Sep 2009 M7.6

2. Sep 2009 M7.6

4. Landslidesand otherforms of masswasting

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

6. 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 have10-45% 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 “Quick-clays”

12. Landslide associated with 1964 Alaska Earthquake Before 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. 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.

24. Venezuela 1999

25. Venezuela 1999

26. Venezuela 1999

27. Venezuela 1999

28. Venezuela 1999

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

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. • Avalanchewent 14 km to with average speed of 270 km/hr

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

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. 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

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

49. Failure of Landslide Dams • Any moderately fast-moving landslide can block a river or stream to create a dam andtemporary 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