The project is an extension of the Thalys high-speed rail link that connects Paris to Amsterdam and Rotterdam. These trips currently take about three or four hours. When the new link is complete, travelers will be able to reach those cities from Paris within two hours on the French high-speed TGV train.

The Dutch Ministry of Transport selected a route crossing the Groene Hart, or Green Belt, in the Randstad region for the high-speed rail link. It is a classic Dutch landscape, dotted with channels, canals, windmills, and an important bird reserve. The Dutch authorities made it clear that the rail exten-sion must not mar the beauty of the region.

Project engineers from Bouygues Travaux Publics SA in Guyancourt, France, and Koop Holding in Groningen, Holland, decided to build the rail route underground, more than 100 feet below the water table, through polder. Polder is the soil, essentially sand and silt, the Dutch reclaimed from the sea with their national system of dikes. This complicated the project, because the loosely structured polder is more sensitive to excavation than denser soil, and could sink because of digging.

The engineers originally decided to bore two separate, parallel tunnels, each 30.5 feet in diameter, and connect the tunnels by means of safety passages excavated every 985 feet. This involved using costly soil freezing techniques to construct the communication passages because of the permeability of the soil. Both the number and size—8.5 feet in diameter—of the communication passages would raise the estimated cost of the project, already budgeted at more than $333 million, and extend its completion date.

The project engineers discarded this plan and designed a single-bore tunnel—5.3 miles long with a 43.6-foot diameter—wide enough for two trains to travel in opposite directions. This would cut the cost and labor required, reduce the size of the access works at each end of the tunnel, and minimize the overall working site area. Just as important, the single-bore tunnel would have less impact on the structure of the polder, and reduce the chance of surface disturbance.

The tunnel would be constructed in trenches protected by cast sheet piles and walls. It required a tunnel-boring machine with a 48.8-foot diameter. The largest the planners knew at the time was a 46.6-foot-diameter digger used to excavate across Germany's Elbe River.

The Groene Hart tunnel digger would have to meet strict environmental constraints, including noise limits of 55 decibels, surface settlement of less than 20 millimeters, and no pollution of the upper water table.

NFM Technologies, a subsidiary of Framatome, was commissioned to design and build the new tunnel-boring machine. Since its founding in 1989, NFM has supplied equipment that excavated metropolitan subway lines in Athens, Copenhagen, Madrid, and Shanghai.

NFM engineers have designed this tunneling machine to dig into sensitive Dutch soil without disturbing the picturesque surface landscape.

The cutting face of the tunneling machine is equipped with teeth and is attached to a rotating drum. Nineteen pairs of hydraulic cylinders powered by 3,500 kilowatts generate more than 18,000 metric tons of force that thrust the head against the tunnel wall and rotate it at 1.4 rpm to cut into the earth.

The loosened soil falls into the drum through openings in the cutting face as the machine advances at 4 centimeters per minute. Scoops in the drum dump the spoil onto a conveyor that transports it to carts at the rear of the machine for disposal.

Because the low-density polder could not counter the upward thrust of the tunneling machine, NFM designed a dual trailer train for the machine to build the tunnel supports as it proceeds. The two trailers are connected by a 230-foot-long beam. Workers remove tunnel lining components from the trailing train and ballast them with a mixture of sand, gravel, and cement. The concrete components are 2 feet thick and are molded to 0.3-mm tolerances.

Once completed, the tunnel will permit two TGV trains to pass in different directions while traveling at more than 187 mph. The two tracks will be separated by a concrete curtain whose upper portion will contain windows. The windows will open to relieve air pressure caused by the train's passage, but can be closed in case of fire. The engineers expect to complete the tunnel by the winter of 2005.

home | features | weekly news | marketplace | departments | about ME | back issues | ASME | site search

© 2001 by The American Society of Mechanical Engineers