Blikra - vajont 2013

DMS

DMS
Lars Harald Blikra & Lene Kristensen, Åknes/Tafjord Monitoring Centre
Mario Lovisolo, Centro Servizi di Geoingegneria (CSG), Italy
Åknes/Tafjord Beredskap IKS

Risk related to large rockslides in Norway

Åknes
Tafjord 1934

Loen 1905 og 1936


Monitored rockslides
in Norway
Handled by two monitoring
centres funded by
government/NVE,
municipalities and counties:
• Åknes/Tafjord Beredskap
(technical and geological)
• Nordnorsk fjellovervåking
(technical)


CONTENT
 Requirements
 Subsurface investigations &
instrumentation
 Åknes and Jettan case
 Conclusions

Regulations in Norwegain building code
• The population safety needs to be taken care of by realtime monitoring, warning and evacuation

• The warning time shall not be shorter than 72 hours and
the evacuation time shall be maximum 12 hours
• A system for daily continuous monitoring of the
conditions, e.g. by measurements of displacements
• Independent monitoring instruments and backup
communication and power systems
• Sufficient technical and geo personnel/competence for
safe and reliable monitoring and warning


(1) Investigations
• Surface
• Subsurface

(2) Monitoring systems
• Surface
• Subsurface
• Driving mechanisms







Geophysics were possible
Core drillings
Borehole logging – televiewer and other sensors
Borehole displacements


INSTRUMENTATION
Inclinometer probe – manual readings
?
Long columns with continuous records
Considerations
•
•
•
•
•


High cost of boreholes
Robustness
Characterization vs real-time warning
Resolution
Flexibility

• Robust technology
• Leading technology
• Large economic savings in a
long-term perspective
The DMS borehole instrumentation:
• Documented to have survived
more than 70 cm movements
Conventional methods: c. 3 cm


DMS

Casing


Method: Differential monitoring system (DMS)

1 m modules


Differential monitoring system, DMS – “Snake”
Real-time , continuous measurements
Steel
modules
with sensors

Flexible centralizers
fixed to the borehole
casing
Flexible
junctions


Subsurface LAB:
• 2D Inclinometer (0,4
mm resolution)
• Pietzometer
• Temperature sensor
• Digital compass
• One systems of 150
m length, total of 304
sensors

Example: Åknes

Deep drillings


SEE ALSO POSTER: GB INSAR
MEASUREMENT AT THE ÅKNES
ROCKSLIDE, NORWAY


Deep core drillings – 200 m deep


N

E

Televiewer image
of the sliding plane

Sliding zone


S

W

N

2 years of
displacements
2011and 2012:
Direction towards
SW

Sliding plane
2009 - 2013:
• Total of 9 cm
movements on
2 sensors


Accumulated
total

Differential
Movement
for each
individual
sensor

DMS time series: 49-51 m depth


Time line for sensor
at 49-50 m depth.
• Displacement
• Water level
• Temperature

Internal changes/
dynamics of the
rockslide controls
the hydrological
system and the
deformations


Lyngseidet


Aktive back fracture:
20-25 m deep, 2,5 cm/year


Active movements


Depth/Size/Volume?


Rock cores


Clay in sliding zone: 45 m depth


Borehole instrumentation to measure movements, temperature and water


DMS movements at 45/46
m depth:
1th of May – 3rd of October

E


Time series February – August 2013
May 2013


Volume: Probably 5-6 mill. m3


DMS and Real-time warning
• Each sensor have threshold values for displacements
• Can follow directly the critical sliding zones
• Sms thresholds: 1 mm/day

• Threshold values for different hazard levels
• Trustable and robust system
• Little noise compared to surface measurements


 Norwegian building codes has strong requirements
for monitoring and permanent handling of the EWS
 Subsurface data gives a knowledge platform for
early-warning and key data for geological
models/volumes needed for evaluation of runout/tsunamis (RISK)
 Borehole instrumentation is a vital part of real-time
monitoring
 DMS instrumentation is a robust in-situ
laboratorium:
• Direct displacement at sliding planes with directions
• Water pressure vs movements
• Temperature/permafrost vs movements


Important that the geoscientific
community clearly tells the
government what knowledge is
needed for reliable monitoring and EW


Blikra - vajont 2013

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Blikra - vajont 2013