Natural Hazards, 2e

1. Natural Hazards, 2e Subsidence and Soils Chapter 7

2. 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

3. 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

4. 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.

5. 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.

6. Karst Topography, cont. Groundwater level drops, leaving behind caves and sinkholes. Sinkholes in large numbers form a karst plain.

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

8. 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.

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

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 the surface.

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.

12. 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.

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

14. 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.

15. Active Lava Tube

17. 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.

18. 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

20. Frost-Susceptible Soils Soils containing water expand when frozen, moving the soil upward. Frost heaving

21. 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.

22. 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.

24. Effects of Subsidence and Soil Volume Change Sinkhole formation Damage highways, homes, sewage facilities, etc. Probably triggered by fluctuations in water table

25. 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.

26. 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.

27. 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.

28. 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.

30. 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.

31. 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.

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

34. 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.

35. 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.

36. 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.

37. 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

38. End Subsidence and Soils Chapter 7