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Landslides
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 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/DFSF > 1 Slope is stableSF < 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 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
shalesandstone
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