ABSTRACT

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.

INTRODUCTION

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. 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.
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   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.
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   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.
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   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.

GENERAL CONCEPT

"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 facilities.

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 dike.

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 the mainland.

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 CAISSONS

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 as follows:

1. 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.
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   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.
3. 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.
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   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.
5. 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.
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   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.

DESIGN CRITERIA

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 are:

Wind effect

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.

Wave effect

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 II scheme.

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 seabed level)

Seismic loading

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 design.

STABILITY VERIFICATION

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.

DURABILITY

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.

GEOTECHNICAL DATA

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.

CONCLUSION

We are convinced that Monaco's future is to look toward the sea, now that its territorial waters have been recognised by France.
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.

References:

1. 1. Ernst G. FRANKEL MIT Proceedings - CitTes Marines 95 - Monaco - A.5.1 1995 page 100
2. 2. RenT BOUCHET - Technical Adviser - Ministry of Public Work of Monaco Proceedings - CitTes Marines 95 - Monaco - 1995 - 1.2 page 14
3. 3. Jerzy WIANECKI - Chief Engineer - CEBTP - Annales - STcuritT des Structures sous l'action du vent - 1983 - fascicule de la page 70 a 99
4. 4. Jean Loup ISNARD - Bureau Vèritas Monaco S.A.M. Proceedings - CitTes Marines 95 - Monaco - 1995 - A.5.3 page 112