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 Long-term 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

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

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

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

32. Rotational Slide Example
Rotational landslide in France

33. 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

34. Next Time
More slides and flows