Mass Wasting
Nancy A. Van Wagoner
Acadia University
Nancy Van Wagoner, Acadia University
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Mass Wasting Defined
 The
down slope movement of material
under the force of gravity
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Agenda
 Factors
controlling mass wasting
 Factors that can change slope stability
 Types of mass wasting
 Examples
 Prediction and mitigation
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Overview
 mass
wasting occurs throughout the world
 The total global property damage from
landslides in a single year equals that
caused by earthquakes in 20 years.
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Factors that control mass
wasting
 Steepness
of the slope
 Orientation of the rock layers
 Strength and cohesion of materials
 Pore water
 Factors acting to change slope stability
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Steepness of the slope
 figure
attached
 factor of safety
 example
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Steepness of Slope
f
d
Resisting Force = R = shear strength
= internal resistance to movement =
n
Angle of the slope
W
fxn
f = friction
Factor of Safety = R/d = F.S.
n = normal force (component of W perpendicular to
slope)
Most building codes require F.S.>1.5
d = driving force (component of W parallel to slope)
W = weight of the block = mass x gravity)
Problem: calculate d for 1000 kg
block for slope angles of 60 degrees
and 30 degrees
Orientation of rock layers
(figure 10-20)
 dip
slope vs rocks dip perpendicular to the
slope
DIP SLOPE = Rock layers are dipping
or inclined in the same direction as the
slope.
VERY UNSTABLE
Dip of rocks is perpendicular to the slope =
better, more stable
Strength and cohesion of
materials
 strength
= ability of material to resist
deformation
 cohesion = ability of particles to stick
together
 examples
– unweathered granite vs poorly indurated
sedimentary rock
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Pore water
 angle
of repose = maximum slope or
steepness at which loose material remains
stable
 The angle of repose for dry sand is about 35
degrees
 Damp sand achieves slopes up to 90 degrees
 Wet sand (saturated) has little strength, and
almost no slope
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Pore water (continued)
 why
the difference in the slopes?
– water is a polar molecule, able to attract grains of sand
by surface tension and hold them together if the pore
water pressure is less than zero
– If the pore water pressure exceeds zero, the pressure
exerted by the water, floats the particles away from each
other
Factors acting to change
slope stability (Triggering
Events)
 Change
in the abundance of pore water
 Earthquakes
 Slope modification and undercutting
 Volcanic eruptions
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Change in abundance of pore
water
 increase
pore water pressure
– decrease cohesion of particles
– increases the weight of the slope
 ways
of changing the amount of pore water
– rain
– housing development
 water
lawn
 build swimming pool
 septic field
– change in groundwater level
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Water (continued)
 Liquefaction
- the transformation of
material to a liquid-like mass
– results from a increase in water content
 may
be associated with ground shaking
 Expansive
clays (shrink-swell soils)
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Earthquakes and other shocks
 can
lead to liquefaction
 earthquakes frequently generate landslides
– Grand Banks under water slide (turbidity
current)
– Peru 1970
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Slope modification and
undercutting
 road
construction
 natural processes
– streams
– waves
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Volcanic eruptions
 deposit
unconsolidated debris rapidly
 may be associated with melting of glacier
and/or rain
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Types of Mass Wasting
 Can
occur slowly or rapidly
– imperceptible (cm/yr) to rapid (400 km/hr)
3
main types (figure 10-23)
– Flow
– Slide
– Fall
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Flows - 3 types
 Unconsolidated
material moves as a viscous
fluid
– creep (slow)
– solifluction (slow)
– debris flows, earthflows, mudflows (fast)
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 Creep
(figure 10-24 to 10-27)
– about 1 cm/yr
– results from alternate expansion and contraction
of surface materials due to
 freezing
and thawing
 wetting and drying
– may notice bent rock layers, tree trunks curved
at the base, tilted fence posts or grave stones
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 Solifluction
(fig. 10-29)
– important in frigid zones
 high
latitude or high elevation when ground has a
layer of permafrost
 summer or spring
– upper layers of permafrost may melt saturating the upper
surface
– water can’t percolate downward because of ice below
 earth
flows slowly downward on icy layers below,
even on gentle slopes
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 Debris
flow, Mudflow, Earth Flow, Lahar
– fluid motion of water saturated debris
– occur everywhere including
 semi-arid
environments
 volcanoes
– viscous, able to float cars
– fast, up to 100 km/hr
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Semi-arid
flows (continued)
– infrequent/high precipitation rainstorms
– loose debris
– little vegetation
– easily saturated by rain
– acquires consistency of concrete,
– moves down slope
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volcanic
mudflows (Lahars)
–instant deposition of ash
–usually associated with rain and/or
melting ice caps
–example: Nevado del Ruiz
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Landslides - two types
 slump
 rock
slide, or rock avalanche
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 Slump
– downward slipping of a mass of rock or
unconsolidated material, moving as a unit,
along a curved surface
– see figure 10-23, note
 slump
block
 surface of fracture
 slump scarp
Slump example
 Portuguese
Bend-Abalone Cove Landslide
 Southern California, coastal community
 area = 10’s of square miles
 very fancy homes, million $ range
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Portuguese Bend geology
 shale,
dipping seaward overlain by poorly
consolidated Portuguese Tuff
 area of natural sliding, undercut and
oversteepened by waves
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Portuguese Bend urbanization
 accelerated
slumping
 over steepened slopes-terraces, road cuts
 added weight and water to slope
– swimming pools
– irrigation
– sewage
 Result
= ground slumps along plane of
weakness
 150
homes destroyed
 Remediation
= pumping excess
groundwater
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 Rock
slide, rock avalanche- blocks of
bedrock break loose and move down slope
– fastest and most destructive type of mass
movement
 common
conditions
– steep slopes
– dip slope
 common
triggers
– earthquake
– excessive rain and/or melting snow
 lubricates
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and adds weight to the slope
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Rock slide, avalanche
examples
 Gros
Ventre landslide, Wyoming (fig. 10-
31)
 Frank Alberta (p. 268)
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 conditions
Gros Ventre
– slope = 15-20 degrees
 river
cut through sandstone, removing toe of the slope
– Spring 1925
 heavy
rains and melting snow
– increase weight, increase pulling force
– increase pore water pressure, decrease cohesion
– lubricate sandstone/slay contact
 result
– 38 million cubic metres of debris gave way
– descended a vertical distance of 600 m
– rose up 100 m on the opposite side to come to rest in the
river bed, damming the river
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Frank Alberta
 show
diagram
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