Traditional slope stability analyses rely on 2D cross-section models that assume rivers act as groundwater divides. However, these simplifications may misrepresent groundwater flow and affect calculated factors of safety. This study evaluates when such assumptions are appropriate by comparing 2D and 3D hydrogeological models for slopes of meandering rivers in postglacial marine clays. Results show that for models representing one side of a valley, 2D modeling is appropriate or conservative for straight slopes without an underlying till layer, as hydraulic head differences remain below 0.5 m. If a till layer is present, imposing a no-flow boundary beneath the river produces higher simulated heads (up to similar to 3 m) yielding conservative factors of safety at the cut banks for meandering and straight slopes. However, in most cases, when a till layer is present, groundwater flow beneath the river necessitates modelling both valley sides to avoid unrealistic flow paths. In such cases, 2D models that include both sides of the valley remain suitable for straight slopes with underlying till, as well as at the point bar in meandering slopes, regardless of till presence. In all other configurations, 2D models produce lower hydraulic heads and factors of safety by up to 10% relative to 3D simulations.

As part of the 2010 St. Jude landslide study, a detailed investigation of a distinctive high strength piezocone signature within the lower Champlain Sea sediment sequence revealed a mass transport deposit containing red beds. Base on geotechnical and sedimentological properties, this deposit is interpreted as an event layer that records an abrupt change in sedimentation conditions in the western Champlain Sea. Similar high strength signatures have been identified at multiple sites in the St. Lawrence Valley where they appear to influence the location of failure surfaces in some retrogressive landslides. The widespread occurrence of this layer suggests deposition during a single sedimentological event across the western Champlain Sea Basin approximately 11500 years ago, likely associated with a catastrophic outburst flood event of cold and oxygen-rich water from glacial Lake Algonquin through the Ottawa River Valley. The timing of this event could be related to the preboreal oscillation cooling event (similar to 11500 cal year B.P.). Drawing on numerous piezocone profiles from the western Champlain Sea region, a schematic stratigraphic model, including the event layer, is proposed to support subsurface investigations and improve the risk assessment of large retrogressive landslides in this area.

Spreads are a type of landslide in sensitive clay, characterized by horsts and grabens that develop over long, quasi-horizontal shear band due to strain-softening and progressive failure. It is not yet understood what causes the initiation of the horizontal shear band, nor why its direction of propagation is approximately horizontal. The paper suggests that initiation of a horizontal shear band is more likely in deposits that have a high capacity for horizontal unloading in the overlying soil mass without mobilizing active failure relative to the direct simple shear undrained shear strength along the potential shear band. Using the 1994 spread at Sainte-Monique-de-Nicolet, two parametric studies are described studying the influence of various soil strength parameters on the ratio of available horizontal unloading to direct simple shear strength as well as the influence of this ratio on mobilized stresses following a horizontal unloading. The results of a recent phase of geotechnical investigation, used to identify soil input parameters, are described.

Over the last 20 years or so, numerous landslides, mainly flowslides and spreads from the provinces of Quebec and Ontario, have been investigated in detail in various research projects. These studies have recently been synthesized in two papers, one on flowslides and one on spreads. Other types of landslides have also been identified. The aim of the present study is to examine the similarities and the differences between the factors characterizing each type of landslide in sensitive clays, the ultimate objective (not reached yet) being to determine which one could occur at a given location and what would be its characteristics.

The most common highly retrogressive landslides in Eastern Canada sensitive clays are flowslides and lateral spreads. Quebec’s post-glacial sensitive clay deposits are typically exposed to these two phenomena, which generally occur independently, but the well-documented 2016 landslide at St-Luc-de-Vincennes has shown that both processes causing these hazards can occur in the same event. The aim of this article is to draw up an inventory of historical composite flowslide-spread landslides. The exercise enables us to list all the characteristics that are present at these sites when the two phenomena are combined.

Highly retrogressive landslides as well as rotational landslides commonly occur in Quebec’s post-glacial sensitive clay deposits. Due to their scale, debris from these landslides often partially or totally obstruct the watercourses along which they occur, leading to significant water retention upstream. Beyond the immediate hazards posed by the sudden release of these water masses, the resulting temporary dams cause long-term disruptions to the natural environment, sometimes lasting for decades. This paper examines several cases of landslide-induced dams in Québec, highlighting their economic, environmental and societal impacts and exploring the various ways, sometimes catastrophic, but generally gradual, in which water flow was eventually restored.

This study applies cross-correlation analysis to hydraulic head data from a large network of vibrating-wire piezometers installed in sensitive glaciomarine clay deposits across the St. Lawrence and Saguenay-Lac St-Jean Lowlands in Quebec, Canada. The results reveal the presence of hydraulically active fractures near slopes, extending to depths of up to 16 meters. These findings challenge traditional models that assume clay deposits remain unfractured below a shallow weathered zone, commonly referred to as the crust. The presence of fractures facilitates rapid groundwater movement, leading to significant variations in hydraulic head that were previously believed to be attenuated at depth due to the clay’s low permeability. To assess the broader implications, we compared field data with the results of steady-state groundwater flow models that incorporate fracture scenarios. Two slope geometries with contrasting groundwater flow dynamics were analyzed, each under different fracture configurations. The hydrogeological modeling outcomes were then integrated into a slope stability model to examine how fractures influence stability. The results indicate that fractures can enhance hydraulic head by acting as preferential pathways for infiltration. However, they may also lower hydraulic head by accelerating water discharge through the slope face. Consequently, from a hydrogeological standpoint, fractures can stabilize or destabilize slopes depending on the prevailing groundwater flow system. Since this study focuses exclusively on the hydrogeological effects of fractures, future research should explore their coupled hydromechanical impacts.

It is usually assumed that post-glacial marine clay deposits, such as those found in Quebec, are generally intact below a shallow fractured crust (3-5mdepth). However, recent work has shown the presence of hydraulically-active fractures to depths of down to 16 m. In light of this finding, the potential impacts of these fractures on groundwater flow dynamics and slope stability are explored by comparing field data with the results of transient and steady-state groundwater models with and without fractures. Two slope geometries that exhibit contrasting groundwater flow directions and different fracture scenarios were considered. The results of the hydrogeological modelling were then imported into a slope stability model to determine how the hydraulic effects of these fractures impact slope stability. Results show that fractures increase hydraulic head when they act as preferential pathways for infiltration, but can also reduce hydraulic head by acting as pathways for water to more quickly exit the formation within the slope face. Therefore, from a hydrogeological perspective, fractures could improve or reduce slope stability depending on the groundwater flow system. As this study only addressed the hydrogeological impact of the fractures, future work should focus on the coupled hydromechanical impacts of these features.

This study investigated a new predictive model of the shear modulus reduction (G/G(max) - Log (gamma c)) and damping ratio (xi - Log (gamma c)) curves for sensitive eastern Canada clays. The model was established based on experimental measurements of G Gmax and xi performed on 69 sensitive clay specimens from eastern Canada. The compiled database was first analyzed to qualitatively assess the influences of different soil parameters on the measurements. Next, statistical analyses using regression analysis and the modified hyperbolic model were performed. It was found that the G/G(max) and xi values of sensitive eastern Canada clays were influenced mainly by the plasticity index (I-p), mean effective stress (sigma '(m)), and structure, which was quantified by the liquidity index (I-L). Based on the statistical analysis performed, new equations for G/G(max) and xi were proposed. The newly proposed model was compared with previously published models. The comparison clearly showed that the proposed model was more representative of sensitive eastern Canada clays. Consequently, it is suggested that the proposed model is more appropriate for use in the dynamic analysis of sensitive eastern Canada clay deposits as well as other similar soils.

Landslides in sensitive post-glacial marine clays are one of the major geological hazards in Canada, Norway and Sweden. Current hydrogeological conceptual models used for slope stability analyses in these deposits consider simple groundwater flow conditions within a homogenous, isotropic, massive clay deposits, where fractures are surficial features that only exist within a 1-5 m-thick weathered zone. This study uses cross-correlation analysis on hydraulic head data from a large network of vibrating-wire piezometers in clay deposits along the St. Lawrence River and in the Saguenay-Lac St-Jean Lowlands, in Quebec, Canada, to show that hydraulically-active fractures are present to depths of up to 16 m at 4 (possibly 6) of the 7 locations studied. These findings suggest that current conceptual models have a high likelihood of misrepresenting local flow systems, and that further field and modeling work is needed to characterize the extent and influence of these fracture networks.

Vibrating-wire piezometers provide a number of advantages over the traditional hydraulic piezometer design. There are currently many methods and configurations for installing vibrating-wire piezometers, 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 at a field site possessing complex stratigraphy, including glacial and marine sediments. To accomplish this objective, pore pressure data recorded between December 2017 and July 2019 were analyzed. Data indicate that SP, MLSP, and FGB piezometers performed most reliably, because piezometers installed at the same depth with these methods 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, likely caused by preferential flow occurring along the wires of the embedded instruments. The lack of a standard method for mixing cement−bentonite grout at the time of construction likely contributed to the failure of the FGCB installations, as the grout mixture used in this study was likely too viscous to provide a suitable seal.

The assessment of the strain rate effect on the geotechnical properties of soils constitutes an important step toward a more accurate analysis of their response. This study presents the experimental results of monotonic and cyclic simple shear tests performed to examine the strain rate ([Formula: see text]) effect on the behavior of eastern Canada soils. Nine natural soils sampled from different locations in eastern Canada were used in this study. The tests were performed on a simple shear device using a strain-controlled mode. In addition to the obtained experimental results, published data in the literature were used to draw the conclusions of this study. Analysis of the data indicates that the undrained shear strength (τ f ) increases proportionally with the strain rate by approximately 6%–17% per log cycle of [Formula: see text]. The results also show that the secant shear modulus G increases with the strain rate, especially at large strain amplitudes. Moreover, the analysis of the data revealed that the extent of the strain rate effect seems to be correlated with the shear strain amplitude (γ c ) and plasticity index (I p ). A practical application of the outcomes on the backbone curves is given, illustrating the influence of I p and γ c on the effect of strain rate.

An extensive network of multilevel vibrating-wire piezometers (VWP) was recently created to monitor the spatial and temporal variation of pore pressure (and hydraulic head) 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 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 two-dimensional 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 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.

The shear modulus and hysteretic damping of three sensitive clays from the sediments of Champlain Sea were investigated using a combined triaxial simple shear apparatus. The tests were conducted on undisturbed samples and were carried out on a wide range of shear strains from about 0.001% to 1%. The values of the small-strain shear modulus of the tested clays were further confirmed through a series of piezoelectric ring actuator tests. Although the shear modulus and damping ratio of the sensitive eastern Canadian clays follow some classic literature models, the results show that the examined clays exhibited more linear behaviour. Such behaviour may be attributed to their highly structured nature compared to other clays. The compilation of available data on the shear modulus and damping ratio of several sensitive eastern Canadian clays confirmed this trend and showed that some literature models might not be representative.

On 9 November 2016, a landslide in sensitive glaciomarine sediments occurred on a terrace of the Champlain River near the municipality of Saint-Luc-de-Vincennes, Quebec. The particularity of this event is that there are evidences that the movement started as a flowslide and then finished as a spread. The landslide morphology comprises horsts and grabens typical of spreads and also a large quantity of remolded material that flowed out of a pear-shaped crater with a narrow bottleneck, typical of flowslides. The geotechnical investigation of this landslide was performed by the Ministère des Transports du Québec (MTQ) in collaboration with Université Laval, and consisted of light detection and ranging (LiDAR) surveys, drone photography, several boreholes, piezocone tests with pore pressure measurements (CPTUs), field vane tests, and piezometric monitoring. They were used to characterize the landslide, to determine the location of the failure surface, and also to acquire information on the properties of the clay deposit. A combined analysis of the debris and volume calculations was done to reconstruct the different phases of flowing and spreading and their relative chronologies.

The thick sequences of marine clayey deposits that 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 that illustrates the complex groundwater flow system present on site and shows the importance of including hydrogeologic context in slope stability analysis.