Academic Reading — Test 90

Reading passage

For more than two centuries, engineers and politicians had dreamed of a fixed connection beneath the waters that separate Britain from mainland Europe. Several schemes were proposed during the nineteenth century, and a few short exploratory shafts were even dug, yet anxieties about national security repeatedly halted progress. It was not until the late twentieth century that the necessary finance, political agreement and engineering confidence finally came together. Construction of the Channel Tunnel began in 1988, and after six years of intensive labour the rail link was officially opened in 1994. The completed system was quickly recognised as one of the most ambitious civil engineering achievements of its era. The tunnel is not a single bore but a system of three parallel tunnels running between Folkestone in the south of England and Coquelles, near Calais, on the French coast. Two of these tunnels carry trains, one in each direction, while a smaller central tunnel serves as a service and safety passage. The service tunnel is connected to the two running tunnels at regular intervals by cross passages, allowing maintenance staff to reach equipment and providing an escape route should an emergency occur. Pressurised air keeps the central tunnel slightly cleaner than the rail tunnels, helping to prevent smoke from spreading in the event of a fire. Altogether the tunnels stretch for roughly fifty kilometres, of which about thirty-eight kilometres lie beneath the seabed, making it one of the longest undersea sections of railway in the world. The route was chosen with great care, largely because of the geology beneath the Channel. Engineers directed the tunnels through a layer of chalk marl, a soft yet stable rock that is relatively easy to excavate and, crucially, resists the passage of water. Boring through this consistent stratum reduced the risk of flooding and allowed the tunnelling machines to advance steadily. Before digging began, extensive surveys of the seabed were carried out so that the engineers could follow the chalk marl layer as closely as possible. Where the rock was less predictable, progress slowed and additional reinforcement was required, which added to both the cost and the duration of the project. The excavation itself was performed by enormous tunnel boring machines, often abbreviated to TBMs. These machines combined cutting, spoil removal and tunnel lining into a single continuous operation. A rotating cutting head at the front ground away the rock, while behind it the machine installed curved concrete segments to form the permanent lining of the tunnel. Eleven such machines were used in total, working outward from both the English and the French coasts. Guiding them accurately over tens of kilometres demanded sophisticated laser surveying, since even a small error in direction could have caused the two halves of a tunnel to miss one another. When the British and French service-tunnel teams finally met beneath the sea in 1990, the two bores were aligned to within a few centimetres, a remarkable demonstration of precision. Operating a railway deep underground created problems that ordinary surface lines never face. The friction of trains moving at high speed through a confined tube generates a great deal of heat, and without intervention the temperature inside would have risen to uncomfortable levels. To counter this, a cooling system using chilled water pipes was installed along the length of the tunnels to carry the excess heat away. Ventilation equipment supplies fresh air and can be used to control the movement of smoke during an emergency. Power for the trains is delivered through overhead lines, and signalling is managed centrally so that controllers always know the exact position of every train. Drainage pumps remove the water that inevitably seeps in, preventing it from accumulating on the track. The financial story of the project proved as dramatic as its engineering. The tunnel was built by private companies rather than funded directly by the two governments, an arrangement that placed considerable strain on the businesses involved. Costs rose far beyond the original estimates, and the project ended up costing roughly twice what had first been predicted. For many years the company that operated the tunnel struggled with heavy debts, even though passenger and freight services were popular from the outset. Despite these difficulties, the link transformed travel between Britain and the Continent, allowing passengers to journey from London to Paris in a little over two hours and giving freight a fast, weather-independent route that ferries could not match. Today the tunnel is widely regarded as a landmark of international cooperation as much as of engineering skill.