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Chapter 6: Mass Wasting Palos Verdes Hills Landslide 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
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A Classification of Mass Wasting l Nature of material F Rock F Unconsolidated material (debris, mud) l Velocity l Nature of movement
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A Classification of Mass Wasting
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RockDebris Fall (rare)
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A Classification of Mass Wasting Rockfall- Talus
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A Classification of Mass Wasting RockDebris Avalanche
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A Classification of Mass Wasting RockDebris Slide
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A Classification of Mass Wasting Earth Debris Mud Flow
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A Classification of Mass Wasting Slump Creep
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Solufluction
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Solufluction
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Angle of repose: higher with coarser and more angular particles Slope stability: F Driving forces: s Mass s Gravity F Resisting forces: s Inertia s Friction s Cohesion
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What are the problems here?
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F Consolidated material can slide or slump, F Unconsolidated can slump or flow Role of slope F Steeper less stable (incr DF and decr RF) Role of material
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F Climate controls water and vegetation F Vegetation absorbs rain impact and roots incr cohesion F Vegetation adds weight F Logging (clearcut): s Less transpiration more water to soil s Increased snowpack in clearings s Roots rot and lose cohesion s Long vertical run for rill development s Logging roads are perhaps worse than the cuts themselves Role of climate and vegetation
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Role of water F Adds weight (incr DF) F Initially increases cohesion, but decreases after a point (decr RF) F Often saturated zone at surface if above impermeable layer (frozen, clay, rock) F Promotes weathering F Erodes gullies and slope bases F Avalanches are all water
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Role of vibration F Earthquakes, traffic, sonic booms F Can immediate loss of cohesion F Liquefaction = shock + water saturation Ultimate cause: oversteepened slope, overweighted... Immediate cause: earthquake, rain storm...
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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
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Vaiont Dam Disaster
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Logging Swanson & Dryness (1975) Cascade study F Found logging and roads increase landslide erosion by several times if performed on weak unstable slopes F Roads interrupt surface drainage (culverts blocked), alter subsurface movement, redistribute mass
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Urbanization Person A undercuts slope for garage while person B builds house above. Who is responsible? S. California F Wave-cut terraces oversteepened F Now lots of building on lower mtn slopes (most = old landslide deposits!)
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Urbanization: Typical Puget Sound Hill
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Seattle Topography
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Slope Stability
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Slope Stability Maps
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Minimizing the Hazard l Identify potential hazard. F Map unstable areas
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Minimizing the Hazard l Identify potential hazard. F Map unstable areas F Land developers and buyers (YOU) look for what??
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Minimizing the Hazard l If suspect, hire a consulting geologist to evaluate F Don’t build F Minimize load or sink foundation deep to stable material F Drainage control s Divert surface water i Surface drains i Soil cement, asphalt, plastic to cover s Cut-off trench to divert groundwater s Drain internal water at base or even pump (Tukwilla)
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Minimizing the Hazard l If suspect, hire a consulting geologist to evaluate F Grade slope s Cut-and-fill: remove top and fill in at base s Must do with care, since this often causes problems s Benching: series of cut-and-fills with diversion drains F Slope supports s Retaining walls (permeable and/or drained) s Rock bolts l Landslide warning systems F Periodic inspection F Tilt meters F Well monitoring
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Minimizing the Hazard Landslide correction (very tough…better to prevent)
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Snow Avalanches 1. Loose-snow avalanches in fresh cohesionless snow
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Snow Avalanches 2. Slab avalanches above subsurface hoar zones
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Subsidence Slow to rapid vertical sinking or settling 1. Withdrawal of fluid (oil, gas, water)
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Subsidence Slow to rapid vertical sinking or settling 2. Sinkholes F Limestone solution, caves and collapse (also with salt deposits)
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Sink hole- Winter Park, Florida
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Subsidence Slow to rapid vertical sinking or settling 1. Withdrawal of fluid (oil, gas, water) 2. Sinkholes F Limestone solution, caves and collapse (also with salt deposits) 3. Mining
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