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

• Coal, salt, limestone, any mine will collapse
• Underground cavities always seal over time
• Danger related to depth below surface
• 100 or less - Dangerous!
• 100 - 500 damage probable - possible
• over 500 usually safe

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Subsidence Cross Section
WVGES

• Fairmont, WV Many homes damaged
• Shallow Mining about 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

• No wonder those houses were being damaged!

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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
Fairmont, WV - largely undermined
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Cross-section close-up

• Room and Pillar Method
• Mining done in 1800s
• Subsidence started 1980
• Many homes destroyed
• Mining companies gone
• No one to sue
• Insurance does not pay

WVGES
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Subsidence Damage

• Buildings subside unevenly
• No longer square, level, plumb
• Underground utilities destroyed
• Most homes condemned
• Owners bankrupt / no insurance

WVGES
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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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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
• Fortunately no miners undergroud
• Weekend shutdown

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

• Mine subsidence happens often in PA
• Know what your house is built over
• 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
• Rain water infiltrating down through the
  limestone rock unit slowly widens fractures into
  passages
• The result is 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
• Shallow limestone bedrock

WVGES
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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
• Look for shallow depressions
• Consult with the geological survey
• Ask if you are in a karst area

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Water / Oil Production

• These fluids exist within the pore spaces of
  subsurface rock
• They provide pore pressure that resists
  compaction
• When these fluids are removed the pore pressure
  declines
• The decline in pore pressure allows the rock to
  compact
• Compaction of subsurface rock causes surface
  subsidence

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Cross-section close-up

• Heavy oil production and ground water depletion
  can result in extreme subsidence
• In the US heavy oil production in the Gulf
  Coast area.
• In the US ground water depletion in
  California, Nevada, Arizona

USGS
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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
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Other Causes of Subsidence

• Sediment loading on deltas
• Sudden earthquake-related movements
• Magma chamber deflation