Landslides - Landslides and Other Downslope Movements Will...

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10/12/09 Landslides and Other Downslope Movements Will Durant- civilization exists by geologic consent- subject to change without notice. Mass Movement and Slope Stability Downward (downslope) movement of material under influence of gravity. Rapid mass movements: landslides, flows Important variables: slope (how steep), gravity, mass (small part or whole hillside). -Human activity can both increase and decrease hazard because it depends on balance of resisting and driving factors Mass Movement Hazards Caused by earthquakes, rainstorms and snowstorms, hurricanes, wave attack on cliffs, volcanic eruptions, river flooding Can move in minutes or move slowly, causing chronic problems over years Annual losses $1-2 billion in US Loss of life from landslides averages 25-50 per year in US Global Mass Movement Hazards Landslides common in mountainous areas Major landslides have produced huge loss of life. 1970 earthquake-triggered landslide/mudflow in Peru killed 70,000 people Earthquake-triggered landslides in Pakistan from the 2005 earthquake killed thousands (one alone killed at least 1000 people) The lahar (earth flow) from Nevada del Ruiz volcano in Colombia in 1985 killed 23,000 people. Driving Forces Shear stress acting on a slope -Depends on slope angle and mass or weight of material on slope above failure location or plane Increase driving force: -Increase in weight on slope (e.g. development) -Increase water on slope (increases weight) -Steepen slope (e.g. cut into slope for building) Decrease driving force: -Decrease weight, dewater, lower cut slope angle Transient increase in stress (e.g. earthquake, large vehicle) Driving force in Mass Wasting- Fd= downslope component of weight of slope material and W=  weight of slope material- Driving force (F) parallel to the slope is proportional to the slope angle Resisting Forces Shear strength: internal resistance of material to downslope movement Depends on internal friction and cohesion of material. -Angle of repose of material -Effects of water, presence of clays and other weak materials, support of material at base of slope.
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Presence of joints and potential failure planes in rock decrease resisting forces Angle of repose indicates an unconsolidated material’s resistance to sliding- finer rounder  particles can sustain only low slope angle while coarse irregular particles can be more steeply  piled. For example sand cannot be piled high but rocks can.  Role of Water Fluids in pore spaces of materials on a slope have two effects:- a little water can keep things
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Landslides - Landslides and Other Downslope Movements Will...

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