Chapter 6 Landslides

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Chapter 6 Landslides Related Phenomena
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Learning Objectives

• Gain a basic understanding of slope stability and
  mechanisms of slope failure
• Understand the role of driving and resisting
  forces affecting slope stability
• Understand factors that affect slope processes
• Topography
• Climate
• Vegetation
• Water
• Time
• (Gravity)
• (rock type)
• Understand how human use of land affect
  landslides slopes
• Familiarization with identification, prevention,
  warning, correction of landslides
• Appreciation for processes related to land
  subsidence (Part B)

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Terms Definitions

• Landslide
• Rapid downslope movement of rock/soil as a
  coherent mass
• Comprehensive term for any type of downslope
  movement of rock/soil
• Related terms
• Slope failure
• Mass wasting

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Slope Processes

• Slopes
• Common landform
• Appearance as stable/static features
• Typically dynamic, evolving systems
• Downslope movement of material
• Interaction with other systems
• Rivers, stream valleys, floodplains
• Soil formation
• Eolian (wind)
• Volcanoes

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Slope Elements

• Convex slope/crest
• Associated with creep
• Free-face (steep/vertical)
• Associated with rockfalls
• Debris slope
• Angle of repose ? 30o-35o
• Concave slope or wash slope
• Produced by processes associated with running
  water

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Types/Patterns of Downslope Movement
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Patterns of Downslope Movement
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Classification of Downslope Movements
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Debris Flow
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Slope Stability

• Relationship between driving resisting forces
• Driving forces (DF)
• Weight of rock, soil
• Weight of superimposed material
• Vegetation
• Fill
• Buildings
• Resisting forces (RF)
• Shear-strength of slope material acting along
  potential slip planes
• Cohesion
• Internal friction
• Ratio RF/DF Factor of Safety (FS)
• gt1.0 stable
• lt1.0 unstable
• Subject to changed conditions (see example fig.
  6.4)

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Causes of Landslides

• Real Causes
• Driving Forces gt Resisting Forces
• Primary Forces
• Gravity (downward force downslope component)
• Resistance shear strength
• Factors and Causes
• Landslides occur when forces along potential
  rupture surfaces exceed the forces resisting
  movement (i.e., cohesion, frictional)

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Functional Relationships
Relationships between driving force (weight)
resistance force (R)
Resisting force (R) is proportional to shear
strength of material (S) Where shear strength or
resistance (S) S C (sn -u) tan f

• The main driving force is the downslope component
  of the weight (force) of the material above the
  potential slip plane W sin q where
• W
• Weight of material (above plane)
• angle of the plane
• (from horizontal)

The factors in S are u fluid pressure (pore
water pressure) tan f coefficient of internal
friction f angle of internal friction
(frict. resist.) sn normal stress (i.e., normal
to surface or plane of discontinuity C
cohesion of material -----------------------------
------------ C and f depend on material type C,
f, and u vary with water content
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Factors/Controls

• Gravity/Amount of material (above a slip plane)
• Weight (force) downslope component of the weight
  of the slope materials above the slip plane
• Downward
• Normal to surface or plane of discontinuity (sn)
• Angle of repose (slope angle)
• Parallel to surface or plane of discontinuity
• Rock/Material Type
• Structure (layering, foliation, etc.)
• Water
• Others?

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Some Implications, Relationships

• The steeper the angle of inclination from
  horizontal, the larger the shear stress
• Decreases in S ? less strength, i.e., less force
  is required to cause a rupture
• C and q have different values for
• different types of soil or rock materials
• dry materials vs. wet materials
• Angles of frictional resistence (q) of dry rock
  materials vary from q gt40o for some igneous
  rocks, to q 10o-20o for clay.
• For materials like soil and clay-rich rocks, q
  can be smaller by a factor of 2 for saturated
  conditions

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Factors Resulting in Decreased Slope Stability

• Increased pore pressure (affects sn) e.g.,
  Storms, fluctuating groundwater
• Increased water content (reduces C, q)
• Steepening of slopes (affects sn)
• Loading of slopes (affects sn)
• Earthquake shaking (reduces C, q)
• Removal of material from the base of slopes
  (Directly reduces S)
• Rivers, waves, man
• Changes in vegetation
• Change in chemical composition of pore water

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Role of Slope Topography

• Slope angle (slope) correlated with
• Driving force (e.g., gt15 slope)
• Correlation w/ geography
• Extent of landslide activity
• Steep slopes
• Rockfalls, debris avalanches
• Soil slips turn into mudflows (saturated slopes)
• Moderate slopes
• Earthflows/mudflows, debris flow

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Roles of Rock/Soil Type

• Patterns of movement
• Rotational slides (slumps)
• occur along curved surfaces
• Produces topographic benches (see fig.)
• Commonly occur in weak rock types (e.g., shale)
• Translational slides
• Planar
• Occur along inclined slip planes within a slope
  (6.2)
• Fractures in all rock types
• Bedding planes in rock slopes
• Clay partings
• Foliation planes (metamorphic rocks)
• Soil Slips
• Type of translation slide
• Slip plane above bedrock, below soil
• Colluvium

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Slope Material Strength

• Influences magnitude frequency of slides
• Creep, earthflows, slumps, soil slip
• Common on slope of
• Shale
• Weak pyroclastic (volcanic) material
• Less common on slopes of more resistant rock
• Well-cemented sandstone
• Limestone
• granite

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Role of Water

• Almost always involved with slope failure
• Soil water/moisture
• Cohesion (unsaturated vs. saturated effects)
• Chemical weathering effects on slope strength
• Soil shear strength ? 1/(pore water pressure)
• pore water pressure ? water saturation
• Rise in water table (incl. perched) ? reduced
  shear strength
• Unsupported bank storage
• Liquifaction (of clay-rich sediments that loose
  shear strength)

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Role of Climate Vegetation

• Controls nature/extent of ppt., moisture content
• Vegetation effects (dependent on plant type)
• Enhances infiltration/retards erosion
• Enhanced cohesion
• Adds weight to slope
• Transpiration reduces soil moisture

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Climatic Effects

• Semiarid regions or regions with resistant rocks
• Steep slope profiles (free-faces) (6.1a)
• Soil at crest wash slope (not free face)
• Subhumid regions or areas of soft rocks
• Gentler slope profiles (6.1b)
• Soil
• thick at top bottom parts of slope
• Thin in steeper middle portions

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Role of Time

• Forces on slopes change w/ time
• Moisture content
• Water table
• Weathering (reduced cohesion)

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Causes Prevention
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Human Use and Landslides

• Timber harvesting
• Urbanization
• (Irrigation)

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Minimizing Landslide Hazards

• Identification of potential landslides
• Prevention of Landslides
• Drainage controls
• Grading
• Slope supports
• Warning systems
• Landslide correction

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Mitigation/Prevention

• Slope Drainage
• Slope Reduction
• Engineering Methods/Structures
• Grading of slopes
• (see figures)

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Features Recognition of Landslides
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Examples, Case Studies
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Others(Subsidence-In Part B)

• Snow Avalanche
• Subsidence
• Sinkholes
• Salt deposits
• Coal mining

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Causes of Landslides

• Real Causes
• Driving Forces gt Resisting Forces
• Immediate causes (triggers)
• Earthquake shocks
• Vibrations
• Sudden increase in water
• External Causes
• Slope loading
• Steepening
• Earthquake shocks
• Internal Causes Causes that reduce shear
  strength

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