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Landslides, Part 1

Landslides, Part 1. La Conchita slide, Jan 2005. Outline. Definitions Factors important for mass movements Timescales of movement Examples. What are mass movements?. Material moves downslope under force of gravity Occur throughout U.S. and the world

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Landslides, Part 1

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  1. Landslides, Part 1 La Conchita slide, Jan 2005

  2. Outline • Definitions • Factors important for mass movements • Timescales of movement • Examples

  3. 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

  4. Gravity: Key Force 1 lb rock • Material on horizontal surface - no problem because no motion driven by gravity Gravity pulls with 1lb towards center

  5. 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

  6. Factors Influencing Mass Movement • Nature of slope materials • Steepness of slope • Water content • Slope stability

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

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

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

  10. How Creep Works Surface materials expand and contract, resulting in net motion downslope

  11. 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

  12. Evidence of Creep

  13. Evidence of Creep

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

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

  16. Internal Factors • Weak materials • Water • Decrease cohesion • Geologic conditions

  17. 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

  18. Water: Important for strengthening and weakening • Strengthens - surface tension • Weakens by • Pore pressure • Adding weight • Interaction with clays

  19. 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

  20. 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

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

  22. 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

  23. Rock Dip is Important Rock layers dip at angles less than hillslope - conditions ripe for failure

  24. Triggers of Movements • Immediate cause • Rains • Earthquakes • Thawing cycles • Construction • Long-term • Gravity

  25. Types of Faster Movements

  26. 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

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

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

  29. Can trap water, lead to more instability Rotational Slides • Move down and out on curved slip surfaces (sometimes called slumps)

  30. Rotational Slide Example • Rotational landslide in France

  31. 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

  32. Next Time • More slides and flows

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