Landslides

Question 1

What are the main similiaries and differences between earthquakes and landslides?

Answer 1

Earthquakes:
Shear failure
Deep/shallow
Slip on planar fault
Shear stress driven
Technoic process/builds topography

Landslides:
Shear failure
Surface phenomenon
Slip on planar or curved surface followed by flow
Gravity Driven
Erosional process, Destroys topography

Question 2

What happens when soil is saturated with water?

Answer 2

It resembles liquefaction, it becomes much more sucseptible to failure

Question 3

What is the most deadly slide in canada?

Answer 3

Frank Slide, Alberta killing 70, “the mountain that walks”

Question 4

What are the impacts of landslides? What are they govered by?

Answer 4

Human and economic

1) population density (lives and injuries)
2) cost of infrastrucutre ($$$)
3) population preparedness (both)

Question 5

How many geohazards account for death by landslide?

Answer 5

24% in canada

Question 6

What are most common human inpacts? Why are they underestimated?

Answer 6

Low deaths

Underestimated (usually landslides are triggered by volcanos/earthquakes and are attributed to those

Question 7

What are some estimates of economic impacts?

Answer 7

1.4 Billion in canada
3.35 billion in USA
Venesuala 62 million

Question 8

Why is BC susceptible to landslides?

Answer 8

Mountainous
Rain- saturated ground
Complex geology (glacial sediment with elevation gain)
Triggers that cause landslides

Question 9

What is the return period of a landslide that has 20 mil m3 of material?

Answer 9

25-70 years..

Question 10

how do you classify landslides?

Answer 10

type of material
type of movement
rate of movement
name is the 1st 2 combo

Question 11

what are types of materials?

Answer 11

rock
soil/earth
mud
debris (mixture)

Question 12

what are types of motion?

Answer 12

falls (rock breaks lose under gravity)
slides (block moves on a surface)
- curved shape failure (rotational)
- flat failure surface (translational slide)
flows (fluid motion)
-complex movements (combo: slide that becomes a flow)

Question 13

where do falls occur?

Answer 13

steep rock slopes, detaches because of weakness/fractures etc and gravity. very fast

Question 14

what are characteristics of slides?

rotational vs translational

Answer 14

slow or fast, soil/rock/debris
moves as ONE MASS along a surface

Rotational: slump, intermediate speed, weak material (sediment) curved failure plane
curved ‘scarp’ above the slide

Translational: slow-fast, strong material on planes of weakness, cohesive motion along flat surface

Question 15

what are flows?

Answer 15

slow-fast
kind of like lahars
water very important
fluid or plastic flow

Question 16

What is gravity counteracted by on a slope?

How does water effect this?

Answer 16

1) internal strength of material
2) friction of that material on a slope

Water increases weight and reduces friction. It decreases NORMAL FORCE.
Decrease strength, promote landslides

always a fight between friction>< downhill

Question 17

what is slow moving stuff called?

Answer 17

x creep

Question 18

What is the angle of repose?

Answer 18

GOOD: the angle of repose shear stress is exactly balanced by shear strength. Factor of safety is equal to or just above 1.

OR BAD: steepest angle a slope can contain without collapsing.
Depends on material

Question 19

What are the forces involved in the stability of slopes?

Answer 19

Driving force:

• Gravity
• manifests as “shear stress” (T), parallel to the slope. Force of gravity parallel to the slope

Resisting force:
Friction - resistance to sliding (proportional to formal force/stress)
Cohesion - how material holds together
Together they make “shear strength” (Tf)
- which is the ability to resist shearing motion

Question 20

When (mathematically) is a slope stable/unstable?

translational slides

Answer 20

When Driving forces are greater than resisting force, motion/failure occurs. When shear stress is greater than shear strength
Fs = Tf/T (shear strength over shear stress)
When Fs > 1 stable!

Question 21

What happens when you add water into the equation?

Answer 21

normally: Tf=u*normal force (which is frictional resistance)
with water
Tf= u(normal force - pressure of water)

water pressure lowers Tf!!
*** look at equations one more time unless u can write them out no problem

Question 22

how do you calculate stability of slopes in rotational slides?

Answer 22

material itself fails, the strength depends entirely on the material, becuase of the ability to ‘interlock’
- internal frictional strength, cohesive strenght (rotational slides dont happen in solid rock)

Question 23

What is the average material strength of certain materials discussed in class?

Answer 23

Ignoues/metamorhpic: high shear strength
Sedimentary rock: moderate shear strength
Sediments: LOW

Question 24

Whats a cause vs a trigger?

Answer 24

causes (long term) factors that lead to instability of a slope
causes reduce shear strength OR increase shear stress
do not initiate movement

triggers translate instability into motion (can be many causes, but only one trigger)

Question 25

What are external causes of mass movement?

Answer 25

factors that affect stability 1) slope angle 2) undercutting: lower part of slope removed, which could be supporting slope, (rivers/roads/buildings etc) 3) overloading: adding weight to top of unstable slope 4) vegetation: roots bind loose material, removing roots can make unstable, but heavy trees can decrease stability which is overloading 5) climate: high rainfall, increased weathering of rocks, temp (internal causes)

Question 26

what are internal causes of mass movement?

Answer 26

factors inside slope that affect its stability 1) water: in all slopes -adds weight (overloading) -decreases normal stress, decreasing friction and shear strength -increases weathering -acts as medium for flows in sediments -depends on how much water is present in solid rock -water reduces shear strength along planes of weakness, acts as lubricant and reduces effective normal stress -frost wedging/freezing thaw (water gets into cracks, as it freezes it expands, forcing fractures apart) landslideshappen more often in colder temperatrures 2) inherently weak material -fail at low angles -volcanic rock (??), quick clay CLAY: so small (0.002mm) that vanderwalls forces apply (electrostatically) - when in salt water clay is attracted to other clay particles 3) Bad geological structures - bad bedding/fracture orientation - structures angled in an instable direction, layered precariously

Question 27

how does water content in sand effect its angle of repose?

Answer 27

in sand sediment -water helps/hinders cohesion depending on HOW MUCH a) unsaturated: surface tension is an important force with films of water and sand grains b) saturated: grains are forced apart and material flows no water: low/medium angle of repose some water: very high angle (almost verticle) too much water: very LOW ANGLE

Question 28

what are quick clays?

Answer 28

a "house of cards" structure, that when is shaken, becomes compact Salt content is lowered by percolating groundwater *** not very common to cause slides

Question 29

What are some triggers?

Answer 29

Force/event that disrupts equilibrium of a slope and initiates mass movement - earthquakes, snow melt, heavy rainfall, rain on snow, loud noises, vehicles, volcanic eruptions, excavation, skiing, jumping

Question 30

Describe the Oso Landslide

Answer 30

``` Causes? 45 days of heavy rain Trigger: collapse of layers of glacial sediment Name? Protect ppl? *** ```

Question 31

What is the landslide disaster scale?

Answer 31

``` A velocity class based on its speed. 7 classes with ranges from slow to very fast!! 16 mm/year to 5m /second (1-7) ```

Question 32

What are telltale signs that a slope is unstable

Answer 32

Steep loose sediments and rock PISTOL BUTT tress: shows slope is slowly going down (makes trees tilt and they try to right themselves) Tilted things Erosion 1. cracking tilted house elements 2. cracks on slope 3. cracks in retaining walls 4. water pooling at base of slope 5. bulges on slope 6. leaning trees and utility posts 7. sunken road beds

Question 33

What are landslide cause and effects?

Answer 33

Cause: reduce shear strength Trigger: initate motion Effect: fall/slide/flow/complex

Question 34

What is migitation?

Answer 34

Solving the problem, investigating/monitering. | sometimes cheaper to repair than remove cause alltogether :)

Question 35

What are some forms of investigation?

Answer 35

1. looking for previous failures 2. Geostatics: frequency and magnitude diagrams, look at historical record and develop stats for a region (logarithmic plot) 3. Hazard mapping (plan best route for a highway/pipeline etc.) 4. Modeling: see slopes and valleys, see where dangerous areas are

Question 36

what are the 3 paths to mitigate landslides?

Answer 36

1. Avoidance (move away) 2. Prevention: make sure they don't occur when ppl or there or at all 3. Protection: strengthen/armour area that might be affected by landslide if it happens

Question 37

What are avoidance strategies?

Answer 37

Often too expensive - buying property in a town - moving a highway - convincing ppl to leave a home

Question 38

What are prevention strategies?

Answer 38

Removal of material Stabilizing slopes with -resisting forces, retaining walls, gabions -stabilizing by cables anchored in rock that pull layers together Remove water with drainage Rock scaling: removing pieces that will fall

Question 39

What are protection strategies?

Answer 39

Minimize hazard Let them occur but control what happens 1. Rock barrier (i.e. on a highway) 2. Rock net (to keep rocks close to wall and not fly out) 3. ROckfall shed, go ontop of road 4. Rock fences: catch falling rocks and dissipate kinetic energy 5. Debris flow retention structures (water and debris, remove debris from water = no more flow!!)

Question 40

what does frost wedging cause?

Answer 40

rock falls!