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Understanding Mass Wasting and Landslide Hazards

Explore the various classifications of mass wasting events, factors affecting slope stability, role of human activities, and methods to minimize landslide hazards in urban and natural environments.

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Understanding Mass Wasting and Landslide Hazards

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  1. Chapter 6: Mass Wasting Landslide is a general term for rapid downslope movement/failure Mass Wasting is even more general and includes slow movements (creep, slow flows…) Subsidence is a vertical failure usually  closed depression Palos Verdes Hills Landslide

  2. A Classification of Mass Wasting • Nature of material • Rock • Unconsolidated material (debris, mud) • Velocity • Nature of movement

  3. A Classification of Mass Wasting

  4. A Classification of Mass Wasting Rock Debris (rare) Fall

  5. A Classification of Mass Wasting Rock Debris Avalanche

  6. A Classification of Mass Wasting Rock Debris Slide

  7. A Classification of Mass Wasting Earth Debris Mud Flow

  8. A Classification of Mass Wasting Creep Slump

  9. A Classification of Mass Wasting Solufluction

  10. Solufluction

  11. Slope stability: • Driving forces: • Mass • Gravity • Resisting forces: • Inertia • Friction • Cohesion Angle of repose: higher with coarser and more angular particles

  12. Slope stability scenario: what are the problems here?

  13. Role of material • Consolidated material can slide or slump, • Unconsolidated can slump or flow Role of slope • Steeper  less stable (incr DF and decr RF)

  14. Role of climate and vegetation • Climate controls water and vegetation • Vegetation absorbs rain impact and roots incr cohesion • Vegetation adds weight • Logging (clearcut): • Less transpiration  more water to soil • Increased snowpack in clearings • Roots rot and lose cohesion • Long vertical run for rill development • Logging roads are perhaps worse than the cuts themselves

  15. Adds weight (incr DF) • Initially increases cohesion, but decreases after a point (decr RF) • Often saturated zone at surface if above impermeable layer (frozen, clay, rock) • Promotes weathering • Erodes gullies and slope bases • Avalanches are all water Role of water

  16. Role of vibration • Earthquakes, traffic, sonic booms • Can  immediate loss of cohesion • Liquefaction = shock + water saturation

  17. Human Use and Landslides Landslides are a growing problem. 1. Incr population in landslide-prone areas 2. Human activity that affects magnitude and frequency of landslides

  18. Vaiont Dam Disaster

  19. Vaiont Dam Disaster

  20. Logging Swanson & Dryness (1975) Cascade study • Found logging and roads increase landslide erosion by several times if performed on weak unstable slopes • Roads interrupt surface drainage (culverts blocked), alter subsurface movement, redistribute mass

  21. Urbanization Person A undercuts slope for garage while person B builds house above. Who is responsible? S. California • Wave-cut terraces oversteepened • Now lots of building on lower mtn slopes (most = old landslide deposits!)

  22. Drainage Divide

  23. Isaquah Delta

  24. Seattle Topography

  25. Slope Stability

  26. Slope Stability

  27. Slope Stability Maps

  28. Minimizing the Hazard • Identify potential hazard. • Map unstable areas

  29. Minimizing the Hazard • Identify potential hazard. • Map unstable areas • Land developers and buyers (YOU) look for what??

  30. Minimizing the Hazard • If suspect, hire a consulting geologist to evaluate • Don’t build • Minimize load or sink foundation deep to stable material • Drainage control • Divert surface water • Surface drains • Soil cement, asphalt, plastic to cover • Cut-off trench to divert groundwater • Drain internal water at base or even pump (Tukwilla)

  31. Minimizing the Hazard • If suspect, hire a consulting geologist to evaluate • Grade slope • Cut-and-fill: remove top and fill in at base • Must do with care, since this often causes problems • Benching: series of cut-and-fills with diversion drains • Slope supports • Retaining walls (permeable and/or drained) • Rock bolts • Landslide warning systems • Periodic inspection • Tilt meters • Well monitoring

  32. Minimizing the Hazard Landslide correction (very tough…better to prevent)

  33. Snow Avalanches 1. Loose-snow avalanches in fresh cohesionless snow

  34. Snow Avalanches 2. Slab avalanches above subsurface hoar zones

  35. Subsidence (oil, gas, water) Slow to rapid vertical sinking or settling 1. Withdrawal of fluid

  36. Subsidence Slow to rapid vertical sinking or settling 2. Sinkholes • Limestone solution, caves and collapse (also with salt deposits)

  37. Sink hole- Winter Park, Florida

  38. Subsidence Slow to rapid vertical sinking or settling 1. Withdrawal of fluid(oil, gas, water) 2. Sinkholes • Limestone solution, caves and collapse (also with salt deposits) 3. Mining

  39. What is wrong here?

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