Identification of Weak Layers and Their Role for the Stability of Slopes at Finneidfjord, Northern Norway

The 1996 Finneidfjord landslide, which took four human lives in northern Norway, initiated along a weak layer in the fjord-marine sediments before developing retrogressively across the shoreline. The integration of results from sediment cores, free-fall cone penetrometer tests and high-resolution 3D seismic data indicates that the slide-prone layer is a regional bed likely sourced from clay-slide activity in the catchment of the fjord. The sediments in this regional layer are softer and more sensitive than the typical bioturbated, fjord-marine deposits, which explains their role in slope instability. In addition, biogenic gas in the stratified event bed may further affect its geotechnical properties. Similar, fine-grained, stratified beds with comparable origin and properties occur in other Norwegian fjords. They are presum- ably also present along coastlines of other previously glaciated margins, where they could contribute to mass movements

Finneidfjord: a Field Laboratory for Integrated Submarine Slope Stability Assessments and Characterization of Landslide-Prone Sediments: A Review

Sørfjord outside the village of Finneidfjord has a history of landsliding throughout the Holocene. The 1996 landslide – the focus of this study – has many characteristics typical of submarine landslides (well-developed slip plane, outrunner blocks, peripheral thrusting and lateral spreading). Due to its sheltered and accessible location, Finneidfjord has become a natural laboratory for testing high-resolution and multidisciplinary techniques to improve our understanding of landslide development.This study integrates multiple sediment cores, swath-bathymetry surveys, single- and multi-channel 2D seismic data (Topas, boomer, sparker, airgun), very-high-resolution 3D chirp seismics, ocean-bottom seismometer as well as free fall and traditional cone penetration testing (CPTU). The cores have been subjected to both geological and geotechnical laboratory analyses. Of particular interest is the correlation of the regional slip plane as a high-amplitude package of reflections in the geophysical data with the results of the sediment and in situ measurements.Comparison of 3D traces with synthetic seismograms based on multi-sensor core logs show that the most prominent slip plane lies within a thin clay unit sandwiching a sand seam. The slip plane is difficult to identify from CPTU data alone. The top part of this composite unit has in places been eroded under the 1996 mass-transport deposit (MTD). This composite unit’s formation is associated with turbidite deposits from terrestrial quick clay landslides and possibly river floods in the catchment of the fjord. While the MTD is extensively deformed, different flow facies are identified within the landslide body revealing a complex, multi-phase failure. The seismic data were also used to infer physical properties (mean grain size, gas saturation from P-wave attenuation). Interestingly, shallow gas adjacent to the landslide appears not to have played a role in the landslide development.Fjordbed stability is strongly influenced by shallow subsurface structure, with geotechnical properties and lateral continuity of stratified beds acting as primary controls on slide plane depth and failure mechanisms. This study can well form a template for near-shore areas prone to landsliding. Currently, a long-term pore pressure monitoring programme is in progress, after the installation of several piezometers close to the depths of the slip plane close to the shoreline in September 2012

Integration of very-high-resolution seismic and CPTU data from a coastal area affected by shallow landsliding: The finneidfjord natural laboratory

In 1996, a retrogressive landslide claimed four lives near the village of Finneidfjord, northern Norway. Since then, the area has been developed into a prime natural field laboratory for investigating submarine slope instabilities. Research activities adopt an integrated approach inwhichwe aim to characterize the slip planes involved in the landsliding and understanding the sedimentary processes contributing to failure, including human activity (e.g., road works), the presence of quick clay deposits in the near-shore area as well as shallow gas accumulations in the fjord. In this paper, we focus on the spatial variability of soil conditions in general and the slip plane in particular as evidenced from different vintages of seismic data (very-high-resolution 3D chirp data and multi-channel sparker and boomer data) tied with soil samples and 1D in situ geotechnical logs (CPTU) across the landslide complex and shallow gas accumulation zone. The slip planes correspond to composite, thin event beds characterized by subtle variations in lithology. These event beds demonstrate a laterally variable character across the landslide complex, both in the seismic data and particularly in the CPTU response. Through impedance inversion and modelling and using the geological and geotechnical data as calibration, we have developed methods to derive geotechnical properties (e.g., partial gas saturation, lithology, density) from the seismic data. The integrated approach can be used to investigate similar landslides in coastal areas

Assessing offshore geohazards: A multi-disciplinary research initiative to understand shallow landslides and their dynamics in coastal and deepwater environments, Norway

In this manuscript, we present the first results of integrated slope stability studies to investigate smaller-scale mass movement processes in different physio- graphic settings of Norway. These include coastal areas (Sørfjord, Finneidfjord), and pristine open ocean settings in intermediate (Vestera?len) and deep waters (Lofoten) on the Norwegian margin. Triggers, pre-conditioning factors and sedimentary processes associated with these landslides are currently not well constrained.The landslides occur either in clusters or isolated, and have variable geomorphologic expressions and run-out. These smaller landslides appear to be comparatively recent phenomena. However, failures likely happen repeatedly and recurrence may affect coastal communities or jeopardize offshore installations. New data indicate that the landslides developed within thin, gently-dipping sediment units that served as slip planes. Some soil samples from these units may show strain-softening behaviour, higher plasticity and higher sensitivity compared to other units. The slide-prone layers in Sørfjord can furthermore be related to specific depositional processes