Personne : Demers, Denis
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Université Laval. Département de géologie et de génie géologique
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- PublicationAccès libreHydrogeology of a complex Champlain Sea deposit (Quebec, Canada) : implications for slope stability(National Research Council of Canada, 2020-11-25) Young, Nathan Lee; Locat, Pascal; Delottier, Hugo; Cloutier, Catherine; Locat, Ariane; Lemieux, Jean-Michel; Fortier, Philippe; Locat, Jacques; Leroueil, Serge; Demers, Denis; Germain, AlexandraThe thick sequences of marine clayey deposits which blanket the St. Lawrence Lowlands in south-eastern Canada are highly susceptible to landslides. With 89% of the population of the Province of Quebec living in this region, improving our understanding of the mechanisms causing landslides in these sediments is a matter of public security. To accomplish this goal, instruments were deployed at a field site in Sainte-Anne-de-la-Pérade, Quebec, Canada to monitor atmospheric, soil, and groundwater conditions. Field and laboratory measurements of soil geotechnical and hydraulic properties were also performed. Results indicate that the groundwater and pore pressure dynamics at the site cannot be explained using simplified site conceptual models. Further analysis indicates that groundwater dynamics and pore pressures in the massive clay deposits on-site are determined by (i) the highly-heterogeneous nature of the local geological materials (ii) the contrasting hydraulic and geotechnical properties of these materials, (iii) the presence of two unconfined aquifers at the site, one surficial and one at depth, and (iv), the presence of the Sainte-Anne River. These results were used to create a new conceptual model which illustrates the complex groundwater flow system present on site, and shows the importance of including hydrogeologic context in slope stability analysis.
- PublicationAccès libreGeophysical and geotechnical characterization of a sensitive clay deposit in Brownsburg, Québec(Springer, 2017-05-24) Bélanger, Karine; Locat, Ariane; Fortier, Richard; Demers, DenisThe 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 m 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 nonsensitive 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
- PublicationAccès libreThe Saint-Jude landslide of May 10th, 2010, Quebec, Canada : investigation and characterisation of the landslide and its failure mechanism(Ottawa National Research Council of Canada, 2017-09-26) Locat, Pascal; Locat, Ariane; Robitaille, Denis; Leroueil, Serge; Demers, Denis; Lefebvre, GuyA landslide occurred on May 10, 2010, along the Salvail River, in the municipality of Saint- Jude, Quebec. Debris of the landslide was formed of blocks clay having horst and graben shapes, typical of spreads in sensitive clays. A detailed investigation was carried out by the Ministère des Transports, de la Mobilité durable et de l’électrification des transports du Québec in collaboration with Université Laval, with the objective of characterizing this landslide, determining the causes and learning about its failure mechanism. The soil involved is a firm, grey, sensitive lightly overconsolidated clay with some silt. Data from piezometers installed near the landslide indicated artesian conditions underneath the Salvail River. Cone penetration tests allowed to location of two failure surface levels. The first one starting 2.5 m below the initial river bed and extending horizontally up to 125 m and a second one 10 m higher reaching the backscarp. Investigation of the debris with onsite measurements, light detector and ranging surveys, cone penetration tests, and boreholes allowed a detailed geotechnical and morphological analysis of the debris and reconstitution of the dislocation mechanism of this complex spread.
- PublicationAccès libreDevelopment of a long term monitoring network of sensitive clay slopes in Québec in the context of climate change(springer international publishing, 2017-05-24) Locat, Pascal; Cloutier, Catherine; Locat, Ariane; Fortin, Alexis; Lemieux, Jean-Michel; Locat, Jacques; Leroueil, Serge; Demers, Denis; Bilodeau, ChantalThe Government of Québec 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 Québec 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.
- PublicationAccès libreField performance of four vibrating-wire piezometer installation methods(National Research Council of Canada, 2022-01-12) Young, Nathan Lee; Locat, Pascal; Locat, Ariane; Lemieux, Jean-Michel; Locat, Jacques; Mony, Laura; Leroueil, Serge; Demers, Denis; Germain, AlexandraVibrating wire piezometers provide a number of advantages over the traditional hydraulic piezometer design. There are many methods and configurations for installing vibrating-wire piezometers, with the most common being: single piezometers in sand packs (SP), multilevel piezometers in sand packs (MLSP), and fully-grouted multilevel piezometers using either bentonite (FGB) or cement-bentonite grout (FGCB). This study assesses the performance of these four different installation methods for vibrating wire piezometers at a field site possessing complex stratigraphy, including glacial and marine sediments. Pore pressure data recorded between December 2017 and July 2019 were analyzed to accomplish this objective. Data indicate that SP, MLSP, and FGB piezometers performed well. This determination is based on the fact that piezometers installed at the same depth with these arrangements recorded similar pressure variations that were coherent with the hydrogeological setting. Of the two fully-grouted installations using cement-bentonite grout, one installation failed completely due to a hydraulic short circuit, caused either by shrinkage of the grout or flow occurring along the wires of the embedded instruments. While the FGB-type piezometers used in this study worked correctly, the lack of standard methods concerning both the construction of fully-grouted piezometers is concerning. Furthermore, the lack of a standard method for mixing cement-bentonite grout likely contributed to the failure of the FGCB installations. Thus, due to the lack of guidance for both construction and grout preparation, the use of a bentonite grout removes a degree of uncertainty when fully-grouted installation techniques are used.
- PublicationAccès libreThe 1994 landslide at Sainte-Monique, Quebec : geotechnical investigation and application of progressive failure analysis(Canadian Science Publishing, 2014-09-11) Locat, Ariane; Leroueil, Serge; Fortin, Alexis; Demers, Denis; Jostad, Hans PetterIn 1994, a landslide occurred in the municipality of Sainte-Monique, Quebec. The debris of the landslide had graben and host shapes, typical of spreads in sensitive clays. The geotechnical investigation shows that the soil involved is a firm to stiff, sensitive, nearly normally consolidated grey silty clay of high plasticity. This soil exhibits a high sensitivity and a high brittleness during shear and is therefore susceptible to progressive failure. Traditional stability analysis cannot explain this landslide. This gives the opportunity to examine the applicability of progressive failure analysis to this spread. Using the finite elements method, it is demonstrated that the initiation and observed extent of the failure surface are explained by a soil having high brittleness during shear and a large-deformation shear strength close to the remoulded shear strength of the soil. The dislocation of the soil mass can also be explained by the active failure occurring in the soil mass above the failure surface during or shortly after failure propagation. It is therefore numerically demonstrated that progressive failure explains the initiation and the extent of the failure surface of this spread.
- PublicationRestriction temporaireBias in hydraulic head measurements from multilevel vibrating-wire piezometers with excessively-permeable backfill(Springer, 2022-04-22) Young, Nathan Lee; Locat, Pascal; Delottier, Hugo; Lemieux, Jean-Michel; Mony, Laura; Demers, DenisAn extensive network of multilevel vibrating-wire piezometers (VWP) was recently created to monitor the spatial and temporal variation of pore pressures (and hydraulic heads) in the landslide-prone post-glacial marine clay slopes in Québec, Canada. Some of the VWP installations used well-sorted crushed stone as well backfill between bentonite plugs, instead of bentonite pellets or cement-bentonite grout, which could create cause a bias in the hydraulic head measurements due to preferential flow within the backfill (i.e., a hydraulic short circuit). This study uses steady-state 2-D radial coordinate numerical models to quantify the extent of this potential bias, and focuses on the relative importance of the following components: hydraulic conductivity of the crushed stone, length of the backfill intervals, length of the bentonite plugs, magnitude and direction of the vertical gradient, and the degree of vertical and horizontal anisotropy within the clay. Simulation results show that the use of crushed stone as backfill results in measurements of hydraulic head that differ from undisturbed conditions by -0.25 cm to +210 cm, regardless of the values assigned to the parameters of interest. In all cases, the cause of this bias is a series of hydraulic short circuits resulting from preferential flow through the crushed stone intervals.