It has now been a month since the Gotthard Base Tunnel was unveiled in Switzerland. Begun seventeen years ago, the project cuts a level passage beneath the Alps that is fifty-seven kilometers in length, three longer than Japan’s Seikan Rail Tunnel, previously the world’s longest. However, this official length underlies the true scale of the endeavour; there are in fact 152 kilometers of passages, including the two main tunnels and a spider’s web of safety, service, and ventilation sections. In constructing them 131 thousand cubic meters of concrete, 3,200 kilometers of cable, and, most importantly, eleven billion dollars were used- all to craft a more direct rail link between northern and southern Europe, a link that will allow shipping on a massive scale, lowering dependence on polluting road travel.
The new tunnel’s entrance
Other famous megatunnels- for instance, the aforementioned Seikan and the older “Chunnel”- cross beneath bodies of water, but the Gotthard Tunnel snakes below solid rock, 2,300 meters of it at some points. As a result, the former projects did not provide precedent to some of the issues that the tunnel engineers faced during construction- most prominently, the uncertainty about what type of rock lay ahead of them. Even using readings from the best sensors and surveys or the predictions of the best geologists it was impossible to know exactly what strata were layered beneath the slopes. As a result, the tunneling process was one that required caution, exploration, and a huge amount of improvisation
The main means of carving a passage was one that has changed very little in concept for decades: two massive drill heads in each shaft, called TBMs (they match the scale of the rest of the tunnel: thirty feet high, 1200 long) ground away at the rock face, advancing the tunnel ninety feet per day. However, different types of rock provided challenges for the TBM operators. If the rock was harder than average, work had to be delayed while modifications were made to the spinning drill bits, but it was soft rock that really gave the engineers headaches, as it was liable to cave in, crushing workers and machinery. Some sediment layers were firm enough for careful drilling, but the very softest required incremental blasting, with all people and equipment a safe distance from potential cave-ins. Engineers had to improvise not only construction method but design in these sections: the tunnels here were built with a greater diameter than normal, so as the rock settled the sides would collapse to the regular width.
One of the TBMs
Also of note is the innovative feature of the tunnels that allows groundwater drainage, named the umbrella method by engineers. Instead of trying to force water around the tunnel- where irregularities in the rock could have allowed it to collect in pockets, posing a dangerous threat to the shafts- the water is allowed to seep through the specially-designed concrete, within which a waterproof layer allows it to drain down the inside wall of the tunnel and out through pipes at the bottom.
Inserting the waterproof lining
Building the main tunnels was only half of the battle, as the expansive auxiliary network is vital for their usage. Safety is paramount in the Gotthard Base Tunnel, as it is not far from the Mont Blanc Tunnel, where in 1999 a fire, still fresh in Swiss memories, killed 38 people due to poor safety measures. Motorists fled their cars to the safety cubicles on the tunnel sides; however, these failed to keep out the heat of the inferno and served only to trap their occupants. The two-shaft design of the Gotthard Tunnel is partially in response to this tragedy: if there is a fire or gas leakage in one of the shafts, people can escape to safety in the other shaft through one of many cross-passages, which are compartmented like airlocks.
The rail tunnel, while a grand achievement, may find its moment in the sun as the world’s longest to be short-lived. Two longer ones are in an advanced planning stage: one beneath the Gulf of Finland, linking Tallinn and Helsinki, and one twice as long as Gotthard beneath China’s Bohai Strait. Still, the completion of the new alpine colossus is a state-of-the-art accomplishment worth celebrating.
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