Transcript No Slide Title

Chapter 6
Landslides &
Related
Phenomena
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:
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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)
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
Slope Processes
• Slopes:
– Common landform
– Appearance as stable/static features
– Typically dynamic, evolving systems
• Downslope movement of material
• Interaction with other systems
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Rivers, stream valleys, floodplains
Soil formation
Eolian (wind)
Volcanoes
Slope Elements
1. Convex slope/crest
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Associated with creep
2. Free-face (steep/vertical)
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Associated with rockfalls
3. Debris slope
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Angle of repose  30o-35o
4. Concave slope or wash slope
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Produced by processes associated with running water
Types/Patterns of
Downslope Movement
Patterns of Downslope Movement
Classification of Downslope Movements
Debris Flow
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)
– >1.0 = stable
– <1.0 = unstable
– Subject to changed conditions (see example; fig. 6.4)
Causes of Landslides
• Real Causes
– Driving Forces > 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)
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) The main driving force is the
downslope component of the
S = C + (sn -u) tan f
weight (force) of the material
The factors in S are:
above the potential slip plane
= W sin q where
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
W
=Weight of material (above plane)
q= angle of the plane
(from horizontal)
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
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Rock/Material Type
Structure (layering, foliation, etc.)
Water
Others?
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 >40o 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
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
Role of Slope & Topography
• Slope angle (slope); correlated with:
– Driving force (e.g., >15% 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
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)
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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
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
Role of Water
• Almost always involved with slope failure
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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)
Role of Climate & Vegetation
• Controls nature/extent of ppt., moisture content
• Vegetation effects (dependent on plant type)
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Enhances infiltration/retards erosion
Enhanced cohesion
Adds weight to slope
Transpiration reduces soil moisture
Climatic Effects
A. Semiarid regions or regions with resistant rocks
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Steep slope profiles (free-faces) (6.1a)
Soil at crest & wash slope (not free face)
B. Subhumid regions or areas of soft rocks
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Gentler slope profiles (6.1b)
Soil
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thick at top & bottom parts of slope
Thin in steeper middle portions
Role of Time
• Forces on slopes change w/ time
– Moisture content
– Water table
– Weathering (reduced cohesion)
Causes & Prevention
Human Use and Landslides
• Timber harvesting
• Urbanization
• (Irrigation)
Minimizing Landslide Hazards
• Identification of potential landslides
• Prevention of Landslides
– Drainage controls
– Grading
– Slope supports
• Warning systems
• Landslide correction
Mitigation/Prevention
• Slope Drainage
• Slope Reduction
• Engineering Methods/Structures
– Grading of slopes
– (see figures)
Features & Recognition of
Landslides
Examples, Case Studies
Others
(Subsidence-In Part B)
• Snow Avalanche
• Subsidence
– Sinkholes
– Salt deposits
– Coal mining
Causes of Landslides
• Real Causes
– Driving Forces > 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