Landslide causes and landslide hazard assessments

Question 1

What is an example showing stable slopes by McColl 2022?

Answer 1

3 slopes (Dipping strata in stable formation, vegetated regolith on moderate dip, fractured steep slope rock)

Question 2

How do the slopes form McColl 2022 shift to being moderately unstable?

Answer 2

Dipping strata is undercut expsoing bedding planes/ weak layer
Regolith - deforestation and loss of cohesion/ interception
Rock slope - Freeze thaw causing fracture growth

Question 3

WHat happens when a slope shifts from stable to moderately unstable?

Answer 3

Becomes more sensitive to environmental change

Question 4

When a slope becomes actively unstable what might the triggering factor be like?

Answer 4

The event which causes failure might be much smaller or the same size as what the slope has coped with in the past but is now no longer strong enough to resist

Question 5

What are the actively unstable slope types from McColl 2022?

Answer 5

DIpping strata= Rock slide
Regolith = Earth flow
Steep rock - Rock fall

Question 6

What are preconditioning factors?

Answer 6

inherent characteristics to a slope independent of time

Question 7

What are some examples of preconditioning factors?

Answer 7

Lithology
Presence of weak layers
Fractures and faults
Slope orientation
Contrast in geomechanical properties

Question 8

What is the timescale of preparatory factors?

Answer 8

Long term can bring slope to be marginally unstable

Question 9

What are some examples of preparatory factors?

Answer 9

Fluvial/marine/glacial erosion
Glacial retreat (debuttressing)
Weathering (e.g. frost)
Sheet jointing
Deforestation
Human activity at toe
Intrinsic strength degradation

Question 10

What is the timescale of triggering factors?

Answer 10

Short term
Transient (vary with time)
Overlap with preparatory

Question 11

What are some examples of triggering factors for landslides?

Answer 11

Earthquakes
Snowmelt *
Water pressures*
Intense rainfall *
Loading of slope
Slope toe undercut
(either natural or manmade)
(Intrinsic strength degradation)

* resulting in high pore pressure

Question 12

What might slope stability be like overtime for a stable slope with a factor of safety of 1.5?

Answer 12

Slope is stable but partially reduced by rainfall but reocvers with drainage, Erosion at toe causes big reduction in stability with no recovery as material gone, prolonged wet period and earthquake continue reduction until a rainfall event which might be relatibely small causes failure

Question 13

What is intrisic strength degradation?

Answer 13

Preparatory but sometimes triggering
Chemical decomposition (pore fluids, stress corrosion) or stress- strain processes (e.g. frost, root) cause fracture development or growth

Question 14

What scale will intrinsic strength degradation work?

Answer 14

Microscopic: nucleation and growth of
small fractures,subcritical fracture propagation
Small fractures and imperfections, imbalanced stress concentration, further growth of cracks at the tips

Question 15

What scale are earthquake triggered hazards?

Answer 15

Regional scale hazard – Earthquakes and storms can generate many simultaneous events (i.e., potentially tens of thousands) with spatial variation in landslide type, size, frequency

Question 16

What are earthquake triggereing landslides influenced by?

Answer 16

EQ depth, slip, and mechanism; and local geomorphology, slope materials, geology, hydrology, land use
Landslides might not occur in high frequency next to EQ depends on characteristics

Question 17

What was landslide distribution like for Wenchuan 2009?

Answer 17

Most landslides on hanging wall, within 10 km of fault
rupture
Areas of high landslide density along the coseismic
rupture, but also the steep sides of major river valleys
Concentrated in south – where the fault had a more
gently dipping, thrust geometry
In the north, geometry was steeper, with a strike-slip
component – note the much narrower distribution
Most of the landslides were in fractured rock masses

Question 18

How much rain was experinced before the landslides of central italy sept 2022?

Answer 18

419mm in 9 hours

Question 19

How many landlsides occured as a result of the rainfall in central italy sept 2022?

Answer 19

1687

Question 20

What were landslides like from sept 2022 in central italy?

Answer 20

Shallow and fast moving
Channelised – high efficiency
Some hill slopes entirely denuded
Entrainment of debris along course

Question 21

How does rainfall intensity/ duration affect stability?

Answer 21

Intensity over short duration less affect on stability then prolonged rainfall

Question 22

What is hazard zonation?

Answer 22

Divison of land in homogeneous areas or domains and ranking of these areas according to their degrees of actual or potential hazard caused by mass movement

Question 23

Why are landslide inventories needed?

Answer 23

fundamental step as they should give insight into the location of landslide phenomena, the types, failure mechanisms, causal factors, frequency of occurrence, volumes and the damage that has been caused

Question 24

What is needed to gve a temporal chnage insight to landslides?

Answer 24

Based on multitemporal aerial photo interpretation

Based on a particular trigger event (e.g. mapping immediately after typhoon or earthquake)

Question 25

What is landslide susceptability?

Answer 25

likelihood that a phenomenon (in this case a landslide) will occur in an area based on the local
terrain conditions gievn the environmental factors that might influence occurrence