A sinkhole is defined as a depression of ground surface material falling into subsurface pathways. Sinkholes occur both gradually and suddenly, depending on the underlying cause(s) of the problem. A gradual formation of a bowl or sunken area in the ground may indicate a developing sinkhole. The edges of sinkholes are unstable and can collapse as the sinkhole grows to match the underlying cavity. Sinkholes can be shallow or very deep, depending on the originating cause.
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Identification of the type of sinkhole helps determine why the sinkhole occurred. Cover-subsidence and cover-collapse are the three common types of sinkholes. Look for permeable covering containing sand when subsidence occurs. Look for areas with large clay deposits and surface erosion to find collapsed areas causing the common "bowl" phenomenon. Flowing water into joints, fractures or at first contact with rock causes dissolution sinkholes.
Susceptible Rock Formations
Certain rock formations are susceptible to sinkholes. The U.S. Geological Survey states that areas high in limestone, carbonate rock, gypsum, naturally dissolving rock and salt beds are prone to develop underground cavities. These cavities grow until the land above them can no longer be supported, causing a sinkhole. Dissolution sinkholes range in size and shape and occur commonly in Alabama, Florida, Kentucky, Missouri, Pennsylvania and Tennessee.
Improper land or water use practices can lead to sinkholes. Ground-water pumping during construction or when natural water diversion is rerouted lead to potential sinkhole situations. Changing land surfaces, such as adding water-storage ponds, adds substantial weight to the surface, causing an underground collapse in supporting surfaces. When local aquifers become unbalanced, the water pressure underground decreases; the lack of pressure once used to hold the ground surface in place causes the surface to collapse.
Man-made issues also cause sinkholes. Buried garbage or stumps decay, leaving underground cavities. These cavities collapse over time when the weight-bearing material is no longer in place. Underground mining operations brought too close to buildings may cause sinkholes because the weight load has shifted from below ground to above ground, causing the ground to fall inward.
The Pennsylvania Department of Environmental Protection lists several activities that may cause sinkholes, including declining water levels, soil disturbance, concentrated water flow, water impoundments, vibration and heavy loads upon the surface of an underground cavity. Causes of these activities include drought and groundwater pumping, drilling, leaking water pipes where water is injected into the ground, basins and ponds, heavy traffic and blasting. Several causes can unite, contributing to sinkhole formation or growth. Multiple sinkholes in one location are an example of multiple causes or ongoing issues where a sinkhole remains unfilled.
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