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The Rion-Antirion Bridge
Located 250 km west of Athens (Greece), the Rion-Antirion
Bridge will, as of 2004, provide a new crossing over the
Gulf of Corinth. With a reference span of 560 meters,
the Rion-Antirion Bridge ranks as one of the ten largest
bridges: built on four piers with each foundation reaching
a diameter of 90 m, this cable-stayed bridge features
the longest suspended deck in the world (2,252 meters).
The bridge has been designed to withstand the collision
of a 180,000-ton oil tanker traveling at 16 knots, winds
of up to 250 km/hour and earthquakes of magnitude 7 on
the Richter scale.
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Physical centrifuge modelling for large civil engineering structures
Each large civil engineering structure displays
the particularity of constituting a prototype: engineers
are not always given prior examples to serve as
references in predicting the behaviour of their
structures. Moreover, major structures being built
throughout the world tend to be increasingly bold
and often rely on concepts based on latest innovations.
The tools available to the engineer for designing
and sizing an exceptional structure lie in the domains
of numerical and physical modelling. Numerical modelling
provides the transcription, in mathematical terms,
of an idealized approach to conceiving the structure,
whereas physical modelling consists of conducting
actual experiments on a reduced-scale model of all
or part of the structure. A physical model therefore
enables not only validating theoretical models,
but visualizing and quantifying structural behaviour
as well, especially in extreme situations of non-linear
behaviour or failure loading (earthquakes, shocks,
violent storms, etc.).
These physical models still require quite sizable
installations (wind tunnels, vibrating tables, geotechnical
centrifuges, etc.). In order to study foundations
and soil-structure interactions, the rules of similitude
(established for the most part by Edouard Phillips
in 1869) lead to testing reduced-scale models under
macrogravity conditions in a centrifuge. The centrifugal
acceleration must be n times greater than gravity
on earth if the model is built to a 1/n scale.
Site characteristics
The challenge of this project is to build a structure
that connects Peloponnesus to the continent under
extremely adverse environmental conditions: a depth
of 65 m midway across the strait, foundation soils
of mediocre quality (Quaternary alluvial deposits
exhibiting poor characteristics on beds with thicknesses
exceeding 500 m), high seismic risk (an earthquake
of magnitude 7 capable of having its epicenter on
a fault located approximately 8 km away, thereby
causing maximum accelerations of 0.5 G at ground
level). The structure must also be able to adapt
to significant tectonic movements (2 m offsets between
two consecutive supports, both vertically and horizontally).
Foundation-laying principle
The foundation-laying principle represents a breakthrough
innovation for construction in a seismic zone. Each
tower is supported by a 90 m diameter caisson, which
in turn lies on the seafloor whose ground has been
previously reinforced by means of rigid vertical
inclusions. Numbering between 150 and 200 underneath
each support, these inclusions are tubular (2 m
in diameter, 25 to 30 m in length) and driven in
a grid approximately 7 m by 7m. A 3.60 m layer of
ballast is inserted between the upper surface of
the marine clay and the lower surface of the basement
slab enclosing the tops of the inclusions, thus
ensuring no contact between foundation and inclusions
at all times.
This entire layout (gravel plus rigid inclusions)
serves both to control the foundation failure mode
in the case of an exceptional event and to limit
the forces transmitted to the superstructure. The
gravel acts as a fuse and the ground reinforcement
enables avoiding the formation of deep rupture surfaces,
which would induce sizable rotational movements
on a 230 m high tower.
This innovative foundation-laying process was adopted
by the GEFYRA consortium, which was commissioned
to build the bridge and within which the VINCI DUMEZ-GTM
Group was named to coordinate the project.
Centrifuge testing
The tests were conducted on the centrifuge at the
LCPC Nantes Laboratory upon the request of Mr. Alain
Pecker, Director of the firm GEODYNAMIQUE &
STRUCTURE (contracted to design the bridge foundation
system). The clay deposits on site were used for
these experiments. Mixed with water in proportions
to obtain a more fluid material, the clay was first
cleaned of all coarse elements (shells, rocks, seaweed,
etc.) and consolidated into several layers in 89-cm
diameter containers. The inclusions were then driven
into the clay (some inclusions had been fitted with
strain gauges to measure the force absorbed); next,
the ballast and circular foundation were set into
place.
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Placement of the inclusions at 1 G in the
(natural clay) soil mass |
The preparation stage is concluded by installing
the loading devices (servo-jack and electrical motor)
and setting up the instrumentation (monitoring cameras
and a series of sensors to measure: applied forces,
vertical and horizontal displacements of the foundation,
settlement, pore pressure, etc.).
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Submerged foundation and experimental device |
The container was then placed into the centrifuge
and first submitted to a consolidation phase at
100 g for several hours until the requisite consolidation
ratio, as determined according to the "Asaoka"
method, had been reached. A series of static penetrometer
tests were performed in order to verify the mechanical
characteristics of the clay, without having to stop
the centrifuge. The loading tests could then get
underway and the foundation was submitted to various
types of loads of increasing magnitude: horizontal
loads associated with reverse moments and alternating
static and cyclical loading sequences. The final
loading was deliberately pushed up to the point
of causing foundation system failure. Once the centrifuge
had been stopped, the soil block was cut into vertical
cross-sections to determine both the internal deformations
and rupture kinematics..
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Post-test state of the embankment
modelled with sand |
Visualization of failures
after cyclic horizontal loading |
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These experiments enabled
verifying the effective resistance of the foundation
system to loading conditions, in addition to deriving
both the maximum allowable loading diagram and the
rupture mechanisms. The experimental data necessary
for validating the theoretical and numerical approaches
were also obtained. The tests performed on reduced-scale
models helped to optimize the number and position
of reinforcement inclusions.
It may be added that the group of international
experts assigned to verify the design (including
Professor Peck) was on hand to witness a test. Moreover,
the response of the bridge to wind effects was studied
on reduced-scale models in the wind tunnel operated
at the Nantes Testing Center of the CSTB (building
industry research organization) (aerodynamique.cstb.fr)
and the stay cables were tested on the fatigue
testing bench at LCPC's Nantes facility.
For further information:
GARNIER J., PECKER A. (1999). Use of centrifuge
tests for the validation of innovative concepts
in foundation engineering. 2nd Int. Conf. on Earthquake
Geotechnical Engineering, Lisbon, June, 7 p.
PECKER A. (1998). Apport des essais en centrifugeuse
au dimensionnement d'une fondation de pont.
Colloquium entitled: "Mécanique et
géotechnique. Jubilé scientifique
de Pierre Habib". Luong (Ed.) ISBN2 7302
0521 3, pp. 143-153.
PECKER A. (2000). Pont de Rion-Antirion : fiabilité
et conception parasismique des fondations. La sécurité
des grands ouvrages, Presses ENPC, pp. 21-51.
RUSSEL H. (2000). Greek triumph - A bridge is born.
Bridge design & engineering, Fourth quarter,
pp. 29-36.
TEYSSANDIER J.-P., COMBAULT J., PECKER A. (2000). Rion-Antirion, le pont qui défie les séismes.
La Recherche 334, September, pp. 42-46.
ARCHIMEDE-ARTE 2002 Conception du pont Rion-Antirion (5 march). Video. |
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