Subsidence

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Subsidence
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Definition Subsidence

• The sinking, collapse or downward settling of the
  earths surface.

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Categories of Subsidence

• Underground Mining
• Cavern Collapse
• Ground Water / Oil Withdrawal
• Sediment Loading
• Earthquake Movement
• Magma Chamber Deflation

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Underground Mining

• Underground cavities will seal over time
• Danger related to depth below surface
• 100 or less - Dangerous!
• 100 - 500 damage is probable to possible
• over 500 usually safe - but not always

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Subsidence Cross Section
WVGES

• Fairmont, WV Many homes damaged
• Shallow Mining in 1800s 100 down or less
• Room and Pillar Mining with pillar robbery
• Failure where pillars have been removed

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Rocks Below Fairmont Neighborhood
Coal Seam
WVGES
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Cross-section close-up

• Roof failure
• Overburden collapse
• Surface subsides
• Usually a slow process
• Damaging to structures
• Damaging to utilities
• Damaging to aquifers
• Usually not injuring

WVGES
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Map of Undermined Area

WVGES
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Mine Close-up

• Mined in the 1800s
• Subsidence starts in 1980
• Many homes destroyed
• Coal company is gone
• No one to sue
• Insurance does not pay

WVGES
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Subsidence Damage

WVGES
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Distribution of Pennsylvania Coals
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Retsof Salt Mine

• One of the worlds largest mines
• About 75 miles north of Mansfield
• Most of the highway salt NE US
• About 1200 feet below surface
• Roof failure in 1994
• Mine slowly lost to flooding

USGS
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Sinkholes Develop Over Collapse
USGS
Water begins pouring into mine, dissolving salt
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Slow Flooding of the Mine
USGS
The mine was slowly lost to flooding
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What you should know

• Mine subsidence happens often over mines
• Know if your house is built over a mine
• Contact your State Geological Survey for info
• Obtain insurance if appropriate

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Cavern Collapse

• Most caverns occur in areas of shallow limestone
• Limestone is a slightly soluble rock
• Water moving through the limestone slowly widens
  fractures into passages
• Result a cavern network that will collapse
  given enough time

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Sinkholes

• Depressions caused by cavern collapse
• Very damaging to utilities and buildings
• Usually a slow process but rarely it is rapid

WVGES
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Sinkholes

USDA
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Sinkholes

USGS
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Pennsylvania Limestone Outcrops
USGS

• Pennsylvania subsidence problems in these
  areas
• Be careful purchasing home or land

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What You Should Know

• When purchasing a home or land
• Be careful in areas underlain by limestone
• Consult a geologic map for limestone areas
• Look for shallow depressions in the land surface
• Consult with the Geological Survey if needed

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Water / Oil Production

• Fluids within the pore spaces of subsurface
  rock
• They provide pore pressure that resists
  compaction
• Pressure drops when fluids are removed
• Lower pressure allows the rock to compact
• Compaction of subsurface rock causes surface
  subsidence

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Under Ground Fluid Production

• Heavy oil production and ground water depletion
  can result in subsidence
• Example Oil production in the Gulf Coast
  area.
• Example Ground water depletion in CA, NV, AZ

USGS
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Other Causes of Subsidence

• Sediment loading on deltas (New Orleans)
• Sudden earthquake-related movements
• Magma chamber deflation

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Mining Under the Ohio River

Early on a typical morning in 1915, a shift of
coal miners living in Stubenville, Ohio, left
their homes and headed toward the La Belle
Mine.arriving at Shaft Number One, they were
lowered straight down through 200 feet of rock to
reach the mine workings. From the bottom of the
shaft the miners proceeded eastwalking under
the Ohio River.when they finally reached their
working place they were one mile inside of West
Virginia. (Thomas Repine - Mountain State
Geology - 1986)
WVGES
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Detecting Subsidence from Space

• Radar scans from two different dates can be used
  to make a subsidence map
• Amazingly small subsidence features can be
  detected

USGS