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Chapter 5 Mass Movements and Subsidence
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Causes vs. Triggers Cause – situation that leads to failureTrigger – event that produces failure Natural geologic – fractures, layers Physical Natural morphology Heavy rainfall/rapid snowmelt Uplift (tectonic or volcanic) Rapid drawdown Subsurface erosion Earthquake Vegetation loss (fire, drought) Volcanic eruption Freeze-thaw action FloodingHuman Excavation or loading Explosions Deforestation Mining Irrigation or water leakage
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Resolution of forces on a slope Driving force – downhill component Resisting force – depends on underlying material Safety factor = resisting force/driving force if > 1, slope is stable if < 1, failure can occur
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Angle of repose
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Bedding planes on a slope
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How stable is this house?
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Classification of Mass Movements Falls – free fall Rock Debris Slides and slumps Rotational Translational Flows Mudflow or earthflow Creep
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Free fall
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Cause of slope failure 1- steepening of slope 2- removal of toe support 3- addition of mass at top
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Toe Crown Basal surface Head scarp Rotational slide
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Anchorage Alaska March 1964 Earthquake
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Translational slide
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Types of flows – notice differences Debris flow Earthflow
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Creep
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Turtle Mtn/Frank Slide, Alberta 1903
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Mass movements of other material Snow avalanches Submarine slumps – largest mass movements on Earth, but not seen
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Subsidence Downward movement of ground surface Not reversible once it occurs Causes Removal of water or oil Compaction of clay materials (deltas) Solutioning of limestone by CO 2
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Alabama 1972 130 m long - 46 m deep
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