Slopes Flashcards
Worldwide statistics
(Increaing) several 1000 deaths pa
UK statistics
> 18,000 features known
Many dormant/fossil
What is the slope failure?
Downward movement under the influence of gravity
Movement of soil/rock
Landslides
Movement of snow/ice
snow avalanche
Components of slope failure
Source/head/Scarp
Body/Chute/Toe/Fan
What is slope stability
The balance of =
Driving force ( gravity) on weight and slope angle)
Resistance of slope materials on cohesion , cement and friction
What is the Mohr–Coulomb failure criterion
Used to describe the strenght of soils and rocks
How does failure occur
Critically oriented plane when gravitational shear stress/ plane exceeds the shear strength
Equation
τf = c + σ tanφ
τf
sheer strength
σ
effective normal stress on the failure plane
c
cohesion angle
φ
friction angle
Factors contributing to slope failure
Steep slopes
Loading
Weak materials
Weathering
Trigger
Landslide triggers
Water
Seismic activity
human activity
Landslide earthquake correlation
The magnitude and frequency links
Few landslides =
M<4
M9 triggers
landslides up to 500km+ from epicentre
Distances correlate with…
focal depth
Areas correlate with …
magnitude
What was the Wenchuan earthquake
M7.9
triggered.56,000 landslides
What was the Alaska earthquake
1964
M9.2
large masses of material landed on glaciers
Human contributions to landslide hazards
Modification of slopes:
loading,cutting,drainage
Artificial slopes:tips
Land degration, logging
Triggers
Encroachment
Froude and Petley, 2018
land slide hotspots
Incresae in frequency most prevalent in South East Asia
Characteristics of landslides
Volume - single boulder (km3)
Velocity - cm/y 100s m/s
Horizontal travel distance ( small 2x fall
Types of movement
fall/topple
slide
spread
flow
Type of material
rock
soil
snow/ice
Falls and topple
Detachment ( rotation)
Common( rapid, variable volume)
Disruptive
Slides
The movement on a shear surface
Are rotational or translational
Topple in the UK
Oxwich Bay, 2009 and 2018
Rotational slides in the UK
Holbeck Hall, Scarborough , 1993
Transformed into mud flow
Translational slides in the UK
Burton Bradstock 2012
Borunemouth 2016
Failures in weak material - spreads and eartrh flows
liquefraction
gentle slopes
Failures in weak material example
Alaska 1964
Sensitive quick clays
Marine clays
Na+ binding with silicates
- causing salt leaching
sig loss of strength
30 times less resistant
Sensitive quick clay example
Rissa , Norway 1978
Loess weak material example
Derbyshire 2001
Type of flows
Earthflow
Debris flows
Rock/debris avalanches( coarse)
Earthflow
rapid
slow( creep )
Earthflow example
Oso, March 2014
Washington,USA
43 deaths
Rain
Past activity , logging
Debris flow characteristics
Mixture of water and sediment
-rapid flow
channelised
surging flow
denser- transporting larger particles than stream flows
How does debris flow
Particles are suppored by =
-buoyancy ( stream flow, denser fuild
-grain grain collisions
matric strength
-with intense rainfall or rapid snowmelt on steep slopes
Lahars
Associated with volcanic activity or
re-working volcanic debris (tephra)
What happened in Venezuela Dec 1999
Rainstorms induced thousands of landslides along the Cordelia de la Costa Vargas ( northern Venezuela
Land relief = steep mountains with settlements on the alluvial fans.
200 mm rainfall followed by a major storm
Debris flows and flash floods on alluvual fans inundated costal communities
20,000-20,000 deaths
Rock and debris Avanlanches
Km’s movement in mins
Volume to 10s km3
Long runout 10x fall
Recurrence ( twice per decade )
Sturzstoms
Rock and debris Avanlanches
Km’s movement in mins
Volune to 10s km3
Long runout 10x fall
Recurrence ( twice per decade )
Sturzstoms
Hope Slide, BC
48x106 m3
Jan 1965
4 killed
1100 m relief
70-80 deposists
potential trigger- earthquakes?
Hope Slide, BC
48x106 m3
Jan 1965
4 killed
1100 m relief
What happened in the Philippines feb 2006
Guinsaugon rockslide - debris avalanche
15-20 x 10^6
100-140km/h
1112 killed
Cause of the 2006 event
steep slopes
weak rocks along the fault zone
deep troipical weathering
Orientation of rock structures
triggers of the 2006 event
Rainfall ( 500 mm in 3 days
Earthquake M 2.6
Precursors - river dried up ( cracks in ground )
Snow avalanches
masses of snow that rapidly decend steep slopes
contains soil, vetegation and ice
Two types of snow avalanches
Loose snow avalanches
Slab avalanched
Loose snow avalanches
From a point of coesionless surface layer of dry/wet snow, it is released.
The inital failure is analogous to the rotation of landslide but occurs within a small volume
Loose snow avalanche characteristics
saltation’, entrainment of air and snow, flow, turbulence,
▪ gaining velocity, ‘powder’, mixing with air – buoyant clouds
▪ fanning out from point release
▪ Speeds up to 300 km/h
Snow slab avalanches
Release of a cohesive slab over an extended plane of weakness.
Analogous to planar
failure of rock slopes.
AVALANCHE FACTORS
Temperature
▪ Slope gradient
▪ Aspect
▪ Wind direction
▪ Terrain
▪ Vegetation
▪ Snowpack conditions
Avalanche triggers
Artificial triggering
Localized rapid near-surface loading by people or
explosives
▪ Gradual uniform loading duesnowfall
▪ No-loading situation that changes
snowpack properties,
surface warming (natural
triggering or spontaneous
release).
Snow slope stability equation
S = τf (σ, x, t)/ τ (x, t)
S
snow stability
t
time
x
slope in question
τf
snow strength
σ
normal stress
τ
shear stress
Theoretically…
unstable conditions will occur when the stability index S approaches 1.
Strength and load vary spatially and temporally within the snowpack
–> this means that the application
of this critical stress concept for snow slope failure is not straightforward, and snow stability
depends on scale.
Schweizer et al. (2003) – Reviews of Geophysics
Landslide hazards
Impact
Burial
Structural failure
Secondary /multiple hazards
Secondary /multiple hazards
transformations
displacement of water
failiure of landslide dams
What is the Lodalen Norway event
1905
Ramnefjell
Rockforll of 50,000m3 from 500m
Wave of 40 m above lake
61 deaths
What happned in 1936
Rockfall 1 M m3 from 800m
Wave 74 m high
74 deaths
conclusion = potential for repeat events
Main aspects of avalanche hazard mitigation
Awareness
Avoidance
Event modification ( engineering structural development )
Vulnerability modification
Awareness
Forecasting
Hazard Maps
Landslide prediction
Avoidance
non-structural methods ( land use restrictions temporary evacuation and artifical triggering
Event modification
Structural measures to divert and retard and starting zone structures design to prevent avalanche initiation or forest management
Slope stabalisation
slope grading
support ( concrete , walls,vegetation)
PRotection ( nets shelters)
Drainage
Debris flows mitigation
barriers to divert
Regualr dam and basin checks
Engineered channels
Planning
Vulnerability
Planning ( non-conflicting use)
Forecasting ( monitoring and warning
education)
