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Mass Wasting and Hillslopes

. Sliding Threshold when gravity component = friction component, both parallel to slope. Shear Forces are parallel to 2 touching surfaces.If the slab is about to move, then the downhill force = resisting force pointing uphill Downhill force = mass x gravity x sine of dip F

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Mass Wasting and Hillslopes

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    1. Mass Wasting and Hillslopes

    2. Sliding Threshold when gravity component = friction component, both parallel to slope Shear Forces are parallel to 2 touching surfaces. If the slab is about to move, then the downhill force = resisting force pointing uphill Downhill force = mass x gravity x sine of dip F0 = mg sin (dip) (1) a is the same as the dip F0 = mg sin(a)

    3. Your book uses mg = weight "w" Downhill force = mass x gravity x sine of dip F0 = w sin (dip) (1) a is the same as the dip Shear Force = F0 = w sin(a)

    4. Stress has units Force/area mass = r V because r has units mass/volume so F0 = ?Vg sin(a) If calculated over a unit area, F0 / a "force per unit area" F0 / a = s = r(V/a) g sin(a) but V/a has units length sd' = ?gh sin(a) (2) when we use the height h as the characteristic length Equation (2) is the shear stress

    5. Role of water for slabs Friction Force is proportional to Normal Force It is the amount of Force needed to lift the surfaces apart Increased water pressure between the surfaces lifts the upper slab, and it will slip at a lower dip angle.

    7. Water's role for slabs: After Fall

    8. Classification of slope movements

    11. Signs of Soil Creep

    12. Creep Typical Features

    13. Solifluction

    16. Rapid Mass Movement Flows: mixture moves downslope as a viscous fluid Slumps: move downslope along a concave slip surface Slides: move downslope along preexisting plane of weakness as a single, intact mass Falls: rock drops from steep slope

    17. Rapid Mass Movement

    18. Flows Flows with a high water content are faster and more dangerous Debris avalanches- rain- regolith detaches 200 kilometers per hour Lahars Liquifaction- Quick Clay earthquake - increased pore water pressure - grains separate - liquefies instantaneously Mudflow swift slurry- heavy rains Earthflows dry masses of clayey regolith 1-2 meters per hour

    19. Yungay Avalanche

    20. Lahar

    21. Liquifaction - Quick Clay

    23. Slumgullion Earthflow San Juan Mtns, CO Volcanics Dams Lake Fork of the Gunnison

    24. Slides Slumps Mudslides Rock Slides Avalanche and Debris Slides

    25. Slump

    26. Little Hat Mountain Slump, CA

    27. La Conchita Slump Typical urban landslide Preexisting slide masses Development to the edge Lawsuits 9 houses destroyed Property values down

    30. Falls: Rockfall

    31. Angle of Repose For loose materials, the angle of repose dictates the maximum steepness a material can be arranged before it will move downslope Bloom claims: coarser materials tend to have steeper slopes

    33. Slope Stability Slope characteristics such as composition, vegetation, and water content also influence slope stability. Haiti is plagued by slides after many trees were cut down.

    34. Natural Triggers Natural triggers such as: torrential rainstorms 1967 central Brazil Earthquakes 1812 New Madrid, Missouri volcanic eruptions 1980 Mount St. Helens produce damaging mass movements

    35. Human Triggers excessive irrigation clear-cutting of steep slopes slope oversteepening or overloading mining practices can also cause mass movement.

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