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What is a Sinkhole?
Information about Florida sinkhole houses. We buy unrepaired sinkhole houses. 813 232 2400 What is a sinkhole? What causes it? What defines sinkhole damage to a structure? These are all important and valid questions. The American Heritage Dictionary defines a sinkhole as a natural depression in a land surface communicating with a subterranean passage, generally occurring in limestone regions and formed by solution or by collapse of a cavern roof.
The word karst is a term that is associated with sinkholes. Karst landscape is formed from the dissolution of carbonate bedrock. Karst terrain is characterized by the presence of sinkholes, springs and caves. It is typically drained internally where drainage moves rapidly into the ground through a network of fractures, pores and voids in the bedrock. Sinkholes occur naturally due to the dissolution of the soluble carbonate bedrock. The beginnings of sinkhole development, or karstic erosion, begins as slightly acidic groundwater flows through interconnected pores and fractures in the carbonate bedrock (limestone or dolostone). The groundwater becomes slightly acidic as the rainwater passes over decaying plant matter that gives off carbon dioxide. This carbonic acid attacks the carbonate rock and increases the size of the pores and fractures through which it flows. The more the size of the fractures and pores increases, the more the rate of dissolution increases due to the increased volume of flow.
Different types of sinkholes can form depending upon the depth at which the bedrock occurs, what the overburden soils consist of, and how thick they are. Where there is very little or no soil cover above the limestone bedrock, dissolution sinkholes can form. This is a very slow developing type of sinkhole and presents no hazard in the lifetime of a person. The dissolution of the limestone may take hundreds or thousands of years for a single foot. The ones that have formed are seen surfically as broad, saucer shaped landforms. The second type of sinkhole forms where there is a collapse in the bedrock and is called a cavern collapse or a bedrock collapse sinkhole. These are the type of large catastrophic sinkholes that are occasionally seen in news headlines. These form when the roof over a large void or cavern in the bedrock collapses and the overlying ground surfaces collapses to fill the void that has been created. These are rare and only a very small percentage of the sinkholes that develop in Florida are of this type.
The third type of sinkhole is called a subsidence sinkhole, which can then be subdivided into cover collapse and cover subsidence sinkholes. Subsidence implies that the overlying soils subside into pre-existing fractures and cavities in the bedrock. Cover subsidence sinkholes are generally slow developing and can take weeks or months for small amounts of surficial displacement to occur. Cover collapse sinkholes are much more rapid in terms of the rate of surface subsidence. This generally occurs where the bedrock is overlain by a thick sequence of cohesive soils. The cohesive soils overlying a void in the bedrock will gradually ravel or spall into the bedrock cavity and creating a cavity in the soils. The cavity will then collapse when the overlying soils become too weak to bridge the cavity. The overlying non-cohesive soils will then fill the cavity and a surface soil displacement is seen. The fourth type of sinkhole is known as a buried sinkhole. This is a sinkhole that has formed at sometime in the geologic past and has been in-filled with younger sediments. This type can be an engineering hazard because it shows no surficial expression and is subject to re-activation. These buried sinkholes can be in a stable or unstable condition. They may be termed as unstable if they are filled with very soft or loose sediments and changes in the hydrologic system may cause re-activation. They may also be in a completely stable condition and be filled with firm or dense sediments that are often more stable than the surrounding area. Determining that damage to a structure is the result of sinkhole activity is often a complex problem. Generally the obvious open hole left by a "drop out" of soils is not seen. To determine that damage to a structure is the result of a cover subsidence type sinkhole, it must be shown with boring data that there is a very soft or loose strata (raveling zone) in contact with the limestone bedrock that would indicate some type of erosion, downward migration of soils and subsidence has taken place. To infer that the structural damage to a building had occurred as a result of this subsidence, the raveling zone should extend from the bedrock to or near the ground surface. If the raveling zone does not extend all the way to the surface, it must then be determined if the overlying strata will "bridge" the raveling zone and support the structure. The presence of soft or loose zones encountered in a boring, or the loss of drill fluid circulation, does not, in itself, prove that sinkhole activity is taking place. Many times drill fluid circulation will be lost at the interface between differing strata, which is not necessarily an indicator of a void. Soft or loose zones are a common occurrence in boreholes, to say that they are the result of sinkhole activity, it must be shown that the soft or loose zone is in contact with the bedrock and extends upward. Damage to structures are often multi-variate problems. Cover subsidence activity is many times combined with damage from shrink/swell clays, peat or muck. A comprehensive settlement investigation can usually determine the cause or causes of subsidence to a building. What is a sinkhole?
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