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A Methodology for Evaluating Liquefaction Susceptibility in Shallow Sandy Slopes
Une méthodologie pour l'évaluation de susceptibilité à la liquéfaction dans les pentes sableuses
Buscarnera G.
Northwestern University
Whittle A.J.
Massachusetts Institute of Technology
ABSTRACT: The paper illustrates a modeling approach for evaluating the liquefaction susceptibility of shallow sandy slopes. The
proposed methodology consists of two main components: (i) a theoretical framework for undrained stability and (ii) the MIT-S1
constitutive model for simulating the response of sands. In the first part of the paper, the use of a stability index able to capture the
onset of undrained failure in infinite slopes is illustrated. In the second part, the practical significance of the method is discussed by
back-analyzing the series of flow failures in an underwater berm at the Nerlerk site. The reinterpretation of these events in the light of
the theory of material stability confirmed that liquefaction was a plausible mechanism for the failures. In addition, the analyses have
provided a prediction of the spatial distribution of the unstable masses which is compatible with what was observed through
bathimetric surveys conducted after the events. This particular application of the theory supports the idea that realistic constitutive
modeling is crucial for achieving consistent predictions of liquefaction potential under field conditions.
RÉSUMÉ: L'article illustre une approche de modélisation pour évaluer la susceptibilité à la liquéfaction des pentes sablonneuses peu
profondes. La méthodologie proposée se compose de deux éléments principaux: (i) un cadre théorique pour la stabilité non drainée et
(ii) MIT-S1, le modèle de comportement pour la simulation de la réponse des sables. La première partie du document illustre
l'utilisation d'un indice de stabilité capable de saisir le début de la rupture dans des pentes infinies dans des conditions non drainées.
Dans la deuxième partie, les implications pratiques de la méthode sont évaluées par rétro-analyse d'une série de ruptures par
écoulement dans une risberme sous-marine sur le site de Nerlerk. La réinterprétation de ces événements, à la lumière de la théorie de
la stabilité des matériaux, a confirmé que la liquéfaction est un mécanisme plausible pour expliquer ces défaillances. En outre, les
analyses ont fourni une prédiction de la distribution spatiale des masses instables, qui est compatible avec ce qui a été observé par des
mesures bathimétriques menées après les défaillances. Cette application de la théorie soutient l'idée qu’une modélisation réaliste du
comportement est essentielle pour faire des prédictions cohérentes de potentiel de liquéfaction dans des conditions de terrain réalistes.
KEYWORDS: sands, static liquefaction, flow slides, material stability, theoretical analyses, constitutive modeling.
1 INTRODUCTION.
Landslides and slope failures are widely recognized as one of
the major natural hazards affecting both the development of
densely populated areas in rugged terrain and the design of
artificial earthworks [Terzaghi 1957, Sladen et al. 1985b].
Within the general class of slope failures, runaway instabilities
or flow slides represent impressive phenomena that still raise
several open questions.
Even though various studies have been carried out on the
subject [Sladen et al. 1985a, Lade 1993], there is still need for
advanced tools of analysis that can explain catastrophic failures,
evaluate hazard levels in landslide prone areas and define
geotechnical design criteria. The purpose of this work is to
propose a predictive modeling methodology to study flow slide
phenomena. The proposed methodology aims to evaluate (i) the
shear perturbations that can trigger a flow slide, (ii) the spatial
distribution of soil masses prone to liquefaction and (iii) the
characteristics of the post-failure response of the slope.
2 MODELING FLOW SLIDE TRIGGERING.
The evaluation of liquefaction conditions based on geotechnical
criteria typically relies on the combined use of the critical state
theory and empirical observations from laboratory experiments
and in situ tests [Poulos et al. 1985]. Although these methods
provide guidance to assist engineering judgement, they lack
appropriate geomechanical foundations that can be applied to
general cases.
Indeed, the application of stability criteria to field conditions
requires accurate consideration of the realistic static-kinematic
characteristics of the problem at stake. An important example is
represented by shallow slopes, in which initial stress conditions
and kinematics are highly anisotropic and cannot be
appropriately represented through classical triaxial testing. One
of the first approaches to consider the role of soil behavior
through a comprehensive constitutive model was suggested by
di Prisco et al. (1995). In order to study the onset of a flow
slide, these authors considered the geometry of an infinite slope
(Figure 1) and modeled sand behavior through simple shear
simulations.
Figure 1. Reference system for a submerged infinite slope and initial
stress conditions.
The present paper discusses a methodology which is inspired
by this original idea, but tries to link it directly to the critical
x
