Landslides
Learning Objectives
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Understand basic slope processes and the causes of slope failure
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Understand the role of driving and resisting forces on slopes and
how these are related toslope stability
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Understand how slope angle and topography, vegetation, water, and
time affect both slope processes and the incidence of landslides
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Understand how human use of the land has resulted in landslides
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Know methods of identification, prevention, warning, and correction
of landslides
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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
mass.
• Comprehensive term – all inclusive term for any
downslope movement of earth materials
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
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
movement
– Resisting forces include strength of
material
Potential Slip Planes
• Geologic surfaces of weakness in the slope
material – bedding, foliation, fractures
Safety Factor
• The ratio of resisting forces to the driving
forces
– RF/DF
SF > 1 Slope is stable
SF < 1 Slope is unstable
Factors Affecting Slope Stability
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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
below
(earthflow)
Slump
scarp
Debris Flow
• Debris flows are the downslope flow of relatively
coarse material
• > 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
Debris Flow
Rockslide
• Rock moves because there’s nothing holding it back!
• Generally requires a pre-existing low-friction surface...
Rockslide
• 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
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
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
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
forces
Earthflow
• basically a very viscous (thick) debris flow
• slow-moving
– faster in wetter weather
Earthflow
Creep
• very slow
• result of freezing and thawing
Creep
Creep
from D. Schwert, NDSU
Triggers for rapid Mass Wasting
• Rain
• Oversteepening
– cutting at foot of slope
– piling on head of slope
• Deforesting / Devegetating
• Earthquakes