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Sep 2009 M7.6. Sep 2009 M7.6 Landslides and other forms of mass wasting.

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Presentation on theme: "Sep 2009 M7.6. Sep 2009 M7.6 Landslides and other forms of mass wasting."— Presentation transcript:


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

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