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

Unit 12

BRIDGE OR TUNNEL?

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 the busiest shipping routes. The opposite shores of these straits are dotted with villages, towns and cities that need safe and reliable transportation by overland or underground railway and motorway networks.

Each time, considering the problem of individual water-crossing facilities, civil engineers, who plan, design, construct and maintain roads, bridges, tunnels, and similar structures, face many challenges. They always have the choice between bridge and tunnel crossings (fig. 12.1, 12.2).

Figure 12.1 Bridges connecting separated areas

 

Each sort of structure offers its own advantages and has some imperfections. One should bear in mind the influence of strong sea currents, high water depth, large-capacity vessels with large overall dimensions (free shipping needs the headroom of 65 m high), and difficult geological structure of the seabed. For instance, seabed silt is rather soft for pier foundations. Tunnels circumvent difficulties with tides, weather and shipping during construction. Besides, destroyed by a terrorist attack or during battles, bridges can block the shipping channel for weeks or even months before the wreckage could be removed.

 

Some water crossings are a combination of bridges and tunnels. A bridge-tunnel is a facility where a bridge passes into an underwater tunnel. Such unique water-crossings include the Denmark to Sweden link, the Chesapeake Bay Bridge-Tunnel in the United States (fig.12.3), the Tokyo Bay Aqualine, etc. The Tokyo Bay Aqualine is a marine crossing consisting of a tunnel, a bridge and two artificially made islands. Its total length is more than 15 km.

 

The Chesapeake Bay Bridge-Tunnel is a complex including two low-level trestles that are 19 km each; four artificially made islands that provide portals for entering the tunnels; two deeply sunk long tunnels beneath the shipping channels; two high-clearance bridges that are more than 3 km long; causeways and 9 km of approach roads. The length of each tunnel is 1.5 km long. This complex, opened to traffic in 1964, is 37 km long.

The next unique structure is the Oresund Bridge by Georg Rotne, built across a narrow channel between Sweden and the Danish island of Zealand (fig.12.4). Queen Margrethe II and King Carl XVI Gustaf opened the link to traffic on July 1, 2000. The structure is the longest combined road and rail bridge in Europe, and the longest border crossing bridge in the world. The international European route E20 runs across the bridge and through the tunnel connecting Copenhagen and the Swedish city of Malmo. The Oresund Bridge is a striking spectacle as its 204m high pylons with their harp-shaped stay cables are visible from the Swedish and the Danish sides.



The total bridge length is 7,845 metres, its width is equal to 23.5 metres, and its weight is 82,000 metric tons. It has one of the longest cable-stayed main spans in the world equal to 490 metres and a navigational clearance of 57 metres. This fixed link is on two levels and carries two rail tracks beneath four motorway lanes. The railway runs along the lower deck while the motorway runs along the upper deck. People travel over the bridge by cars, buses and trains that run every 20 minutes, and once an hour during the night. The daily volume of traffic across the bridge is 17,000 road vehicles but the toll for driving the fixed link is high.

The bridge ends in the middle of the Oresund Strait (the English name of the strait is the Sound) and passes into an immersed tunnel on an artificially built unpopulated island that is more than four km long and a few hundred metres wide (fig. 12.4b). The tunnel connects this island and the nearest populated part of Denmark. It is 4,050 m long and has two 270-metre gate-tunnels. The tunnel provides a clear passage for boat traffic. Since the structure is actually a bridge and a tunnel, technically it is more correctly to call it the Oresund Link or Oresund Connection. The total cost of its construction, including motorway and railway connections on land is expected to be paid back by 2035.

Examples of water-crossing tunnels built instead of bridges include the Holland Tunnel and Lincoln Tunnel under the Hudson River in New York City, and the Elizabeth River tunnels in the USA.

The advantages of a bridge crossing are 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. Lane capacity in tunnels is also lower when compared to bridges. In addition, underwater tunnels have vehicle restrictions for hazardous cargo, and require 24-hour staffing known as patrol officers for incident management and operation under life-threatening emergency.

The advantages of the tunnel are also obvious. Being deep beneath the water’s surface, tunnels provide unobstructed shipping that is very important for intensive navigation. Besides, the weather does not influence the vehicles running through the tunnel. The design aspect of the tunnel is more attractive because there is no need to accommodate high approach embankments.

The final decision for choosing between the two structures often falls in favour of bridges, for instance, the outstanding bridge across the Bosporus constructed in 1974, and the bridge crossings connecting several Japanese islands in 1985. Nevertheless, the choice fell on a tunnel underneath the English Channel in 1994. It provides a high-speed rail-link reducing the travel time between Great Britain and France to three-and-a-half hours. The cost of this tunnel is enormous.

Currently, tunnels are widely used for rapid communication in states surrounded by water because they offer a much safer way than bridges or ferries traveling through stormy sea. Japan built the Seikan Tunnel because a typhoon sank five ferryboats crossing the Tsugaru Strait in 1954. Having considered a variety of solutions the Japanese government proposed a railway tunnel because any bridge that could withstand severe weather conditions would be too difficult to construct and maintain.

The problem “a bridge or a tunnel” is under discussion for planning future structures across the Straits of Gibraltar and the Bering Straits. The choice falls on a bridge crossing in Europe, and on a tunnel for Alaska due to harsh northern conditions. The idea of constructing an intercontinental bridge or tunnel between Alaska and Siberia is actually not very new as Tsar Nicholas II dreamed of the railway and tunnel that could close the gap between Siberia and North America. Currently this bold idea needs governmental study by both the RF and the USA. A 65-mile-long tunnel, built in three sections, could pass underneath the Big Diomede and Little Diomede islands in the Bering Strait. It could be twice the length of the Channel Tunnel and might cost approximately $10-12 billion. It would be the world’s longest tunnel that could improve a railway network of the Northern Hemisphere and provide efficient transportation corridor for goods and passengers as well as transmission lines and communication by fiber optic cables.

Exercises


Date: 2015-12-11; view: 565


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