Mass Wasting
Objectives
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Definition
Types and Rates
Causes or Factors Involved
Effects
Significance
Definitions
• Mass movement/wasting involves down slope movement
of rock, regolith, and soil under the direct influence of
gravity. The earth materials behave as solids or viscous
masses, and may be consolidated (compacted and
cemented) or unconsolidated (loose and uncemented).
Movement occurs when the force or gravity exceeds the
resisting force (slope stability).
• Angle of repose refers to the natural angle formed by
loose, unconsolidated materials (gravel, sand, soil, etc.).
The angle of repose is the angle at which loose material
sits at rest. Typically 25 - 40 degrees.
Types of Mass Wasting
Types (cont.)
Rates of Mass Wasting
• Rapid Movement (1 m/sec to >100 km/hr)
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Rockfalls
Mudflows
Debris flows
Rock slides or rock avalanches
• Moderate Movement (1 cm/day to 1 cm/sec)
– Slumps
– Earthflows
– Debris slides
• Slow Movement (1 mm/yr to 1 mm/day)
– Creep
– Solifluction
Causes/Factors
• Oversteepening of Slopes/Slope Gradient
– By humans (road building, etc.)
– Natural processes (i.e. tectonics, stream and wave erosion, etc.)
• Addition of water to a slope
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Adds weight (~8 lbs/gal)
Lubricates loose material, reduces friction
Adds pressure called pore water pressure
Reduces cohesion by displacing air and destroying surface
tension
• Removal of vegetation
– Decreases soil cohesion when roots decay
– Plants adsorb water, decreasing saturation limit of the slope
material
Causes/Factors (cont.)
• Overloading
– weight of buildings or other structures
– Dumping, filling, or piling up material
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Geology
– rocks dipping (incilned) in the same direction as the slope
– Bedding planes and Fractures serve as zones of weakness
• Triggering Mechanisms
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Excessive amounts of water
Earthquakes
Volcanic eruptions
Vibrations
Noise
What Can Be Done?
• Slope Dewatering
• Slope Reduction
– Cut and Fill
– Benching
• Stabilizing Structures
– Retaining Walls
– Rock Bolts
Summary
• Mass wasting is the movement of earth
materials downslope under the influence of
gravity
• Controls and Triggers include: water,
oversteepened slopes, removal of vegetation,
earthquakes, rock type, land use types, etc.
• Types include: slumps, slides, flows, and falls
• The rate of mass movement may be rapid,
moderate, or slow depending on the type of
material
Mass Wasting Video Links:
http://www.youtube.com/watch?v=jHrPwzbqNyM&feature=related
http://www.youtube.com/watch?v=4PF4Y3P88GQ&feature=related
http://www.youtube.com/watch?v=0FRf6FF6KxA&feature=related
http://www.youtube.com/watch?v=B4XjmIAxOfA&feature=related
http://www.youtube.com/watch?v=AiczROnfGbc&feature=related
http://www.youtube.com/watch?v=KGh4g399wpc&feature=related
http://www.youtube.com/watch?v=pJF-RhL4TvE