PowerPoint Presentation - Lecture 13 - Mass Wasting

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                   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
                 Mass Wasting

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

• 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
     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
• Landslides are commonly complex
  combinations of slding and flowage
   – Upper slump block
   – Lower flow
             Forces on Slopes

• The stability of a slope expresses the
  relationship between resisting forces and
  driving forces
• 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
  – Resisting forces include strength of
           Potential Slip Planes

• Geologic surfaces of weakness in the slope
  material – bedding, foliation, fractures
                Safety Factor

• The ratio of resisting forces to the driving
   – RF/DF
   SF > 1 Slope is stable
   SF < 1 Slope is unstable
      Factors Affecting Slope Stability

•   Type of earth material
•   Slope Angle and Topography
•   Climate
•   Vegetation
•   Water
•   Time
             Rotational Slides

• Sliding occurs along a curved slip plane
           Translational Slides

• Sliding occurs on a planar surface or on a
  slip plane
             Slump (a type of slide)

• Indicators:
   – Scarp
   – “Hummocky”
     terrain on and

                   Debris Flow

• Debris flows are the downslope flow of relatively
  coarse material
• > 50% of particles in a debris flow are coarser than
• 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
Debris Flow
• Rock moves because there’s nothing holding it back!
• Generally requires a pre-existing low-friction surface...
• like a clay layer, once it’s wet...
         “Earthquake Lake”, MT
• 28 deaths in 1959, triggered by earthquake
          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
      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
              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
   – 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
               Role of Water

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

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

• Physical and chemical weathering can
  weaken slope materials decreasing resisting

• basically a very viscous (thick) debris flow
• slow-moving
   – faster in wetter weather
• very slow
• result of freezing and thawing

        from D. Schwert, NDSU
    Triggers for rapid Mass Wasting

• Rain
• Oversteepening
  – cutting at foot of slope
  – piling on head of slope
• Deforesting / Devegetating
• Earthquakes

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