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Mass Wasting
Sculpturing Earth’s Landscape
What is Mass Wasting?
• The downslope movement of material under the
influence of gravity
• May occur at rates of only a few inches per year
– result in little damage and no loss of life
• May occur at over 100 mph
– result in great loss of life and property damage
Mass Wasting – Factors Controlling Mass Wasting
• Angle of Repose
• Pore Water
• Material
• Orientation of Rock Layers
• Vegetation
Mass Wasting - Factors
• Angle of Repose
– The angle at which
loose material sits at rest
– Major determining
factor in slope stability
– Every material and
slope has an inherent
angle it becomes
unstable.
– Typically 25°-40°
Angle of repose
• In general, coarser grained, poorly sorted, and
angular rocks have a higher angle of repose
Mass Wasting - Factors
• Pore Water
– Water held within the
void spaces
– May provide surface
tension
When Saturated:
– Adds weight
– Provides lubrication
Pore Water
• No water
– grains held together by frictional forces
• Some water
– Grains held together by surface tension
• Too much water
– Provides lubrication and grains move freely
Mass Wasting – Factors
• Material
– Loose soil and sediment are
more prone to slope failure
– Sand material have greater
pore spaces
– Horizontal layers are more
stable than loose sediment
–Massive rocks tend to be the
most stable
Mass Wasting – Factors
• Orientation
– pertains to layered rocks
– When layers dip in the
same direction as slope,
failure is more likely
– Horizontal layers are more
stable
–Most stable is when rock
layers dip into slope
Mass Wasting - Factors
• Vegetation
– roots form an anchor for soil
– reduces amount of water in pore spaces
Mass Wasting - Factors
Mass Wasting - Types
• Slow Movement
– Creep
• Rapid Movement
– Rock fall
– Slump
– Rock slide
– Debris Flow
Mass Wasting – Types (slow)
• Creep
– Gradual downhill movement
of slope material
– Result in tree trunks to curve
at base
Mass Wasting – Types (fast)
• Rock fall
– Free fall of detached
pieces of material of any
size
– May occur as a result of
freeze-thaw
– May occur by the
loosening action of plant
roots
Mass Wasting – Types
• Slumps
– A mass of material sliding
along a curved, rotational
surface (shaped like a spoon)
– Scarp is crescent-shaped
cliff at upslope end
– Bulge is usually at toe
where soil accumulates
Mass Wasting – Types
• Rockslides
– Also called “landslides”
– Blocks of rocks and soil
move down a welldefined plane (sliding)
– Among the most
destructive
– May be triggered by
rain, melting of snow, or
earthquakes
Mass Wasting – Types
• Debris Flows
– Downslope movement of
a viscous (thick) fluid
– Commonly occur in
volcanic areas
– Typically follow drainage
patterns (valleys)
Mass Wasting – Classification Types
Mass Wasting Potential
Road Cut
Brush Fires
Volcanoes
Heavy Rains
Debris Flows
Nevado del Ruiz
Monterey Hills
Mass Wasting Hazards
Mitigation - Identification
Mitigation – Identification
• Studying aerial photographs to
determine previous landslides
• Field investigations of
potentially unstable slopes
Mitigation - Prevention
• Steep slopes can be
graded into gradual slopes
• Retaining walls can be built
• Drain pipes to reduce pore
water