2. Sep 2009
M7.6

4. Landslides and other forms of mass wasting

5. Particles create an angle of repose based on their size and angularity
35°
40°
45°
Fine sand
Coarse sand
Angular pebbles

6. Surface tension binds particles
More cohesive
Less cohesive
Damp sand
Dry sand
Water-saturated sand
Surface tension binds particles
Dry particles are bound only by their size and friction.
Saturated particles are separated by water, which acts as a lubricant, allowing them to flow.

7. Water can fill pores in soil
Cohesion: force holding soil grains together
Loose soils have 10-45% pore space
Small amount of water increases cohesion
Too much water pushes grains apart, reducing cohesion

8. Frictional Resistance Prevents Sliding
Gravitational Force pressing down on slope
Friction is the ‘Roughness’ of slippage surface
Area of contact does not affect friction coefficient
Slide occurs when gravitational force exceeds frictional resistance

9. Slope Material influences sliding
Loose materials slide easier:
Soil
Loose sediment
Soft sedimentary rocks such as clay or shale

10. Clays can increase chance of landslide
Clays absorb water and expand to weaken rock
Kaolinite: soaks up water
Smectite: forms from volcanic ash, with open structure between layers that fills with water  swelling soils

11. Mud in bays, lakes is likely to fail
Figures 8-7, 8-8
“Quick-clays”

12. Before earthquake Landslide associated with 1964 Alaska Earthquake
Sand and
gravel
Clay
Clay
Water-saturated
sandy layer

13. After earthquake
Pre-earthquake profile

14. Landslide Triggers Oversteepening Earthquakes Rainfall
Volcanic eruptions

15. 1. Oversteepening decreases stability
Steeper slopes are less stable
Slope angle is increased when
Fill is added above
Construction of homes with magnificent views
Slopes are undercut below
Erosion at base of slope, by waves at coast
Excavation of road at base of slope

16. Oversteepening decreases stability

17. 2.Earthquake
loosens large masses
of rock
Earthquake

18. Denali Earthquake 2002

19. 3. Adding Water Water reduces strength of slope
Heavy or prolonged rainfall saturates soil,
Human actions add water to slopes
Lawn-watering, crop irrigation
Leaking water/sewer pipes, swimming pools
Filling reservoir behind dam

20. …which quickly loosen…
Rain has soaked
fine-grained
permeable soils,…
…which quickly loosen…
Water-
permeable
soil
Water-
impermeable
rocks
…and flow downhill.

21. Rain soaks muds and rubble…
Clear-cut slopes
…resulting in
a flow of mixed
mud, rock, and
surface debris.
Shale
Jointed
bedrock

23. Irrigation caused this slide in So. Cal.
Figure 8-1

24. Venezuela 1999

25. Venezuela 1999

26. Venezuela 1999

27. Venezuela 1999

28. Venezuela 1999

29. 4. Eruption causes landslides
Snow and ice
A volcanic eruption has
melted snow and ice that
soaks volcanic ash over
impermeable lavas.
Water-permeable
volcanic ash
Water-
impermeable
lava
The resulting mud
moves quickly
downhill.

31. Types of Landslides Landslides are classified by: -Material type
-Movement type
-Movement velocity
Velocity can range from
<1 mm/yr to 100 m/sec

32. Rockfall: Extremely rapid
Ice wedging prepares
rocks to loosen and
fall away.
Individual blocks
free-fall down slope.

34. Rockfall creates talus slopes of loose rock

35. Debris Avalanches: extremely rapid
Triggered by Peru earthquake
Can begin as rockfall, but become larger and run further.
Avalanche went 14 km to with average speed of 270 km/hr
Figure 8-18

36. Debris Avalanche An earthquake has loosened large masses of rock…
…that flow downhill
at high velocity on a
cushion of air.
Earthquake

38. Debrisflows, mudflows, and LAHARS: Rapid
Snow and ice
A volcanic eruption has
melted snow and ice that
soaks volcanic ash over
impermeable lavas.
Water-permeable
volcanic ash
Water-
impermeable
lava
The resulting mud
moves quickly
downhill.

40. Rockslide: moderately rapid
Frost wedging has
loosened jointed
bedrock layers…
…that move
downhill as
a unit.

41. Translational Slide: goes along existing weakness
This one cost taxpayers $400 million

42. Slump: Slow to moderate
Scar
Unconsolidated material
slowly slides as a unit.

43. Scar

45. Creep: extremely slow Gravestones and fence posts lean Building
foundations
shear and crack
Trees grow with
curved trunks
Road cracks
Power poles
lean
Creep: extremely slow

47. Soil Creep Slow, downslope movement of soil and weak rock
Involves near-surface movement by alternate expansion and shrinkage of soil
Figure 8-27

48. Snow Avalanches: usually rapid
Trigger for avalanche could be
Weight of skier crossing slope
Vibrations of snowmobile
Movement of glacier
Changes in temperature
Earthquake
Figure 8-30

49. Failure of Landslide Dams
Any moderately fast-moving landslide can block a river or stream to create a dam and temporary lake before eventually failing
Time before failure and size of flood depends on
Size, height and geometry of dam
Material making up dam
Rate of stream flow, how fast lake rises
Use of engineering controls (artificial breaches, spillways or tunnels)
Dams from mudflows, debris flows and earth flows are noncohesive and erode quickly

50. Failure of Landslide Dams
Most landslide dams fail when water overflows and erodes spillway that drains lake
If dam-failure flood incorporates significant sediment, can turn into debris flow – much more dangerous
Useful dams can be constructed on top of landslide dams
Rockfalls or rock slides are most stable
1928 St. Francis high-arch concrete dam failed – built on toe of old landslide

51. Mitigation of Damages from Landslides
Damages can be extremely costly
Not covered by most insurance policies
In U.S., landslides cost more than $2 billion and cause deaths per year
Globally, cost more than $20 billion, cause about 7500 deaths per year
Major landslide disasters increase with growth in population in dangerous areas

52. Mitigation of Damages from Landslides
Figure 8-39