CONNECTION Steel ball-and-socket joint attaches floating dock to land base. (Photo courtesy of Dragados)

The 160,000-tonne floating main breakwater is "a prototype with a new level of

specification," says Fernando Ortiz, project director with the construction consortium leader Grupo Dragados, Madrid. But the team was surprised by the unusually dense reinforcing and design complexities imposed by the 12-day sea tow from the Spanish casting yard. "People were not always confident about the possibility of finishing this job," Ortiz says.

The project is a year late and currently pegged at $135 million-more than double the original estimate. The consortium and Monaco's Ministry of Public Works are working through claims.

But away from negotiations, the consortium has overcome the technical challenge of casting the 352-m-long, 28-m-wide, 19-m-deep prestressed box. With construction in Algeciras done, workers are prepping the breakwater for the tow to Monaco.

At the tiny but affluent enclave, the structure will join a smaller, French-built caisson to form the two wave walls outside the existing harbor at Condamine. From the south shore, the main box will reach diagonally across the port approach providing shelter from the east. It will be connected by a 650-tonne, steel ball-and-socket joint to one of several caissons enclosing 8,000 sq m of reclaimed land at Fort Antoine. The rest of the breakwater, anchored by chains, will float with just 3 m of freeboard.

Projecting from the opposite shore of the harbor entrance, the smaller box will end on the leeward side of the main breakwater, completing the new wave barrier. Because of shallower water, this smaller box will be propped from below at points 25 m and 65 m from its offshore end. Forming the props are vertical extensions at the ends of a 46-m-long, 17-m-deep concrete caisson, now resting on an underwater earth embankment 40 m below the surface.

PONTOON WALL DESIGN. The deep water and a weak sea bed at Port Condamine's entrance led the Ministry's designer, Paris-based Doris Engineering S.A, to plan the unusual pontoon-type sea walls, says the firm's project director François Sedillot.

SAFE New wings (tinted) will protect harbor (right). (Photo left courtesy of Bouygues, right courtesy of Doris Engineering)

Doris, which is also the owner's project manager and supervisor, produced drawings detailed enough for bidding the main design-build contracts for the breakwaters and associated works, both awarded in summer 1999.

First off the starting block late that year was a consortium including Paris-based Bouygues S.A., with Groupe Vinci and others. The team began work at La Ciotat, Marseilles, ultimately building seven caissons. Most of them went to enclose the land reclamation for the main breakwater's anchorage, forming part of that contract. The consortium also built the smaller breakwater and its seabed caisson support at the drydock.

The French breakwater is 145 m long, 30 m wide at its base, 11 m deep and will be positioned with 2 m above water. Because of its profile, it is smaller and more effective at calming waves than a rectangular version, claims Bouygues Offshore S.A., which developed the concept. Basically, a 5-m-long sloping projection at base slab level breaks oncoming waves. The perforated main vertical wall behind dissipates water energy through turbulence.

SETTING SAIL. The French team flooded the La Ciotat drydock in mid-May, and now is preparing the breakwater for its 200-km tow to Monaco later this month. The Spanish drydock will be flooded by then, according to the schedule. Once in deep water, the structure will take on additional ballast ahead of the planned delivery in July or August. The Spanish team, also including Fomento de Construcciones y Contratas S.A., with minor partners, began work at Algeciras in 2000.

(Illustration courtesy of Doris Engineering)

Designed to float with 16 m below sea level, the main breakwater is twin skinned. Its internal walls and slabs, all around 50 cm thick, are set several metres back to create ballast chambers. The whole box is stiffened with cross walls and bulkheads. For about half its length, precast floors will provide parking for 400 cars on four levels. The other half will contain two floors of boat stores. To improve wave-breaking performance, the box's base slab projects 8 m on either side.

(Illustration courtesy of Doris Engineering)

In shape and dimensions, the finished structure remains virtually unchanged from Doris' design, says Sedillot. The contractor's responsibility was to engineer the breakwater for temporary loading at sea. And it had to detail the rebar and prestressing, which was more daunting than first expected, he concedes. "It's...very congested in terms of density of reinforcing and prestressing," he says. The prestressing density of 80 kg per cu m of concrete is at the top end of the scale, he adds.

It was tough to design the box as a ship, accommodate the tight mesh of steel and pour concrete through it as well, says Ortiz. And the unusually high 100-year design life didn't help. The structure is "so new and so special" that he hopes for a sympathetic hearing during cost negotiations.

But Sedillot's response may not be encouraging. "From our experience in the North Sea and offshore structures in general, we have seen more difficult structures," he says.