Landslides In Sensitive Clays Research Articles

Numerical modelling of downward progressive landslides in sensitive clay

Landslides in sensitive clays are catastrophic events that threaten life and property. When plastic strain occurs in sensitive clays, there is a significant reduction in the undrained shear strength. Consequently, even minor triggers can lead to multiple progressive landslides, causing extensive devastation. Investigating landslides in sensitive clay presents challenges, necessitating robust numerical methods to comprehend failure modes and collapse behaviors, particularly in the post-failure phase. Although several numerical methods have been utilized for simulations, most have primarily focused on upward progressive landslides. In this study, we adopted the nodal integration-based particle finite element method to simulate downward progressive landslides triggered by surcharge loads from an embankment near the crest in sensitive clay on a long slope. We also considered nonlinear strain softening in sensitive clays. The effects of the strain softening rate and strength gradient of the sensitive clay layer on the failure mechanisms and destruction extent were investigated. The simulation results reveal that a downward progressive landslide initiates with a localized flow slide surrounding the embankment and usually involves a combination of various failure modes.

A practical approach for predicting landslide retrogression and run-out distances in sensitive clays

In the present study a novel approach of practical interest is proposed to perform a predictive analysis of the landslides in sensitive clays. Specifically, this approach allows a prediction of the landslide triggering along with the consequent distances of retrogression and run-out. To this end, a simple limit equilibrium solution is proposed to deal with the first two steps (prediction of the landslide triggering and the subsequent retrogression distance). A calculation code based on the material point method (MPM) is instead employed for evaluating the run-out distance of the unstable soil mass that was defined in the previous step. Operative values of the undrained shear strength are also suggested for each of the aforementioned steps. The proposed method is very attractive from a practical viewpoint because it is simple to use, not time-consuming and requires few geotechnical parameters of easy experimental determination as input data. The method is applied successfully to several well-documented case studies available in the literature, demonstrating also its reliability for engineering applications. Three of these landslides are analysed in more detail to illustrate the application of the present method to real cases.

Remoulding energy as a criterion in assessing retrogressive landslides in sensitive clays: a review and its applicability to Eastern Canada

Remoulding energy as a criterion in assessing retrogressive landslides in sensitive clays: a review and its applicability to Eastern Canada

Analysis of a landslide in sensitive clays using the material point method

Slope movements are generally classified into four different phases: pre-failure, failure, post-failure and eventual reactivation. In engineering applications, the pre-failure and failure phases are usually analysed using traditional numerical techniques, such as the finite-element method and the finite-difference method. However, these methods are often based on the assumption of small deformations and consequently are unsuitable for analysing the slope behaviour during the post-failure stage, which is usually characterised by very large deformations. To overcome this shortcoming, the material point method (MPM) is employed in the present study. Specifically, MPM is used to perform an analysis of a landslide in sensitive clays that occurred at Saint-Jude (Quebec, Canada) in 2010. To assess the accuracy of the analysis, the final profile and the displacement magnitude detected after the event are compared with those obtained by the numerical simulation. The results provided by MPM are in satisfactory agreement with field observations. The failure mechanism and the development of the failure surface within the slope are also reproduced successfully. These results also show that MPM is an attractive method for analysing the kinematics of landslides in sensitive clays, requiring also a limited number of conventional geotechnical parameters as input data.

Surface displacement expression of progressive failure in a sensitive clay landslide observed with long-term UAV monitoring

Surface displacement expression of progressive failure in a sensitive clay landslide observed with long-term UAV monitoring

A nonlocal Eulerian-based finite-element approach for strain-softening materials

The dependency of finite element (FE) results on mesh size is a major concern for the numerical analysis of strain-softening materials. The local methods of strain regularization rely on the shear strains of a solitary point. However, the nonlocal methods incorporate strain-softening, including the strain in surrounding soil elements, which show less mesh dependency. Previously, nonlocal methods were mostly implemented in Lagrangian-based FE programs and simulated the response for small to moderate strain levels. However, many geotechnical problems, such as large-scale landslides in sensitive clays, involve extremely large deformation. This study presents the implementation of the “original” and two modified nonlocal methods in a Eulerian-based large deformation FE program using a relatively simplified approach where simple soil models, such as von-Mises criteria for undrained behaviours, can be used. Two biaxial compression tests are simulated by using the nonlocal Eulerian-based FE program, and the results are compared with a nonlocal Lagrangian-based FE analysis and a nonlocal Material Point Method (MPM) of simulation, respectively. Among the three, the modified nonlocal methods, especially the over-nonlocal method, show a better performance in mesh convergence analysis. Several approaches have been proposed to minimize the computational costs, as nonlocal modelling is generally computationally expensive.

Stratigraphic profiling, slip surface detection, and assessment of remolding in sensitive clay landslides using the CPT

A multi-year cone penetration testing program was conducted at a landslide subject to episodic retrogression in Mud Creek, Ottawa, to assess whether a hand-operated mobile cone penetration test (CPT) could yield new insights into the current degree of remolding under progressive failure in metastable areas of a landslide where conventional tracked rigs are unable to gain access. The mobile CPT rig permitted tests to be performed through the entire thickness of the Champlain Sea deposit at a penetration rate of 0.5 cm/s, with similar results to tests performed at the standard 2 cm/s. Measurements of pore pressure varied considerably with cone size, with the magnitude of pore pressure response decreasing with cone size. The elevation of the slip surface was identified in the tip resistance as the point of transition between the remolded soil above the slip surface and the intact soil below the slip surface, whereas a further 0.5 m of penetration was required to elevate pore pressures to values indicative of the intact soil behaviour. In situ measurements of shear strength of corresponding layers between the intact and remolded profiles to be compared indicated that the soil above the slip surface had remolded to 50% of its fully remolded strength.

Modeling of Initial Stresses and Seepage for Large Deformation Finite-Element Simulation of Sensitive Clay Landslides

AbstractGroundwater seepage and an increased lateral earth pressure coefficient at rest (K0) increase the potential for triggering large-scale landslides in sensitive clays. After the failure is tr...

GIS based assessment of rainfall-induced landslide susceptibility of sensitive marine clays: a case study

ABSTRACT Landslides are frequent occurrences in Ottawa, Canada because sensitive marine clays (also known as Leda clay or Champlain Sea clay) dominate the landscape. These landslides are often triggered by rainfall in the region. A geographic information system (GIS) based modelling approach has been developed to assess and predict rainfall-induced landslides in the sensitive clays of the Ottawa region. The Transient Rainfall Infiltration and Grid-based Slope-stability (TRIGRS) model has been used in a GIS framework to investigate the influence of rainfall On shallow landslides over the Ottawa region, with respect to time and location. The assessment result maps show that the steep slopes of sensitive clays are more prone to landslides. Furthermore, susceptibility to landslides in these areas increases with long periods of intense rainfall. The proposed GIS based model is validated by comparing the predicted areas of landslides induced by rainfall with previous shallow landslides that had occurred in the Ottawa region. The results show that there is a good agreement between the predicted areas of landslides and the previous landslides reported. The proposed GIS–TRIGRS combined modelling approach can be therefore considered as a potential tool for assessing and predicting rainfall induced landslides in sensitive marine clays in Ottawa.

Modelling of mobility of Rissa landslide and following tsunami

Landslides in sensitive clays pose a major threat to life, property and the environment. The lack of warning signs and the extreme mobility of quick clays increase the risk. When occurring along fjords, lakes or large rivers, the landslides can generate destructive tsunamis. Historical records show that 45% of the shoreline landslides in marine sediments in Norway triggered tsunamis with run-up height of 1 to 15 m. To improve the management of the hazard and risk due to landslides in sensitive materials, the profession needs to develop tools for modelling the mobility of landslides and their tsunamigenic potential. The paper back-calculates the 1978 Rissa landslide, one of the largest quick clay landslides in Norway. Because of the quantity of data available, the Rissa landslide provides a unique benchmark to study the mobility of the quick clay, and the tsunami following the landslide. The paper describes a new approach to model landslide mobility and tsunami run-up, and presents the results of the analyses and a comparison of the results with the observations. The back-calculated mobility (runout distance, flow velocity and debris thickness) and wave run-up on the lakeshore agreed well with the measurements.

Geotechnical centrifuge modelling of retrogressive sensitive clay landslides

Sensitive clay landslides represent a significant geohazard due to their well-known potential for extensive retrogressive failures, on the scale of multiple hectares, which could encompass surrounding communities and infrastructure. Interpretation of retrogression mechanisms is often limited as only forensic investigations are possible. This work presents the results of a physical modelling study to examine retrogressive failures, analysis of each failure episode, and interpretation of the results using published relationships. Five novel centrifuge model tests were conducted under a defined range of undrained shear strength and slope angle conditions. The models are constructed of a sensitive cement–soil mixture that allows for a consistent contractile material with bespoke shear strength. Results indicate the observed retrogression distance correlates with Taylor’s stability number. The addition of a 5° slope angle to invoke a static shear stress on the model provoked notably larger retrogression distances. Post-test undrained shear strength measurements quantified softening of the material along the failure surface. Stability analyses on each failure episode captured the observed failure geometry and factor of safety. Results indicate that the geometric parameters of a slope, specifically the slope angle, may be able to explain a component of the scatter for relating Taylor’s stability number with retrogression distances.

Effects of geometry and soil properties on type and retrogression of landslides in sensitive clays

Flowslide and spread are two common types of landslides in sensitive clays. Empirical criteria, based on single or multiple soil properties and slope geometry, have been proposed for a rough assessment of potential landslide type and retrogression distance. A large variation has been found in the comparison of retrogression distance between empirical equations and field data. In the present study, flowslides and spreads are simulated using a Eulerian-based large-deformation finite-element (FE) method. In addition to strain-softening, a strain-rate-dependent undrained shear strength model that elevates the strain rate effects on the shear strength of liquefied clay flowing at high speed is used. In flowslides, a higher rate of increase in undrained shear strength with depth reduces the depth of subsequent slides, and thereby the retrogression distance. The maximum retrogression occurs for a uniform shear strength profile. The increase in the ratio of horizontal to vertical stress, resistance to downslope movement of the debris and decrease in soil brittleness and slope steepness change the failure pattern from a flowslide to a spread.

Simulating retrogressive slope failure using two different smoothed particle finite element methods: A comparative study

Various smoothed particle finite element methods (SPFEMs) have been developed to simulate large deformation problems, but their efficiency and accuracy in simulating progressive landslides in sensitive clays have remained unclear. In this study, a series of numerical analyses are carried out to investigate the development of retrogressive landslides by two SPFEMs (an edge-based strain smoothing PFEM, ESPFEM, and a node-based strain smoothing PFEM, NSPFEM) in view of their outstanding performance in large deformation analysis. A strain-softening Mohr–Coulomb (MC) model is adopted to simulate the behaviour of sensitive clays during landslide, assuming a Poisson's ratio of 0.49 to ensure undrained conditions. The influence of mesh density on the development of retrogressive failure is evaluated for two SPFEMs. Numerical analyses of three mesh sizes (0.2 m, 0.15 m and 0.12 m) are carried out sequentially, with all results demonstrating that (1) the spread retrogressive landslides with horsts and grabens can be achieved by both SPFEMs with the adopted soil model, (2) run-out and retrogression distances decrease as mesh density increases for both methods, (3) retrogressive collapse occurs earlier for ESPFEM but is delayed for NSPFEM with increased mesh density, (4) NSPFEM allows faster calculations and reduces mesh dependency problems when compared with ESPFEM and (5) the increase of shape factor can accelerate retrogressive evolution of landslides. Finally, a real landslide in sensitive clay at Sainte-Monique, Quebec, is simulated to demonstrate ESPFEM's computational efficiency and accuracy.

Overview of Retrogressive Landslide Risk Analysis in Sensitive Clay Slope

Sensitive clays are known for producing retrogressive landslides, also called spread or flowslides. The key characteristics associated with the occurrence of these landslides on a sensitive clay slope must be assessed, and the potential retrogressive distance must be evaluated. Common risk analysis methods include empirical methods for estimating the distance of potential retrogression, analytical limit equilibrium methods, numerical modelling methods using the strength reduction technique, and the integration of a progressive failure mechanism into numerical methods. Methods developed for zoning purposes in Norway and Quebec provide conservative results in most cases, even if they don’t cover the worst cases scenario. A flowslide can be partially analysed using analytical limit equilibrium methods and numerical methods having strength reduction factor tools. Numerical modelling of progressive failure mechanisms using numerical methods can define the critical parameters of spread-type landslides, such as critical unloading and the retrogression distance of the failure. Continuous improvements to the large-deformation numerical modeling approach allow its application to all types of sensitive clay landslides.

On casting clay specimens of bespoke shear strength and sensitivity for landslide modelling

Sensitive clay landslides are a geohazard often exhibiting flow-like retrogressive behaviour. Soils with high sensitivity experience significant strain softening − a key characteristic of these types of slope movements. In this paper, an experimental method using high early strength cement mixed with clay soil is investigated to cast repeatable samples of a bespoke shear strength and sensitivity while reducing specimen preparation time. The addition of cement bonds from hydration reactions was observed to encourage the development of a metastable soil structure with sufficiently high moisture content to exhibit both a high peak strength and a low remoulded strength. Various cement and water content mixtures were examined, with either kaolin clay or a naturally sensitive clay from Mud Creek, Ontario and Portland type 1 or type 3 cement. Undrained shear strength testing was measured with the Swedish fall cone and the miniature lab vane. Soil−cement mixtures developed shear strength of up to 60 kPa with sensitivities from 4 to 16 within a 7 d curing time. This paper reports lessons learned from the mixing, curing and testing of the sensitive material, as well as the results from a geotechnical centrifuge experiment examining retrogressive sensitive clay landslides.

Geotechnical investigations of an earthquake that triggered disastrous landslides in eastern Canada about 1020\u2009cal BP

Geotechnical studies are carried out to investigate the location and magnitude of a prehistoric earthquake in eastern Canada. Previous studies identified 12 sensitive clay landslides of about 1020 cal BP in the Ottawa-Gatineau region, including one of the most disastrous slope failures in eastern Canada. The landslides were hypothesized to have been triggered by an earthquake. In the current study, three of those landslides are investigated to understand their failure mechanisms. The study sites form a triangle of lateral distances of about 38, 38 and 35 km. Micro-seismic surveys, cone penetrometer tests, vane shear tests, soil sampling and laboratory testing are conducted to collect geotechnical data. Slope stability models are constructed to calculate threshold ground accelerations required to trigger the landslides. The ground accelerations are used with Ground-Motion Prediction Equations to triangulate the earthquake. The results indicate an earthquake of a minimum moment magnitude of 5.9 near Eardley, Quebec.

A case study and implication: particle finite element modelling of the 2010 Saint-Jude sensitive clay landslide

Modelling of landslides in sensitive clays has long been recognised as a challenge. The strength reduction of sensitive clays when undergoing plastic deformation makes the failure proceed in a progressive manner such that a small slope failure may lead to a series of retrogressive failures and thus to an unexpected catastrophic landslide. The clay in the entire process may mimic both solid-like (when it is intact) and fluid-like (when fully remoulded, especially for quick clays) behaviours. Thereby, a successful numerical prediction of landslides in sensitive clays requires not only a robust numerical approach capable of handling extreme material deformation but also a sophisticated constitutive model to describe the complex clay behaviour. In this paper, the particle finite element method (PFEM) associated with an elastoviscoplastic model with strain softening is adopted for the reconstruction of the 2010 Saint-Jude landslide, Quebec, Canada, and detailed comparisons between the simulation results and available data are carried out. It is shown that the present computational framework is capable of quantitatively reproducing the multiple rotational retrogressive failure process, the final run-out distance and the retrogression distance of the Saint-Jude landslide. Furthermore, the failure mechanism and the kinematics of the Saint-Jude landslide and the influence of the clay viscosity are investigated numerically, and in addition, their implications to real landslides in sensitive clays are discussed.

The energy reduction factor (FER) to model sensitive clay flowslides using in situ geotechnical and rheological data

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.

Implementation of a large deformation finite element modelling technique for seismic slope stability analyses

Post-slide investigations show large displacement of failed soil mass in many earthquake-triggered landslides. An Eulerian-based finite-element modelling (FEM) of large deformation of soil, for pseudostatic and dynamic loadings, is presented in this study. The Eulerian FEM is compared with Lagrangian-based explicit and implicit finite-element (FE) modelling approaches. The dynamic FE modelling of two hypothetical slopes for eight earthquake acceleration–time histories show that the failure surfaces develop progressively, which cannot be modelled using the traditional limit equilibrium method (LEM). A large plastic shear strain concentration (i.e. shear band formation) occurs when the strain-softening behaviour of soil is considered. The similarities and differences between the results of dynamic and pseudostatic FE analyses based on estimated pseudostatic coefficient from acceleration–time records are presented. The duration of an earthquake influences the failure process and displacement of the failed soil mass. The displacement of the toe obtained from FEM is compared with Newmark's simplified approach. The developed Eulerian-based FE modelling technique has been used to simulate large-scale landslides in sensitive clays due to earthquake loading [1].

Large-deformation finite-element modelling of earthquake-induced landslides considering strain-softening behaviour of sensitive clay

Large-scale landslides in sensitive clays cannot be explained properly using the traditional limit equilibrium or Lagrangian-based finite-element (FE) methods. In the present study, dynamic FE analysis of sensitive clay slope failures triggered by an earthquake is performed using a large-deformation FE modelling technique. A model for post-peak degradation of undrained shear strength as a function of accumulated plastic shear strain (strain-softening) is implemented in FE analysis. The progressive development of “shear bands” (the zone of high plastic shear strains) that causes the failure of a number of soil blocks is simulated successfully. Failure of a slope could occur during an earthquake and also at the post-quake stage until the failed soil masses come to a new static equilibrium. Upslope retrogression and downslope runout of the failed soil blocks are examined for varying geometries and soil properties. The present FE simulations can explain some of the conditions required for different types of seismic slope failure (e.g., spread, flowslide or monolithic slides) to be triggered, as observed in the field.