Latin America’s first underwater tunnel

The design and construction of the tunnel beneath the Coatzacoalcos River, in the state of Veracruz, is the first project of its kind in Mexico and the very first project in Latin America to be built out of reinforced concrete, using pre-tensioned concrete caissons first sunk into the riverbed and then connected to one another.

The tunnel will link the centre of the city of Coatzacoalcos, Mexico's major industrial port, with the Allende district, home to important petrochemical plants. The two locations are situated on either side of the river's mouth, so one factor taken into account in this project was to avoid obstructing navigation on the Coatzacoalcos River while construction was under way.

The project involves the construction of a road a total of 2,280 metres long, which runs through the underwater tunnel for 805 metres of its length. The road has four lanes that are 3.5 metres wide in the outdoor sections and 3.75 metres wide inside the tunnel.

The underwater tunnel is designed to last 100 years. It is built out of reinforced-concrete caissons pre-tensioned longitudinally to withstand seismic stress. The caissons are placed on the riverbed at a depth of 30 metres below sea level and then topped with a smooth, flat covering layer and sediment.

The Coatzacoalcos underwater tunnel is a real technological innovation for the construction industry, because it uses the immersed-tunnel method, a procedure that enables builders to precast reinforced-concrete tunnel sections in a drydock, prepare them for floatation, tow them out and lay them on the sea floor. It resembles the pipe-jacking process used to build steel pipelines for petroleum products, and it requires no special subsoil drilling machinery.

FCC is experienced at building this kind of major infrastructure at a dry site and then shipping it out to its definitive site by water, whether the infrastructure floats on its own or requires aid. Some such projects have posed special challenges, like the dock built in a drydock in Algeciras Bay in southern Spain and then towed all the way to its final position in the harbour at Monaco.

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