Geohazard Assessment related to Submarine Instabilities in ...
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Geohazard Assessment related to Submarine Instabilities in Bjørnafjorden, NorwayC.S. Forsberg, B. Carlton, M. Vanneste, C.F. Forsberg, S. Knudsen, F. Løvholt, T. Kvalstad, G. Sauvin – NGI H. Kjennbakken*, M.A. Mazhar**, S. Degago – SVVH. Haflidason – UiB
Currently:* Norconsult** COWI
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Outline
IntroductionAvailable data1D analyses2D analysesLandslide run-out analysesAge-datingSummary
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Intr
oduc
tion Framework: Ferry-free Coastal Highway (E39)
© NPRA
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Bjørnafjorden – Side-anchored floating bridgeIn
trod
uctio
nAnchoring of four pontoons with eight anchors each 32 anchorsRedundancy: two anchors in one cluster out of play
Images © NPRA
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Bjørnafjorden – Multi-span suspension bridgeIn
trod
uctio
n2 towers supported by floating platforms anchored with 16 tethersRedundancy: Two tethers out of play
Images © Baezeni/NPRA
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Implications for design (Risk)In
trod
uctio
nMass movements (and their consequences) are the main geohazardFairly unique situation and some complications:
Many possible impact pointsSource areas can be found all over, from all anglesRelocation and avoidance not realistic
Image © Baezeni/NPRAImage © NPRA
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Key Data AvailableDa
ta
Geophysical data:• Multibeam echosounder seafloor topography
• Sub-bottom profile data sub-seafloor information
Geotechnical data:• 5 locations with CPTU
and/or sampling data
1 age dating location
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Bathymetry & slope angleDa
ta
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Bathymetry & sediment thicknessDa
ta
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Bathymetry & mapped landslide featuresDa
ta
UtløpsområdeKildeområde
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1D analyses – Screening Tool1D
ana
lyse
s
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2D Plaxis Analysis, incl. Retrogression2D
Ana
lyse
s
Bedrock/stiff material
Marine clay
Water
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2D Plaxis Analyses2D
Ana
lyse
s
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Landslide Back-analysisLa
nd sl
ide
runo
ut
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Back-analysisLa
nd sl
ide
runo
utNo rheological parametersSome guestimates:
Evacuation ~ 20,000 m2
Deposition ~ 45,000 m2
Run-out ~ 300-350 mHeadwall ~ 5 mThickness < 10 mVolume (displaced) ~ 100,000 m3
Bedrock/stiff materialMarine clay
Water
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Land
slid
e ru
nout Landslide dynamics – Back-analysis
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Landslide dynamics – Quasi-3DLa
nd sl
ide
runo
ut
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Landslide dynamics – forward modellingLa
nd sl
ide
runo
ut
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Age dating resultsCore descriptionsEvents 1 & 2: debris flow or mud flow deposits
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SummaryAs part of the geohazard assessment we conducted:1. 1D static and pseudo-static and fully dynamic slope stability analyses2. 2D slope stability analyses for 40 profiles 3. Back-analysis of a palaeolandslide;4. Landslide dynamic simulations calibrated using results from the back
analysisResults show that the soils in many locations are unstable and could cause debris flows with long runout distancesGeophysical data support this interpretationNeed more information about the timing and triggering mechanisms of the landslides
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Acknowledgments
The audience for kindly listening
Dof SubSea for geophysical data acquisition
NPRA (SVV) for permission to publish this work
NGI colleagues: Dieter Issler, Jihwan Kim, Noel Boylan, Amir Kaynia
GMT – Generic Mapping Tools
NGI’s R&D funds for developing in-house routines (screening tool, landslide dynamics, …)