GEO The Åknes rock slope. Content. Dr. Vidar Kveldsvik NGI

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1 GEO 4180 The Åknes rock slope Dr. Vidar Kveldsvik NGI Content Background on large rock slides Triggers Stability analysis Risk mitigation (risk reduction) The Åknes rock slope 1

2 Background Landslides due to massive rock slope failures represent a major geological hazard in many parts of the world. Volumes 10 5 to m 3. Low-frequency /high magnitude events which occur mainly in areas of high relief Rock avalanching is extremely rapid (>30m/s) flow-like movements of large volumes (>10 6 m 3 ) which are beeing increasingly fragmented during the runout process. High mobility and long runout make rock avalanches some of the most destructive geological processes on the Earth s surface. Background Swash height (m) v max = (2gh sw ) 0.5 v max = maximum velocity g = gravitational acceleration h sw = swash height v max = 20 m s -1 v max = 130 m s Volume (10 6 m 3 ) 2

3 Background Failure may often be preceded by observable slope deformation: Growth and widening of tension cracks Increased rock fall activity Increased disaggregation of the intial failure mass on the slope Background Secondary processes from massive rock slope failures include: Landslide dams Landslide generated waves (tsunamis) 3

4 Background The 600m high Usoi Dam on Lake Sarez in Tajikistan Usoi dam and Lake Sarez Scarp of the landslide The volume of the landslide was 2.2 km 3 Background Artist s depiction of tsunami at Geiranger 4

5 Background 540 moh. Alaska 1958 Background Historical: ~ present From Blikra et al. (2006) 5

6 Background Background Åknes Large rockslide 6

Background Loen, 1905 Western Norway 1900 s: 3

$of fracture asperities Progressive failure in$

7 Background Loen, 1905 Western Norway 1900 s: 3 rockslides causing tsunami Tafjord, 1934 Caused 175 fatalities Loen, 1936 Triggers Earthquake (>M6?) Water pressure and its fluctuations Erosion Permafrost thaw Weathering and breakdown of fracture asperities Progressive failure in intact rock bridges 7

8 Stability analysis Stability analysis 8

9 Stability analysis Stability analysis 9

10 Stability analysis Material properties Intact rock properties (σ c, E, ν, σ t) Often more important: shear strength parameters of fractures Stability analysis τ σ n tan JRC log JCS + ϕ σ ' n = ' 10 r and ϕ r = ϕb r R Where τ: peak shear strength σ n : effective normal stress JRC: joint roughness coefficient JCS: joint wall compressive strength φ r : residual friction angle. φ r = φ b (basic friction angle) for fresh unweathered joints r: Schmidt hammer rebound on a joint surface R: Schmidt hammer rebound on intact rock 10

11 Stability analysis τ ci + σ n tanϕi = σ ' n tan JRC log JCS + ϕ σ ' n = ' 10 r φ a Stability analysis 11

12 Stability analysis Stability analysis 12

Stability analysis JCS r τ = σ ' + n tan JRC

reduction: What is an Early Warning System?

risk management system for detecting and

13 Stability analysis JCS r τ = σ ' + n tan JRC log ϕr and ϕ 10 r = ϕb σ ' n R 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. 13

14 Risk reduction: Elements of an EWS An early warning system will normally have a minimum of 5 components: 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. 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 14

15 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 15

16 Risk reduction: Early Warning Systems Risk reduction: Early Warning Systems Ground water level From Blikra (2008) Extensometers Laser From Blikra (2008) 16

17 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 unstable rock slopes 17

18 The Åknes rock slope Artist s depiction of tsunami at Geiranger 18

19 Flooding in Hellesylt with run-up 25 35m 2D experimental setup PIV PIV Surface elevation measured at gauges 1-3 Velocity field measured 19

20 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 Surface elevation Run-up 20

surface: Permanent GPS network with 8 antennas Total station with 30 prisms Ground-based radar with 8 reflectors (radar

21 Overview of the Åknes rock slope Assumed max. slide area ~650,000m 2 Dip 35deg ~600m ~20m 8-15cm/year ~1m 2-3cm/year ~180masl 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 21

22 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 22

Monitoring: overview Early warning centre: now

Treshold values need to be defined and updated

tsunami hazard zone Phone messages Evacuation

23 Monitoring: overview Early warning centre: now in operation 24hrs a day Alarm tresholds criteria 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 hazard zone Phone messages Evacuation procedures and routes The police responsible for the evacuation 23

$Displacements across the upper tension fracture: 1993-2007$

24 Displacements across the upper tension fracture: Displacements across the upper tension fracture:

25 Displacements per year - horizontal component shown on a possible block model : GPS, Tot. stat., Extensometers : Photogrammetry : Photogrammetry More displacements in the NW part from 1961 to 1983 than later LISA Radar 25

$tension fracture (4) Ground water level From$

26 LISA Radar results Displacements and block boundaries Displacements across the upper tension fracture (4) Ground water level From Blikra (2008) Extensometers Laser From Blikra (2008) 26

27 Displacements in the upper borehole 1 st interval of measurements 2 nd interval of measurements 2D Resistivity Geological and geotechnical investigations Geophysical surveys: resisitivity, georadar and seismic 27

28 2D resistivity: Interpretation of depth of unstable rock mass Boreholes Core logging Samples for lab testing Optic televiewer and borehole logging Instrumentation 28

$loss Results of field mapping: fracture orientation$ $Fol. frac.$

29 Geological model Core logging (1) Crushed and core loss Results of field mapping: fracture orientation Fol. frac. downslope the upper tension fracture: mean dip 32deg. Fractures non-parallel with the foliation 29

30 Geological model Block model: DDA analysis on possible block boundaries based on all three displacement data sets Profile for stability analyses The area of Block 11 is 201,000m 2 and a major part moved insignificantly from 2004 to 2007 Borehole location Big question: does Block 10 move???? 30

31 Stability analyses: static UDEC model of the whole slope Stability analyses: static One major conclusion from the numerical modelling: Instability at great depth agrees with the back-calculated limiting friction angle of theunstablearea Instability at 120m later indicated by borhole measurements 31

Stability analyses: dynamic UDEC model Earthquakes with return periods of

period of 1000 years is likely to trigger a slide to great depth at the

32 Stability analyses: dynamic UDEC model Earthquakes with return periods 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 32

33 References?????? 33

RAPSODI Risk Assessment and design of Prevention Structures for enhanced tsunami DIsaster resilience

RAPSODI Risk Assessment and design of Prevention Structures for enhanced tsunami DIsaster resilience Possible NGI contributions related to tsunami modelling activities Finn Løvholt and Carl B. Harbitz