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GEO 4180The Åknes rock slopeDr. Vidar KveldsvikNGI
Content• Background on large rock slides• Triggers• Stability analysis• Risk mitigation (risk reduction)• The Åknes rock slope
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Background
Landslides due to massive rock slope failures represent a major geological hazard in many parts of the world.
Volumes 105 to 1010 m3.
Low-frequency /high magnitude events which occur mainly in areas of high relief
Rock avalanching is extremely rapid (>30m/s) flow-likemovements of large volumes (>106 m3) which are beeingincreasingly fragmented during the runout process.
High mobility and long runout make rock avalanches some ofthe most destructive geological processes on the Earth’ssurface.
Background
10
100
1000
10000
0.1 1 10 100 1000 10000 100000
Volume (106 m3)
Swas
h he
ight
(m)
vmax = (2ghsw)0.5
vmax = maximum velocity g = gravitational acceleration hsw = swash height
vmax = 20 m s-1
vmax = 130 m s-1
3
Background
• Failure may often be preceded by observableslope deformation:– Growth and widening of
tension cracks– Increased rock fall activity– Increased disaggregation of
the intial failure mass on theslope
Background
Secondary processes from massive rock slope failuresinclude:
Landslide dams
Landslide generated waves (tsunamis)
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Usoi dam and LakeLake Sarez
The volume of the landslide was 2.2 km3
Scarp of the landslide
BackgroundThe 600m high Usoi Dam on Lake Sarez in Tajikistan
Artist’s depiction of tsunami at Geiranger
Background
5
Alaska 1958
540 moh.
Background
Background
From Blikra et al. (2006)
Historical: ~1700 - present
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Background
Åknes
Large rockslide
Background
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Loen, 1905
Tafjord, 1934
Western Norway 1900’s:
3 rockslides causing tsunami
Caused 175 fatalities
Loen, 1936
Background
Triggers
• Earthquake (>M6?)• Water pressure and its fluctuations• Erosion• Permafrost thaw• Weathering and breakdown of fracture asperities• Progressive failure in intact rock bridges
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Stability analysis
Stability analysis
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Stability analysis
Stability analysis
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Stability analysis
• Material properties– Intact rock properties (σc, E, ν, σt)
– Often more important: shear strength parameters of fractures
⎥⎦
⎤⎢⎣
⎡+⎟⎟⎠
⎞⎜⎜⎝
⎛×= r
nn
JCSJRC ϕσ
στ'
logtan' 10
Where
τ: peak shear strength
σ’n: effective normal stress
JRC: joint roughness coefficient
JCS: joint wall compressive strength
φr: residual friction angle.
φr = φb (basic friction angle) for fresh unweatheredjoints
r: Schmidt hammer rebound on a joint surface
R: Schmidt hammer rebound on intact rock
⎟⎠⎞
⎜⎝⎛ ×+−=
Rr
b 2020r ϕϕand
Stability analysis
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⎥⎦
⎤⎢⎣
⎡+⎟⎟
⎠
⎞⎜⎜⎝
⎛××=×+= r
nnini
JCSJRCc ϕσ
σϕστ'
logtan'tan' 10
φa
Stability analysis
Stability analysis
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Stability analysis
Stability analysis
13
⎥⎦
⎤⎢⎣
⎡+⎟⎟⎠
⎞⎜⎜⎝
⎛×= r
nn
JCSJRC ϕσ
στ'
logtan' 10 ⎟⎠⎞
⎜⎝⎛ ×+−=
Rr
b 2020r ϕϕand
Stability analysis
Risk reduction: What is an Early Warning System?
• In common usage, an EWS is a component of a risk management system for detecting and dealing with an anticipated natural or man-made hazard.
• Early warning systems are not restricted to natural hazards and disasters. They are applicable to any activity or situation that may create a problem that must be dealt with.
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Risk reduction: Elements of an EWS
• Knowledge of and means of forecasting the danger faced• Information from technical monitoring and visual
observations• A response plan• Dissemination of meaningful warnings to population at risk• Public awareness and preparedness to respond to the
warning.
An early warning system will normally have a minimum of 5 components:
Risk reduction: Available Technology for EWSs
• Sensors and sensing technology• Communication technology• Data collection systems as well as data
processing, reporting and analyzing software• Forecasting methods and modelling tools
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Risk reduction: Principal activities in an EWS
Risk reduction: The key to a successful EWS
• The key to a successful EWS is to be able to identify and measure the relevant precursors to the event.
• For example, typical precursors for an impending landslide event are:– Intense rainfall– Earthquakes and ground vibrations– High rate of slope movement– Rapid increases in pore water pressure– Erosion at the toe of the slope
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Risk reduction: Early Warning Systems
Risk reduction: Early Warning Systems
Ground water level From Blikra (2008)
Laser
From Blikra (2008)
Extensometers
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Risk reduction: Early Warning Systems
Risk reduction: Other measures than EWS, evacuation, etc.?
Protection barriers may be applicable for ”small large” rock slides.
Drainage may stabilize even very large unstablerock slopes
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The Åknes rock slope
Artist’s depiction of tsunami at Geiranger
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Flooding in Hellesylt with run-up 25 – 35m
2D experimental setup
• Surface elevation measured at gauges 1-3• Velocity field measured
PIV PIV
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Ongoing 3D laboratory experiments• Coast and Harbour
Research Laboratory at SINTEF, Trondheim
• Instrumentation based on numerical simulations and 2D experiments
Results from initial numerical modelling – assumed worst case scenario
Run-up
Surface elevation
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Overview of the Åknes rock slope
~20m
~1m
8-15cm/year
2-3cm/year
~600m
~180masl
Dip≈35deg~650,000m2
Assumed max. slide area
N
Monitoring systems• At the slope surface:
• Permanent GPS network with 8 antennas
• Total station with 30 prisms
• Ground-based radar with 8 reflectors (radar located accross the fjord)
• Five surface rod extensometers
• Surface crackmeters
• Surface tiltmeters
• Two single lasers measuring distances across the upper tension crack
• 8 geophones: micro-seismic network
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Monitoring systems
• Climate station:
• Temperature• Precipitation• Two snow-depth sensors• Wind speed• Ground temperature
Monitoring systems
• In boreholes:
• Two 50 m long DMS systems with 50 inclinometers• One 100m long DMS (not installed yet)• Pietzometers, conductivity and temperature sensors in 3
boreholes
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Monitoring: overview
Early warning centre: now in operation 24hrs a day
Alarm tresholdscriteria based on:• Total displacements
• Velocity inn defined time periods
• Acceleration
• Treshold values need to be defined and updated
Sirens in all the villages located in the tsunami hazardzone
Phone messages
Evacuation procedures and routes
The police responsible for the evacuation
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Displacements across the upper tension fracture: 1993-2007
Displacements across the upper tension fracture: 2004-2007
25
Displacements per year - horizontal componentshown on a possible block model
2004 -2006: GPS, Tot. stat., Extensometers
1961-1983: Photogrammetry
1983-2004: Photogrammetry
More displacements in the NW part from 1961 to 1983 than later
LISA Radar
26
LISA Radar results
Displacements and block boundaries – Displacements across the upper tension fracture (4)
Ground water level From Blikra (2008)
Laser
From Blikra (2008)
Extensometers
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Displacements in the upper borehole
1st interval ofmeasurements
2nd interval ofmeasurements
Geological and geotechnicalinvestigations
Geophysicalsurveys: resisitivity, georadar and seismic
2D Resistivity
28
2D resistivity: Interpretation of depth of unstable rock mass
Boreholes
• Core logging• Samples for lab testing• Optic televiewer and borehole
logging• Instrumentation
29
Geological model – Core logging (1)
Crushed and core loss
Results of field mapping: fracture orientationFol. frac. downslope the upper tensionfracture: mean dip 32deg.
Fractures non-parallel with the foliation
30
Geological model
Block model: DDA analysis on possible blockboundaries based on all three displacement data sets
Profile for stabilityanalyses
The area of Block 11 is 201,000m2 and a major part moved insignificantly from 2004 to 2007
Borehole location
Big question: does Block10 move????
31
Stability analyses: static
UDEC model of the whole slope
Stability analyses: static•One major conclusion from the numericalmodelling: Instability at great depth agrees withthe back-calculated limiting friction angle ofthe unstable area
•Instability at 120m later indicated by borholemeasurements
32
Stability analyses: dynamicUDEC model
Earthquakes with returnperiods of 100 and 1000 years
The analyses indicate that an earthquake with a return period of 1000 years is likely to trigger a slide to great depth at the present ground water conditions and that the slope will remain stable if it is drained.
An earthquake with a return period of 100 years is not likely to trigger a slide at the present ground water conditions.
Drainage
33
References??????