Mass Movements
Mass Movements
• Smith Chapter 8
Mass Movements
• Downslope movement of large volumes of
surface materials under gravity
Mass Movements
• Vary in speed
– rapid movements can kill
– slower movements can be costly
• soil creep, subsidence
Mass Movements
• Vary in materials
– Landslides
• Rockfalls
• Mudflows
– Avalanches
Mass Movements
• Triggered
– weather
– earthquake, volcano
– vibration
Mass Movements
• Risk increasing as land-hunger forces
development of unstable & steep slopes
Germi, Iran
Mass Movements
• Early 1970s: produced around 600
deaths/year
• Late 1980s: several thousand annually
• Most deaths in Third World
– USA: annual death toll 25-30
Mass Movements
• Annual losses in the billions US$ each year
– especially in cities
• US$1-3 bn/year in the USA
– Appalachian, Rocky Mtns, Pacific
• US$500 million in LA area from landslides
in an el Nino year
Mass Movements
• Most deaths on Pacific Rim
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associated with seismic activity
high population densities
heavy typhoon rainfall
rapid economic development
Mass Movements
• Increased by shanty-town settlement
– on steeper, less-stable slopes
Caracas, Venezuela
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1950s: fewer than 1 urban landslide/year
City expands rapidly, esp. in 1970s
Shanties spread out onto hills
1980s: 35-40 urban landslides/year
High landslide hazard: Guatemala City
La Paz,
Bolivia
Avalanches
• Common on snow-covered slopes steeper
than 20 degrees, less than 60 degrees
• Move at 80m/s (288 km/h)
• Settlement needs to avoid the runout tracks
Avalanches
• Frequent but usually harmless
– USA 7,000-10,000 avalanches/year, only 1%
cause harm
• Threat to unwary travellers, badly-sited
settlements
• Risks increased by alpine tourism
development
Lake Tahoe CA
Avalanches
• More avalanche deaths in Europe
– higher population density
– Switzerland 25-30 deaths/year
Intense Landslide Risk
• High mountains
– Areas of seismic shaking
– High relief
• Soils made erodible by deforestation
• Thick deposits of fine-grained looselypacked material
• Areas of high rainfall
Ganges delta
Unstable slopes, Nepal
Rohtang pass, India
Auguas Calientes, Peru
Rail line serving Maccu Piccu
Huascaran Peru
• Monument to the
dead of Yungay
Yungay school bus
Frank Slide AB, 1903
• Rockfall caused by natural weathering of
anticlinal rocks of Turtle Mountain
• Triggered by spring freeze-thaw
• Undercut by mining
• Destroyed Frank townsite
• Killed 70
Frank slide AB
Hope slide BC
Soil Mechanics
• Slopes fail when shear strength of slope is
exceeded
– excessive loading
– cohesion failure of slope materials
Soil Mechanics
• Different configurations of slide
– rotational
– translational
Lake Merced 1967
Lake Merced 1967
Ground subsidence
Debris flows
• Slope materials become fluidised
– saturated
– follows natural drainage channels
– great destructive force
• Very common in the wet tropics
China: Guangxi
Brazil 1991: 15 killed
Bhutan after monsoon
NZ west coast
NZ landslide hazard
How to trigger a landslide
• Make slope steeper
– undercut it
– add material
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Add weight to the slope
Add water to the slope (very effective)
Remove vegetation
Provide vibrations and shocks
Aberfan 1966
• Coal-mine waste dump collapses on Welsh
village
• Destroyed several homes and primary
school during morning assembly
• Killed 140+,
– including 111 children
Aberfan 1966
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Hazard noted in 1920s
Mine dump not engineered
Receiving wet slurry in 1966
Village and primary school in harm’s way
School principal had complained of the
dangers, her protests were ignored
Reducing landslide risks
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Careful settlement siting
Slope stabilisation
Slope drainage modification
Slope vegetation
Revetments, slope freezing (temporary)
Reducing avalanche risks
• Trigger avalanches at safe times
– smaller avalanches
– with explosives, artillery
• Defensive structures
• Careful settlement siting
Avalanche
defences