Jean-Sébastien L'Heureux

Multidisciplinary investigation of a shallow near-shore landslide, Finneidfjord, Norway

The 1996 landslide near Finneidfjord, Norway, involved the displacement of c. 1 x 106 m3 of sediment. Failure initiated offshore and developed in a retrogressive manner, back-stepping 100 – 150 m inland, and removing a 250 m long section of the main North-South highway. The landslide caused the loss of four human lives, and may have been triggered by human activity (e.g., blasting for road works and/or placement of fill along the shore). Acquisition of an extensive and multi-disciplinary data set, including high-resolution swath bathymetry, 2D/3D seismic data, multiple short (up to 6 m) and two long (12 m and 14 m, respectively) sediment cores, and in situ Free-Fall Piezocone Penetrometer (FF-CPTU) profiles complemented with geotechnical laboratory data, has afforded detailed analysis of both the landslide morphology and stratigraphic controls. Using regional 2D parametric sub-bottom profiler (TOPAS) profiles and a targeted decimetre-resolution 3D Chirp seismic volume (950 m x 140 m), we focus on post-failure material transport/deposition, correlating the failure plane against one of several regionally extensive packets of high–amplitude, composite reflections. In seismic reflection data, the slide plane lies within a distinct, thin (< 0.5 m) stratigraphic bed of lower acoustic impedance than the background sedimentation (indicated by high amplitude reverse-polarity top reflection), which is extensively deformed or completely scoured by motion of the overlying material. Within the body of the landslide, two different flow facies are identified. Inversion of these broadband (1.5 – 13.0 kHz) seismic data has allowed the calculation of remote physical properties (using acoustic quality factor, Q), affording a depth and spatial assessment of the relationship between morphology and grain size. These remote physical properties have been correlated against high-resolution geotechnical data from core logs and FF-CPTU profiles, identifying the slide plane as a weak, laminated, clay-rich bed. This combined geophysical/geotechnical assessment of the landslide morphology and internal architecture supports previous work indicating a complex, multi-stage failure. These combined data illustrate how seafloor stability is strongly influenced by shallow subsurface structure, with the geotechnical properties and lateral continuity of stratified beds acting as a primary control on slide plane depth and failure probability.

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

Sorfjord 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.

Case Studies of Offshore Slope Stability

Submarine slides represent a significant threat for offshore infrastructure and for coastal communities. This paper presents four case studies where the stability of underwater slopes was investigated. In two cases, the slopes were near shore, whereas for the third and fourth cases, the slopes were in deepwater far from shore and at locations of large oil and gas exploitation projects. Two cases show the importance of integrating information from the geosciences to establish a realistic model of the slope and seabed. Submarine slides can be due to natural on- going processes or to human activities, or can be triggered by external processes such as earthquakes. Assessing a slope instability hazard requires information about the frequency of the sliding events and triggers, the soil conditions and the morphology of the area. Other than man-made activity causing underwater slides close to shore, earthquakes are probably the most common trigger of offshore slope instability. Three scenarios of earthquake-induced slope failure should be analyzed: 1) Failure occurring during the earthquake, where excess pore pressures generated by the cyclic stresses degrade the shear strength; 2) Post-earthquake failure due to increase in excess pore pressure caused by seepage from deeper layers; and 3) Post-earthquake failure due to creep and reduction of the shear strength. Soils with significant strain- softening are most susceptible to failure during earthquake shaking. The paper out- lines a procedure for calculating the annual probability of earthquake-induced failure for a submarine clay slope. The main challenges for improved hazard and risk assess- ment are not only related to the probabilistic or risk analysis aspects of the offshore geohazards, but also to reducing the uncertainties in the parameters in the analysis.

Geophysical Investigations of Quick-clay Slide Prone Areas

Quick clay is known to be a hazard in formally-glaciated coastal areas in, e.g., Norway, Sweden and Canada. The properties of quick clays are reviewed in order to find a suitable, integrated and multi-disciplinary approach to accurately identify the occurrence of quick clay and map their extent both vertically and laterally. As no single geophysical method yields optimal information, one should combine a variety of methods with geotechnical data for an in-depth quick clay assessment of a given site. Such integrated approach allows moving towards a 2D or pseudo-3D site characterization for quick clays. The integrated approach is applied in practice on two Norwegian quick-clay sites. The collected data and preliminary site characterization will illustrate the high diversity of quick-clay grounds as well as the complexity related to an integrated approach.