Surface processes:
Mass movements
Reading:
This week: 8 and 9
Next week: 9 and 11
Environmental Geology – Mass movements
Mass wasting
plate tectonics vs. gravity
Cost in U.S.
• $1.5 bill/year
• 25-50 deaths/year
Environmental Geology – Mass movements
1
Slope stability
gravity vs. friction
Important factors:
• Sediment type (size, angularity)
• Fluid content
• Vegetation
• Triggering events
Environmental Geology – Mass movements
Slope stability
Angle of repose
Î maintained by mass wasting
Environmental Geology – Mass movements
2
Slope stability
Effects of fluids – a little
Surface tension binds sediment particles
together
Environmental Geology – Mass movements
Slope stability
Effects of fluids – a lot
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
3
Slope stability
Vegetation
Root systems bind together
sediment particles
When fire removes that
vegetation erosion proceeds
rapidly
Here erosion has removed material
from beneath the layer held together by
vegetation
…a trigger of sorts
Environmental Geology – Mass movements
Slope stability
Triggering - Earthquakes
Stress of passing seismic waves can reduce
inter-particle friction below the threshold to
slide
Í Southern CA:
• Many slides
resulted from
Northridge
earthquake
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
4
Slope stability
Triggering – Rain storms
Rapid influx of fluid into pore spaces increases
pore pressure causing slope failure
Í Brazilian mudflow following
heavy rainfall
Swollen rivers rapidly
undercut slopes
Environmental Geology – Mass movements
Failure in rock
Freeze-thaw
process
breaks up
Weak layers with high silt/clay
content can fail during earthquakes
Environmental Geology – Mass movements
5
Types of mass movements
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
Rock mass movements
Rock fall:
freeze-thaw
high velocity!
talus slopes
Rock slide:
freeze-thaw,
undercutting
high velocity
debris piles
Environmental Geology – Mass movements
6
Unconsolidated mass movements
debris: soils, broken up rock, vegetation and often human construction, cars etc
Creep:
• gradual movement of
surface soils
• 1 to 10 mm/yr
• causes structures to tilt
Environmental Geology – Mass movements
Unconsolidated mass movements
Earthflow:
• Fluid mass movements
• speeds up to km/hr
• fine grained soils
Debris flow:
• Fluid mass
movements
• speeds up to
km/hr
• rock fragments
supported by
muddy matrix
Environmental Geology – Mass movements
7
Mud flows
Armero Columbia 1985
• flows of finer
muds and
sands
• significant
volumes of
water
• very high
velocities
(km/hr)
How do these relate to other natural
hazards we have discussed?
Environmental Geology – Mass movements
Slumps and debris slides
Movement of unconsolidated units
Environmental Geology – Mass movements
8
Natural causes of landslides
Rainstorms, undercutting of
slopes, earthquakes
1982 Thistle, Utah
Environmental Geology – Mass movements
Human acceleration of landslide potential
• removing vegetation
• steepening slopes
• adding construction to
slopes
Environmental Geology – Mass movements
9
The Vaiont Dam, Italy
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
Preventative measures
Slope reduction
Environmental Geology – Mass movements
10
Preventative measures
Retention structures
Thick low wall have
been more effective
than thin high
coverings
Rock bolts are
very effective at
stabilizing rock
surfaces
Fences
catch
small
debris
Environmental Geology – Mass movements
Preventative measures
Fluid removal
• Prevent absorption with waterproof
covering
• Drain pore fluids – some soils will drain
well, others will not
Environmental Geology – Mass movements
11
Recognizing hazard
1.
The historic record: landslides
are recurrent events
2.
Tilted structures, cracked
construction
Environmental Geology – Mass movements
12