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Read the text and give your reasons for making a choice between a bridge and a tunnel.






Vast bodies of water, all over the world, represent natural obstacles to communication. People have been striving to overcome water obstacles by bridging the gap between continents, islands, etc. The English Channel separating Great Britain and Europe, the Straits of Gibraltar between Spain and Africa, the Bosporus between Europe and Asia, the Bering Straits connecting the Eurasian and American continents are of great economic importance as it is used as a shipping route. The shores of these straits and Japanese islands are dotted with villages, towns and cities, and might provide the missing link for the rapid increase in overland railway networks and motorway systems. Some water crossings are a mixture of bridges and tunnels, such as the Denmark to Sweden link and the Chesapeake Bay Bridge-Tunnel in the eastern United States. Examples of water-crossing tunnels built instead of bridges include the Holland Tunnel and Lincoln Tunnel between New Jersey and Manhattan in New York City, and the Elizabeth River tunnels between Norfolk and Portsmouth.

Builders always have the choice between bridge and tunnel crossings (fig. 12.1). Each sort of structure offers its own advantages and has some imperfections. One should bear in mind the influence of strong sea currents, great water depth, large-capacity vessels with great overall dimensions (the headroom must be about 65 m high for free shipping), and the difficult geological structure of the seabed. Seabed silt is rather soft for pier foundations, and besides, the risk of seismic activity may jeopardize the foundations. Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather and shipping during construction, aesthetic reasons, and for weight capacity reasons (it may be more feasible to build a tunnel than a sufficiently strong bridge).

The advantages of a bridge crossing may be the following: low construction costs in comparison with tunneling, though sometimes the reverse may be true; low maintenance costs in comparison with tunnels which require costly water discharging, ventilation, illumination, etc.; the longer the sub aqueous tunnel is then heavier outlays are required.

The advantages of the tunnel are:

1. Unobstructed shipping is very important for intensive navigation. Tunnels are much safer as compared to bridges because the piers must withstand berthing impact. Being deep beneath the water’s surface, tunnels do not interfere with navigation. Besides, the weather does not influence the traffic.

2. The design aspect of the tunnel is more attractive because there is no need to erect high approach embankments.

The final decision for choosing between the two structures depends on the fact that in some cases bridges are more convenient solutions which were realized in such outstanding structures as the bridge across the Bosporus constructed in 1974, the bridge crossing connecting some Japanese islands in 1985, and the bridge spanning the strait separating Denmark and Europe. Nevertheless, the choice fell on a tunnel underneath the English Channel. In 1994, the railway tunnel from Great Britain to France was put into operation. It provides a high-speed rail-link with shuttle trains reducing the travel time between the two countries to three-and-a-half hours. Needless to say, that the cost of this tunnel is enormous. Currently tunnels are widely used in states surrounded by water for instance; in 1987, some Japanese islands chose tunnels for safe and rapid communication. Making their well-reasoned choice, the engineers have to be up to different challenges. The problem “a bridge or a tunnel” is being discussed for planning future structures in Italy, across the Straits of Gibraltar, and across the Bering Straits. The choice falls on a bridge crossing in Europe, and on a tunnel for Alaska due to harsh northern conditions.

Exercises:






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