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Subsidence, in the context of underground mining, is the
lowering of the Earth's surface due to collapse of bedrock
and unconsolidated materials (sand, gravel, salt, and
clay) into underground mined areas.
There are two types of subsidence: (1) pit, also called
sinkhole or pothole, and (2) sag or trough.
Pit subsidence is characterized by an abrupt sinking of
the surface, resulting in a circular steep-sided, crater
like feature that has an inward drainage pattern.
Sag subsidence is a gentle, gradual settling of the surface.
It is associated with pillar crushing or pillar punching of
deeper mines. Sag-subsidence features may fill with water
if the surface of the subsidence intersects the water table.
Pit-subsidence features generally do not hold water because
the pit drains into the underlying mine.
Mine subsidence is controlled by many factors, including height
of mined-out area, width of unsupported mine roof, thickness of
overburden, competency (strength) of bedrock, pillar dimensions,
hydrology, fractures/joints, and time. The vertical movement of
subsidence is proportional to the height of the extraction area.
The vertical movement decreases with increasing depth or
thickness of overburden, especially bedrock. As the roof
rock sags, ruptures, and eventually collapses into a mined-out
area, the roof rock rotates, twists, splinters, or crumbles as
it falls, resulting in incomplete compaction. In other words,
the mine void is not completely filled during a mine-roof
collapse. Because bedrock collapses with incomplete compaction,
the deeper the extraction area, the smaller the vertical movement
is at the surface.
The length of time for mine subsidence to occur increases with
increasing depth of mining and increasing competency of overburden.
The type and amount of roof support in addition to pillars of coal
left in the mine also affect subsidence. Most early underground
mines in Ohio used wooden timbers as additional roof support. Steel
I-beams were used in Ohio mines as roof support beginning in the
early twentieth century. By the mid-twentieth century, roof bolting
was another type of roof support being used in Ohio mines. With
time following abandonment of an underground mine, these types of
roof support eventually rot or deteriorate, allowing subsidence to
occur. Because of the complexity of the variables which contribute
to mine-related subsidence, no acceptable system exists which is
capable of accurately predicting the time or amount of subsidence
in a variety of geological settings, especially for mines that have
an irregular pattern of room-and-pillar mining.
* GEOFACTS No. 12 published by the Ohio Department of Natural
Resources Division of Geological Survey
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