Dominique Turmel

Analysis of a large rock slope failure on the east wall of the LAB Chrysotile Mine in Canada: back analysis, Impact of water infilling and mining activity

Abstract A major mining slope failure occurred in July 2012 on the East wall of the LAB Chrysotile mine in Canada. The major consequence of this failure was the loss of the local highway (Road 112), the main commercial link between the region and the Northeast USA. LiDAR scanning and subsequent analyses were performed and enabled quantifying the geometry and kinematics of the failure area. Using this information, this paper presents the back analysis of the July 2012 failure. The analyses are performed using deterministic and probabilistic limit equilibrium analysis and finite-element shear strength reduction analysis modelling. The impact of pit water infilling on the slope stability is investigated. The impact of the mining activity in 2011 in the lower part of the slope is also investigated through a parametric analysis.

Upstream Migration of Knickpoints: Geotechnical Considerations

Multiple knickpoints have been observed in recent years on the lakefloor of Wabush Lake, Labrador, Canada. Two or more sequential high-resolution geophysical surveys have revealed that these knickpoints tend to migrate updip, i.e. upstream relative to the turbidity currents that overflow them. In order to understand the different processes accountable for the migration of these knickpoints, laboratory studies have been performed to reproduce the sedimentological setting and hydrodynamics of Wabush Lake. Knickpoints are topographic depressions, characterized by a steep slope and a flatter profile downslope and upslope. In the present case they are initiated by morphodynamic interaction between an erodible bed and an overriding turbidity current. A set of 22 experiments have been performed in a facility able to run continuous turbidity currents. Knickpoints created in laboratory are up to 1 cm deep and migrate upstream at variable speed. Experimentation results show that migration of the knickpoints is controlled by two factors: erosion by the turbidity current and a landsliding process in the knickpoint head scarp. Knickpoint migration has also been observed when no turbidity current was present. These results show that this phenomenon is governed by not only hydraulics, as previously described in the literature, but must also be studied from a geotechnical point of view.

Morphodynamic and Slope Instability Observations at Wabush Lake, Labrador

Since 1964, Iron Ore Company of Canada (IOCC) has deposited iron tailings resulting from mining operations into Wabush Lake, Western Labrador. Bathymetric surveys were carried out between 2004 and 2008 as part of the overall environmental IOCC plans to maintain safe disposal strategies of tailings into the lake. In this paper, we present the evolution in distribution and morphological characteristics over the last 4 years of the tailings overlying lacustrine sediments and bedrock. In 2004, a high definition multibeam survey of Wabush Lake was carried out for the first time and revealed lake floor features, including a fine network of channels and a delta foreslope with well-developed knickpoints. Other features such as older subaqueous mass movement scars and an esker were also identified. The delta foreslope channels, in 2004, led into a deeper long channel connected with the deepest part of the lake where turbidite deposition took place. A second bathymet-ric survey was carried out in 2006. Many of the features seen on the 2004 map were already buried by the advancing delta front. Results indicate that the 2004 channel system was almost completely obliterated with the generation of many new others. Development of the knickpoints is also observed where some are still present and new ones are discovered. A third bathymetric survey conducted in 2008 demonstrates a different spatial arrangement of features. Well developed wider channels and new knickpoints are observed on the foreslope delta. The sequential analysis of 2004, 2006 and 2008 surveys proved to be a useful tool to evaluate: (1) the rate of infilling of the lake, where accumulation took place and by which mechanisms (2) the evolution of the foreslope delta gradient and, (3) the evolution of instability areas. Our results indicate that these recently developed techniques are useful tools for monitoring underwater tailings disposal and stability.

Observations of Coastal Landslide-Generated Tsunami Under an Ice Cover: The Case of Lac-des-Seize-Îles, Québec, Canada

On April 15 2014, a landslide occurred on the east shore of a lake in the municipality of Lac-des-Seize-Iles, about 100 km north of Montreal, Quebec, Canada. The coastal landslide, with a length of 94 m and a width of 55 m, had a volume of about 30,500 m3 of sandy to gravelly till. It was likely triggered by a significant amount of water infiltration caused by a heavy rain coincident with an accelerated snow melt. The displaced mass spread on the lake floor and triggered a tsunami that broke and partly lifted the ice cover. Water and ice damaged several seasonal residences and boathouses on the periphery of the lake, in an area extending 450 m north and 500 m south of the landslide debris location. Observations on aerial photographs taken shortly after the event revealed the existence of radial structures on the ice cover in the affected area resulting from the evolution of the tsunami wave. Investigations carried out on east and west shores showed evidences of net marks on tree trunks explained by a maximum inundation height which can be as much as 1.8 m directly in front of the landslide position on the west shore.

The 1908 Disaster of Notre-Dame-de-la-Salette, Québec, Canada: Analysis of the Landslide and Tsunami

The landslide of 1908 in Notre-Dame-de-la-Salette, Quebec, was the deadliest event occurring in sensitive clays of Eastern Canada, causing 33 deaths. Of these, 26 are associated with the tsunami generated impact of water and ice on the opposite bank. A LiDAR survey of the sector, and a geotechnical investigation were carried out respectively in 2009 and 2010 to characterize this landslide. Covering an area of 6.5 ha, the soil mass carried is estimated at nearly 1.2 million m3. The paper describes the event, reports the results of the investigation and discusses the tsunami caused by the debris of the landslide. The tsunami approach includes modeling both the kinematics of the slide and the wave.

Parametric Analysis of the Mobility of Debris from Flow Slides in Sensitive Clays

Sensitive clays are prone to various types of landslides. Among these are flow slides that are able to affect hectares of land. Moreover, debris from these flow slides has a high mobility with run out distance of hundreds of meters, even in relatively flat areas, are quite common. In the context of hazard mapping, mobility of the debris is also an important factors to consider. In this context, a parametric analysis using simplified geometries is undertaken in order to evaluate the run out characteristics of these flows (such has the length of the run out area and the lateral spread of the debris) as a function of the rheological parameters such as the yield stress and viscosity. In order to proceed with the parametric analysis, a newly developed 3D numerical model was used.

Tsunamigenic landslides in Québec

This paper will present some of the known cases histories in Québec where a tsunami wave was or could have been generated by a landslide, either subaerial or subaqueous: the 1908 Notre-Dame-de-la-Salette slide, which is the one that caused the greatest number of casualties (34), the La Grande (1987) slide and the Cap Trinité rockslide (Saguenay Fjord). This analysis will provide an opportunity to illustrate the destructive power of these events. The analysis of those in quick clays will show that the actual tsunami mechanics is very much influenced by the fact that the rupture surface is nearly horizontal. It will be shown that the presence of an ice cover can be a significant aggravating factor associated with the generation of a tsunami wave. Finally, some discussion will be made regarding the approach which could be followed to take this hazard into account as part of a landslide hazard assessment procedure in Québec. RÉSUMÉ Cet article a pour but de présenter des cas historiques connus, où un tsunami a été ou a probablement été généré par un glissement de terrain, subaquatique ou subaérien, dans la province du Québec : le cas de Notre-Dame-de-la-Salette de 1908, causant le plus grand nombre de décès (34) ainsi que le cas de celui du glissement de La Grande en 1987 et de la chute de l’éboulis rocheux du Cap Trinité (fjord du Saguenay). Cette analyse permet d’illustrer la puissance destructrice de ces événements. L’analyse des cas survenus dans les argiles sensibles va démontrer que la mécanique entourant les tsunamis est grandement influencée par le fait que la surface de rupture est presque horizontale. Il sera démontré que, dans deux cas particuliers, la présence d’un couvert de glace peut devenir un facteur aggravant significatif associé à la génération de tsunami. Finalement, une discussion sera faite concernant l’approche qui pourrait être suivie afin de considérer la formation de tsunami dans l’évaluation des zones susceptibles aux glissements de terrain au Québec.

Earthquake Induced Landslides in Lake Éternité, Québec, Canada

Lake Eternite, located between the Upper Saguenay Fjord and the St. Lawrence River has registered many submarine slides caused by at least one earthquake. Landslides are mostly rooted in the gyttja (Holocene sediments). Mapping of landslides revealed a total of 128 scars over an area of only 3.2 km2. A larger proportion of the landslide scars are located on the SE and NW facing slope which may support an epicentre location for the strongest earthquake (1663?), to the NW or NE of the lake. The preliminary numerical analysis of the site effects caused by topography on local preferred seismic amplification is not conclusive enough to support the observations made for landslides. Associating landslides to specific earthquakes will only be possible with further investigations, including coring of various features including rupture surfaces. The study also revealed interesting slide morphologies developed in homogeneous sediments, providing excellent examples for future modelling of similar events.

Knickpoint Migration Induced by Landslide: Evidence from Laboratory to Field Observations in Wabush Lake

Wabush Lake is characterized by a nearly constant sediment input, resulting from the deposition of mine tailings. Five bathymetric surveys were conducted in this lake, in order to understand the sedimentation pattern over more than 12 years. Of the morphologies studied, submarine channel and knickpoints were traced out and documented. A physical model of Wabush Lake was also constructed in order to understand specific morphologies, such as the knickpoints.

Kinematic analysis of the 7250 cal BP Betsiamites landslide

On the North shore of the St. Lawrence Estuary (Québec, Canada), near the Betsiamites river delta, a large subaerial-submarine landslide complex was mapped using multibeam bathymetry, LiDAR and photogrammetric data. Previous analysis of this landslide complex revealed that at least four different landsliding events occurred to form the present morphology, in which over 2 km3 of material have been mobilized. Of particular interest here is the Betsiamites 7.25 kyr cal BP landslide that is entirely submarine, and mobilized about 1.3 km3 of material deposited over an area of 54 km2. This is, to date, the largest landslide scar identified on the St. Lawrence estuary seafloor. Previous studies showed that this landslide may have been triggered by a strong earthquake that liquefied a sandy layer interbedded between clayey layers. Submarine landslides that mobilized that kind of volume within a short time may have triggered tsunamis. In order to be able to evaluate the tsunami potential of the 7250 cal BP landslide, a kinematic analysis needs to be done. This kinematic analysis is presented in this paper. The pre-failure and postfailure topographies are available via a morphologic analysis of the multibeam data. The morphology of the deposit is well described by the present morphology and an analysis of seismic profiles. Relations between geotechnical properties (from samples taken in cores) and empirical rheological relationships are used to determine the rheology of the materials. The BING model, that simulates the propagation of the mass using bilinear or Herschel-Bulkley rheology, and that takes into account the presence of water, is used in order to simulate the post-failure behavior of the Betsiamites submarine landslide. able to model the tsunami wave generated by this landslide, a kinematic analysis must be undertaken. In this paper we describe the general morphology of this landslide and present pertaining geotechnical data. The kinematic analysis is then done using a 2D model.