Landslides

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Landslides
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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

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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

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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

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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

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

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Forces on Slopes

• The stability of a slope expresses the
  relationship between resisting forces and driving
  forces

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• 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

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Potential Slip Planes

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

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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

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Factors Affecting Slope Stability

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

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Rotational Slides

• Sliding occurs along a curved slip plane

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Translational Slides

• Sliding occurs on a planar surface or on a slip
  plane

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Slump (a type of slide)

• Indicators
• Scarp
• Hummocky terrain on and below (earthflow)

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Slump
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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
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Rockslide

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

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Rockslide

• like a clay layer, once its wet...

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Earthquake Lake, MT

• 28 deaths in 1959, triggered by earthquake

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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

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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

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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

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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

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Role of Climate

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

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Role of Time

• Physical and chemical weathering can weaken slope
  materials decreasing resisting forces

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Earthflow

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

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Earthflow
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Creep

• very slow
• result of freezing and thawing

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Creep
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sandstone
shale
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Creep
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Triggers for rapid Mass Wasting

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

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