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A simplified procedure to assess the dynamic stability of a caisson breakwater
Une procédure simplifiée pour évaluer la stabilité dynamique d’une digue en caissons
Madrid R., Gens A., Alonso E., Tarrago D.
Dep. of Geotechnical Engineering and Geosciences, Technical University of Catalonia, Barcelona, Spain
ABSTRACT: The paper describes a simplified method of analysis used to evaluate the stability of a caisson breakwater to sea wave
actions. An intensive laboratory program was performed in order to evaluate the static and dynamic characteristics of the foundation
soil. Anisotropic and isotropic consolidated cyclic triaxial tests and cyclic simple shear tests were used to define the cyclic interaction
diagram for the foundation soil. The possibility of foundation cyclic mobility due to wave loading and their effect on the breakwater
stability was examined combining the cyclic interaction diagram with the results of finite element analysis. The potential reduction in
soil strength is then incorporated into a conventional stability analysis. The procedure is illustrated by a specific application to a
caisson breakwater that is part of the extension works of the Barcelona Harbour.
RÉSUMÉ : L’article décrit une méthode simplifiée pour évaluer la stabilité d’une digue verticale sous l’action de la houle. Les
caractéristiques statiques et dynamiques de la fondation ont été évaluées à l’aide d’un programme intensif de tests en laboratoire, qui
inclut des essais triaxiaux cycliques isotrope et anisotrope et des essais de cisaillement simple cycliques dans le but d’établir le
diagramme d’interaction cyclique du sol. La possibilité d’une mobilité cyclique de la fondation sous l’action de la houle et son effet
sur la stabilité de la digue ont été examinés en combinant le diagramme d’interaction cyclique ainsi obtenu avec une analyse
numérique par Éléments Finis. La réduction potentielle de la résistance du sol est ensuite incorporée dans une analyse de stabilité
conventionnelle. La procédure est illustrée par une application spécifique à une digue en caissons qui fait partie des travaux
d'extension du port de Barcelone.
KEYWORDS: cyclic tests, interaction diagrams, liquefaction, caisson breakwater, wave loading, stability.
1 INTRODUCTION
Two new breakwaters and a large container area, immediate to a
new quay, are the main development works of the ongoing
extension of Barcelona harbour. A plan view of the new
breakwaters and quays is shown in Figure 1.
Breakwaters have a total length of 6.8 km. The East
breakwater is of a rubble mound type whereas the South
breakwater involves two different types: rubble mound and
vertical caissons. This paper refers to the caissons section that
has a total length of 1.7 km constructed in water depths that
range from 20m to 25m. Most of the foundation soil
immediately under the breakwaters consists of weak sediments
of clayey silts and silty clays belonging to the pro-deltaic
deposits of the Llobregat River.
The paper describes summarily the main geotechnical
features of the foundation ground with special attention given to
undrained strength parameters. The bases for the static design of
the breakwater are then briefly presented. Finally, a description
of the cyclic resistance of the foundation soil is described in
terms of an interaction diagram; this information is then used in
a simplified assessment of the stability of the breakwater under
storm conditions incorporating the potential strength reduction
due to cyclic loading.
2 SOIL PROFILE CHARACTERISITCS
A representative soil profile at the location of breakwaters is
shown in Figure 2. It consists of: i) upper silts and clays, brown
and grey in colour, although dark colours occasionally appear
when organic matter content increases. The thickness of this
deposit underneath the breakwaters is about 50 m. Sandy and
silty sand inter-stratifications, were often found, specially in the
upper levels of the layer. ii) an intermediate layer of gravels and
sands, whose thickness is about 7 m; some silt partings were
also detected. iii) a lower level of clays whose identification
properties are similar to the upper clay unit, although it is a
denser soil. The maximum thickness of this layer is 14 m. iv) a
lower layer of gravels and sands; it includes several clays and
sands stratifications.
Figure 1. Plan view of the new breakwaters and new container areas of
the Barcelona harbour. The location of the caisson breakwater is
indicated.
Closer to the coast line, an upper deltaic sand deposit of
increasing thickness, laid on top of the upper stratum of soft
silty clays, appears. As it would be expected from a deltaic
environment the transitions between this sand deposit and the
upper clays are neither sharp nor regular. This sand deposit is 15
m thick at the shore line but it practically disappears at the
breakwater location and it is not considered further in this
paper.
