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HOW DO SINKHOLES DEVELOP?
There
are many detailed, scholarly reports that are available for those
desiring a more thorough and highly technical discussion of this
topic. What follows here is intended only to be a brief “consumer
friendly” summation of sinkhole development.
Sinkholes are depressions or holes in the land surface. They can be
shallow or deep, small or large, but all are the result of the
dissolving of the underlying limestone. Hydrologic conditions,
including lack of rainfall, lowered water levels, or conversely,
excessive rainfall in a short period of time (especially after a
drought) can all contribute to sinkhole development. New
construction, new roads, and any diversion of water flow are also
common culprits.
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Solution
activity within the limestone is greatest along localized fractures,
joints, or bedding planes, since these features represent
preferential paths that concentrate the flow of water into the
formation. This activity is accelerated where the volume of flow
increases. In Florida, especially, infiltration rates are very high;
sometimes upwards of 75%, and this increased volume transports soil
into the voids and speeds the dissolving of the limestone rock. The
dissolution of the limestone can be accelerated even more if the
rainwater permeates through vegetation. This makes it more acidic.
Obviously, the closer to the surface the limestone is, the more
likely that a sinkhole can occur, although there are many that are
very deep. Despite many so-called insurance company experts, many
sinkholes occur below layers of clay.
Sinkholes occur naturally, and it is estimated that the majority of
all lakes in Florida are a result of sinkhole activity. Sinkholes
are common where the rock below the land surface is limestone and
other carbonate rock, which can naturally be dissolved by water. As
the rock dissolves, the ceiling of the cavern becomes thinner. When
the land above the cavern becomes too heavy for the ceiling to
support, a depression or collapse of the land surface can occur. The
damage can be minor or significant depending upon what structure is
immediately above the sinkhole activity and how much ground is
disturbed. Abrupt collapse sinkholes have become more common over
the past several decades, proportionate to the increased activity of
humans, which involves building, withdrawal of ground water,
diversion of surface water, and etc. Even the “repair” of some
sinkholes by grouting can cause more damage and create new sinkholes
by the added weight to the soil and from the diversion of the
current water flow.
THREE
GENERAL CLASSIFICATIONS OF SINKHOLES:
The three general types of sinkholes -- subsidence, solution, and
collapse -- generally correspond to the thickness of the sediments
overlying the limestone of the Floridian aquifer system. The
sediments and water contained in the unsaturated zone, surficial
aquifer system, and the confining layer above the Floridan aquifer
are all collectively referred to as overburden. Collapse sinkholes
are most common in areas where the overburden is thick, but the
confining layer is breached or absent. Subsidence sinkholes form
where the overburden is thin and only a veneer of sediments is
present overlying the limestone. Solution sinkholes form where the
overburden is absent and the limestone is exposed at land surface.
I. COLLAPSE SINKHOLES:
Collapse sinkholes are the most dramatic of the three sinkhole
types; they form with little warning and leave behind deep,
steep-sided holes. One mechanism for the formation of a collapse
sinkhole is illustrated below. Notice that the geologic conditions
include soluble bedrock (such as limestone) covered by relatively
thick deposits of sediments. This type of sinkhole can form
naturally but is often affected by human activities. The progression
of a collapse sinkhole is illustrated in figures 1-3 below.
FIGURE 1: There
is no evidence of land subsidence, but small to medium size
cavities have already formed in the rock matrix. Water from
surface percolates through to rock, and the erosion process
begins. |
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FIGURE 2:
Cavities in the rock matrix continue to
grow larger but remain filled with water. This water pressure
helps to support the thinner, weaker roof of the enlarged
cavity. |
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FIGURE 3:
As the water level drops during the dry
season, or is lowered due to pumping it out, the weight of the
overburden exceeds the strength of the cavern roof, and the
overburden collapses into the cavern, forming a sinkhole. |
II.
SUBSIDENCE SINKHOLES:
The
progression of a subsidence sinkhole is illustrated below in figures
4-6. Rainwater percolates through overlying sediments (usually thin)
and reaches the limestone, dissolving the rock and gradually
weakening its structural integrity. Gradually subsiding sinkholes
commonly form where slow dissolution takes place, mostly along
joints in the limestone. These sinkholes tend to form naturally and
are not greatly affected by human activities.
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FIGURE 4: Initially
the limestone contains fractures and small cavities that have
formed by dissolution, but no subsidence has occurred.
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FIGURE 5: Small
cavities and cracks grow larger as time progresses, and water
moving through the rock erodes the rock matrix. Sediments from
above are carried by groundwater to fill the voids forming in
the rock. |
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FIGURE 6: Sediments
from the upper layers continue to fill in the openings in the
limestone, causing a depression at the land surface. If water
collects in the depression, a new lake is formed. |
III.
SOLUTION SINKHOLES:
If the
overburden is thin or absent, the surface of the limestone bedrock
is broken down by erosion from wind and surface water. A bowl-shaped
depression, or solution sinkhole, naturally forms slowly and
continuously as chemical and physical processes erode the rock.
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