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Sensitive clays, mostly found in Eastern Canada and Scandinavia, are prone to various types of landslides; among them are flowslides that may affect hectares of land. Debris from those slides has high mobility, with runout distances that may extend over hundreds of meters in relatively flat and non-channelized terrain. Most studies on the mobility use apparent rheological properties to back-calculate the runout behavior of these landslides. This works well for back-analyses of landslides but is of little help in a context of hazard mapping, where prospective analysis of future landslides is necessary. In this study, a new factor, using the destructuration index concept, is introduced to estimate the remaining energy available as kinetic energy for runout after the remolding of the sensitive clay: the energy reduction factor (FER). Using the FER allows the use of real rheological data acquired with a rheometer to analyze the post-failure behavior of a flowslide in sensitive clays. This method is applied on a sensitive clay landslide which took place in Quebec and where rheological and geotechnical data are available. [ABSTRACT FROM AUTHOR]

A landslide occurred on 10 May 2010, along the Salvail River, in the municipality of Saint-Jude, Quebec. Debris of the landslide was formed of 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 was 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 the location of two failure surface levels: the first one starting 2.5 m below the initial river bed, 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. [ABSTRACT FROM AUTHOR]

Fragmentation is one of the mechanisms involved in rock avalanches. Quantifying the associated energy during a rock avalanche can help to assess the influence of fragmentation on post-failure mass movements. In this paper, in situ block size distributions of the intact rock mass and the debris deposits are presented and analyzed for nine rock avalanches, five in the Canadian Rocky Mountains and four in the European Alps. Degrees of fragmentation are estimated from these data. Two methods are examined to assess fragmentation energy, one based on the comminution theory, and one on the blasting energy used in the mining industry. The results show that, for the studied rock avalanches, there is a relationship between the reduction in diameter ratio, Rr = D50/d50 (where D50 and d50 are the mean diameter of the intact rock mass and the mean diameter of the debris, respectively), and the potential energy per unit volume normalized with respect to the point load strength of rock (γHG/σc), where γ is the unit weight of the material, HG is the vertical distance between the centres of gravity of the mass at the start and end positions, and σc is the point load strength). For the cases studied, fragmentation energy calculations average 20% of the potential energy. An empirical relationship between Rr and γHG/σc has been established and is used in the definition of a disintegration index (ID). This index seems to reflect the physics of the disintegration process, since it accounts for the fact that the reduction in diameter ratio is a function of the dissipated energy and the strength of the rock. These factors have long been known to affect fragmentation but have never been presented in a coherent manner for rock avalanches. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This study investigated a new predictive model of the shear modulus reduction ( G G max − Log (γ c)) and damping ratio (ξ − Log (γc)) curves for sensitive eastern Canada clays. The model was established based on experimental measurements of G G max and ξ 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 ξ values of sensitive eastern Canada clays were influenced mainly by the plasticity index (Ip), mean effective stress (σ m ′) , and structure, which was quantified by the liquidity index (IL). Based on the statistical analysis performed, new equations for G G max and ξ 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. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Much of Eastern Canada, Sweden, Norway, and parts of the Northeastern United States were flooded by the sea at the end of the last ice age, resulting in the deposition of thick clay deposits that are prone to landslides. Since landslides represent one the largest natural hazards to human life and property in these areas, understanding the mechanisms that trigger landslides is a matter of public security. Previous research examining slope stability in these deposits has assumed that small cracks (i.e., fractures) only exist in the upper 1–5 m of the clay. Evidence presented here shows that fractures can extend to depths of greater than 15 m, and are present in many hillslopes. These fractures enable water to rapidly flow through a material that is otherwise nearly impermeable, ultimately influencing the processes that control landslide initiation. This work uses statistical analyses of groundwater data from a network of wells across an 800‐km transect of the province of Quebec to show that groundwater flow systems in massive clay hillslopes are much more complicated than originally thought, and that methods used for conducting current slope stability analyses need to be updated to properly assess landslide risks in these deposits. Key Points: Fractures were identified beneath the upper 1–5 m‐thick weathered zone of the post‐glacial marine clays at multiple locations in QuebecFractures allow large variations in hydraulic head at >10 m depth to occur almost concurrently with variations observed near the surfaceFractures control groundwater flow in low‐permeability media, but their hydraulic effects are not considered in slope stability analyses [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 (--> γ ˙ -->) 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 (τf) increases proportionally with the strain rate by approximately 6%–17% per log cycle of --> γ ˙ -->. 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 (γc) and plasticity index (Ip). A practical application of the outcomes on the backbone curves is given, illustrating the influence of Ip and γc on the effect of strain rate. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: In 1903, the eastern slope of Turtle Mountain (Alberta) was affected by a 30 M m3-rockslide named Frank Slide that resulted in more than 70 casualties. Assuming that the main discontinuity sets, including bedding, control part of the slope morphology, the structural features of Turtle Mountain were investigated using a digital elevation model (DEM). Using new landscape analysis techniques, we have identified three main joint and fault sets. These results are in agreement with those sets identified through field observations. Landscape analysis techniques, using a DEM, confirm and refine the most recent geology model of the Frank Slide. The rockslide was initiated along bedding and a fault at the base of the slope and propagated up slope by a regressive process following a surface composed of pre-existing discontinuities. The DEM analysis also permits the identification of important geological structures along the 1903 slide scar. Based on the so called Sloping Local Base Level (SLBL) an estimation was made of the present unstable volumes in the main scar delimited by the cracks, and around the south area of the scar (South Peak). The SLBL is a method permitting a geometric interpretation of the failure surface based on a DEM. Finally we propose a failure mechanism permitting the progressive failure of the rock mass that considers gentle dipping wedges (30°). The prisms or wedges defined by two discontinuity sets permit the creation of a failure surface by progressive failure. Such structures are more commonly observed in recent rockslides. This method is efficient and is recommended as a preliminary analysis prior to field investigation. [Copyright &y& Elsevier]

Analysis of morphology, failure and post-failure stages of the Palos Verdes debris avalanche reveals that it may have triggered a significant tsunami wave. Our analysis of the failure itself indicates that the slope is stable under aseismic conditions but that a major earthquake (with a magnitude around 7) could have triggered the slide. A post-failure analysis, considering the debris avalanche as a bi-linear flow, shows that peak velocities of up to 45 m/s could have been reached and that the initial movement involved a mass of rock less than 10 km wide, 1 km long and about 50–80 m thick. Initial wave height estimates vary from 10 to 50 m. Tsunami waves propagating to the local shoreline would be significantly smaller. Such a range demonstrates our lack of proper knowledge of the transition from failure to post-failure behavior related to mass movements. Further investigations and analyses of terrestrial and submarine evidence are required for a proper hazard assessment related to tsunami generation in the Los Angeles area. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

On 25 November 1988, after the Saguenay earthquake, a landslide occurred in the municipality of Saint-Adelphe. The soil profile indicated that the deposit was composed of a stiff clay crustal layer overlying sensitive plastic clay with a soft-to-stiff consistency. A geotechnical investigation was carried out in situ and in the laboratory and included the use of a new seismic simulator to develop a geotechnical model of the Saint-Adelphe clay. The model was incorporated in a finite-difference slope stability analysis before and during the earthquake. The results showed the development of plastic zones and the generation of pore water pressure, but the global safety factor remained above unity. A post-seismic analysis that utilized a strain-softening behavior model showed the initiation and propagation of the plastic zone, as well as the development of a failure surface close to the observed failure surface. Therefore, it is proposed that the Saint-Adelphe landslide could be explained by a progressive failure mechanism. [ABSTRACT FROM AUTHOR]

The concept of the spectral pseudo-static procedure has been developed at the Université de Sherbrooke as an alternative to the conventional pseudo-static approach for the seismic stability analysis of clayey slopes. The destabilizing effect of an earthquake is approximated, in the new approach, by an inertial force that hyperbolically varies with depth while being proportional to the maximum acceleration of the seismic event. Its results have been rigorously verified against available static and dynamic laboratory tests, and have been extensively validated by a series of simulations performed using the computer code FLAC. Good agreements have been achieved between the results of the spectral pseudo-static procedure and complete numerical analyses, in terms of the computed safety factors and the critical slip surfaces. This implies that the spectral pseudo-static approach can be integrated into available limit equilibrium (LE) software providing a useful tool to examine the effect of dynamic soil characteristics on the proposed seismic coefficient profiles. The herein-reported study extends the previous endeavors to examine and quantify the effect of plasticity index (PI) (0%, 15%, 30%, 50%, and 100%) on the proposed formula of seismic coefficient profile using the same numerical modelling and assumptions. Original analyses were carried out considering earthquakes compatible with the seismicity of Quebec City (zone 4), and they have been repeated in the current study for earthquakes compatible with other regions having different seismicity (zones 2, 3, and 5). Based on the results of the current analyses, side-formulas were established between the spectral pseudo-static coefficient on ground surface for any value of soil plasticity index (PI) and the corresponding coefficient originally used in the main formula (PI = 30%). [ABSTRACT FROM AUTHOR]

For more than 10 years, digital elevation models (DEM) produced by light detection and ranging (LIDAR) technology have provided new tools for geomorphologic studies and especially for landslide studies. In particular, terrestrial laser scanning (TLS) provides a great versatility of use. TLS can be used either for monitoring purposes or in an emergency situation that necessitates a rapid DEM acquisition for assessing a hazard. Using three examples we demonstrate the usefulness of TLS for landslide volume quantification, profile creation, and time series analysis. These case studies are landslides located in sensitive clays of eastern Canada (Quebec, Canada) or small rotational slides in river banks (Switzerland). [ABSTRACT FROM AUTHOR]