Natural Hazards and Disasters Chapter 9 Sinkholes, Land Subsidence and Swelling Soils - PowerPoint PPT Presentation

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Natural Hazards and Disasters Chapter 9 Sinkholes, Land Subsidence and Swelling Soils

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  1. Natural Hazards and Disasters Chapter 9 Sinkholes, Land Subsidence and Swelling Soils

  2. Shrinking Ground Groundwater has been critical for agriculture, mining and municipal uses in southern Arizona for more than 100 years Natural replacement rates do not match rate of demand for rapidly growing metropolitan areas Fall of more than 180 m in groundwater levels has led to ground subsidence, fissures up to 6 m deep and 9 m across Imported water from Colorado River flows through concrete channels, themselves damaged by fissures

  3. Types of Ground Movement Ground movements are not as dramatic as earthquakes or volcanoes, but cause far more monetary damage Deform and effectively destroy roads, utility lines, homes, other structures Sinkholes form when overlying ground collapses into underground cavities Land subsidence occurs when sediment becomes more closely packed, through groundwater or petroleum extraction, or earthquake shaking Swelling soils form from alteration of volcanic ash to clays

  4. Karst Topography • What is Karst topography (KT)? • The most common soluble rock is limestone • Others include dolomite, gypsum, and salt • Unique landscape features exist where limestone strata exists at or near the surface • These processes were first studied on the Krš Plateau in Slovenia (Karst Plateau) • Features created by ground and surface water are common, and can be confused with KT, but true KT is rare, due to the conditions that must be met in order to create such landscapes • Some glacial features can also be confused with KT

  5. Regions, Climate, and Time • KT is generally absent or not well developed in arid climates • Where it is found in arid climates, it is a remnant from when the area was more humid • The role of climate is uncertain • Amount and distribution of rainfall are probably key factors • KT landscapes do not appear to evolve on a temporal continuum, but rather, are locally specific • Nevertheless, they do exhibit specific characteristics

  6. Where is it found?

  7. Karst in the U.S.

  8. Karst Features • We see KT expressing itself in two places • Surface features, which are due to… • Underground: limestone caverns and caves • Surface features • Disappearing streams • Sinkholes (dolines) • Haystack hills (hums) • Tower Karst if they are high enough • Valleys and valley sinks • Potholes

  9. Surface Features Tower Karst or Hums Potholes

  10. O’Leno State Park, Florida Sinkholes Hums Disappearing Stream Potholes

  11. Surface Features of KT • Limestone-dominated landscapes are usually absent of surface water due to the high permeability of the rock • In spots where shale and/or clay meets limestone, swallow holes or potholes develop, and streams disappear into them…disappearing streams • If the calcium carbonate solution continues to erode the limestone, potholes hundreds of feet deep develop

  12. Sinkholes Ground may suddenly collapse into sinkholes tens to hundreds of meters across Can damage houses and roads Can drain streams, lakes and wetlands Can channel contaminants directly into underground aquifers

  13. Processes Related to Sinkholes • Limestone must contain 80% or more calcium bicarbonate • Complex patterns of joints in the limestone are needed for subsurface drainage patterns • An aerated zone between the ground and the water table must be present • Vegetation cover necessary to supply acids to enhance the processes

  14. By the Numbers Formation of Calcium Bicarbonate H2O (rain water) + CO2 (carbon dioxide) = H2CO3 (carbonic acid in water) Carbonic acid reacts with limestone to form calcium bicarbonate: H2CO3 + CaCO3 = [Ca++ + 2HCO3-]

  15. Processes Related to Sinkholes • Limestone dissolves in slightly acidic rainwater, at rate of millimeters per thousand years • Slightly faster rate in humid areas  sinkholes and caverns more common in tropical climates • Air pollution makes rain more acidic, increases rate of dissolution • Highest rate of dissolution occurs at level of water table • Water precipitating down (above water table) can form stalagmites and stalactites • Landscape of limestone dissolution: karst

  16. Types of Sinkholes Dissolution (Solution) Cover Subsidence Cover Collapse (Bedrock Collapse)

  17. Types of Sinkholes • Dissolution (Solution): acidic groundwater seeps through soil, dissolves underlying limestone along fractures, which widen to lumpy or jagged Karst surfaces

  18. Types of Sinkholes • Cover Subsidence: where deep, permeable sediment sits atop limestone, numerous gradual sinkholes form

  19. Types of Sinkholes

  20. Types of Sinkholes • Cover Collapse (Bedrock Collapse): where deep, impermeable (clay) sediment sits atop limestone, soil cavities grow large, become unstable and collapse suddenly into steep-sided sinkhole – most dangerous • Roof collapse • Groundwater lowering

  21. Types of Sinkholes Truck Stuck in Sinkhole

  22. Areas That Experience Sinkholes Slowly dissolving carbonate rocks underlie more than 40% of humid eastern U.S. Fluctuations of groundwater level can lead to sinkhole formation (much lower groundwater level during last ice age) Greatest potential for sinkholes is where surface water percolates into ground, recharging aquifers Least potential for sinkholes is where water is being discharged to surface

  23. Areas That Experience Sinkholes Limestone weathering produced Mammoth Caverns in Kentucky, and more than 60,000 nearby sinkholes

  24. Areas That Experience Sinkholes In urban areas, storm drains and leaky water mains can contribute to sinkhole formation Construction can lead to sinkhole development by increasing load on ground surface, de-watering foundations, drilling wells Underground mining of salt or gypsum creates artificial cavities that can be enlarged by groundwater dissolution

  25. Land Subsidence • Occurs when ground settles as result of changes in fluid levels underground • Occurs across large regions, so less obvious or dramatic than sinkholes • Small faults cause some areas to drop more than others • Fissures form • Areas are dropped closer to sea level so more vulnerable to flooding • Caused by variety of human activities • Extraction of groundwater • Drainage of organic or clay-rich soils • Melting of permafrost

  26. Mining Groundwater and Petroleum Most common cause of ground subsidence Rainwater soaks into ground and travels through aquifers, from which groundwater can be pumped as source of fresh water for many communities If pumpage rate exceeds recharge rate (from precipitation)  mining groundwater Aquifers consist of loosely packed sand and gravel with water filling pore spaces Withdrawal of water allows sand and gravel to pack more tightly, take up less space Once pore space has collapsed, can not be expanded again  most ground subsidence is permanent Problems in California, Arizona, Texas, Venice, Italy

  27. Mining Groundwater and Petroleum

  28. Drainage of Organic Soils • When groundwater levels drop: • Organic-rich soils (such as peat) are exposed to aerobic (oxygen-rich) water instead of anaerobic water (without free oxygen) • Allows bacteria to oxidize organic matter  decomposes mostly to carbon dioxide • Problems in California’s Central Valley, Florida Everglades, Mississippi River delta

  29. Drying of Clays • Clay soils are particularly prone to subsidence when they dry out • Especially true of randomly oriented marine clays with high porosity (quick clays) • Can collapse when jarred by earthquake or heavy equipment, or flushed with fresh water • Leaning Tower of Pisa sits atop marine clay • Leda clay causes landslides throughout St. Lawrence River valley in Quebec, Canada

  30. Permafrost Thaw and Ground Settling Arctic climates have in-ground temperatures that are below freezing year round  permafrost If ice thaws, water escapes pore spaces, ground settles Forest fires, removal of vegetation can lead to permafrost thawing Building structures on permafrost must take into account possibility of thaw and ground surface deformation

  31. Permafrost Thaw and Ground Settling A Lake on Fire

  32. Swelling Soils Smectite clays expand when water soaks into interlayer spaces of mineral structure (same process as subsidence when clays dry out) Appearance of surface soils as popcorn clay Expansion and contraction cause cracking of foundations, walls, chimneys, driveways Wet clay is extremely slippery in dirt roads Variable rates of swelling occur when some areas get wetter than others

  33. Swelling Soils

  34. Case In Point

  35. Case In Point

  36. Case In Point Excessive Mining Causes Roof Collapse: Genesee Valley, New York State Retsof Salt Mine had been active since 1880s 1994: section of shale roof rock collapsed into excavation Hole allowed groundwater to sink into mine, filling its cavities, flooding mine Sinkholes as large as 60 m opened across region New mine began excavating few years later

  37. Case In Point • Subsidence Due to Groundwater Extraction: Venice, Italy • Built at sea level on subsided Brenta River delta • Subsidence mostly caused by extraction of groundwater from delta sediments • Load of buildings squeezes water out of sediments • Groundwater is pumped to surface for domestic and industrial uses • Collapsed pore spaces in sediments cause ground to subside

  38. Case In Point Subsidence Due to Groundwater Extraction: Venice, Italy High tides of winter storms compound problem Drainage pipes carry effluent back into city Pumping groundwater under city now prohibited Subsidence has mostly stopped Catastrophic flooding occurs periodically, and will occur more frequently with global warming and rise in sea level Giant flood protection project under construction

  39. Case In Point Subsidence Due to Groundwater Extraction: Mexico City, Mexico Population over 20 million Lies in dry lakebed Aquifer has been depleted since early 1900s Collapse of porous lake bed sediments has caused subsidence of up to 8.5 m under central city Continuous pumping necessary to prevent flooding during summer rains

  40. Case In Point Differential Expansion over Layers of Smectite Clay: Denver, Colorado Underlain by flat-lying sedimentary rocks including Pierre Shale, smectite-rich clay Clay layers swell when wet, creating broad waves that twist, crack and bend structures Solution has been excavation and homogenization of clay layers and non-clay-bearing layers to depth of at least 3 m