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Mass Movements. Introduction. Explain why slopes fail under force of gravity Explain factors influencing slope failure Describe types of mass movements and why they usually occur Discuss importance of mass movement processes in shaping landscapes and associated hazards. Introduction. - PowerPoint PPT Presentation
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Mass Movements
Introduction
• Explain why slopes fail under force of gravity• Explain factors influencing slope failure• Describe types of mass movements and why
they usually occur• Discuss importance of mass movement
processes in shaping landscapes and associated hazards
Introduction
• Rock, soils, and other materials move downhill– Sometimes rapidly,
sometimes slowly– Pose threats to people– Modify the landscape
Slope Stability and Mass Movement• Gravity is main force driving materials
downslope• Slopes have threshold of steepness
– Angle of repose is steepest angle at which a given material will remain stable
– `sand 33-35 degrees, boulders as steep as 45
• Frictional strength of material resist movement– Rough surfaces greater frictional strength– Increased weight of material increases
resistance
Force of Gravity• Two components– Shear stress –pushes
block parallel to slope– Other component pushes
block into slope• Stress is perpendicular
to slope surface• The greater the weight
of block the more resistance to movement
Cohesion
• Besides shear stress, cohesion also resists motion
• Is tendency of materials to stick together to resist movement
• Cements and roots can increase cohesion
Water Pressure
• Increases chance of failure• Pressure of pores filled with water reduces
frictional resistance
Forms of Mass Movement
• Mass movement – movement of earth materials by gravity– Mass wasting is term also used
Creep and Solifuction
• Slow almost imperceptible mass movement– Revealed by tilted trees, fence
posts– Soil particles rise with freeze or
wetting, settle lower on slope when warmed or dry
– Flow of soil occurring mostly cold high latitudes
Slides• Blocks of rocks moving on
distinct plane– Landslide is general term– Slump is movement on a
curved plane– Rockslides – large blocks
break loose, move rapidly downslope
• Important factors in slides– Groundwater pressure– Oversteepening slopes
Flows• Fluid movement of loose earth
materials– Earth flows are slow moving
flows of mostly fine grained or clay-rich soil/sediment
– Debris flows – fast-moving mixture of sediment and water• Mudflows are debris flows
of mostly muddy sediment• Debris flows involve water
Flows
Rockslides
• Mass of bedrock moving rapidly downslope
• One of most dangerous mass movement types
Fall
• Rocks break off and fall to base of slope/cliff– Talus
accumulates at base
U.S. Map of Landslide Risk
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