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Landslides lectures 1&2 (introduction) Flashcards

1
Q

What is the evolution of landslide hazards like?

A

Can have a long evolution or can be rapid from beginning to end with variability

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2
Q

What is a landslide?

A

Generic term for a wide range of processes that result in downward and outward movement of slope forming materials

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3
Q

What are the variables that affect landslides?

A

Driving force (gravity, height)
Gradient (incline)
Materials (rock, soil)
Size (highly variable)
Type of movement
Velocity (speed)

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4
Q

How does material have an affect on gradient needed for landslide?

A

Clays are able to move on a shallower slope due to platy structure while most others need steeper

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5
Q

Why is velocity of landslide so important?

A

Allows for planning slow creep can be prepared for but extremely fast must be known of beforehand or no hope

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6
Q

Where does a landslide distribution map from 2018 show?

A

The majority of landslides are clustered in mountainous regions i.e., Himalayas

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7
Q

Approximately how many people are killed by landslides globally per year?

A

14000

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8
Q

What is the cost of landslides to industrialised countries per year?

A

6 billion euro (including survey, damage and recovery)

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9
Q

What is the cost per year for recovery from UK landslides?

A

£10 million

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10
Q

Why is the study of landslides important?

A

Property destruction
Loss of arable land
Loss of historic landmarks
Threat to coastal paths
Threat to infrastructure (rail and gas pipes)

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11
Q

What is an example of a small scale landslide that had a big impact on rail connection?

A

November 22 around Exeter cutting off the South west

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12
Q

What are some examples of the most deadly landslides?

A

Vajont, Italy 1963
Huascaran, Peru 1970

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13
Q

How many deaths resulted form the 1963 Vajont landslide?

A

2000+

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14
Q

What were the series of events that caused the Vajont landslide?

A

A reservoir was built in the valley once filled this increased the water pressure in the mountain with cracks noticed at the mountain peak but due to political pressure filling resumed until the mountain collapsed into the reservoir causing it to overtop

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15
Q

How many people died as a result of the Huascaran landslide 1970?

A

18000 people

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16
Q

What is mass movement?

A

Mass wasting is a natural phenomenon by which rock, soil and/or debris move downwards due to the action of gravity

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17
Q

What is usually responsible for rock fall?

A

Joint sets in the rock that create weaknesses

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18
Q

What are the 2 divisions of rock fall?

A

Fall
Topple (block and flexural)

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19
Q

What are the characteristics of rock fall?

A

Detachment, fall, rolling and bouncing of fragmented rock
Very little dynamic interaction between fragments
Short runout
Small volume
Very rapid

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20
Q

What is an example of a rockfall?

A

2 trucks destroyed on a Swedish highway in May 2006

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21
Q

What is the geomorphic setting of rockfall?

A

Steep narrow gorges
Steep valley flanks
Upper reaches of cliffs

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22
Q

What is rock fall by topple?

A

Forward rotation and overturning of rock columns or plate separated by steep fractures - can be induced by roots or ice prying open joint

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23
Q

What will the structures of Block and Flexural toppling be?

A

Block - well defined basal structures
Flexural - No basal structure instead bending of rock

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24
Q

What are the characteristics of Toppling falls?

A

Slow beginning but can evolve to extremely rapid
Small or large scale
Require fractured or intensely foliated rock mass

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25
What are the different types of Slide?
Rotational Planar Wedge Compound
26
What is a planar rock slide?
Sliding of a rock mass on a planar rupture surface Large volume Visible where detachment occurred
27
What is rotational rock slide?
Surface rupture that is curved- sliding of rock mass of cylindrical rupture surface (typical in clays)
28
What is Wedge rock slide?
Sliding of a mass of rock on a rupture surface formed by 2 planes with a downslope orientated intersection
29
What are irregular slides?
Sliding of a rock mass on an irregular rupture surface consisting of a number of randomly orientated joints separated by segments of intact rock (rock bridges - which if fail will lead to slide)
30
How will rotational slides occur in soil?
Common in saturated low permeability soils (clays and silts) Slow to rapid velocity dependant on water content
31
How will planar slides occur in soil?
Weak layer or discontinuity inclined at an angle greater then friction angle When PDC deposit of colluvium veneer on string substrate Coarse material (gravel and sand) slope between 30-60 Where seepage accumulates (water on concave slopes)
32
What are rock avalanches?
Highly mobile granular flows Extremely massive and rapid Flow-like motion of fragmented rock from large slide or fall
33
What are the characteristics of rock avalanches?
Large (>1million cubic meters) Rapid rock disintegration as moves downslope Rapid (20-130m/s) Long runouts (10s of km) Bulk is dry Highly mobile
34
What have rock avalanches been known to do due to their high mobility?
Flow down one side of the valley and up the other overtopping the far wall
35
What is a reason for why rock avalanches are so mobile?
Cushion of saturated water material entrained from the flow path and liquefied under the weight of rock debris (high pore pressure)
36
What is an example of a rock avalanche?
From Piz Cengalo Switzerland where it flowed down valley and destroyed town of Bondo
37
What are debris flows?
Very rapid to extremely rapid surging flow of saturated debris in a steep channel Very erosive with growing volume from entraining material and water in flow path
38
What are the characteristics of debris flows?
Occur periodically in the same gullies (widespread in mountains) Very erosive Mobilise large boulders Steep confined channels Deposit material on debris fans
39
What are debris avalanches?
Very rapid to extremely rapid shallow flow of partially or fully saturated debris on a steep slope without confinement in an established channel
40
What are the characteristics of debris avalanches?
Found anywhere in steep slope with soil Triggered by impact of rock fall or soil slide Extremely rapid (>20m/s)
41
In what types of soil will debris avalanches occur?
Residual soil Colluvium Pyroclastic Organic veneer
42
What is mountain slope deformation?
Large-scale gravitational deformation of steep, high mountain slopes manifested by scarps, benches, cracks, trenches and bulges but lack a defined rupture surface Extremely slow (unmeasurable movement rates)
43
What are some of the specific morphological structures that occur from mountain slope deformation?
Doubled ridge Scarps Counterscarps Trenches Bulging (at bottom of landslide)
44
What are the characteristics of mountain slope deformation?
Deep seated (very thick) Ubiquitous on mountain slopes High internal deformation Morpho-structures Size comparable to slope Low displacement rate (mm/yr) Secondary landslides inside deformed mass
45
Using the map on slides (not needed) what would type of landslide would occur in the upper reaches and the midslope?
Upper-reaches likely rockfall Midslope - channels and vegetation (soil) so debris flows
46
What is present in all slopes?
Destabilising stresses
47
What will affect whether driving stresses are capable of triggering slope failure?
The relative magnitude of the resisting stresses
48
How can slope stability be assessed?
By considering the balance of driving and resisting stresses
49
How do shear strength and shear stress affect slope stability?
Strength - prevent failure / sliding Stress - tries to destabilise/ move downslope
50
What is the factors of safety equation?
Forces resisting slide (shear strength)/ forces tending to slide (shear stress)
51
What are the different tolerances for factor of safety?
Stable hillslopes >1.5 Conditionally stable hillslopes =1-1.6 Unstable hillslopes < or equal to 1
52
What are the driving stresses in the factors of safety equation?
Weight component acting parallel to the slope Water pressure
53
What are the resisting forces in the factor of safety equation?
Weight component acting normal to the slope Ground shear strength (friction and cohesion)
54
What will increasing the slope angle do to shear stress/ normal stress ratio?
Increase it
55
What processes can steepen slopes increasing the shear stresses?
Glaciers carving valleys steepening sides and lowering floor when it retreats nothing to support rock at new steepness River erosion in areas of tectonic uplift when an oversteeped slope might have support eroded Human activity might expose a weak layer or take rock past its steepness point
56
What things increase a materials cohesion?
Roots that bind the regolith together Cementation of regolith Electrostatic forces generated by clay
57
What does friction depend on?
The nature of the material
58
What material characteristics affect friction?
Grain size and Grain shape (dictate interlocking)
59
What is the angle of internal friction?
Maximum angle a hillslope material attains before failure
60
What is the angle of friction for sand?
~30*
61
What is more cohesive soil or a block of rock?
A block of rock showing that joints and weaknesses in the rock mass cause failure
62
What is the role of water in slope stability?
Water pressure in the pores modifies shear strength
63
What is the effect of a high pore water pressure (U)?
Creates buoyancy force reducing normal stress (makes slope more unstable)
64
What will adding water do to the angle of friction?
It will reduce the angle of friction
65
What is the crown of a landslide?
upper part which separates stable from unstable and may or may not have failed yet
66
What is the head of a landslide?
The amount of material separated from the crown
67
How does the difficulty of identifying the flanks of a landslide increase the hazard?
Dont know the boundaries so cant estimate volume
68
What are the zones of depletion of accumulation for landslides?
Depletion - area where material lost Accumulation - area of increased material (deposition)
69
What can be done for very slow and slow moving landslides?
Maintenance - million pound tunnels to drain water form slope and reducing speed
70
What can be done for moderate to rapid landslides?
Evacuation anything quicker then this without warning means little to do
71
How can the mobility of mass movement be characterised?
Efficiency - H/L = the ration between height change and the runout distance
72
What will increase the efficiency of mass movements?
Water saturation vs drier masses Larger volume flows Channelised flows
73
What is the relationship between landslide volume and runout distance?
Greater volume will have a greater runout distance
74
What is a potential example of the transition of landslides?
Brienz debris slide - "debris avalanche" - debris flow
75
What is colluvium?
Loose material deposited by a previous debris slide
76
What problem is associated with the Brienz debris flow?
Accumulation near the scarp so still unstable material that could be reactivated by storms or other conditions

Geological natural hazards (21 decks)

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