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CREATION ART

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    Saturday, January 12, 2008

     

    Sinkhole Formation

    One cannot call sinkholes a work of art but they are a part of the processes of this planet. The only sinkholes that may be called beautiful are those ancient ones that have filled with water and formed a lake, or those which have collapsed into caves that eventually became lovely, fascinating places having their own odd beauty.
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    http://www.dnr.mo.gov/geology/geosrv/geores/sinkholes_formation.htm

    Sinkhole Formation

    A sinkhole (also called a doline) is a depressed area usually formed by solution of surficial bedrock or collapse of underlying caves. The surface expression of a sinkhole is typically a conical depression or area of internal drainage. Sinkholes range in size from several square yards to hundreds of acres. They may be quite shallow or may extend hundreds of feet deep. Sinkholes are places where there is rapid recharge (replenishing) of groundwater from the surface and, therefore, are areas of potential groundwater contamination. For this reason, managing surface water and waste disposal in sinkhole-prone areas are important to maintaining good groundwater quality.

    The diagrams below conceptually illustrate the stages of sinkhole formation. Actual conditions in nature may be very different than those illustrated. For instance, the rock and soil layers may be thicker or thinner, the fracture and cave passage may be larger or smaller, and the surfaces are likely to be much more irregular in shape.


    stage 1

    stage 2

    Stage 1
    For a sinkhole to form there must be an opening in the bedrock surface that allows overlying soil to move downward into a cave passage. This stage illustrates a solution-widened fracture in the bedrock choked with soil.

    Stage 2
    Soil that collected in the cave passage in Stage 1 has been carried away by flowing water.



    stage 3

    stage 4

    Stage 3
    Soil that collected in the fracture or bedrock opening collapses into the cave or is washed into the cave by water movement from the soil into the cave.

    Stage 4
    Additional soil movement or collapse causes a void to form at the bedrock surface.



    stage 5

    stage 6

    Stage 5
    The void enlarges and moves upward in the soil profile, a process known as stoping.

    Stage 6
    Eventually the void enlarges until only a thin layer of soil remains at the surface.



    stage 7

    stage 8

    Stage 7
    Finally the thinned soil roof can no longer support itself and creates a surface collapse that may or may not choke the hole in the bedrock. Typically the initial appearance is a steep-sided hole at the surface several feet deep with a floor of soil that used to be at the surface.

    Stage 8
    If the bedrock throat of the sinkhole remains plugged with the collapsed soil, the surface hole may fill with other eroded soil.

    In some instances the unstable, steep-sided surface hole may widen into a conical depression, like the upper portion of an hour glass. If the throat of the sinkhole remains open, surface water will drain readily. If the throat becomes plugged with soil, water may pond temporarily or permanently in the depression, forming a sinkhole lake. The entire process may repeat itself by starting over.



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    http://www.mme.state.va.us/Dmr/OUB/Brochures/sink.html

    For complete article, access above url.

    Excerpts from above article:

    SINKHOLES


    In Virginia the formation and modification of sinkholes (also known as sinks, dolines and dolinas) is a natural process in areas underlain by limestone and other soluble rock. The location and rate at which sinkholes form can be affected by man's activities. Sinkholes are basin-like, funnel-shaped, or vertical sided depressions in the land surface. In general, sinkholes form by the subsidence of unconsolidated materials or soils into voids created by the dissolution of the underlying soluble bedrock. The rock exposed in a collapsed sinkhole is usually weathered and rounded, but some sinkholes contain freshly broken rock along their steep sides. Freshly broken rock may indicate that the sinkhole has formed by the collapse of a cave (naturally occurring) or a mine (man made). Where sinkholes and caves have formed by the dissolution of soluble rock, such as limestone, dolomite, and gypsum, surface water is uncommon and streams may sink into the ground. This type of topography, formed by dissolution, is referred to as karst terrain. In karst terrain, sinkholes are input points where surface water enters the groundwater sustem.



    Potential Sinkhole Problems
    There are three types of potential problems associated with the existence or formation of sinkholes: subsidence, flooding, and pollution. Sinkholes are the result of differential subsidence of the land surface. The term subsidence is commonly used to imply a gradual sinking, but it also can refer to an instananeoust or catastrophic collapse. Sinkholes result from various mechanisms (Sowers, 1976), including consolidation from loading, consolidation from dewatering, hydraulic compaction, settling as materials are removed by groundwater flow, stoping or raveling of materials into a void, and instantaneous collapse into a void. Although the formation of sinkholes is a natural process in karst terrains, man-made modifications to the hydrology of these areas commonly results in the acceleration of this process.

    [...]Patterns of pumping of high yield wells over extended periods of time can result in large as well as rapid drawdowns of the water table. Where such rapid and large drawdowns occur in unconsolidated materials, sinkhole collapse can be catastrophic and subsidence can be extensive over the area subject to the drawdown (Foose, 1967 and 1968). Sinkhole formation can also occur above solutionally enlarged fractures, which have fomed caves or "mudseams". Water-table drawdowns can cause soil voids to migrate along solution features eventually leading to sinkhole formation at a distance from the well.

    Sinkhole subsidence is associated also with soil piping. Water leaking from culverts, or other drainage structures can create a void beneath the drainage structure by compaction or internal scour of the soil. This reduction in support can result in displacement of the leaking structure and an increase in leakage or breakage. The void may increase in size to the extent that the soil has insufficient strength to support itself with subsequent failure leading to the fomation of a steep sided collapse sinkhole. The recognition of water mark stains on the fracture surfaces and joints of drainage structures are indicators of this type of sinkhole formation.

    (end excerpts)

    [A large sinkhole of the soil piping type opened up in a neighbor's lawn and under her front sidewalk. City workers discovered a leak in the underground water main had washed away the soil and caused the sinkhole. My friend's husband almost caused more collapse of the surface soil and grass . He narrowly escaped falling into the deep cavity by stepping too close to the hole. The soil and grass at the edge of the hole formed a shelf under which the cavity belled out. The shelf could not support his weight and broke away. NEVER get near the edge of a sinkhole!]

    Wet soil weighs more and has less strength than if it were dry. If the strength of the wet soil is insufficient the soil arch will fail. A number of adjacent voids may coalesce to form a large void. If the strength of the dry soil is sufficient to support the soil arch, the wet soil failure will proceed only to the dry soil above the former water level. Subsequent periods of extended heavy rainfall may wet the soil sufficiency to reduce its strength below that necessary to support the soil arch and failure would propagate to the surface and form a steep-sided collapse sinkhole (Figure 1). Patterns of pumping of high yield wells over extended periods of time can result in large as well as rapid drawdowns of the water table. Where such rapid and large drawdowns occur in unconsolidated materials, sinkhole collapse can be catastrophic and subsidence can be extensive over the area subject to the drawdown (Foose, 1967 and 1968). Sinkhole formation can also occur above solutionally enlarged fractures, which have fomed caves or "mudseams". Water-table drawdowns can cause soil voids to migrate along solution features eventually leading to sinkhole formation at a distance from the well.

    Sinkhole subsidence is associated also with soil piping. Water leaking from culverts, or other drainage structures can create a void beneath the drainage structure by compaction or internal scour of the soil. This reduction in support can result in displacement of the leaking structure and an increase in leakage or breakage. The void may increase in size to the extent that the soil has insufficient strength to support itself with subsequent failure leading to the fomation of a steep sided collapse sinkhole. The recognition of water mark stains on the fracture surfaces and joints of drainage structures are indicators of this type of sinkhole formation.

    [Figure 1] Disposal of storm water in sinkholes or shallow dry wells can induce subsidence. Adjacent to the drainage input additional sinkholes may form. Subsidence results from a combination of factors, which may include hydraulic compaction, soil piping, and increases in the range of fluctuation of the water table.

    The collapse of a void created by underground mining activities is another mode of sinkhole formation. Voids, created by the solution mining of salt and the conventional underground mining of gypsum, limestone, and coal, have collapsed to form sinkholes in Virginia.

    Sinkhole flooding can develop from a number of conditions, but two man-made conditions are the most common causes in Virginia: the plugging of natural sinkhole drains by sediment and the overwhelming of natural snkhole drains by increases in runoff due to artificial surfaces. Inadequate erosion control during construction can result in the plugging of natural sinkhole drains by sediment-laden runoff. The accompanying restriction of subsurface drainage causes an increase in ponding or flooding. Increased runoff from roads, parking lots, and structures is the most significant cause of sinkhole flooding. Much of the precipitation that would have percolated through a vegetated soil cover is introduced rapidly into surface and subsurface (input through sinkholes) drainage networks. Increases in runoff have been reported to range from 48 percent for area of suburban housing to 153 percent or more for industrial or commercial areas (Aley and Thomson, 1981). Such increases in runoff can exceed the drainage capacity of natural sinkhole drain and result in ponding or flooding. In severe cases, excessive runoff can overwhelm the capacity of the natural subsurface drainage systems of sinkholes, causing water to back-up and flood snkholes up-system (Crawford, 1981). An example of an overwhelmed natural subsurface drainage conduit occurred in Virginia in November, 1985. A stream of water estimated with a peak flow of 50,000 gallons per minute was observed flowing from a normally dry sinkhole during this major storm event(D. W. Slifer, 1988, personal commmication).

    The pollution of groundwater resources is an ever present problem in karst areas. Sinkholes have long been used as dumps for waste materials. The dumping of solid wastes, such as dead animals, garbage, and refuse, into sinkholes is a major hazard to groundwater resources. It is also prohibited by existing State law (Code of Virginia, Title 10, chapter 12.2, section 10-150.14). Liquid wastes dumped into sinkholes can enter the gromdwater system undiluted through the underground drainage routes or conduits. An excellent principle is to never put anything in a sinkhole that you would not want in your drinking water.




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