unit 3 presentation - strength and assessment
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This work is supported by the National Science Foundation’s Transforming Undergraduate Education in STEM program within the Directorate for Education and Human Resources (DUE-1245025).
sS = Shear Stress is the sum of all of the Driving Forces
sN = Normal Stress is only a part of the Resisting Forces Besides the normal stress (gravitational component),
the other components of the resisting force depend on the material’s shear strength:
Normal Stress: holding block in place
Shear Stress: pulling block down slope
W = FG
FN q
Besides the normal stress (gravitational component), the other components of the Resisting Force depend
on the material’s shear strength:Internal Friction (friction between material particles)
Cohesion(how well material sticks together)
Pore Pressure (degree of saturation in unconsolidated materials)
ANGLE OF REPOSE
What seems to be the relationship between angle of
repose and grain size?
The maximum angle possible between a non-cohesive material and a horizontal plane.
If a material is standing at the angle repose, which of the following is true?(a) shear strength > shear stress(b) shear strength < shear stress(c) shear strength = shear stress
ANGLE OF REPOSE
Freeman, Environmental Geology
In most sediments, angle of repose is ~30-45°
INTERNAL FRICTION
Rounded grains Angular grains Measured in the lab!
Discuss with a partner 2-3 factors that may influence internal friction.
COHESION
Non-cohesive granular material
Cohesive granular material
Measured in the lab!
COHESION
Materials αr Repose angle (°)
σr Cohesive index
Height heap (mm) Flow properties
Clay 71.7 2.3 53.2 Extremely poor
Polymer 33.7 0.1 14.6 Good
Lactose 56.4 1.4 27.9 Very poor
Metallic 43.8 0.2 17.9 Passable
Flour 66.4 2.1 48.4 Very poor
ROLE OF WATERSurface tension between grains increases cohesion
Pore Pressure:• Opposes Normal Force• Depends on degree of
saturation• Higher saturation,
reduces the normal (resisting) force)
DRIVING VS. RESISTING FORCES: REVIEW
Driving Forces: SHEAR STRESSA component of the gravitational force (dependent on mass) pulling the mass down a slope (FS = shear force)
Resisting Forces: SHEAR STRENGTH A component of the gravitational force (FN = normal force) AND forces related to material strength (cohesion, friction,
pore-pressure)
W = FG
Shear Strength: holding block in place FN
Shear Stress pulling block down slope
q
Driving Forces:Resisting ForcesFS
When Driving Forces > Resisting ForcesMass wasting occurs!
The block slides down the slope.
Shear Strength: holding block in place
Resisting Forces
When Driving Forces = Resisting ForcesThe block stays in place. This is the angle of
repose for a given material.
FN
FS Shear Stress pulling block down slope
Driving Forces:
THE SAFETY FACTOR!
The Safety Factor (SF) is the ratio of Resisting Forces to Driving Forces. Mathematically:
Driving ForcesResisting ForcesSF =
When SF > 1 THIS IS SAFE!When SF < 1 THIS IS UNSAFE!
FACTORS INFLUENCING DRIVING AND RESISTING FORCES
Let’s hear some of the things from your lists that increase the driving force or decrease the resisting force. We’ll make a list on the board.
Given what is on our list, what are some geologic processes that might play a role?
Given what is on our list, what are some anthropogenic processes that might play a role?
And thus, mass wasting potential (“The safety factor!”)
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WHERE IN CANADA…RELIEF
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WHERE IN CANADA…PRECIPITATION
http://www.nrcan.gc.ca/
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WHERE IN CANADA…VOLCANOES
http://www.nrcan.gc.ca/
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WHERE IN CANADA…EARTHQUAKES
http://www.nrcan.gc.ca/
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WHERE IN CANADA…LANDSLIDES
http://www.nrcan.gc.ca/
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CANADA…FATALITIES/POPULATION
http://www.nrcan.gc.ca/
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WHERE IN CANADA…AVALANCHES
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