Fig' 8'CO

1
Fig. 8.CO
LANDSLIDES
2
Internal heat drives mountain-building so
.. gravity acts to tear the mountains down
through MASS WASTING or MASS MOVEMENTS this
may be slow subtle OR sudden rapid
avalanche
deslizamientos
3
Fig. 8.01
Areas of high landslide risk
4
Fig. 8.01 p.182
Downslope pull of gravity overcomes friction (or
shear strength)
Example Vaiont River flows through Italian Alps.
Geology is buckled sequence of sedimentary rocks
- synclinal fold Pore pressure of groundwater in
rocks caused clay to swell, Dam failure in 1960.
5
Fig. 8.02
The effect of slope geometry on slide
potential Downward pull of gravity the same (mass
is the same) but the greater the shearing stress
on steeper slope. An increase in pore pressure
can decrease frictional resistance
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Fig. 8.03
max. angle of stability
ANGLE OF REPOSE for larger particles is often
34º seen in diverse geologic structures eg
volcanic cinder cones sand dunes
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Fig. 8.04
Uniform slope due to sands angle of repose
Volcanic cinder cones in Haleakala Crater, Maui
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Fig. 8.03 p.183
Tablacha Dam, Peru Reactivated slide block
composed of foliated metamorphic rocks Government
stabilized the block using drainage
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Fig. 8.05
Slump in Pacific Palisades area, California -
common feature
10
Fig. 8.06
Instability in expansive clay, Colorado NOTE
Clay takes in 20 of its volume of water
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Fig. 8.07
Heavy rains trigger landslide mudflow in Brazil
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WATER - powerful erosive agent but ALSO seep
along bedding planes of layered rock reduce
friction landslide FROST WEDGING expansion
contraction of water washing into
fractures FROST HEAVING expansion of wet
soil as it freezes loosens soil on thawing,
susceptible to landsliding
Sudden, rapid landslides need a trigger
rainfall
EXAMPLE Volcanic dome in current eruption on
Montserrat, large eruptions initiated by intense
rainfall
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Fig. 8.09
Landslides often triggered by earthquakes
Seismic waves pass through rocks breaks apart
soil particles
In Peru (1970), Magnitude 7.7 earthquake killing
18, 000 people burying 2 towns of Yungay
Ranrahica
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Fig. 8.10
Earthquake triggered landslide in El Salvador,
2001
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Fig. 8.13
Accumulation of talus from rockfalls of
columnar-jointed lava flow
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Fig. 8.14
Niagara Falls-US / Canada border gradually
moving upstream due to rockfall activity
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Fig. 8.15
Rockslide in Karakoram Valley, Pakistan
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Dhaulagiri Peak, Nepal
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Fig. 8.16
Slumps - occur in rock or soil
Often rotation accompanying downslope movement
Scarp formed at top of slide - near vertical face
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Fig. 8.17
Snow avalanche initiated triggered by skier
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Fig. 8.18
FLOWS In a flow, material moved is not coherent
moves in chaotic disorganized style with mixing
of particles in fluid style. Many types of flow
eg pyroclastic flow (volcanic), mudflows
(saturated with water) earthflows (movement of
soil). Flow involving
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Fig. 8.19
Puget Peak debris avalanche following 1964
earthquake
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Fig. 8.21
HUMAN ACTIVITY effects slope stability 1/
Clearing stabilizing vegetation 2/ Construction
oversteepens the slopes
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Fig. 8.22
steep, unvegetated slope
triggers failure of steep slopes
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Fig. 8.25
Floodwaters triggering huge debris flows
Debris flows through lowest 3 floors
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Debris flow deposits in large channel. Largest
boulder was 400 tonnes.
THINK ABOUT HIGH ENERGY OF FLOW !!!
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Fig. 8.27
Avalanche protection structure in Canadian Rockies
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Slope stabilization through slope reduction
removal of unstable material A/ BEFORE Road-cut
leaves unsupported slope B/ AFTER Material
removed to reduce slope angle load
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Fig. 8.29
A/ Retaining wall of concrete blocks on slope
cut B/ Side view of retaining structure (Lake
Tahoe, Nevada)
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Fig. 8.30

• Concrete sheets on steep rubbly hillside
  (Montana)
• B. Slipping buckling road repair below

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Fig. 8.31
Improved drainage enhances slope stability
BEFORE Water trapped in wet soil causes
movement, pushing down wall
AFTER Water drains through pipe, allowing wall
to keep stable
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Fig. 8.32
Rock Bolts stabilize a slope - steel cables
anchored in cement
Rocks bolts stabilizing a slope in the Bighorn
Mountains
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Fig. 8.33
Granite domes of Yosemite - curved slabs of rock
at the surface, ready to fall.
Large block crashed down, shattering on impact
leveling trees
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Fig. 8.34
AVALANCHE-PRONE AREA Lack of vegetation on
unstable slopes is an indicator of failure
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Fig. 8.35
SOIL CREEP Gravitational bending of rock
layers near surface (bending of trees etc)
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Fig. 8.36
Evidence for slope instability includes broken
walls slumped yards
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Fig. 8.37
Spectacular landslide in Puget Sound (1996)
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Landslide monitoring site, washington. Note
proximity of house pool to fault scarps.
Seasonal fluctuations in rainfall seasonal
fluctuations in pore pressure. Useful for
predicting landslides
39
Soufrière Hills volcano - collapses triggered by
intense rainfall