The station does not have a name, but somebody has sprayed a number on the concrete wall: 9703. This is the distance in metres from this very place back into daylight, to the southern tunnel entrance in the municipality of Bodio. It takes the narrow gauge railway as long as half an hour to transport both workers and material inside the mountain. During their journey, the men have been dozing a bit. Now, they start walking, plodding their last few metres into the belly of a machine by crossing iron footpaths and climbing stairs up and down. They enter a steel monster, a stone-flinging giant: the TBM, short for ‘tunnel boring machine’ that was christened “Heidi” by the workers.  

Infernal noise
Heidi has just come to a standstill, since two engineers are replacing a leaking hydraulic hose. Then one of them gives the „all clear“ to the workers in the control stand of the tunnel boring machine – “pronto!” -  finished. This is the last word we will hear for a long time. Everybody who is too slow in putting their rubber ear plugs in and not fast enough in getting a firm hold on the iron platform with their legs wide apart will panic once the noise and the vibration start. The machine is making an onslaught on the mountain. 58 roller bits are attacking the rock with a vengeance, crushing what has lasted for more than 300 million years, each of them throwing a weight of 26 tons against the hard gneiss of the Central Alps. 

 The longest railway tunnel in the world
In Switzerland, tunnelling has a long tradition and its many vehicle tunnels make it look like a Swiss cheese with lots of holes in it. Merely adding up the number of tunnels currently in operation gives you a total of nearly 900. Since 2002, however, the Swiss have not just been digging another hole into the ground, but been working on the longest railway tunnel in the world that will probably cost as much as 4.6 billion euros: Over a distance of 57 kilometres, the St. Gotthard base tunnel is to link the municipality of Bodio in Ticino with Erstfeld that is located in the canton of Uri, shortening travel time between Zurich and Milan to two hours and 40 minutes, i.e. by around one full hour. This construction is a bold undertaking; its necessity leaps to the eye as soon as you look at a European road map. North and south of the Alps, there is a dense network of traffic arteries. But so far, there has only been one large route for motor traffic connecting the commercial centres of Basel, Zurich and Lucerne: the St. Gotthard road tunnel opened in 1980. Every day, about 3500 lorries are squeezing through this bottleneck. 

 The tunnel will bring peace and clean air to the valleys
The „New Alpine Transversal” railway (NEAT) is to shift freight transport from the road to the rail in 2015 and will, thus, bring a more peaceful life and better air to the Reuss and Leventina valleys. While it is true that there is also a railway tunnel crossing the Gotthard, it is impossible to make the traffic flows of the 21st century switch to an old tunnel that was opened at the end of the 19th century. Its portals are located at an altitude of 1200 metres. An additional engine has to push the trains that are struggling their way up from the Milanese or Zurich lowlands from behind. Moreover, due to being avalanche-prone, this stretch is frequently closed during the winter season.   

 The geology of the mountain massif is by and large unknown
In contrast, the new tunnel crosses the mountain massif on the ground floor. Its highest point is at an altitude of only 550 metres above sea level, roughly at the same altitude as the city of Munich. 57 kilometres – a routing that also harbours a few risks. Since a mountain cannot be x-rayed. Only after completion of the last breach will the scientists know more about the geology of the Gotthard.  The managers of Alptransit AG, a subsidiary of the Swiss Railways (SBB), hope that in nine years’ time the first trains will roll through the tunnel. In order to meet the schedule, boring, blasting and excavation work is simultaneously performed in five locations: Apart from the entrance portals in Bodio, i.e. in the south, and near Erstfeld on the northern part of the Gotthard, there are tunnel accesses ‘from above’ that are being built in Faido in Ticino, Sedrun in the canton of Grisons as well as Amsteg in the canton of Uri. This is called “intermediate points of attack" by the tunnel builders.

 Two-pronged attack
Heidi, the tunnel boring machine, has already been ploughing its way northward for three years. It is drilling the western tunnel, while „Sissi“, its spitting image, is approaching from the north, carving out the eastern tunnel. The base tunnel consists of two parallel tubes, as at high speeds, railways generate a strong pressure wave that is building up in front of them. If two passenger trains met at a speed of 250 km/h, their respective pressure waves would hit the other train like a rock, severely damaging it. Every 325 metres, the tunnels are linked via galleries: Thus, the passengers may escape to the respective twin tunnel in the event of an emergency.
On a good day, the tunnel boring machines drive the tunnel east and west 40 metres, while they are only making five or six metres on a bad day. The harder the rock, the more effective their performance. They are fastest when going through granite gneiss. Using their drill bits, Heidi and Susi are cutting stone chippings no larger than a breakfast plate out of the „face“ of the tunnel. A conveyor belt transports the waste several hundred metres back, loading it onto large tippers.

 Sparkling silvery stone
At the top end of each TBM, there are only six workers operating its instruments and controls. The men are preparing the concreting of the tunnel wall. They are driving rock anchors into the rock that has just been laid open: stainless steel pipes of a length of up to four metres are pushed into drilled holes and then “inflated” by means of water under enormous pressure in order to fix them. Reinforcing grids are mounted on these anchors to secure this newly bored part of the tunnel. Everything is controlled by a computer. The gneiss that has just been removed is sparkling like silver in the light of the site lamps. You feel an urge to stroke it, because it looks so smooth. Then shotcrete pumps wrap it in a grey mass. 

 A structure of superlatives
For a human being it is hardly possible to take in the figures associated with this construction. A tunnel boring machine weighs more than 3000 tons. Thinking of it, what should you picture to yourself? The weight of a giant container with 2400 VW Golf vehicles stacked inside? With an output of 3500 kilowatts, i.e. nearly 500 HP, this more than 400 metre long colossus is boring into the rock. More than 13 million cubic metres of waste will be transported out of the tunnel by conveyor belts and wagons in the course of the entire construction period. This corresponds to five Pyramids of Cheops next to each other. Nonetheless, there are no rubble mountains rising into the sky in the valleys of the cantons of Uri and Ticino. About one fifth of the waste is ground to sand and gravel in stone mills by the workers, whereupon it partly returns into the mountain in the form of concrete admixtures. The remainder of the material is used for filling old quarries in Ticino, levelling depressions in the terrain in Sedrun and heaping up six isles in Lake Lucerne. 

Planning and construction require utmost precision
When planning the line, the engineers were facing a tricky problem: How is it possible to transfer a route that was ascertained overground as exactly as possibly to the level of the tunnel? Even though the Swiss have relied on a lot of international know-how, for instance by buying the tunnel boring machines from a German company and recruiting the tunnel builders from a dozen different nations – this assignment requiring ultimate precision was not entrusted to any third party. Hilmar Ingensand, professor at the Swiss Federal Institute of Technology Zurich, ranks among the group of experts entrusted with underground routing. First of all, Ingensand explains, the surveyors defined fixed points whose coordinates were known and „had been checked again and again“: concrete pillars near the tunnel portals. 

 Surveys conducted with the help of GPS
Based on these fixed point coordinates above ground, the surveyors were then able to ascertain three-dimensional coordinates underground and to exactly define the direction of the tunnel boring machines. A red laser beam indicates the computed direction into which the tunnel is driven: Wherever this beam hits the rock, the tunnel boring machine continues its onslaught on the rock. After having advanced a few metes, the laser beam is readjusted; in each case, the last location of the machine is referred to as current reference point. "We are making our way through the mountain at the speed of a tunnel boring machine,” says Ingensand. 

The route plotted for the Gotthard base tunnel is not a dead straight line, but takes a slight “S” curve. This way, the highest peaks of the region are avoided, just like the geologically tricky zones – as far as they are known, of course. Long before the giant construction machines advanced towards the Gotthard base tunnel, Yves Bonanomi, armed with a hammer and a map in his backpack, was hiking through the mountains. The Swiss geologist was knocking off a bit of rock here and there. 

Soft, crumbly rock is giving the borer a hard time
The surface of the Gotthard massif allows geologists to make an informed guess as to the conditions deep down in the mountain: Due to the formation of the fold mountains, the strata in this region folded up vertically. There was a high probability that the formations Bonanomi discovered on the mountain ridge during his reconnaissance mission would also exist inside the mountain. 
However, the conditions in the so-called Piora Zone (depression) about six kilometres north of Faido posed a problem to the scientist and his colleagues. What we have here is dolomite of a grainy, sugary texture. "The material is crumbling in your hand“, says Yves Bonanomi. If the tunnel builders had come up against this „swimming mountain“, this would not only have hampered the progress of the work, but jeopardised the entire tunnel construction project as such.  

This is why the reliability of the geological forecast was of such tremendous importance: Would this grainy, sugary dolomite go down deep into the mountain? Or was it only a wedge-shaped area? Seismic measurements as well as first test borings had presented a contradictory picture. When a boring team tried to drill into the Piora Zone, they were hit by a jet of water mixed with tiny little stones that was ejected at high pressure. The drilling rig was catapulted out of the hole. A fiasco was looming.
After this test boring, pessimists thought that crossing the Piora Zone would only be feasible by incurring enormous costs. Others presumed that the dangerous rock meal wedge would not continue down to the base tunnel – and they were right. The border between the sugary dolomite and firmer rock is about 200 metres above the level of the new tunnel. 

Geological disturbance in the Tavetscher Sub-Massif
In this respect, the all-clear was given. Then, in autumn 1995, the tunnel builders reached a section that really turned out to be critical. This disturbance zone in the Tavetscher Sub-Massif – which is located below the winter sports resort of Sedrun – is roughly one kilometre wide and consists of tectonically ground rock. Geologists call this mixture „kakirite“. The rock mass is so soft that it can be scraped off with bare hands. Kakirite is too pasty for tunnel boring machines. It has to be blown up by conventional means and must be scraped out by excavators in extreme cases. 

South Africans provided the solution
Starting in Sedrun, a South African company experienced in excavating gold mines bored two vertical, 800-metre long accesses to the future tunnel floor. Two shaft lifts are operating vertically and have also taken Yves Bonanomi down into the mountain several hundred times. He arrives in an underground hall that smells of ammonia. "There has just been a blast", Bonanomi explains. 98 per cent of SL 700, an explosive, consist of acetic acid – while one per cent is made up of ammonium nitrate. The remaining one per cent is a matrix foaming up the explosives when they are filled into the drilled holes in order to provide a good contact with the rock.

35 degrees Celsius inside the mountain massif
After moving a few kilometres south, the pungent smell gets stronger. Front-end loaders are dumping fresh rubble into an open railway wagon. Huge ventilation pipes release water-cooled wind for blowing the dust away from the construction site. Without cooling, temperatures in this area would exceed 35 degrees Celsius: temperature rises with increasing depth, on average by 3 degrees Celsius for every 100 metres. In the Tavetscher Sub-Massif the mountains rise up to an altitude of 1300 metres. The rock in this section is extraordinarily soft and ductile. Once a stretch of tunnel is driven, this rock expands, narrowing the tunnel. This is why, right from the start, the tunnel builders are driving larger holes into the rock than in locations with hard, rigid stone; in addition, they also set up steel girders for immediately securing the tunnel.

A method taken from German hard coal mining
Under high pressure, the ring-shaped girders subside a little bit, gradually sliding into each other. They carry up to 100 tons per square metre – the weight of a locomotive. Once the mountain has stabilised, a concrete shell is cast. A time-consuming process. On average, the length of tunnel driven in the Tavetscher Sub-Massif every day only adds up to one single metre. 

Unforeseeable consequences
Nobody is able to predict how these inroads into the Gotthard will impact upon the surface of the Gotthard massif in the long run. However, there is some evidence for change. For instance, according to recent measurements, the mountain massif above the road tunnel has subsided by twelve centimetres in the course of the years. Experts explain that this process is related to „crevasse water“ that escapes into the tunnel: Once the water saturated gaps close, the volume of the mountain goes down.

"We are doing well in terms of schedule"
Should the mountains towering the Gotthard base tunnel also decline, the Swiss might be exposed to a tremendous danger, since there are three reservoirs above the tunnel. For this reason, the Swiss Federal Office for Water and Geology has subjected these areas to permanent monitoring.  Should the terrain actually subside, the dammed up water will be drained before the walls burst and the floods hit the valleys. 

In the meantime, more than half of the 153 kilometres of tunnels and shafts have been bored or blown out. "We are doing well in terms of schedule“, says Yves Bonanomi. He hopes that he will be able to report the next cut-through of the Gotthard base tunnel as early as in 2006:  By autumn Heidi is expected to have bored the 16 kilometre subsection between Bodio and Faido.

By courtesy of  GEO Journal / Text from Philipp Mausshardt

Mit freundlicher Genehmigung der Zeitschrift GEO / Text von Philipp Mausshardt