Mass wasting
plate tectonics vs. gravity
Surface processes:
Mass movements
Reading:
This week: 8 and 9
Next week: 9 and 11
Cost in U.S.
• $1.5 bill/year
• 25-50 deaths/year
Environmental Geology – Mass movements
Environmental Geology – Mass movements
Slope stability
Slope stability
Angle of repose
gravity vs. friction
Important factors:
• Sediment type (size, angularity)
• Fluid content
• Vegetation
• Triggering events
Î maintained by mass wasting
Environmental Geology – Mass movements
Environmental Geology – Mass movements
1
Slope stability
Slope stability
Effects of fluids – a little
Effects of fluids – a lot
Surface tension binds sediment particles
together
Too much fluid increases pore pressure
and pushes sediment particles apart
reducing friction
Clays: can absorb water into their crystal
structure turning them into gel-like
sediments
Environmental Geology – Mass movements
Slope stability
Slope stability
Vegetation
Root systems bind together
sediment particles
Environmental Geology – Mass movements
Triggering - Earthquakes
When fire removes that
vegetation erosion proceeds
rapidly
Stress of passing seismic waves can reduce
inter-particle friction below the threshold to
slide
Í Southern CA:
• Many slides
resulted from
Northridge
earthquake
Here erosion has removed material
from beneath the layer held together by
vegetation
…a trigger of sorts
Environmental Geology – Mass movements
Î Nevados Huascaran,
Peru debris flow:
• Resulting from a
mag 7.7 earthquake
in 1970
• Flow travelled at
600 mph
• Buried 2 towns,
18,000 dead
Environmental Geology – Mass movements
2
Slope stability
Triggering – Rain storms
Failure in rock
Rapid influx of fluid into pore spaces increases
pore pressure causing slope failure
Í Brazilian mudflow following
heavy rainfall
Swollen rivers rapidly
undercut slopes
Freeze-thaw
process
breaks up
Weak layers with high silt/clay
content can fail during earthquakes
Environmental Geology – Mass movements
Types of mass movements
Environmental Geology – Mass movements
Rock mass movements
Rock fall:
freeze-thaw
high velocity!
talus slopes
Rock slide:
freeze-thaw,
undercutting
high velocity
debris piles
Characterized by
• material involved (rock or debris)
• speed of the movement (cm/yr to km/hr)
• slide or flow (one unit or fluid-like)
Environmental Geology – Mass movements
Environmental Geology – Mass movements
3
Unconsolidated mass movements
Unconsolidated mass movements
debris: soils, broken up rock, vegetation and often human construction, cars etc
Earthflow:
• Fluid mass movements
• speeds up to km/hr
• fine grained soils
Creep:
• gradual movement of
surface soils
• 1 to 10 mm/yr
• causes structures to tilt
Environmental Geology – Mass movements
Mud flows
Armero Columbia 1985
• flows of finer
muds and
sands
Debris flow:
• Fluid mass
movements
• speeds up to
km/hr
• rock fragments
supported by
muddy matrix
Environmental Geology – Mass movements
Slumps and debris slides
Movement of unconsolidated units
• significant
volumes of
water
• very high
velocities
(km/hr)
How do these relate to other natural
hazards we have discussed?
Environmental Geology – Mass movements
Environmental Geology – Mass movements
4
Natural causes of landslides
Human acceleration of landslide potential
Rainstorms, undercutting of
slopes, earthquakes
1982 Thistle, Utah
• removing vegetation
• steepening slopes
• adding construction to
slopes
Environmental Geology – Mass movements
Environmental Geology – Mass movements
Preventative measures
The Vaiont Dam, Italy
Slope reduction
1960 slide:
• Small slide raised
awareness and initiated
monitoring
October 1963:
• Landslide filled
reservoir causing dame
to be over topped
• Flooded many towns
down stream, 3000
drowned
Environmental Geology – Mass movements
Environmental Geology – Mass movements
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Preventative measures
Preventative measures
Retention structures
Fluid removal
• Prevent absorption with waterproof
covering
Thick low wall have
been more effective
than thin high
coverings
• Drain pore fluids – some soils will drain
well, others will not
Rock bolts are
very effective at
stabilizing rock
surfaces
Fences
catch
small
debris
Environmental Geology – Mass movements
Environmental Geology – Mass movements
Recognizing hazard
1.
The historic record: landslides
are recurrent events
2.
Tilted structures, cracked
construction
Environmental Geology – Mass movements
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