1. Chapter 8
Subsidence and Soils

2. Learning Objectives
Know what a soil is and the processes that form and maintain soils
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

3. Learning Objectives, cont.
Recognize linkages between subsidence, soil expansion and contraction, and other hazards, as well as natural service functions of karst
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. Soil and Hazards
Soil
Solid earth material that has been altered such that it can support rooted plant life
Any solid earth material that can be removed without blasting
Soil is related to hazards (i.e., soil types and landslides)
Soil is produced through weathering
physical and chemical breakdown of rocks
Changed by residual or transported activity of soil organisms
Soils can stay in place or be transported to new locations

5. Soil and Hazards, cont. Soil development depends on:
Climate
Topography
Parent material (the rock or alluvium from which the soil is formed)
Time (age of the soil)
Organic processes (activity of soil organisms)
In an area, soils form distinct layers
Soil Profile

6. Soil Horizons Layers in a profile are soil horizons
O horizon entirely of plant litter and other organic material
A horizon both organic and mineral material
E horizon, zone of leaching, light-colored layer leached of iron-bearing components
B horizon, zone of accumulation, materials moved downward top layers
Bt horizon enriched in clay minerals
Bk horizon calcium carbonate
K horizon carbonate fills the pore spaces and the carbonate often forms in layers parallel to the surface
Caliche is irregular accumulation or layers of calcium carbonate

7. Soil Horizons, cont.
C horizon parent material partially altered by weathering processes
R horizon, unaltered parent material, consolidated bedrock
Hardpan, hard (compacted) soil horizon
composed of compacted and/or cemented clay with calcium carbonate, iron oxide, or silica
Impermeable

8. Figure 8.1

9. Soil Color
Can be an important diagnostic tool for analyzing a soil profile, but can be misleading
O and A horizons are dark
E horizon is white
B horizon varies from yellow-brown to light red-brown to dark red
K horizon may be almost white
Soil color can also indicate drainage

10. Soil Texture
Depends on proportions of sand-, silt-, and clay-sized particles
Clay diameter < mm
Silt mm > diameter > mm
Sand > diameter > 2.0 mm
Gravel, cobbles or boulders > 2.0 mm
Figure 8.2

11. Relative Soil Profile Development
Soil chronosequence: series of soils from youngest to oldest to give information about the recent history of an area
Weakly developed soil: A directly over a C horizon (without B)
Few hundred to several thousand years old
Moderately developed soil: A overlying an argillic Bt that overlies the C horizon
More than 10,000 years old (at least Pleistocene)
Well-developed soil: B is more red, more translocation of clay to the Bt horizon, and stronger structure
Between 40,000 and several hundred thousand years and older

12. Water in Soils
Saturated – all the pore spaces in a block of soil are completely filled with water
Unsaturated otherwise
Moisture content – amount of water in a soil
Important to strength of soil and potential to shrink and swell
Water flow
Saturated flow if all the pores are filled with water
Unsaturated flow otherwise (more common)

13. Soil Classification Soil taxonomy Engineering classification of soils
By soil scientists
Based on physical and chemical properties of soil profile
Useful for agricultural and land use
Engineering classification of soils
Used by engineers (and for hazards)
Based on particle size or the abundance of organic material

14. Table 8.1

15. Table 8.2

16. 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

17. 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

18. 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

19. Sinkholes Solutional sinkholes Collapse 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

20. Figure 8.9

21. 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

22. 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

23. 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

24. 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

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

26. 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

27. 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

28. Figure 8.13

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

30. 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

31. 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

32. Figure 8.17

33. 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

34. Effects of Subsidence and Soil Volume 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

35. Effects of Subsidence and Soil Volume 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

36. Effects of Subsidence and Soil Volume 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

37. Figure 8.22

38. 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

39. 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

40. 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

41. Figure 8.26

42. 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.

43. 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

44. Minimizing Subsidence and Soil Volume Change
Artificial fluid withdrawal – groundwater mining
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

45. Minimizing Subsidence and Soil Volume Change, cont.
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

46. 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

47. Subsidence and Soils Chapter 8
End
Subsidence and Soils
Chapter 8