Natural Hazards, 2e

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Natural Hazards, 2e

• Subsidence and Soils
• Chapter 7

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Learning Objectives

• Understand the causes and effects of subsidence
  and volume changes in the soil
• Know the geographic regions at risk for
  subsidence and volume changes in the soil
• Understand the hazards associated with karst
  regions
• Recognize linkages between subsidence, soil
  expansion and contraction, and other hazards, as
  well as natural service functions of karst

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Learning Objectives, cont.

• Understand how humans interact with subsidence
  and soil hazards
• Know what can be done to minimize the hazard from
  subsidence and volume changes in the soil

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Introduction to Subsidence and Soil Volume Change

• Subsidence is ground failure characterized by
  sinking or vertical deformation of land
  associated with
• Dissolution of rocks beneath the surface
• Thawing of frozen ground
• Compaction of sediment
• Earthquakes and drainage of magma
• Soil volume change results from natural
  processes.
• Changes in water content of soil
• Frost heaving
• These are probably not life threatening, but are
  some of the most widespread and costly natural
  hazards.

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Karst Topography

• Common type of landscape associated with
  subsidence.
• Rocks are dissolved by surface or groundwater.
• Evaporites, rock salt, and gypsum are dissolved
  by water.
• Carbonates, limestone, and dolostone are
  dissolved by slightly acidic water.
• Acid comes from carbon dioxide from plant and
  animal decay.
• Common in humid climates.

Figure 7.3a
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Karst Topography, cont.

• Groundwater level drops, leaving behind caves and
  sinkholes.
• Sinkholes in large numbers form a karst plain.

Figure 7.3b,c
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Sinkholes

• Groundwater dissolves soluble rock, creating
  fractures and caves.
• Dissolving continues to form larger caves and
  fractures.

Figure 7.4a,b
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Sinkholes, cont.

• Collapse sinkholes form when top of the sinkhole
  falls because groundwater levels drop.
• Solutional sinkholes form when the bedrock
  continues to be dissolved.

Figure 7.4c
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Cave Systems

• Cave systems are formed when dissolution produces
  a series of caves.
• Related to fluctuating groundwater table.
• Groundwater seepage causes flowstone,
  stalagmites, stalactites.

Figure 7.5
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Tower Karst, Disappearing Streams and Springs

• Tower karst is created in highly eroded karst
  regions.
• Disappearing streams are streams that disappear
  into cave openings.
• Springs are places where groundwater naturally
  flows into the surface.

Figure 7.6b
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Thermokarst

• In polar or high altitude regions, permafrost
  exists.
• Soil or sediment cemented with ice for at least 2
  years
• When permafrost thaws, it can create land
  subsidence.
• Extensive thawing creates uneven soil called
  thermokarst.

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Sediment and Soil Compaction

• Fine sediment
• Sediment collapses when water is removed.
• Common on river deltas.
• Flooding replenishes sediment, thwarting
  collapse.
• Collapsible soils
• Dust deposits, loess, and stream deposits in arid
  regions are bound with clay or water-soluble
  minerals.
• Water weakens bonds, causing soil to collapse.
• Organic soils
• Wetland soils contain large amounts of organic
  matter and water.
• When water is drained or soil is decomposed,
  these soils collapse.

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Earthquakes

• In subduction boundaries, when fault is locked,
  land can become uplifted.
• After an earthquake, the land subsides.

Figure 7.7
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Underground Drainage of Magma

• Magma uplifts the land during an eruption, and
  afterward, land subsides.
• Lava tubes form when molten lava drains out from
  underneath cooled surface lava.

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Active Lava Tube
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Lava Tube Cave Mojave Preserve, CA
Image Credit Jacquelyn Hams
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Expansive Soils

• These soils expand during wet periods and shrink
  during dry periods.
• Common in clay, shale, and clay-rich soil
  containing smectite.
• Can produce cracks and popcorn-like texture.

Figure 7.8c
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Expansive Soils, cont.

• Can produce wavy bumps in surfaces
• Can create tilting and cracking of sidewalks and
  foundations
• Can create tilting of utility poles and headstones

Figure 7.8d
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Figure 7.10
Figure 7.9
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Frost-Susceptible Soils

• Soils containing water expand when frozen, moving
  the soil upward.
• Frost heaving

Figure 7.11
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Regions at Risk for Subsidence and Soil Volume
Change

• Landscapes underlain by soluble rocks,
  permafrost, or easily compacted soil and
  sediment.
• Soils that contain large amounts of smectite clay
  are susceptible to shrinking and swelling soils.
• Soils containing silt are susceptible to frost
  heaving.

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Regions at Risk for Subsidence and Soil Volume
Change, cont.

• Climate controls the amount and timing of
  rainfall and duration of freezing temperatures.
• Sinkholes are common in humid climates.
• Expansive soils are common in areas with wet and
  dry seasons.
• Collapsible soils are found in arid and semi-arid
  climates.
• Areas with extensive, below-freezing temperatures
  can host frost heaving.

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Distribution of Karst Topography in U.S.
Figure 7.12
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Effects of Subsidence and Soil Volume Change

• Sinkhole formation
• Damage highways, homes, sewage facilities, etc.
• Probably triggered by fluctuations in water table

Figure 7.13
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Effects of Subsidence and Soil Volume Change,
cont.

• Groundwater conditions
• Caves create direct access between surface and
  groundwater.
• This access can make water vulnerable to
  pollution, especially during drought and when
  sinkholes are used as landfills.

Figure 7.14
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Effects of Subsidence and Soil Volume Change,
cont.

• Melting of permafrost has caused roads to cave
  in, airport runways to fracture, railroad tracks
  to buckle, and buildings to crack, tilt, or
  collapse.

Figure 7.15
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Effects of Subsidence and Soil Volume Change,
cont.

• Coastal flooding and loss of wetlands
• Along the Mississippi Delta, this has contributed
  to the sinking of New Orleans.
• Wetlands that protect the city from surges are
  eroding.

Figure 7.16
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Damage Caused by Soil Volume Change

• Responsible for billions of dollars of damage
  annually to highways, buildings, and structures.
• Frost action on roads costs 2 billion each year.
• Damage caused by soil volume change exceeds the
  cost of all other natural hazards combined.

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Figure 7.17
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Links to Other Natural Hazards

• Can be an effect of earthquakes, volcanoes, and
  climate change
• Climate change adds to the drying of soils and
  altering of groundwater table.
• May cause flooding and mass wasting
• Frost heaving and swelling soils cause creep.
• Areas subsiding due to groundwater mining are
  most susceptible to flooding.

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Natural Service Functions

• Water Supply
• Karst regions contain the worlds most abundant
  water supply.
• Aesthetic and Scientific Resources
• Caves and karst landscapes are beautiful.
• Caves attract visitors.
• Caves and karst provide research for scientists.
• Unique Ecosystems
• Many species of animals can live only in caves.
• Caves also provide shelter for other animals.

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Human Interaction

• Withdrawal of fluids
• Pumping fluids such as oil, natural gas, water,
  groundwater, etc., decreases fluid pressure,
  causing rocks to subside.

Figure 7.20b
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Figure 7.21
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Human Interaction, cont.

• Underground mining
• Coal mine structures have collapsed.
• Water is used to dissolve and pump out salt,
  leaving behind cavities.
• Flooding in salt mines can also cause sinkholes.
• Melting permafrost
• Global warming and building practices
• Restricting deltaic sedimentation
• Construction of dams, levees, etc.

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Human Interaction, cont.

• Altering surface drainage
• Draining soils for agriculture
• Draining wetland soils for development
• Adding water for irrigation
• Poor landscaping practices
• Adding or removing plants changes water levels,
  contributing to shrinking and swelling soils.

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Minimizing Subsidence and Soil Volume Change

• Artificial fluid withdrawal
• Restricting oil and water pumping.
• Injection wells add water when oil is pumped.
• Regulating mining
• Prevention of damage from thawing permafrost
• New engineering of buildings and pipelines on
  permafrost.
• Reducing damage from deltaic subsidence
• Controlled flooding could rebuild marshes.

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Minimizing Subsidence and Soil Volume Change

• Managing drainage of organic and collapsible
  soils
• Limit irrigation and modify land surface
• Prevention of damage from expansive soils
• Design of subsurface drains, rain gutters, and
  reinforced foundations
• Construct buildings on compacted fill

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End

• Subsidence and Soils
• Chapter 7