International Geologiical Congress - Oslo 2008

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GHZ-11 Rock slope movements and early warning of catastrophic failure and related tsunamis

 

Numerical analysis of the Åknes rock slope based on measured displacements and geotechnical data

 

Vidar Kveldsvik, Norwegian Geotechnical Institute (Norway)
Herbert Einstein, Massachusetts Institute of Techology (United States)
Bjørn Nilsen, Norwegian University of Science and Technology (Norway)
Lars Harald Blikra, Norwegian Geological Survey (Norway)
Lars Harald Blikra, Norwegian Geological Survey (Norway)
 

 

The unstable 650,000m2 Åknes rock slope (Western Norway) poses a hazard as a sudden failure may cause a destructive tsunami in the fjord. In this study the slope was divided into blocks based on displacements measured at the slope surface. Discontinuous deformation analysis (DDA) showed that 3 - 4 blocks in the upper half may be considered as potential sub-areas that may fail catastrophically. The lower half may be divided into 2 - 3 blocks, but more limited data introduce more uncertainty into block definition. The Universal Distinct Element Code was used for 2D stability analyses. By varying fracture geometry, fracture friction and ground water conditions within reasonable limits based on site specific data a number of possible models were compared. The conclusions show that models that were unstable to great depths were in closer agreement with shear strength parameters derived from an earlier study than models that were unstable to smaller depths. The length (depth) of the outcropping fracture, along which shear displacements are shown to occur, plays an important role. A (shallow) slide at 30m, in which displacements have been documented by borehole measurements, will reduce the stability at greater depths. Increased ground water pressure is demonstrated to be less critical for very deep slope instability. The results of the DDA and UDEC modelling will be useful for planning of future investigations, interpretation of the subsequent results, further development of the early warning system and in the tsunami modelling.

 

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