Sinkholes and their nature
Sinkhole is a mainly natural phenomenon that reveals itself as a depression in the ground caused by underground streams1. Such sort of surface subsidence can develop in the meter of a few days or even hours and causes unexpected and serious damage. According to The United States Geological Survey more then 6,500 sinkholes have occurred since 19502. The damage caused by this phenomena can’t be precisely estimated, however, there is no doubt that counts millions of dollars. Annual expenses connected with the sinkholes are measured by millions of dollars.
In order to have a wider view on the problem one should find out the origin of the sinkholes. Mostly, they occur in the arias with the limestone bedrock. There are two main types of limestone. The first one was formed on the ocean flour and is mostly compressed of sea animals and shells. In the course of time it may be exposed on the surface through the erosion. However, most of the world limestone is formed without organisms. Almost all water contains calcium, so the large deposits of limestone are formed through the evaporation. Since limestone contains high percentage of calcium carbon it dissolves easily in the water, and can be easily remover by the underground steams. Unsaturated water that circulates underground absorbs calcium carbon, destroys the bedrock and forms wide voids. In case water removes so much limestone that it weakens the upper layers, a sudden or gradual collapse occurs. Through this way forms a sinkhole from three feet to a half-mile across.
However, it is far from common knowledge that sinkholes also form in evaporate rocks. Gypsum and salt dissolve in water, respectively 150 and 7,500 times better then limestone. Sinkholes formed through the solution of gypsum and salt cause even more dramatic effects. One should note, that unlike carbon sinkholes that are formed by natural processes, evaporate sinkholes can also be resulted by human activities like mining rock salt, drilling or construction works.
Natural sinkholes
Natural sinkholes are widely spread all over the world. The topography of sinkholes in abandons is called “karst terrane” after the name of the Karst region in Slovenia where this phenomenon is rather common.
There are four basic prerequisites for forming a sinkhole. First of all there should be a deposit of limestone, gypsum or salt lying on the relatively small depth. Second, water, unsaturated with CaSO, NaCl or other minerals should be present. Third, after being saturated the water needs an outlet for the escape. And finally, the energy is needed to move the water. As soon as water dissolves the significant amount of the mineral and forms the permanent pass for escape, the process accelerates and cave enlarges. Sinkholes appear almost in all regions rich in limestone or evaporate rocks, however, naturally, they are more common in the wetter regions then in dry states. Perhaps, the most pronounced examples of gypsum and salt sinkholes in Northern America can be found in the Permain Basin that covers large parts of Texas, New Mexico and Oklahoma.
The Permain Basin
It is one of the larges depositories of evaporates and carbonate rock in the world. The basin contains about 290-230 years old and 500 – 1,500 miters sick accumulation of evaporates that were formed during the Permain Period by the inland sea. Because of the abundance of evaporates and underground drainage sinkholes from just a few to 100 meters across are common in this aria. In some parts they are so widespread and dangerous that it is in common practice to fence them in order to prevent people and cattle from falling in. During the storm in 1918 the sinkhole 25 meters across and 20 meters deep was formed burying some 23,000 cubic miters of ground. Numerous circular, steep walled or vertical sinkholes are situated at the sight of Bottomless Lakes State Park; most of them are from 50 to 100 meters across and 30-60 metes deep. They are mostly situated at the floodplain of Pecos River. The water in the lakes contains a high percentage of salt that speaks for their origin3.
Another sight within the basin with gypsum sinkholes lies in western Onlahoma and north-central Texas. Despite their relatively small size their abundance is so wide that it causes serious restrictions in the land use.
Another important and well-known gypsum-karst features lie in Blaine Formation. Those are two well-known caves and a salt-water spring. One of the caves is a The D.C. Jester Cave located in Oklahoma. It is the longest gypsum cave in western world with the length of the passes that reaches 10,065 meters and multiple sinkholes passageways with the opening to the surface. The other one is Alabaster Cavern with maximum width and height of passages 18 and 15 meters respectively.
In most parts of the basin were found the evidence of sinkholes as well as other karst features of different periods of time. The site gives a broader view on the natural phenomenon of the sinkholes and helps to understand processes leading to the terrain depression and destruction deeper.
Sinkholes an human activity
The development of the sinkholes may be either induced or enhanced by human activities. Construction, mining or petroleum activities as well as direction of water through or above evaporate rocks may have unpredictable effect. However, in this cases gypsum sinkholes are rather uncommon. On the other hand sinkholes resulted by salt solution appear to be a major problem. Drilling through bedded salt, or opening dry mines can enhance the process of sinkhole development. These activities may enable unsaturated water to the salt bedrock and cause its dissolution. The concentration of salt in unsaturated water may reach 30 percent (10 times more then normal sea water). In case the significant amount of water was enabled, a catatonic collapse or land subsidence can result.
The factor that has strong affect on the sinkholes formation is solution mining. Solution mining in conducted by introducing unsaturated water into a salt mass, dissolving it from the brine, recovering the brine and evaporating water afterwards. The process usually involves creation of one or several cavities filled with brine. The size of the cavity normally depends on the thickness of salt deposit and variates from 10-100 meters long and 10-600 meters high. In the course of time, the cavity becomes too large and the roof may collapse. Since 1981 10 areas of solution mining collapse were reported. Mainly those were mines from 50 to 75 years old with old engineering constructions. Cargill sink (Kansas), Grand Saline sink (Texas), Grosse lie (Michigan) and Bayou Choctaw (Louisiana) are four main well-documented sinkholes resulted by solution mining.
Cargill sink formed in one of the solution mines developed by Cargill, Inc. Salt has been mined in the aria since 1888, but on October 21, 1974 it was reported that the crater on surface reached diameter on 60 meters in four hours. The sinkhole stabilized at the size of 90 miters across and 15 miters deep. The volume of the sinkhole is estimated at about 70,000 cubic miters. The part of the track of the Missouri-Pacific Railroad was left suspended above the water accumulated in the sinkhole after the collapse. Since the aria is reach in salt deposits overlain at about 110 meters depth and the location of early mines is unknown, the risk of the development of new holes is rather high4.
In 1976 another sinkhole developed in the city of Grand Saline, Texas. Grand Saline sink of 15 miters across and 8,500 cubic meters of volume formed at the mine that produced bride for 25 years. As the result of the collapse the house was endangered and a pipeline was broke and the sewage flown straight into the sinkhole polluting groundwater.
Several sinkholes developed as the result of solution mining at the depth of 325 meters on Grosse Ile near Detroit in 1971. It is estimated that the aria affected by the phenomenon is about 37,000 square meters. The total volume of subsidence is about 1,2 million square meters. However, the report made shortly after the collapse stated that the subsidence had no significant effect on the environment.
In 1954, at the Bayou Choctaw salt dome, the drilling rig, that was producing brine fell in the sinkhole and was never seen again. The lake that formed after the collapse still exists and is about 250 meters in diameter. This large and deep collapse was caused by the lack of protective oil blanket.
Another example of artificial influence on the depression of lend is the formation of sinkholes over dry salt mines. They can form above underground mines and are the result of collapse of overburden into the subsurface interstice. Basically, sinkholes develop as the result of unsaturated water inflow into the voids. The significant amount of water can dissolve large amount of salt rather rapidly. It causes uncontrolled enlargement of the rooms, destruction of salt supporting pillars, desolation of salt on the roof and evoke collapse of overburden. These processes do not only cause sinkholes but also make dry salt mine behave as a solution mine.
Petroleum related activities can also cause unintentional dissolution cavities through drilling on or through large underground deposits of salt. The cavities created by such activities can reach the size of those created by solution mining and if the roof is shallow enough the collapse can happen.
Despite the fact that the process of natural sinkholes formation is beyond human control we do understand the nature of their formation. The facts given above open a view on the problem and the ways of its salvation not only in the mining activities but also in construction works that we face in our everyday life. One can’t avoid building and living at the sights with the risk of sinkhole formation, but at least the measures can be taken to minimize that risk. Karst arias require detailed geological mapping and safe engineering solutions. The best results can be achieved by not letting unsaturated water into or above large salt deposits through the construction of engineering systems.
Solution, room-and-pillar salt mines as well as petroleum related facilities should be designed with the use of latest developments in the field of mining safety.
However, in wild nature, once a sinkhole stabilized, a beautiful lake is formed. It becomes a source of life for numerous creatures and plants.
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