Modeling of large-deformation behaviour of marine sensitive clays and its application to submarine slope stability analysis

Abstract

Post-slide investigations suggest that many large-scale submarine landslides occur through marine sensitive clay layers. A nonlinear mathematical model for post-peak degradation of undrained shear strength of sensitive clay is proposed based on experimental results. A method for estimation of model parameters is presented. Incorporating the model, an analytical solution is developed to examine possible mechanisms of large-scale submarine landslides. Analyses are performed for mild infinite slopes where the failure initiates from a "fully weakened zone" of soil having undrained shear strength lower than the shear stress acting parallel to the slope. The driving force, in excess of resistance, generated from the fully weakened zone is then transferred to the surrounding soil elements resulting in shear band formation due to strain-softening behaviour of sensitive clays. When the length of the fully weakened zone is greater than a critical length, catastrophic shear band propagation (self-driven without any additional external force) occurs, which could result in large-scale offshore landslides. A simple design chart is developed to calculate the critical length. Compared with a 2005 study by Puzrin and Germanovich based on a linear post-peak shear strength degradation model, the present study gives a conservative estimation of critical length for catastrophic shear band propagation.Key words: sensitive clay, large deformation, submarine landslides, shear band, undrained shear strength.Résumé : Les enquêtes post-glissement de terrain suggèrent que de nombreux glissements de terrain sous-marin à grande échelle se produisent à travers les couches d'argiles marines sensibles. Un modèle mathématique non linéaire de la dégradation post-pic de la résistance au cisaillement non drainée de l'argile sensible est proposé sur la base de résultats expérimentaux. Une méthode d'estimation de paramètres du modèle est présentée. Incorporant le modèle, une solution analytique est développée pour examiner les mécanismes possibles de glissements de terrain sous-marin à grande échelle. Les analyses sont effectuées pour des pentes infinies douces où l'échec est entamé à partir d'une « zone totalement affaiblie » du sol ayant une résistance au cisaillement non drainé inférieure à la contrainte de cisaillement agissant parallèlement à la pente. La force d'entraînement, au-delà de la résistance, générée à partir de la zone entièrement affaiblie est transférée aux éléments de sol environnant aboutissant en la formation de bandes de cisaillement en raison du comportement d'adoucissement à la déformation des argiles sensibles. Lorsque la longueur de la zone entièrement affaiblie est supérieure à une longueur critique, une propagation d'une bande de cisaillement catastrophique (auto-conduit sans aucune force externe supplémentaire) se produit qui pourrait entraîner des glissements de terrain en mer à grande échelle. Un graphique de conception simple est développé pour calculer la longueur critique. Par rapport à une étude réalisée e...

Numerical modeling of combined effects of upward and downward propagation of shear bands on stability of slopes with sensitive clay

“…The parameters in Table I are consistent with previous studies on landslides in sensitive clays [8, 9, 14-16, 24, 56, 63]. Further discussion on this post-peak s u degradation model is available in Dey et al [18,53,60] 2. FE RESULTS Figure 4(a) shows the equivalent plastic strain (PEEQVAVG) at Δ = 0.5 m for the 0.5-m mesh.…”

“…The parameters in Table are consistent with previous studies on landslides in sensitive clays . Further discussion on this post‐peak s u degradation model is available in Dey et al …”

“…After that a linear relationship (line de ) is used to decrease s u to a constant s u(ld) at δ ld . Equation fits well with post‐peak stress–displacement behavior of sensitive clays reported from laboratory tests , which has been presented elsewhere . Assuming simple shear condition, the plastic shear strain is calculated as δ p / t s , where t s is the thickness of the soil element or shear band.…”

Frictional heating lubrication for submarine landslide

“…Even for snow avalanches, "weak layer" was also well recognized (Heierli et al 2008). As far as we know, there are several types of "weak layers" observed in field investigations, for example, (1) sandy layers where gas hydrates dissociate (Sultan et al 2004b); (2) consolidated clayey areas of high sedimentation rates (Dugan and Flemings 2000;Sultan et al 2004a); (3) sandy silt layers that can liquefy during earthquakes (Kokusho and Kojima 2002); (4) rapid accumulation events, e.g., turbidity currents or debris flows, over a clay-rich seabed (Hansen et al 2011); and (5) sensitive clay layers; i.e., clayey sediments with a "strain-softening" behaviour Dey et al 2016). The "strain-softening" implies that just a small distortion can cause a tremendous deformation of soil (like a "collapsing" or "contracting" of soil texture) leading to a significant reduction of the porosity, which for undrained conditions will induce an increase of pore pressure there and therefore decrease the soil strength (Thakur 2011).…”

“…Normally, just a small shear distortion can significantly reduce the soil strength of an extra sensitive clay. Due to this "strain-softening" property, the existence of sensitive clay layers is very possibly a main cause of many large-scale submarine landslides that occurred on nearly flat continental slopes, e.g., the Storegga slide Gauer et al 2005;Kvalstad et al 2005;Dan et al 2007;Locat et al 2014;Dey et al 2016). For this reason, many scientists had proceeded to study sensitive clay.…”