Landslides 2

Question 1

does the scarp tell us about what slide it is?

Answer 1

not rlly!
-> have to look at the failure surface

Question 2

What is the angle of repose?

Answer 2

• shear stress is exactly balanced by shear strength
• F_s is equal or just above 1.0
• IOW the steepest angle a slope can maintain without collapsing

Question 3

Driving force in slope stability?

Answer 3

Gravity
- manifests as shear stress (T)
- component of force of gravity parallel to slope

Question 4

Shearing

Answer 4

motion from side to side across a plane

Question 5

Resisting forces in stability of slope

Answer 5

Shear strength (T_f)

• Friction
• Cohesion

Question 6

What is shear strength?

Answer 6

the slope’s ability to resist shearing motion

• friction
• cohesion

Question 7

What is friction proportional to?

Answer 7

normal force

Question 8

Cohesion

Answer 8

how the material holds together

Question 9

Friction

Answer 9

resistance to sliding

Question 10

shear strength vs shear stress

Answer 10

strength
- resisting force, opposite to direction of motion

stress
- driving force, direction of motion

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Question 11

What force prevent slopes from failing? Examples?

Answer 11

resisting forces (shear strength)
- frictional strength
- cohesive strength

Question 12

Shear strength of most sediment

Answer 12

low

Question 13

Shear strength of most sedimentary rock

Answer 13

moderate

Question 14

Shear strength of most crystalline rock

Answer 14

high (if cohesive)

Question 15

to model stability of any slope, we compare

Answer 15

resisting forces (shear strength)
to
driving forces (shear stress)

Question 16

motion occurs when ___ is greater than ___

Answer 16

shear stress (drive) greater than shear strength (resist)

Question 17

Factory of safety (F_s)

Answer 17

ratio of Shear Strength / Shear Stress

aka
resistance / drive
T_f / T

Question 18

When will a slope fail (F_s) ?

Answer 18

F_s <= 1.0

• IOW, fails as soon as it reaches 1!
• stable slope means F_s alot greater than 1

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Question 19

Do we want our slope to be near the angle of repose?

Answer 19

no! means it is nearly unstable

Question 20

The angle of repose varies depending on what?

Answer 20

material

Question 21

Cause

Answer 21

factors leading to instability of a slope
- long term
- reduce shear strength OR increase shear stress of a slope

Question 22

Do causes initiate movement?

Answer 22

no!!

Question 23

Triggers

Answer 23

factors that translate instability into motion
- short event

Question 24

Can there be many causes?

Answer 24

yes

Question 25

Can there be many triggers?

Answer 25

no
- there is only ever one trigger

Question 26

Do we always know what the trigger is?

Answer 26

its possible we won’t know

Question 27

Cause vs Trigger

Answer 27

cause: makes slope susceptible to movement

trigger: initiates failure

Question 28

What are some external causes of mass movement?

Answer 28

• slope angle (steeper slope = more motion)
• undercutting
• overloading
• vegetation
• climate

Question 29

What are external causes

Answer 29

factors outside of slope that affect stability

Question 30

Undercutting

Answer 30

• lower part of slope is removed
• removes material supporting the slope

Question 31

Overloading

Answer 31

adding weight
- buildings, roads, landslides, trees, etc

Question 32

What external factor is a cause in the most landslides?

Answer 32

slope angle
-> slope is requirement while climate is catalyst

Question 33

Does climate always affect slope?

Answer 33

nah, it’s everywhere but doesn’t always affect slope

Question 34

How does vegetation affect slope stability?

Answer 34

roots bind loose material
- but heavy trees can increase instability cuz overloading

Question 35

How does climate affect slope stability?

Answer 35

if average temp and rainfall is high

• atmospheric rivers
• more water increases weathering of rocks
• freeze/thaw = internal causes…