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4 resisting factors that add stability to mass movements:
- material strength/cohesion of soils
- vegetation
- slope angle
- small amount of water (adhesion+cohesion)
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2 driving factors that decrease stability of mass movements
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slope stability equation
- forces pulling slope downward [gravity, water saturation]...
- ...must be matched or exceeded by forces holding slope in place [cohesion, friction, angle of repose]
- F driving ≤ F resisting
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role of water in mass movements:
- a little water adds stability through cohesion (surface tension attraction of water molecules)
- too much water decreases stability because of the effect of water pressure
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angle of repose
- the maximum angle at which a slope will be stable
- ~30˚ for most slopes
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2 types of SLOW mass movements:
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4 types of FAST mass movements:
- rock falls
- slides/slumps
- flows
- subsidence/sinkholes
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freeze/thaw creep of mass movement
- when water in soils freeze, they expand; inflates slope
- when thaws, it contracts; gravity pulls slope down
- signs of creep: bent trees, bent rock layers
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lateral spreading of mass movement
spreading of ground surface due to strong rock layers sinking into weaker ones
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fall of mass movement
vertical transport of material, mostly free-fall
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slides, of mass movement
- failure occurs on a surface
- planer surface is a slide (transitional or daylighting)
- curved surface is rotational
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flow of mass movement
a mass movement where the material behaves in a fluid-like manner
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long-runout landslides
- sturtzstroms
- runout is significantly longer than the fall height
- possible causes: water lubrication?, heat generates steam?, friction melting of material?, acoustic fluidization?
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subsidence; mass movement
lowering of the ground surface due to withdrawal of underground fluids [oil, water]
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sinkhole; mass movement
- ground surface collapses into an underground cavity
- typically form in areas with limestone bedrock [carbonic acid dissolution]
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