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Canada (161,886)
Geography (106)
GG231 (31)
Rob Milne (27)
Chapter 5

Chapter 5 - Mass Wasting/Landslides

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Rob Milne

Risks and Disasters – Chapter 5  The Frank Slide o Turtle Mountain (mountain that moves) o April 29, 900m*650m*150m slab crashed into valley o Formed a small lake, buried approx. 2km of Canadian pacific railway and 3km of Frank and Grassy Mountain Railway o Frank was fully evacuated, but people moved back o 1911, people were evacuated, today, has about 300 people o Another large landslide could occur on the south peak of mountain o Contributing Factors  Cause – internal/external factor that reduces stability of slope and brings it to point of failure  Trigger – event that sets off landslide  Geology  Water infiltrates the mountain and over time decreases stability of the slope  Glaciations  Erosion eroded shales on lower slope, steepened overall slope of mountain reducing strength  Mining  Coal mining created large openings in the mountain  Weather  Heavy rain falls and increase in water exerts pressure on rocks o Lessons learned  Large landslides cannot be prevented  Geology is important  Human activity can trigger landslides  Introduction to Landslides o Mass wasting/landslides – down slope movements of rock or sediment due to gravity o Can be slow/fast and involve small/large sediment or rock o Types of landslides  Four variables to classify  Mechanism of movement (flow, slide, fall, topple, complex movement)  Type of material (rock, consolidated sediment, organic soil)  Amount of water  Rate of movement  Fall – bouncing of rock or blocks of sediment from face of a cliff  Slide – down slope movement of coherent block of rock or sediment along discrete failure plan  Slump – type of slide in which failure plane is curved upward  Flow – slow to rapid down slope movement of sediment in which particles move semi- independently of one another, common with aid of water  Debris flows – mixture of mud, debris and water, range in consistency  Creep – very slow rock flow, mm to dozens of cm per year  Sacking – special type of creep involving movement of large rock masses, up to many billions of cubic metres along ill-defined, deep failure planes  Topple – slow creep-like movement in which rock mass pivots about a point  Rock avalanches – high-velocity flows of fragmented rock  Subaqueous landslides – underwater landslides, slump or slide on submerged slope of delta or edge of continental shelf o Forces on slopes  Driving forces – moves rock or sediment down a slope  Largest driving force – weight of slope material and amount of water  Resisting forces – opposes movements 1 Risks and Disasters – Chapter 5  Shear strength – resistance to failure by sliding or flow along potential slip planes o Potential slip planes- surfaces of weakness in slope material  Slope stability evaluated by computing factor of safety (FS)  Ratio of resisting factors to driving forces o If <1 then stable o If =1 then failure can be expected  Role of material type  Material can affect type and frequency o Mineral composition o Degree of cementation or consolidation o Presence of planes of weakness  Planes of weakness can be dangerous if they are inclined more than 15 degrees and intersect/parallel to slope of hill or mountain  Slides have two basic movement patterns o Rotational – curved slip surfaces  Produces small topographic benches that tilt upslope o Translational – planar slip surfaces  Common type is debris avalanche  Failure plane generally at base or in colluviums o Mixture of weathered rock and other debris below soil  Role of slope and topography  Influenced by slope steepness and topographic relief o Steeper – greater the driving forces  Topographic relief – height of hill or mountain above the land below  Role of climate  Weather characteristics of a region over years or decades  Influences amount and timing of water that erodes hill slopes and the type/abundance of hillside vegetation  Arid and semi arid climates o Sparse vegetation, thin soil, exposed bare rock o Mass movements include rock falls, debris flows and shallow soil slips  Subhumid and humid climates o Abundant vegetation, thick soil cover slopes o Deep complex landslides, soil creep, rockslides, slumps and debris flows  Role of vegetation  Provides protective cover reducing impact from rain  Plant roods add strength and cohesion  Adds weight to slope, increasing slope failure  Role of water  Soil slips and debris flows happen during rainstorms  Slumps develop months following deep water infiltration  Erosion of slope toe reduces mass of resisting material, increasing slope failure  Contributes to liquefaction, causing granular sediments to lose strength and flow as liquid  Thaw flow slides – occur during warm spells in summer when seasonal melted layer is thickest  Role of time  Forces acting on slope change with time  Weathering of rocks reduces cohesion and strength 2 Risks and Disasters – Chapter 5  Soil water acidic because it reacts with carbon dioxide, creating carbonic acid  Geographic regions at risk from landslides o Landslides occur wherever there are significant slopes o Mountainous areas have higher risk for landslides o
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