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