PowerPoint Presentation - Geophysics 189 Natural Hazards

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Landslides, Part 1
La Conchita slide, Jan 2005
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Outline

• Definitions
• Factors important for mass movements
• Timescales of movement
• Examples

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What are mass movements?

• Material moves downslope under force of gravity
• Occur throughout U.S. and the world
• Often triggered by earthquakes, volcanoes,
  rainstorms, human activities
• In U.S., average 1.5 billion, 25 deaths per year

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Gravity Key Force
1 lb rock

• Material on horizontal surface - no problem
  because no motion driven by gravity

Gravity pulls with 1lb towards center
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Gravity Key Force

• Downslope component leads to slides
• Just need some energy or trigger to get it started

1 lb rock
Gravity force split into components perpendicular
to surface, and parallel to surface
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Factors Influencing Mass Movement

• Nature of slope materials
• Steepness of slope
• Water content
• Slope stability

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Angle of Repose

• Angle of Repose
• the maximum angle at which a pile of
  unconsolidated particles
• can rest, increases with grain size

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Classification of mass movement is based on
dominant material, fluid content, and velocity
of movement.
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Creep

• Slowest motion, very widespread
• Downhill motion of soil and uppermost bedrock
  layer, rates of few mm/year

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How Creep Works
Surface materials expand and contract, resulting
in net motion downslope
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Creep - Expansion and Contraction

• Ways to get expansion
• Water in pores freezes - expands in volume
• Abundance of clay minerals - absorb water easily
• Heating of ground surface
• Ways to get contraction
• Thawing
• Drying
• Cooling

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Evidence of Creep
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Evidence of Creep
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More Speed!

• More substantial, higher velocity movements -
  important for hazards
• Several internal and external factors important
  for increasing the odds of movement

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• External Factors
• Steepen slope
• Remove support at base
• Waves, streams, people
• Add mass to top
• Sedimentation, human effects

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Internal Factors

• Weak materials
• Water
• Decrease cohesion
• Geologic conditions

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Weak Materials

• Clays most common problem very abundant
• Sheetlike structure can absorb, release water
• Some clay minerals lead to large slope failures
• Quickclay mix of clay, water, salt that can
  collapse from vibrations (blasts), rain
• Example Canada

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Water Important for strengthening and weakening

• Strengthens - surface tension
• Weakens by
• Pore pressure
• Adding weight
• Interaction with clays

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Importance of Water Content
Saturated sand flows easily because of
interstitial water
Surface tension in damp sand increases cohesion
Dry sand is bound only by friction
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Other water weakening methods

• Weight - excess water adds mass on the slope
• Clay interaction - can attach to clays because of
  positive/negative charges on water molecule

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Decrease in Cohesion

• Occurs through erosion
• Rocks expand as they reach surface (less pressure
  than at depth)
• Open fractures, allows water to enter

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Geologic Conditions

• Slopes can be weak due to pre-existing geologic
  conditions
• Ancient slide surfaces
• Slope of rock layers relative to slope of
  hillside
• Structures such as joints, faults, clay layers in
  rocks

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Rock Dip is Important
Rock layers dip at angles less than hillslope -
conditions ripe for failure
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Triggers of Movements

• Immediate cause
• Rains
• Earthquakes
• Thawing cycles
• Construction
• Long-term
• Gravity

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Types of Faster Movements
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Falls

• Hard rock splits along joints, weak zones
• Detaches from cliff, free falls to ground
• Can shatter, leading to dust clouds
• Triggers rain, frost, earthquakes

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Rock fall - Yosemite 1996
162,000 ton mass fell off valley wall Large tree
kill, thick dust blanket 1 death
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Slides

• Movement above failure surface
• Can be rotational
• Curved slip surfaces
• Or translation
• Fail along planar surface

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Rotational Slides

• Move down and out on curved slip surfaces
  (sometimes called slumps)

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Rotational Slide Example

• Rotational landslide in France

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Translational Slide

• Move down and outward on planar surfaces of
  weakness
• Can be joints, faults, clay layers, etc
• Can go as
• Block slides
• Debris slides
• Lateral spreading

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Next Time

• More slides and flows