Academic Reading — Test 27

Reading passage

When the Channel Tunnel was constructed beneath the sea separating England and France during the late 1980s and early 1990s, the project relied upon some of the most sophisticated tunnelling equipment ever assembled. The link is, in fact, not a single passage but three parallel tunnels: two large running tunnels that carry trains, and a smaller central service tunnel positioned between them. To excavate this network, engineers commissioned eleven enormous tunnel boring machines, frequently abbreviated to TBMs. These machines were not bought ready-made from a shelf; each was designed and built specifically for the geological conditions expected on either side of the Channel, and the British and French teams approached the task with notably different equipment. A tunnel boring machine is, in essence, a mobile factory that advances slowly through rock. At its front sits a rotating circular cutting head, studded with hardened steel discs and picks that grind away the rock face as the head turns. Behind this head, the excavated material, known as spoil, is gathered and carried backwards along conveyors. The machine braces itself against the surrounding ground and then thrusts forward using powerful hydraulic rams, inching ahead by roughly a metre and a half before resetting and pushing again. The largest of the Channel machines measured over two hundred metres from the cutting head to the trailing equipment at the rear, and the cutting heads themselves were close to nine metres in diameter for the running tunnels. Crews lived and worked aboard these machines for long shifts, since the workplace travelled forward with the excavation itself. The geology of the two coasts dictated much of the engineering. Beneath the English side, the tunnels were driven mainly through a layer called chalk marl, a relatively soft, water-resistant material that proved ideal for boring. Because this chalk marl tended to keep groundwater out, the British machines could operate as so-called open machines, without elaborate sealing against water pressure. The French side presented a far greater challenge. There the rock was more fractured and heavily saturated with water, so the French engineers required closed machines capable of withstanding considerable water pressure and even of being sealed off entirely should the sea break in. One French machine was specifically designed so that it could be flooded and later pumped out without being destroyed. As each machine advanced, it left behind a finished tunnel wall. The lining was assembled from curved segments, made of either reinforced concrete or, where conditions were most demanding, cast iron. A mechanical arm called an erector lifted each heavy segment into position, and a complete ring of these segments was bolted together to form a strong, watertight collar around the bore. This lining work happened continuously and in close coordination with the forward thrust of the machine, so that excavation and construction proceeded almost as a single combined operation rather than as separate stages. Navigation underground demanded extraordinary precision. The two national teams were boring towards one another from opposite coasts, and the service tunnel breakthrough in December 1990 was the moment when British and French workers first met beneath the seabed. Laser guidance systems continuously checked each machine's position and alignment, comparing its actual path against the surveyed route so that operators could make tiny corrections. When the two halves of the service tunnel finally joined, the error in their alignment was a matter of only a few centimetres over many kilometres of tunnelling, a remarkable demonstration of underground surveying. The fate of the machines after they had finished their work was almost as striking as their performance. Several of the British TBMs, having completed their drives, could not practicably be reversed back along the narrow tunnels they had just created. Rather than attempt to dismantle them in the cramped bore, the engineers steered some of these machines downward, away from the route, and abandoned them in the ground, where they remain entombed to this day. Others were taken apart and recovered. The French machines, which had begun their journeys from a large access shaft, were more readily dismantled and brought back to the surface. Taken together, the boring fleet had carried out one of the most ambitious civil engineering exercises of the twentieth century, joining two countries by a route that lay, at its deepest, some seventy-five metres below the bed of the sea.