone both downward and upward. The
downward flow occurs due to gravity and the upward flow occurs because
fluids tend to flow towards areas of lower pressure.
Subsidence can be caused by any process that results in lowering of the
water table. So, drought, dry seasons, and excessive withdrawal of
groundwater by humans can cause the water table to move to deeper levels
and result in subsidence.
Most subsidence occurs as a result of hydrocompaction, discussed
previously under mass
wasting processes. Hydrocompaction occurs when the sediments
loose water. Since lowering of the water table involves loss of
water, hydrocompaction often occurs.
Hydrocompaction means that water absorbed on and within clay minerals are
removed by withdrawal or drying and the clays shrink. Shrinkage of
clays results in less volume, so the surface will subside as the clays
become more tightly compacted.
Hydrocompaction also occurs when organic rich sediment like peat is
subjected to loss of water. For example, in southern Florida, swamp
with organic rich soils containing both peat and clay minerals was drained
for agricultural use. In 1912 drainage canals were installed
throughout the swamp to drain water. This resulted in lowering the
water table and hydrocompaction of the soil led to some subsidence.
In 1942 further drainage of the area was undertaken by installing pumps to
pump out the groundwater. Subsidence due to hydrocompaction
increased again. By 1980 the land surface had subsided about 2.8 m
from its original elevation.
After Coch (1995)
Most hydrocompaction is an elastic deformation process. Recall that
elastic deformation is reversible, so that when the clays or peat dry out
they compact, but when they become wet once again, they expand.
Compaction, however, can become inelastic, which is not reversible.
In such a case as the pores are closed by compaction, they cannot be
restored when new fluids are pumped in.
The rate of subsidence relative to rate of fluid withdrawal can sometimes
show when material passes from elastic compaction to inelastic
compaction. If the rate of fluid withdrawal is large yet the rate of
subsidence is small, this is usually an indication of elastic
compaction. If, however, there is a large amount of subsidence with
only small amounts of fluid withdrawal, inelastic compaction is likely
For example, in the Tucson Basin of southern Arizona, prior to 1981 the
ground surface dropped about 3 mm for every meter of lowering of the water
table. Since 1981, for every meter lowering of the water table a 24
mm lowering of the land surface has been observed. This 8-fold
increase in the rate of subsidence relative to the level of the water
table likely indicates that inelastic compaction is occurring. If
so, then the subsidence cannot be reversed by raising the water table.
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.
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.
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.
Construction of buildings and streets adds weight to the region and
further compacts the sediment.
Often the areas have to be drained in order to be occupied. This
results in lowering of the water table and leads to hydrocompaction.
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.
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
For subsidence caused by sudden collapse of the ground to form sinkholes,
several measures can be taken. First, geologists can make maps of
areas known to be underlain by rocks like limestone, gypsum, or salt, that
are susceptible to dissolution by fluids. Based on knowledge of the
areas, whether active dissolution is occurring or has occurred in the
recent past, and knowing something about the depth below the surface where
these features occur, hazard maps can be constructed.
Once these areas have been identified, detailed studies using drill holes,
or ground penetrating radar can be used to locate open cavities beneath
the surface. These areas can then be avoided when it comes time for
decisions about land use.
In areas where there is a possibility of sudden collapse, one should be
aware of any cracks that form in the ground. especially if the cracks
start to form a circular or elliptical pattern. Such ground cracking
may be an indication that a collapse event is imminent.
In areas located above known mining operations or former mining
operations, maps can be constructed based on knowledge of the actual
locations of open cavities beneath the surface. Such maps can then
be used as a guide for land use planning. Currently laws are in
place to prevent active mining beneath urban areas, but these laws did not
always exist, and older mines could still cause problems.
Where fluid withdrawal is the main cause of subsidence, information on the
rate of fluid withdrawal should be determined and combined with studies of
the material in the subsurface based on sampling with drill core
methods. If subsidence is suspected or observed, human activities
can be modified to prevent further subsidence. For example new
sources of water can often be found, or waste water can be treated and
pumped back into the ground to help maintain the level of the water table,
maintain fluid pressure, or re-hydrate hydrocompacting clays and
Fluid withdrawal problems are complicated in the United States where laws
are in conflict. Rights to withdrawal of an underground resource
like water or oil usually take precedence over the rights to sue for
damages that might result from subsidence.