Jean-Sebastien L’Heureux

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.

The 1978 Quick Clay Landslide at Rissa, Mid Norway: Subaqueous Morphology and Tsunami Simulations

The 1978 landslide at Rissa is the largest to have struck Norway during the last century and is world-famous because it was filmed. Swath bathymetry data and seismic reflection profiles reveal detailed information about the subaqueous morphology of the mass-transport deposits (MTD). Results show that the landslide affected nearly 20% of the lake floor and that it exhibits a complex morphology including distinct lobes, transverse ridges, longitudinal ridges, flow structures and rafted blocks. The rafted blocks found at the outer-rim of the MTD travelled a distance of over 1,000 m in the early stage of the landslide on an almost flat basin floor. Simulation of sediment dynamics and tsunami modelling show that the rafted blocks most likely triggered the flood wave with a recorded maximum surface elevation of 6.8 m.

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.

Towards geophysical and geotechnical integration for quick-clay mapping in Norway

Quick clay is a known hazard in formerly-glaciated coastal areas in e.g., Norway, Sweden and Canada. In this paper, we review the physical properties of quick clays in order to find a suitable, integrated and multi-disciplinary approach to improve our possibilities to accurately identify the occurrence of quick clay and map its extent both vertically and laterally. As no single geophysical method yields optimal information, one should combine a variety of geophysical methods with geotechnical data (in situ measurements using Cone Penetration Testing (CPTU), Seismic CPTU (SCPTU) and Resistivity CPTU (RCPTU); laboratory tests) for an in-depth quick-clay assessment at a given site. In this respect, geophysical data are used to fill the gaps between geotechnical boreholes providing ground-truth. Such an integrated and multi-disciplinary approach brings us closer to 2D or pseudo-3D site characterization for quick clays and as such, an improved assessment of the potential hazard they pose. The integrated approach is applied in practice on two Norwegian quick-clay sites. The first site, Hvittingfoss, was remediated against potential landslides in 2008 whereas the second one, Rissa, was the scene of a major quick-clay landslide in 1978, quick clays being still present over a large area. The collected data and preliminary site characterizations illustrate the high diversity as well as the complexity and clearly emphasize the need for higher resolution, careful imaging and calibration of the data in order to accomplish the assessment of a quickclay hazard.

An In-Situ Free-Fall Piezocone Penetrometer for Characterizing Soft and Sensitive Clays at Finneidfjord (Northern Norway)

The identification and characterization of weak layers, potentially acting as detachment planes, are key elements in submarine landslide research. In this study, the MARUM Free-Fall Piezocone Penetrometer (FF-CPTU) was used to characterize soft and sensitive clays at Finneidfjord, a site with historical landslide events. The FF-CPTU measurements are compared with standard, industry Cone Penetration Testing (pushed CPTU) data in order to verify and validate the penetration rate effect by using an empirical closed-form solution to convert dynamic properties to quasi-static ones. The quasi-static properties and sedimentological/laboratory results across the weak layers show significantly lower values for the CPTU parameters (qt and fs) and undrained shear-strength (su), build-up of excess pore-water pressure (∆u) as well as a normally-consolidated behavior in comparison with the surrounding sediment. These findings allow us to develop a 2D model of the sub-surface in which the weak horizon is mapped, and to demonstrate that the light-weight FF-CPTU instrument is a powerful, versatile device for the geotechnical evaluation of submarine mass movements and their consequences.

The 1930 Landslide in Orkdalsfjorden: Morphology and Failure Mechanism

The 1930 landslide and associated 15-m high tsunami along the shore of Orkdalsfjorden, mid Norway, killed one person and caused major damage to port facilities. The combination of witness testimony with swath bathymetry data, high resolution seismic reflection data and gravity cores show that the failure propagated rapidly (up to 25 m/s) and progressively over a clay layer in a retrogressive manner. The volume of sediment evacuated downslope of the 8–12 m high and 3 km long headwall amounts 18.5 · 106 m3 during this event. The transformation of the failed mass into a sediment gravity flow caused subsequent slope failures on the opposite side of the fjord and the breakage of submarine cables at distances of 3 and 18 km away from the initial landslide.

Landslides in Sensitive Clays – From Geosciences to Risk Management

Landslides in sensitive clays represent a major hazard in the northern countries of the world such as Canada, Finland, Norway, Russia, Sweden and in the US state of Alaska. Examples of catastrophic landslides in sensitive clays that impacted populations are numerous: e.g., Saint-Jean-Vianney in 1971 (Tavenas et al. 1971; Potvin et al. 2001), Rissa in 1979 (Gregersen 1981; L’Heureux et al. 2012), Finneidfjord in 1996 (Longva et al. 2003), Kattmarka in 2009 (Nordal et al. 2009) and St-Jude in 2010 (Locat et al. 2012). In order to respond to the societal demands, the scientific community has to expand its knowledge of landslide mechanisms in sensitive clay to assist authorities with state-of-the-art investigation techniques, hazard assessment methods, risk management schemes, mitigation measures and planning.

Runout of Landslides in Sensitive Clays

An essential part of landslide hazard and risk assessment is the estimate of the runout distance of the landslide masses. There is, however, little guidance available today on the estimation of the landslide runout in sensitive clays and no suitable model exists for predicting runout in sensitive clays. A new empirical model for the runout estimation is presented in this paper. The new model is based on empirical data, and is recommended for use in Norway until further research on analytical models becomes available. The recommended empirical procedure is based on the historical landslides in sensitive clays in Norway. The paper discusses the implementation of the proposed empirical models in a calculation tool called GeoSuite Toolbox as a part an ongoing R&D project GeoFuture II.

Landslides Along Norwegian Fjords: Causes and Hazard Assessment

A collection of 28 well-known historical and near-shore landslide data is analysed in order to better understand the key factors governing mass-wasting processes along Norwegian fjords. The distribution of near-shore slope failures in Norway is linked to the occurrence of thick marine deposits. Compared to those found along deltas and on the steep side-walls of fjords, slope failures in bays and inlets have more often endangered coastal populations and infrastructures due to their extensive retrogression. Factors such as the presence of a weak layer, unfavourable groundwater conditions and stream erosion are found to often contribute to the failure of slopes. However, the dataset shows that more than 60% of historical failures along Norwegian fjords are related to human activity. This enhances the need for a methodology integrating both on- and off-shore data for mapping the hazard and risks associated to such natural processes in Norway.

Controls on the Dimensions of Landslides in Sensitive Clays

The dimensions of landslides are subject to geometric, material and external controls. Geometric controls include: slopes and orientation of the ground surface and bedding planes, valley width and geometry, depth of failure, plus the presence and incision depth of bounding streams. Material controls include undrained and remoulded shear strength, sensitivity, stability number, remoulding index, rapidity, and the mobility of the depleted mass. Dense networks of trees can influence mobility. The position of layers prone to landsliding in the slope is important.