SH

© 2012 Pearson Education, Inc.
Lecture Presentation
Chapter 8
Subsidence
and Sinkholes

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
 Results in karst topography
 Thawing of frozen ground
 Compaction of sediment
 Earthquakes and drainage of magma
 Soil volume change result from natural processes
 Changes in water content of soil
 Frost heaving
 These are probably not life threatening, but is one of the most
widespread and costly natural hazards

Karst
 Common type of landscape
associated with subsidence
 Rocks are dissolved by surface
or groundwater
 Evaporites, rock salt and
gypsum, dissolved by water
 Carbonates, limestone and
dolostone and marble,
dissolved by slightly acidic
water
 Acid comes from carbon
dioxide from plant and animal
decay
 Common in humid climates
Figure 8.8a

Karst Topography, cont.
 Rocks are dissolved and groundwater level drops,
leaving behind caverns and sinkholes
 Pits in that are near surface
 Sinkholes in large numbers form a karst plain
Figure 8.8b, c

Sinkholes
 Solutional sinkholes
 Acidic groundwater becomes concentrated in
holes in joints and fractures in the rock
 Water is drawn into a cone above the hole in
the limestone
 Collapse sinkholes
 develop by the collapse of material into an
underground cavern

https://www.youtube.com/watch?v=e-DVIQPqS8E

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 8.10

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 at the surface
Figure 8.11

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

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

Earthquakes
 In subduction boundaries, when fault is locked,
land can become uplifted
 After an earthquake, the land subsides
Figure 8.12

Underground Drainage of Magma
 Magma uplifts the land during an eruption,
afterwards land subsides
 Lava tubes form when molten lava drains out
from underneath cooled surface lava

Expansive Soils
 These soils expand during wet periods and
shrink during dry periods
 Common in clay, shale and clay-rich soil
containing smectite
 After expansion, soils can have cracks and
popcorn-like texture
 Often will produce wavy bumps in surfaces
 causing tilting and cracking of sidewalks,
foundations, utility poles and headstones

Frost-Susceptible Soils
 Soils containing water expand when frozen moving the
soil upward
 Frost heaving
Figure 8.16

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

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 semiarid
climates
 Areas with extensive below freezing
temperatures can host frost heaving

https://www.youtube.com/watch?v=yZkqNHGu4ys
https://www.youtube.com/watch?v=Cp5S6WWZsWc

Effects of Subsidence and Soil Volume
Change
 Sinkhole Formation
 Common in Florida and
Pennsylvania
 Damage highways,
homes, sewage facilities,
etc.
 Probably triggered by
fluctuations in water
table
 High groundwater
enlarges underground
caverns and fills with
water. Removal of water
causes roof to collapse.
Figure 8.18

Change, cont. 1
 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 8.19

Change, cont. 2
 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 8.20

Change, cont. 3
 Coastal flooding and loss of wetlands
 Along the Mississippi delta this has contributed to sinking
of New Orleans
 Wetlands that protect the city from surges are eroding
 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

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

Natural Service Functions
 Water supply
 Karst regions contain the world’s 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

Human Interaction
 Withdrawal of fluids
 Pumping fluids such
as oil, natural gas,
water, groundwater,
etc. decreases fluid
pressure causing
rocks to subside
Figure 8.25b

Human Interaction, cont. 1
 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.

Human Interaction, cont. 2
 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

Perception and Adjustment to Hazard
 Few people are aware of subsidence and soil
volume change hazard
 People who live in dramatically affected areas
are more aware than others
 Best solution is to avoid building in vulnerable
areas through:
 Geologic and soil mapping
 Surface features
 Subsurface surveys

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