Tore J. Kvalstad

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 presumably also present along coastlines of other previously glaciated margins, where they could contribute to mass movements.

Origin of shallow submarine mass movements and their glide planes—Sedimentological and geotechnical analyses from the continental slope off northern Norway

Submarine landslides are often characterized by a basal surface of rupture parallel to the stratigraphy, in which downslope movement is initiated. However, little is known about the sedimentology and physical properties of the sediments within these surfaces. In this study, we present a multiproxy analysis of the sediments collected from a giant piston core penetrating a shallow submarine mass transport deposit, in combination with high-resolution seismoacoustic data to identify and characterize the basal glide plane and the weaker sediments in which movement was initiated. The initial phase of instability consists of a single fracture that formed due to the downslope movement of a mostly intact slab of sediments. The 16 m long core, comprising mostly undisturbed massive and laminated ice-rafted debris-rich clay penetrated this slab. The base of the slab is characterized by a high-amplitude semicontinuous reflection visible on the subbottom profiler data at about 12.5 m depth, interpreted to originate from the glide plane on top of a plumite deposit. This plumite has dilative behavior with pore pressure decrease with increasing shear strain and high undrained shear strength. Movement probably started within contouritic sediments immediately above the glide plane, characterized by higher sensitivities and higher water contents. The occurrence of the mass movements documented in this study are likely affected by the presence of a submarine landslide complex directly downslope. The slide scar of this landslide complex promoted retrogressive movement farther upslope and progressive spreading of strain softening along the slide base and in the slide mass. Numerical models (infinite slope, BING, and retrogressive slope models) illustrate that the present-day continental slope is essentially stable and allow reconstruction of the failure processes when initiated by an external trigger.

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 physiographic settings of Norway. These include coastal areas (Sorfjord, Finneidfjord), and pristine open ocean settings in intermediate (Vesteralen) and deep waters (Lofoten) on the Norwegian margin. Triggers, pre-conditioning factors and sedimentary processes associated with these landslides are currently not well constrained.

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.

Submarine Landslides and Their Consequences: What Do We Know, What Can We Do?

The threats posed by submarine landslides to human civilization are the disappearance of valuable land near the shoreline, the destruction of seafloor installations like cables, pipelines or oil wells, and – most importantly – the devastation of coastal areas by landslide-generated tsunamis. Assessing and mitigating these hazards almost invariably implies the estimation of risk in situations where the probabilities associated with different scenarios are difficult to quantify. However, substantial progress has been made in the understanding of the geological processes and physical mechanisms operating at different stages of a submarine landslide event. This paper briefly reviews the state-of-the-art and points out why knowledge and methods from several disciplines of the physical sciences need to be combined to find solutions to the geotechnical engineering challenges from submarine landslides. A number of references to relevant case studies are also provided.

Effects of gas hydrates dissociation on clays and submarine slope stability

Gas hydrate dissociation is often considered as a precursor or triggering factor for submarine slope failures occurring in relatively deep waters where the bulk of the gas hydrate is found in fine-grained sediments. However, there are actually relatively few studies that focus on the effect of gas hydrate dissociation on the behavior of clays, and very few on what physically happens to clay during and after the dissociation process and how gas hydrate dissociation affects the geotechnical properties of clays. In this paper, we illustrate the effects of hydrate dissociation in clays from laboratory strength tests (direct simple shear) combined with visualization including very-high-resolution 3D imaging (computed tomography), using R11 as the hydrate forming fluid in both laponite and Onsøy clay. The test results reveal that the hydrate dissociation creates bubbles in the surrounding clay matrix and around pipe/well models. In addition, we use CO2-saturated water as the pore fluid in soft clay, and test results show that cracks may develop, allowing gas migration to take place after reducing back pressure in an oedometer cell. Direct simple shear tests show that the undrained shear strength decreases by up to ∼15% due to this process. The test results were then implemented in a 2D finite element model to assess the influence of hydrate dissociation on submarine slope stability. We chose a slope segment west of Svalvard—an area where methane gas bubbles escape from the seabed. The gas bubbling in this area is likely due to climate-controlled hydrate-dissociation (warming of bottom water masses). In the finite-element model, we include the change of methane hydrate stability zone (MHSZ) with time as well as the hydrate-dissociation-induced failure zone, which may be a potential leakage pathway. The numerical study indicates that the hydrate dissociation caused by bottom water warming is unlikely to be the main cause generating a leakage pathway or failure plane. However, the hydrate dissociation causing the reduction in shear strength facilitates a potentially unstable condition. The results imply that the hydrate dissociation may contribute to slope failure as a secondary driver, but are unlikely the main driving force. The aim of this study was to improve our understanding of the physical processes of gas expansion, migration and effect of hydrate dissociation through visualization and a finite element model. In addition, this study discussed methods to detect gas hydrate through a case study, and it was found possible to predict average gas hydrate saturation at sites where the sulfate-methane transition depth is known.

Statistical Analysis of Well Logs Compared with the Geotechnical Data in Storegga Slide Area

Storegga Silide has been studied intensively due to the development of the big Ormen Lange gas field. Both marine and glacial deposits were involved in this slide, and marine layers were the sliding planes. Data from different wireline logs in the Storegga Slide area are analyzed using principal components and cluster statistical methods to characterize the two different kinds of sediments. The results show that the marine layer with high water content, high clay content and low strength can be differentiated from the glacial deposits. Moreover, the analysis from log response are compared with the physical parameters from the geotechnical boreholes, a good correlation exists in the dataset.