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Progressive failure of slopes with sensitive clay layers
Rupture progressive de pentes comportant des couches d'argile sensible
Dey R., Hawlader B.
Memorial University of Newfoundland, St. John’s, Canada.
Phillips R.
C-CORE, St. John’s, Canada.
Soga K.
University of Cambridge, Cambridge, UK.
ABSTRACT: Progressive failure of slopes can trigger large scale landslides. The presence of sensitive clay layers is one of the main
reasons for progressive failure of a slope. The whole soil mass involved in a potential landslide might be of sensitive clay, while in
some cases there exist only thin layers of sensitive clay interbedded with relatively strong soils. The movement of a slope might be
initiated due to the presence of a weak soil layer, where the shear stress is increased or soil strength is reduced by various triggering
factors. Once the failure/movement is initiated in a small zone, the imbalanced force is transferred to the surrounding soil in which
slip surface might propagate in the form of a shear band through the sensitive clay layer even though the sensitive clay layer is
relatively thin. The propagation of shear band in sensitive clay is also associated with post-peak strain softening behaviour of soil. In
this study, upward progressive failure due to river bank erosion has been modelled using nonlinear post-peak strain softening
behaviour. It is shown that the pattern of propagation of shear band varies with soil type and slope geometry.
RÉSUMÉ : La rupture progressive de pentes peut déclencher des glissements de terrain de grande envergure. La présence de couches
d'argile sensible est l'une des principales raisons de la rupture progressive d'une pente. La masse de sol impliquée dans un glissement
de terrain potentiel pourrait être totalement constituée d'argile sensible, alors que dans certains cas, il existe seulement des couches
minces d’argile sensible intercalées entre des sols relativement résistants. Le mouvement d'une pente peut être initié en raison de la
présence d'une couche de sol peu résistant, dans laquelle le taux de cisaillement augmente ou sa résistance diminue en raison de divers
facteurs de déclenchement. Lorsque la rupture / le mouvement sont initiés dans une petite zone, le déséquilibre de force se développe
vers le sol environnant, dans lequel la surface de glissement peut se propager sous forme d'une bande de cisaillement à travers la
couche d'argile sensible, même si la couche d'argile sensible est relativement mince. La propagation des bandes de cisaillement dans
l'argile sensible est également associée au ramollissement post-pic du comportement du sol. Dans cette étude, la rupture progressive
amont due à l'érosion des berges a été modélisée par le ramollissement non-linéaire post-pic du comportement. Il est montré que le
faciès de propagation des bandes de cisaillement varie en fonction du type de sol et de la géométrie de la pente.
KEYWORDS: Sensitive clay, progressive failure, spread, shear band propagation, strain softening.
1. INTRODUCTION
Large landslides in soft sensitive clays are common in Eastern
Canada and Scandinavia. Most of the onshore slides which
occurred in soft sensitive clay have been reported as progressive
in nature (Bernander 2000, Locat et al. 2008, Quinn 2009, Locat
et al. 2011). The presence of strain-softening clay layers is one
of the main reasons for progressive failure of a slope.These
slides could be in the form of multiple retrogressive,
translational progressive or spreads (Tavenas 1984, Karlsrud et
al. 1984). Failure might be initiated in a fully stable and/or
marginally stable slope depending on the nature of triggering
factors. Failure could propagate either in upward or downward
direction and the movement of the slope might be initiated due
to the presence of a weak soil layer, where the shear stress is
increased or soil strength is reduced by various triggering
factors. Large landslides in sensitive clays classified as spread
(Cruden and Varnes 1996) might be triggered by erosion near
the toe of the river bank slope (Quinn et al. 2007, Locat et al.
2008). Numerous spread failures such as Sköttorp landslide in
Sweden (Odenstad 1951) and the landslides occurred in Quebec
including 1989 Saint-Liguori landslide (Grondin and Demers
1996), Saint-Ambroise-de-Kildare landslide (Carson 1979),
Saint-Barnabé-Nord slide (Locat et al. 2008) have been reported
to be triggered by erosion at the toe of the slope (Bernander
2000, Locat et al. 2008, Quinn et al. 2007, Locat et al. 2011),
although it is very difficult to identify the true disturbing agents
which caused these spread failures.
Progressive failure might occur in drained as well as
undrained conditions. Bjerrum (1967) explained upward
progressive failure initiation in an intact slope containing
overconsolidated plastic clays and clay shales and considered
the failure as drained. Sensitive clays from Eastern Canada and
Scandinavia show strain softening behavior under undrained
loading which has been considered as one of the main reasons
for developing progressive failure (Bernander 2000, Locat et al
2008, Quinn 2009, Locat et al 2011). Hence undrained
condition is considered in this study for analyzing spread
failure.
During Ormen Lange field development, numerical
simulations have been carried out by Norwegian Geotechnical
Institute (NGI) using PLAXIS software to analyze the potential
of retrogressive sliding due to strain softening effect in mild
clay slopes (NGI 2001). Anderson and Jostad (2007) conducted
numerical analyses of progressive failure of slope by modeling
the shear band as an interface element using the NGI finite
element (FE) code BIFURC. Quinn (2009) also demonstrates
the use of linear elastic fracture mechanics concept in
progressive failure of slopes.
This paper describes a numerical technique which can be
used to analyze the spread or upward progressive failure of a
slope typically occurs in river banks due to toe erosion.
2. PROBLEM DEFINITION
The geometry of the slope modeled in this study is shown in
Fig. 1. The river bank has a slope of 30
to the horizontal. A
thick crust of overconsolidated clay near the face of the slope
and below the ground surface is assumed. For simplicity the
