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2016
Conference Paper
Titel
First Results of the Geotechnical In Situ Investigation for Soil Characterisation Along the Upper Slope Off Vesterålen: Northern Norway
Abstract
High-resolution geophysical data reveal the presence of several spatially-isolated, small-scale landslides along the gently dipping (~3-4°) upper slope off Vesterålen, Northern Norway. Dynamic slope stability analysis suggests that seismicity may be largely responsible for the occurrence of these slope failures. The landslides are clustered in two groups, with one group of parallel features with their headwalls in ~500 m water depths. The second group is found in ~800 m water depths. We present first results of geotechnical in situ Cone Penetration Test (CPTU) data and TOPAS sub-bottom profiles collected during two cruises in summer 2013/2014. We obtained a total of six static CPTU profiles penetrating the top 20 m of soil. Three of these were taken across one of the landslide complexes (SL3) from the slide scar down to the depositional area. The other three are reference sites in the adjacent undisturbed areas. The combination of geophysical and geotechnical data allows us to divide the well-stratified glacio-marine slope deposits into three different sediment-mechanical units, and reveals the occurrence of mechanically weaker zones (MWZ). These zones are interbedded by coarser layers with high values of cone tip resistance. The occurrence of sensitive fine-grained material may be responsible for the loss of strength in the deeper portion. One-dimensional pseudo-static stability analysis attests that the Vesterålen slope is stable except for exceptionally large earthquakes, that induce a peak-ground acceleration (PGA) of 0.224 g or larger to the MWZ. The depth levels of the MWZ correspond well with the slide planes of the landslides.