Numerical modeling of progressive failure and its application to spreads in sensitive clays

Les étalements sont de grands glissements de terrain survenant dans les argiles sensibles. Les méthodes d’analyse de la stabilité utilisant la méthode à l’équilibre limite donnent des coefficients de sécurité élevés et ne peuvent s’appliquer à ces glissements. Le mécanisme de rupture progressive expliquerait l’initiation et la propagation de la surface de rupture et la dislocation du sol en horst et en grabens, typiques aux étalements. Une méthode numérique est présentée afin d’identifier les paramètres influençant la rupture progressive et de valider son application aux étalements. Cette méthode évalue les contraintes présentes initialement dans le talus et modélise l’initiation et la propagation de la rupture progressive. Il est démontré que les hautes pentes, fortement inclinées ayant un coefficient de pression des terres au repos élevé sont susceptibles à la rupture progressive et que la surface de rupture se propage sur une grande distance. La rupture est favorisée par un sol ayant une grande fragilité lors du cisaillement. Une faible résistance à grande-déformation du sol favorise une grande distance de propagation. Les argiles de l’est du Canada, pouvant présenter une forte sensibilité et une grande fragilité lors du cisaillement, sont donc susceptibles à la rupture progressive et celle-ci explique l’occurrence d’étalements dans ces sols.Spreads are a type of large landslide occurring in sensitive clays. Stability analyses using the limit equilibrium method give factors of safety that are too large and are therefore not applicable to this type of landslide. The progressive failure mechanism is believed to explain the initiation and propagation of the failure surface and the dislocation of the soil mass in horsts and grabens, typical of spreads. A numerical method is presented to identify the parameters influencing progressive failure and to validate the application of this mechanism to spreads. The method evaluates the stresses acting in the slope before failure and models the initiation and propagation of the progressive failure. It is demonstrated that high, steep slopes, with a large earth pressure ratio at rest, are more susceptible to progressive failure and the failure surface propagates over a large distance. Failure is more likely to occur when soil with high brittleness is involved. Soil with low strength at large deformation induces failure propagation over a larger distance. Eastern Canadian clays can exhibit high sensitivity and large brittleness during shear and are susceptible to progressive failure, which explains the occurrence of spreads in these soils

The 1994 landslide at Sainte-Monique, Quebec : geotechnical investigation and application of progressive failure analysis

In 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.Un glissement de terrain est survenu en 1994 dans la municipalité de Sainte-Monique, Québec. Les débris présentaient des formes de horsts et grabens typiques des étalements dans les argiles sensibles. L’investigation géotechnique démontre que le sol impliqué dans ce glissement est une argile silteuse grise normalement consolidée, sensible, de forte plasticité et ayant une consistance ferme à raide. Ce sol présente une sensibilité et une fragilité lors du cisaillement élevées et peut donc être susceptible à la rupture progressive. Les analyses de stabilité traditionnelles n’arrivent pas à expliquer ce glissement. Ceci offre donc l’opportunité d’examiner l’application du concept de la rupture progressive sur cet étalement. À l’aide de programmes d’éléments finis, il est démontré que l’initiation et l’étendue de la surface de rupture observée peuvent être expliquées par un sol ayant une grande fragilité lors du cisaillement et une résistance à grandes déformations près de la résistance du sol remanié. La dislocation de la masse de sol en horsts et grabens est expliquée par la rupture active survenant dans la masse de sol au-dessus de la surface de rupture pendant ou peu après la propagation de la rupture. Il est donc démontré numériquement que la rupture progressive explique l’initiation et l’étendue de la surface de rupture d’un étalement

The Saint-Jude landslide of May 10th, 2010, Quebec, Canada : investigation and characterisation of the landslide and its failure mechanism

A 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.Un glissement est survenu le 10 mai 2010 le long de la rivière Salvail, dans la municipalité de Saint-Jude au Québec. Les débris de ce glissement étaient formés de blocs d’argile ayant la forme de horst et de grabens, typique des étalements dans les argiles sensibles. Le Ministère des Transports, de la Mobilité durable et de l’électrification des transports du Québec et l’Université Laval ont réalisé l’investigation détaillée de ce glissement de terrain, dans le but de le caractériser, d’en déterminer les causes et d’en apprendre d’avantage sur le mécanisme de rupture. Le sol impliqué est une argile sensible grise avec un peu de silt, de consistance ferme, légèrement surconsolidée. Les piézomètres installés à proximité du glissement indiquent des conditions artésiennes sous la rivière Salvail. L’utilisation du piézocône a permis de localiser deux niveaux de surfaces de ruptures. L’un à 2,5 m sous la position initiale de la rivière, s’entendant horizontalement sur 125 m, et l’autre 10 m plus haut, allant jusqu’à l’escarpement arrière. L’investigation des débris par mesures prises sur le terrain, levées de télédétection par laser, piézocônes et forages a permis une analyse géotechnique et morphologique détaillée de ces derniers et la reconstitution du mécanisme de dislocation de ce glissement complexe

Development of a long term monitoring network of sensitive clay slopes in Québec in the context of climate change

The 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.

Field performance of four vibrating-wire piezometer installation methods

Vibrating 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

Hydrogeology of a complex Champlain Sea deposit (Quebec, Canada) : implications for slope stability

The 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

Radial consolidation response upon the application and removal of vacuum and fill loading

This Note presents a laboratory study using a Rowe cell to compare the consolidation responses upon vacuum pressure and fill load application and removal. The influences of the duration of application and removal of fill load and vacuum pressures on radial consolidation were investigated using excess pore-water pressure, axial strain, and overconsolidation ratio. It is shown that the appropriate removal time for vacuum pressure can be determined based on excess pore pressure responses

Analytical model for vacuum consolidation incorporating soil disturbance caused by mandrel-driven drains

When vacuum preloading is applied with vertical drains, the rate of consolidation can be increased, and the stability of an embankment is enhanced due to the inward lateral movement. The aim of this study is to develop an analytical solution for vacuum preloading that accurately captures the more realistic variations in compressibility and permeability in actual ground conditions as a result of drain installation. The soil samples were obtained from various locations after drain installation to determine the characteristics of soil surrounding the vertical drain in terms of compressibility and permeability. The main differences between the proposed and conventional models are described by considering the stress history and preloading pressure. The effect of pre-consolidation pressure and the magnitude of applied preloading are examined through the dissipation of average excess pore pressure and associated settlement. The analysis of a selected case history employing the writers\u27 solution indicates improved accuracy of the predictions in comparison to the field measurements