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All lanes lead to Tangier

Tangier sea wall - Africa's largest port is rapidly taking shape in northern Morocco. NCEI reports on the construction of a new hub for international shipping.

Lying 35km east of Tangier, right on the Strait of Gibraltar, the new Tangier Mediterranean Port could hardly be better placed to win business from international shipping lines.

The port is on the crossing point of north Atlantic and Mediterranean trade routes. As well as providing an import and export route for goods entering and leaving Morocco, it will serve as a 'hub' for the transhipment of goods en route from one part of the world to another.

The port will handle some of the world's biggest ships. An estimated 25Mt of goods will pass through it each year. It will handle approximately 3M TEU containers - 20ft-equivalent units, the standard by which containers are measured. To give a sense of scale, all of the UK's ports together handle just over 7M TEU each year. It will also have a 12ha tank farm for storage of petroleum products.

The port is part of a larger regional development scheme, costing some $963M.

Associated works include construction of a railway line and new toll road network (see box). The normally quiet province is buzzing with activity as contractors race to complete the port and transport infrastructure ready for commencement of shipping operations next year.

French contractor Bouygues Construction won the $277M design and construct contract in June 2003, in a joint venture with Italian firm Saipem. Designer is Halcrow and the client is the Special Tangier Mediterranean Agency - part of the Moroccan government.

The 36 month construction programme includes work on two breakwaters, one of which is built from 40 precast reinforced concrete caissons in water up to 20m deep. It also involves 4,577,000m 3 of dredging and 660,000m 3 of rip-rap installation.

In 2005, Bouygues won an additional $39.6M contract, adding design and construction of a new oil tanker berth and bulk goods quay to its scope of works.

Bouygues won the contract on the strength of its Halcrowinspired breakwater design that created 18ha more operating water within the port than allowed in the schematic design.

In shallow water the breakwater is fairly conventional, composed of 7,500 Accropodes laid to form a huge partially submerged bund. In deeper water of up to 20m, however, the Accropodes were replaced with a wall of massive concrete caissons. By providing a defensive structure with sheer walls, rather than battered slopes, ships will be able to manoeuvre closer to the breakwater than had previously been envisaged.

As well as freeing up space, the design presents a lower environmental impact by reducing the scale of the sea-bed ground works required and the volumes of the materials needed.

It also offered significant time savings during construction.

The design evolved following analysis of ocean currents and swell. The caissons in particular were subjected to physical modelling to test their stability and perfect their four-leafed clover geometry.

Research into concrete was also carried out to find a low shrinkage mix that would minimise cracking and remain impermeable and resistant to chloride attack over the port's 100-year design life.

The caissons are the height of 10 storey buildings and in plan are approximately 40m 2. Forty are needed to complete the breakwater wall. They have been cast at a rate of one every week in a fabricating yard on land.

Only the base and first 9m of the caisson walls are constructed before they are transferred to the water for completion - weighing in at 3,200t, the part-built caissons would be impossible to shift if construction were taken any further.

Transporters are used to move the caisson walls. Once in dry dock, a further 15m of height is added, bringing the total weight up to 6,000t.

Caissons are towed to site and sunk into position with the aid of GPS. To avoid battling with the currents that race through the Strait of Gibraltar, this is done during the period of slack water around low tide.

Once in place, the caissons are ballasted with 13,500 m 3 sand. Cast insitu, concrete is poured to bring the breakwater wall up to a total height of 35m.

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