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65 Cards in this Set
- Front
- Back
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Mass wasting
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general term for any type of downslope movement of rock or sediment because of gravity
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landslide
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includes rock fall, debris flows, slowly moving bodies of coherent rock
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variables for landslide classification
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Type of movement
Type of material Amount of water Rate of movement |
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Falling
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free fall and bounding of rock or blocks of sediment from the face of a cliff
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Sliding
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downslope movement of a coherent block of rock or sediment along a discrete failure plane
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Slumping
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Particular type of sliding in which the failure plane is curved upwards
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creep
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very slow flow of rock or sediment
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topples
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slow creep-like movements in which rock masses pivot about a point
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subaqueous landslides
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complex combination of sliding and flowage
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Types of falls
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rock fall
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Slope stability
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relation between driving forces and resisting forces
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driving forces
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move rock or sediment down a slope
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resisting forces
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oppose movement
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largest driving force
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weight of the slope material and the amount of water it contains
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Shear strength
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resisting force
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Slope stability/factor of safety
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ratio of resisting forces to driving forces. If FS is equal to 1, the driving forces equal resisting forces and a slope failure is expected.
Shear strength/shear stress Resisting forces/Driving forces Equal to or greater than 1 than the slope is stable |
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Rotational slides
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Slumps
Curved slip surfaces |
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Translational slides
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Planar slip surfaces
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Slope stability influenced by
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slope and topography
that is, slope steepness and topographic relief |
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role of vegetation
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Provides a protective cover that reduces the impact of falling rain. Allows rainwater to infiltrate into the slope while retarding surface erosion
Plant roots add strength and cohesion to slope materials. They act like steel rebar in concrete Vegetation adds weight to a slope, which can increase the likelihood that the slope will fail |
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Water affects slow stability in 3 ways
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Many landslides such as shallow soil slips and debris flow happen during rainstorms when slope materials become saturated
Other landslides such as slumps develop months or even years following deep infiltration of water into a slope Erosion of the toe of a slope by a stream reduces the mass of resisting material and thus decreases the slope's stability |
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liquefaction
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Water saturated silts and sand can lose their strength and flow as a liquid
Quick clay failures Leda clay is sensitive to liquefaction. "house of cards" structure leads to this |
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Freezing of water in fractures in rock
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Can destabilize slope and trigger rock falls
Water increases in volume when frozen, exerts force along fractures in rock |
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Role of time
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driving and resisting forces change seasonally as water table fluctuates
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Three factors expected to increase incidence of landslides
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Urbanization and development will increasingly expand into landslide prone areas
tree cutting will continue in landslide prone areas Changing global climate patterns will result in increases in precipitation in some regions |
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Water may Increase likelihood of a landslide by
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causing expansion of clay mineral
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Fastest moving to slowest
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Rockfall
Debris flow Slumb Creep |
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Quick clay problems are common in what part of Canada?
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Quebec
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Rockfall refers to
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Individual rocks that free fall from the face of a cliff
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Gravity influences
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shear stress
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Component of gravity acting parallel to the slope
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Shear stress
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Liquefaction occurs when pore water pressure becomes
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great enough to eliminate grain-to-grain contacts
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Landslide trigger is external stimulus that
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Causes a near-immediate response by rapidly decreasing strength
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Trigger frequency
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How often an event occurs that sets off a landslide
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Tensioning a rock anchor stabilizes a slope by
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Increasing normal stress, and therefore increasing frictional strength
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Effective stress
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-Intergranular stress
-Grain to grain contact forces resulting in frictional strength -Total stress minus as pore water pressures |
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Small amounts of water makes slopes
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more stable
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Increased pore pressures
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weaken slopes
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Sturzstroms are combo of
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Falls and translational slides
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Quick clays lead to
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spreads
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Main force or "prime mover" of landslides
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gravity
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Can causes trigger landslides?
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yes
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Are there usually multiple causes for a landslide event?
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yes
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One or many triggers?
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only one trigger
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Gp
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Component of gravity that acts perpindicular to the slope.
-Helps hold an object or slope materials in place on the slope -when slope is steeper Gp decreases |
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Gt
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Component of gravity that acts tangential to the slope surface, or parallel to and down the slope
-pulls object or materials in the down-slope direction -causes shear stress -When slope is steeper, Gt increases |
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Shear strength
(frictional resistance & cohesion) affected by |
Type of soil or rock
-condition of geologic materials (how eroded) -Presence of weak surfaces (joints or faults) -Pore water pressure within slope |
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Effective stress (intergranular stress)
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Normal stress Minus pore water pressure
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Normal stress
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Weight of the material itself acting perpindicular to the slope.
-Increasing normal stress (by anchoring) helps |
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Erosion
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transport of material away from its source
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Water causing/hindering landslides
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-Excess increase slope's mass/shear stress
-reduces friction, reducing shear strength -freezes and then expands -Ex sand-water helps cohesion |
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Ways humans can increase likelihood of landslide
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-Excavation of a slope at its toe
-Loading a slope at its crest -deforestation -irrigation -mining -water leakage from utilities |
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flows occur when
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large volume of water present in a mixture of soil and debris
-material flows as chaotic, viscous mixture |
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Debris flow
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Rapid flow of fully saturated debris in a steep channel,
-form debris fan at the mouth of channel or gulley, cuz it spreads out |
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debris avalanche
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Rapid shallow flow of partially to fully saturated debris on a steep slope that is not confined to an established channel
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Return period
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Time span of data record/number of cases of magnitude M
Ex-109 years/3 events RP=36 years |
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Portuguese Bend
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Illustrates how people can increase landslide hazard
-Road building and changes in sub surface drainage reactivated ancient landslide |
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Hope Slide 1965
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Canada's largest historic landslide
-huge mass of rock slid, then fragmented and drove a wave of muddy debris up opposite valley wall -Trigger is unknown -Cause was joints in rocks |
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Leda clay found
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St Lawrence Lowland of Quebec and Ontario
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Liquefaction and buildings
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Building built on sand will lose support, causing structural damage
-Upwards pressure applied by movement of liquefied soil through the crust can crack weak foundation slabs and enter buildings through service ducts -buildings on pile foundations may lose support from adjacent soil and then buckle |
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La conchita contributing factors
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Presence of steep, high slopes
-presence of weak rocks -presence of numerous historic landslides -periodic and intense rainfall |
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Lituya bay, alaska
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Earthquake triggered a rockslide that went into the bay and displaced seawater
-triggered tsunami |
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grand banks newfoundland
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Earthquake triggered submarine landslide,
Set off tsunami that fucked shit up |
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debris classification
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unsorted soil
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Types of movement (classification of landslides)
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falls
topples slides spreads flows complex |
