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Landslides

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Scarp 'Hummocky' terrain on and below (earthflow) Slump. scarp. Debris Flow. Debris flows are the downslope flow of relatively coarse material ... – PowerPoint PPT presentation

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Title: Landslides


1
Landslides
2
Learning Objectives
  • Understand basic slope processes and the causes
    of slope failure
  • Understand the role of driving and resisting
    forces on slopes and how these are related
    toslope stability
  • Understand how slope angle and topography,
    vegetation, water, and time affect both slope
    processes and the incidence of landslides
  • Understand how human use of the land has resulted
    in landslides
  • Know methods of identification, prevention,
    warning, and correction of landslides
  • Understand processes related to land subsidence

3
Mass Wasting
  • Definition mass wasting refers to a downslope
    movement of rock or soil as a more or less
    coherent mass.
  • Comprehensive term all inclusive term for any
    downslope movement of earth materials

4
Slopes
  • Weak and Soft Rock
  • Form slope with 3 segements
  • Convex upper part
  • Straight central segment
  • Concave lower part
  • Hard Rock
  • Form free face with talus slope at base

5
Important types of mass wasting
  • Slide downslope movement of coherent block of
    earth material
  • Slump is sliding along a curved slip plane
    producing slump blocks
  • Fall rocks fall from vertical face
  • Flow Downslope movement of unconsolidated
    material in which particles move about and mix
    within the mass
  • Subsidence is the sinking of of a mass of earth
    material below the level of surrounding material

6
  • Landslides are commonly complex combinations of
    slding and flowage
  • Upper slump block
  • Lower flow

7
Forces on Slopes
  • The stability of a slope expresses the
    relationship between resisting forces and driving
    forces

8
  • Driving forces forces which move earth
    materials downslope
  • Downslope component of weight of material
    including vegetation, fill material, or buildings
  • Resisting forces forces which oppose movement
  • Resisting forces include strength of material

9
Potential Slip Planes
  • Geologic surfaces of weakness in the slope
    material bedding, foliation, fractures

10
Safety Factor
  • The ratio of resisting forces to the driving
    forces
  • RF/DF
  • SF gt 1 Slope is stable
  • SF lt 1 Slope is unstable

11
Factors Affecting Slope Stability
  • Type of earth material
  • Slope Angle and Topography
  • Climate
  • Vegetation
  • Water
  • Time

12
Rotational Slides
  • Sliding occurs along a curved slip plane

13
Translational Slides
  • Sliding occurs on a planar surface or on a slip
    plane

14
Slump (a type of slide)
  • Indicators
  • Scarp
  • Hummocky terrain on and below (earthflow)

15
Slump
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18
Debris Flow
  • Debris flows are the downslope flow of relatively
    coarse material
  • gt 50 of particles in a debris flow are coarser
    than sand
  • Movement may be very slow or very fast, depending
    on topographic conditions
  • Mudflows, debris avalanches, and debris flows
  • Small to moderate magnitude events, occasional
    large magnitude events

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Debris Flow
21
Rockslide
  • Rock moves because theres nothing holding it
    back!
  • Generally requires a pre-existing low-friction
    surface...

22
Rockslide
  • like a clay layer, once its wet...

23
Earthquake Lake, MT
  • 28 deaths in 1959, triggered by earthquake

24
Role of Earth Materials
  • Slopes formed by weak rocks such as shale or have
    thick soil deposits typically fail by rotational
    slides
  • Slopes formed by hard rocks typically fail by
    translational slides
  • Soil slips occur above bedrock and fail by
    translational slides

25
Role of Slope and Topography
  • Hillslope angle is a measure of the steepness of
    a slope slope gradient
  • Steeper slope increased driving forces
  • Steep slopes associated with rockfalls
  • Subarid to arid environments

26
Role of Vegetation
  • In subhumid to humid environments, vegetation is
    thick and abundant
  • Landslide activity includes deep complex
    landslides, earthflows, and soil creep.
  • Vegetation influences slope stability by
  • Providing a cover that cushions the impact of
    rain falling on slopes and retards erosion on
    surface
  • Vegetation has root systems that tend to provide
    an apparent cohesion which increases resistance
    to landsliding
  • Vegetation adds weight to the slope increasing
    the driving forces

27
Role of Water
  • Water can affect slope stability by
  • Shallow soil slips can develop during rainstorms
    when slopes become saturated
  • Slumps or translational slides can develop months
    or years after slope is saturated
  • Water can erode the base or toe of a slope
    decreasing slope stability

28
Role of Climate
  • Climate influences the amount and timing of water
    in the form of water or snow
  • Influences type and amount of vegetation

29
Role of Time
  • Physical and chemical weathering can weaken slope
    materials decreasing resisting forces

30
Earthflow
  • basically a very viscous (thick) debris flow
  • slow-moving
  • faster in wetter weather

31
Earthflow
32
Creep
  • very slow
  • result of freezing and thawing

33
Creep
34
sandstone
shale
35
Creep
36
Triggers for rapid Mass Wasting
  • Rain
  • Oversteepening
  • cutting at foot of slope
  • piling on head of slope
  • Deforesting / Devegetating
  • Earthquakes

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