Monaco, the smallest country in the world after
the city of Vatican, has no longer space available to expand. Thirty
years ago Monaco regained one seventh of its territory over the
sea. Which other nation has done so? Today, given the depth of its
surrounding seabed, Monaco must imperatively consider the most advanced
offshore technolgies to create artificial islands in order to give
Monaco the maximum possible expansion area.
The concept of an offshore artificial island,
located close to the dike of Fontvieille, the latest Monegasque
district won from the sea, results from a reflection over the urbanization
of our territory, which identifies itself with the city of Monaco.
Today, no further real estate development is possible on land.
Already 30 year ago, in order to create a new
22 hectares development area after backfilling, it was decided to
build a dike, a kilometer long in 40 meter of water depth off Fontvieille.
Today, this reclaimed area is fully built and
the need for additional space is there again. Expanding further
the Fontvieille's district toward water depth rapidly increasing
to 60/80 meters is not feasible, therefore the concept of an island-based
tower becomes attractive. We believe the solution of the future
is to create space on multiple levels rather than horizontally.
Several international architects have considered
Island projects at various locations around Monaco, particularly
Mr. Kikutake, a Japanese architect specialist in "marine cities".
Our project, at this particular location in front of Fontvieille,
calls upon well established offshore technologies and is, in our
opinion, fitting very well within the typology of this unique location.
Lessons learnt from the construction
of the Fontvieille dike
As already mentioned the Fontvieille dike was
built on a berm of rock dumped on the seabed, over which reinforced
concrete caissons were placed . The caissons fabricated in Genoa
(Italy), some 150 km from Monaco, were surface-towed to location
and flooded in place.
We were involved in the project at the time, and our main task was
to adapt the Italian structural design of the caisson to French
norms and standards.
We like to emphasise that, having been involved
with this project since its inception and for over 30 years, we
have learnt a few lessons and drawn some conclusions:
- 1. At the time we had difficulties with the concept
that after building such a dike the enclosed water behind still
had to be backfilled to provide a constructible area.
2. The method of designing the foundations of the various building
erected on this land filled area renforced our scepticism. Effectively,
the foundations were designed to reach solid grounds, on average
25 meters deep, though the various backfilled materials. As an
example, we can mention the Louis II stadium for which the below-ground
foundation structures represented some 50 per cent of the total
above-ground civil work.
3. When seismic design standards were introduced later in Monaco,
the question was raised of the dike ability to substain seismic
induced loads. Fortunately, a group of experts, formed of three
independent teams respectively French, American and Italian, concluded
that in the event of seismic tremors, there was no risk of soil
liquefaction underneath the dike.
4. Finally the economic aspect, resulting from the previous considerations
is overwhelmingly in favour of our artificial island concept,
based on offshore construction methods.
Professor Frankel (1), of the Boston MIT, stated
that the cost of offshore platform, per m2, has been halved over
the last 10 years and that today's investment in offshore facilities
is in the order of US$ 200,000 per man (for 50 m2). This is comparable
to the cost of land acquisition per m2 of floor in Monaco today.
The Fontvieille's development has demonstrated
that, beside the significant inconvenience imposed upon the local
residents, the overall cost of the dike and buildind foundations
were higher than the above estimate.
We admire the major offshore projects undertaken
worldwide and we endorse the studies performed by the Ministry of
Public Work of Monaco under the management of Mr. RenT Bouchet (2)
for a new dike and the Fontvieille II scheme.
However, we believe that our project "La Tour
de la Mer" (The Sea Tower) provides a feasible and realistic alternative
to "Fontvieille II" for expanding Monaco toward the sea.
"La Tour de la Mer" is a buildind rising 390
m above the sea. It will be built on three artificial islands, each
consisting of a circular concrete caisson with a diameter in the
order of 100 m.The three caissons will rest on the seabed at respectively
-90 m, -85 m and -60 m. Arranged in a three point star shape, the
caissons will receive the loads of the Tower legs and will tranfer
them to the cretaceous seabed.
At sea level piers of reinforced concrete,
supported by the caissons at one end and tripods at the other end,
will support concrete slabs. The pre-stressed concrete slabs will
provide for atifical beaches, harbours and other leisure facilities.
The three legs of the tower, also arranged
as a three point star, will be supported by the caissons and will
converge in a single platform at elevation 66 m. From this level,
still in a star shape, the tower will rise to respectively elevations
336 m, 360 m and 390 m for the tallest section.
Every fifth floor above the level +66 m, a
"garden slab" will provide a green environment for the residents
and the necessary stiffening of the structure to resist the torsional
stresses induced by wind or seismic loads.
The floors above 66 m will provide accomodation
while the floors below will be dedicated to office space and leisure
Within the caissons parking facilities as well
as utilities for the day to day running of the tower will be accommodated.
A suspension bridge anchored to the tower structure
and some 9 meters above sea level, will link the tower to the Fontvieille
The road bridge will allow vehicles to reach
the Tower parking facilities and importantly it will also provide,
through its structure, for all the necessary technical links with
A FEW NUMBERS
The areas dedicated to leisure activities are
estimated at 50,000 m2, office space at 100,000 m2, while accomodation
will be in the order of 200,000 m2. The garden areas will be 40,000
m2, while the new marina created between the dike and the tower
will represent some 70,300 m2.
The concept is partially derived form the North
Sea Ekofisk Central Complex outer wall and other offshore concrete
structures. A watertight double wall ring is built firstly in a
dry dock and later in a floating mode using the conventional "slip
forming" method. The ring caisson is surface-towed to its final
location and sunk in place. A seal will enable emptying the inner
volume of the caisson, thus providing a dry environment for the
later stages of construction.
The construction steps are briefly described
- Step 1: At a dry dock the base of a double wall
ring, some 100 m in diameter, is built to an elevation sufficient
to provide a positive buoyancy. The base of the watertight structure
is designed with a sealing system for future installation purpose.
The double wall structure provides the necessary buoyancy, strength
and stiffness for the caisson.
Step2 : The dock site is flooded and the ring caisson towed to
a protected location (deep harbour or bay) with sufficient water
depth for the structure final draft.
- The caisson is built by the "slip forming" method
to its ultimate height, that is several meters above sea level
when in place. The double wall, strengthened by a cellular structure,
is flooded step by step to ensure the caisson stability and to
facilitate the construction work. This is commonly done for example
in the Norwegian fjords.
Step 3: The seabed at the site offshore Fontvieille is prepared,
for example by dredging, in order to remove unconsolidated materials
and to provide a solid and levelled base to receive each caisson.
- Each caisson is surface-towed from the construction
yard to its final location and sunk accurately in place by additional
controlled flooding of the double wall cells. Offshore positioning
allows today an accuracy in the order of 1 meter or even better.
Step 4: When the caisson is in place the sealing system is activated
and the inner volume of the caisson can be emptied. The concrete
foundations are then built in a dry environment. Inside the caisson
the three dimensional steel structures required to transfer the
loads of the tower legs to the seabed are erected, thus achieving
continuity of the two structures. Within each of the three caissons,
various facilities and parking areas can be accomodated on several
levels between the transfer structures, since the total height
available will be 50 meters or more. Thereafter, the construction
work above sea level is carried out in a more conventional manner,
although the site is located several hundred meters offshore.
This conceptual design already takes into account
the stability design criteria that such structure shall meet.
It is of course essential that the implementation
of the various design criteria be carefully coordinated as it is
always the case for major civil engineering projects.
Taken individually the three main criteria
Mr. Jerzy Wianecki (3), Chief Engineer of the
CEBTP, when advising on the design of the Louis II stadium canopy
made suggestions regarding the aerodynamic of such major structures.
Taking these suggetions into account we will
in parallel, calculate the wind load applied to the overall caissons
andsuperstructure system and perform aerodynamic model tests in
wind tunnel. In accordance with the French norms, the building must
be designed for a wind speed of 200 km/hour. This generates, at
the foot of the tower, a shear load in the order of 7,000 tonnes
and a tilting moment of 1,000,000 tonnes/meter at sea level. This
wind load is to be combined with wave loads.
On this matter, we obtained the advise of Mr.
Bouchet (2), Technical Adviser to Monaco's government, who has carried
out, for many years, studies related tot Monaco offshore projects
such as the new dike of the Hercule Harbour and the Fontvieille
For a water depth in the order of 70 meters
and for a heavy structure, we have to account for the full wave
loading over the caissons. The criteria is the maximum wave increased
by 10 per cent for a return period of 100 years.
The wave height is 8 meters and the resulting
load is 10 tonnes per m2 applied between -10 m and sea level
The combined wind and wave loads are:
- total shear load: 36,000 tonnes
- combined tilting moment: 3,3 million tonnes/meter (applied at
The overall structure design shall meet Monaco's
requirements, particularly a maximum acceleration of aN = 3,0 m/s2
applied to the substratum.
In compliance with these rules, we calculated
a shear load of 85,000 tonnes at the base of the caissons and a
tilting moment of 23 million tonnes/meter. It is clear that seismic
loading is by far the most stringent criteria for the structural
Seismic loading is by far worst case. The weight
alone of the Tower provides a stability coefficient of 1,8 which
can be improved by anchoring or increasing the weight of the caissons.
Unless specific preventive measures are taken,
reinforced concrete is subject to ageing in sea water.
The experience gained offshore with the giant
concrete platforms wil be very useful although the IslandTower shall
be designed for at least 100 years against the usual offshore design
life of 30 years.
Bureau Veritas (4) (France) has developed specifications,
in line with the DnV Norwegian standards, which guarantee a design
life of 100 years for such application.
A survey performed for the Monaco Publics Works
enables us to establish a profile locating the sands and scree layers
above the parent cretaceous rock through the unconsolidated materials.
We are convinced that Monaco's future is to
look toward the sea, now that its territorial waters have been recognised
Our project is the recognition of this strong concept.
Over the last 700 years, under the Grimaldi's
rule, we have seen Monaco prosper from the sea. At the end of the
last century Prince Albert Ist, a great oceanographer, built the
oceanographic museum, a temple to the sea. The building clings to
the "Rocher" of Monaco, but already has its feet in the sea. Following
this example, Prince Rainier III has won one seventh of Monaco territory
over the sea.
Today, our project follows the same logic.
It is a human and technical challenge that Monaco shall take on
at the beginning of this third millennium.
- 1. Ernst G. FRANKEL MIT Proceedings - CitTes Marines
95 - Monaco - A.5.1 1995 page 100
- 2. RenT BOUCHET - Technical Adviser - Ministry
of Public Work of Monaco Proceedings - CitTes Marines 95 - Monaco
- 1995 - 1.2 page 14
- 3. Jerzy WIANECKI - Chief Engineer - CEBTP - Annales
- STcuritT des Structures sous l'action du vent - 1983 - fascicule
de la page 70 a 99
- 4. Jean Loup ISNARD - Bureau Vèritas Monaco S.A.M.
Proceedings - CitTes Marines 95 - Monaco - 1995 - A.5.3 page 112