Ariane Locat

Weak Layers: Their Definition and Classification from a Geotechnical Perspective

Weak layers play a major role in the development of many large submarine landslides. A definition of a weak layer is proposed here using a geotechnical perspective: a layer (or band) consisting of sediment or rock that has strength potentially or actually sufficiently lower than that of adjacent units (strength contrast) to provide a potential focus for the development of a surface of rupture. Such a layer or a band can follow stratigraphic horizons, but this is not a requirement. From this it is proposed to define two types: inherited and induced weak layers. In addition, weak layers can develop in strain softening sediments where progressive failure can generate a surface of rupture without the need to invoke the role of excess pore pressures.

Landslides in Sensitive Clays

Landslides in Eastern Canadian sensitive clay deposits are generally located along river or stream valleys. The stratigraphy of these deposits can be simplified as an impervious clay deposit between two pervious boundaries, a weathered fissured crust at the top and a coarse till layer at the bottom. The paper examines the evolution of the ground water regime as the valleys are formed by erosion and discusses the impact of the valleys formation on the requirements for slope stability analysis and determination of shear strength. A large data bank of laboratory test results are then presented and treated to arrive at a determination of the shear strength parameters based only on the preconsolidation pressure.

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.

Linking Geotechnical And Rheological Properties From Failure To Post-Failure: The Pointe-Du-Fort Slide, Saguenay Fjord, Québec

The Pointe-du-Fort submarine mass movement likely took place at the time of the with a run out distance of 1070m and a final flow thickness of 10-15m resting on a slope of 1.4 degrees. The slide involved about 1.95 Mm 3 of clayey sediments from an original slope of 24 degrees. The slide took place in normally consolidated sediments composed of stratified low organic Laflamme Sea clay at the base overlain by progressively more organic rich recent sediments. In situ strength testing and sampling on the tidal flat, morphological analysis and remolded strength of the debris lobe can be related to rheological tests to model the mobility of the debris. For the first time, it has been possible to link the mobility of a submarine slide with the characteristics of the sediments at the time of failure with no need to consider water content increase to explain the observed mobility.

Failure Mechanism of Spreads in Sensitive Clays

Through detailed case studies from the literature it is suggested that a sensitive clay spread is formed by propagation of a failure surface in an intact slope and dislocation of the soil mass in horsts and grabens. It is proposed that the initiation and propagation of the failure surface can be explained by progressive failure mechanism. According to this failure mechanism, failure is initiated near the toe of the slope and the strain-softening stress-strain behaviour of sensitive clays is used to redistribute shear stress along the quasi-horizontal shear zone. The failure propagates inside the deposit reducing the horizontal stress. Active strength of the soil may be mobilised, explaining the dislocation of the soil mass above the shear zone in horsts and grabens. A numerical procedure is used to back calculate the 1994 spread at Sainte-Monique, Quebec, Canada, involving slightly over-consolidated sensitive clay. The initiation and extent of the failure surface observed on site are explained by a soil having large brittleness during shear and large-deformation shear strength close to the remoulded shear strength of the soil.

Geophysical and Geotechnical Characterization of a Sensitive Clay Deposit in Brownsburg, Quebec

The results of a geophysical and geotechnical investigation in a sensitive clay deposit affected by numerous landslide scars in Vases Creek Valley near Brownsburg, Quebec, Canada are presented herein. The main objective of this investigation was to assess the suitability of electrical resistivity measurements in marine clay deposits for mapping out areas prone to flowslides. In addition to a 1.6 km-long electrical resistivity tomography (ERT) carried out perpendicular to the axis of the Vases Creek Valley, six piezocone penetration tests and five boreholes with sampling were also performed along the geophysical survey line. Moreover, standard geotechnical parameters and pore water salinity, as well as electrical resistivity of undisturbed clay samples were measured in the laboratory. According to the correlations found between the remoulded shear strength, the pore water salinity and the electrical resistivity, clay samples with salinity below 6.2 g/l are characterized by remoulded shear strength below 1 kPa and electrical resistivity above 2.8 and 10 Ωm measured respectively in the field and in the laboratory. In such conditions, sensitive clay deposits can be prone to flowslides if all other criteria are also met. Based on this resistivity limit value, only one small area of non-sensitive clay was identified in the interpretative stratigraphic cross-section assessed from the field investigation. Otherwise, the deposit is entirely composed of sensitive clay. The ERT is a promising geophysical tool for the delineation of areas prone to large landslides in eastern Canada.

Development of a Long Term Monitoring Network of Sensitive Clay Slopes in Québec in the Context of Climate Change

The Government of Quebec recently initiated the deployment of a vast groundwater pressures monitoring network in postglacial marine clays to document their variations in time and improve our understanding of the relationship between failure initiation and climate in clay slopes. This project aims at evaluating the impacts of climate change on clay-slope stability and how it can be integrated in landslide risk management to improve public safety. Hydrogeological data will be acquired at sites located throughout the Quebec Province’s post-glacial clay deposits to create a public georeferenced index of typical hydrogeological conditions. The project goes beyond the characterization of groundwater pressures and their variations in clay slopes. Indeed, slope deformation will be measured at several sites. Also, two sites in flat terrain will be instrumented in order to evaluate mechanical properties of clay layers in simple 1-D conditions and groundwater recharge. The unsaturated clay crust in slopes susceptible to superficial landslides will be characterized and instrumented. The current lifetime of the monitoring project has been set to a period of 25 years.

Landslide in Sensitive Clays – From Research to Implementation

Sensitive clays, when provoked by manmade or natural causes, have led to several landslide disasters throughout history. This has been reminded by the recent catastrophic landslides at e.g. St. Jude (2010), Lyngen (2010), Kattmarka (2009), Byneset (2012), Skjeggestad landslide (2015) and Sorum (2016). In the last 40 years there has been approximately 1 or 2 slides per decade with a volume ≥ 500,000 m3 (Thakur et al. 2014). Alone, the collapse of Skjeggestad bridge in Norway in 2015 resulted in damages for over several millions of dollars and was associated to a landslide in sensitive clay. Since landslides in sensitive clays possess huge destructive capabilities, there is a need for accurate assessment and prediction of landslide potential in such materials. However, this is not a straightforward task due to the complexity associated with characterization, identification, mapping and testing of such materials.

Reply to the discussion by Olsen and Stuedlein on “Use of terrestrial laser scanning for the characterization of retrogressive landslides in sensitive clay and rotational landslides in river banks”Appears in Canadian Geotechnical Journal, 47(10): 1164–1168.

In their discussion about aerial laser scanning (ALS) and terrestrial laser scanning (TLS), Olsen and Stuedlein present some issues than we did not address in our paper (Jaboyedoff et al. 2009a). Our study deals with compact landslide bodies and we had to overcome many related problems, such as shadowing, vegetation, site accessibility, and safety issues. In contrast, Olsen and Stuedlein deal with linear, clean features (coastal cliffs), which enable a completely different way of working. Nonetheless, their remarks are relevant and they were not addressed in our paper because it was beyond the scope of our study. Studies on landslide volumes and mechanisms do not in general need a high accuracy, while it is crucial for landslide movement monitoring. In their discussion, Olsen and Stuedlein give some examples of TLS applications in landslide studies in addition to those highlighted in our article. Most of them relate to coastal erosion and rockfalls in cliffs, and only a few relate to landslides. For the sake of completeness of the literature overview, TLS has also been widely used for