The world’s longest train tunnel opened to passenger and freight traffic in early June. The Gotthard Base Tunnel in Switzerland, first envisioned more than 70 years ago, is expected to reduce the travel time for freight and passengers between northern and southern Europe and lessen train travel’s environmental impact on the Alps.

The 35-mile-long tunnel took 17 years to construct, and involved twin 1,200-foot-long drilling machines with 30-foot-tall bits burrowing through rock formations. The new tunnel will allow a train to travel at more than 150 mph during portions of its journey. The deviation between the two tunnels at breakthrough was 3 inches.

(Click to enlarge) Cross-section of the tunnel and topography.“For such a large scale project with so many different construction aspects to be managed, there is no question that the logistical effort was huge,” says Gabriel Walton, assistant professor of underground construction and tunneling at the Colorado School of Mines.

“The difficulty of the tunneling process should not be understated either, though, as they faced squeezing ground, bursting ground and everything in between, all of which present unique technical challenges,” he says.

Long Time for a Long Tunnel

To the casual observer, 20 years might seem like a long time to construct a train tunnel. Yet a peek below the surface of how difficult a process building the Gotthard Base Tunnel was makes it seem remarkable that it was completed even in that time frame.

“When considering the construction time of the project overall, it must be recognized that this project is truly unique with respect to the length and depth of the excavations,” says Walton. “Although it seems like a long time relative to what people are used to, it is in large part due to the unprecedented scale of the work.”

The amount of excavation was extraordinary, says Georgios Anagnostou, professor of tunneling at the Swiss Federal Institute of Technology in Zurich and co-editor of the book, Tunneling Switzerland. More than 93 miles of tunnels were excavated as were two multifunction underground stations. “In addition, the intermediate attacks in Faido and Sedrun necessitated the construction of an access tunnel and a 2,600-feet-deep access shaft, respectively,” he says.

The tunnel excavation, which lasted from 1999-2011, would have taken longer without the many years of planning and the preliminary investigation of the underlying geological structure, according to Simon Loew, professor of engineering geology at the Swiss Federal Institute of Technology in Zurich.

(Click to enlarge) The tunnel bore lasted from 1999-2011 and was off by 3 inches. Image source: AlpTransit“These lasted about 10 years and included detailed geological mapping of surface outcrops and landforms, drilling of many shallow and a few deep boreholes (around Sedrun), comparison with previous excavations in the same geological units, laboratory testing of rock samples and computer modeling,” Loew says. “The general geological structure was properly predicted, but the behavior was sometimes different than expected.”

While the detailed studies gave the Gotthard tunnel engineers a reasonably good idea of what to expect, planners built extra time into their plans for unexpected events. And the tunnel engineers had plenty, Anagnostou says.

“There have also been geological surprises: squeezing ground during the tunnel boring machine (TBM) drives; two bigger incidents due to faults and geothermally altered rock (each causing a standstill of about five months); and extremely adverse conditions (squeezing, rock bursting) in the Faido multifunction station. Even if these specific events were unexpected, the overall construction time and budget had reserves for geological uncertainty,” Anagnostou says.

Another drag on the Gotthard tunnel construction schedule involved the logistics of using TBMs. The enormous machines must be assembled and torn down on site. While they are the most efficient way to quickly cut through many types of hard rock, they must be disassembled and moved if they encounter loose rock, a process which can take as long as five months. This happened twice at Gotthard.

To Drill or Bore: Geological Risks

Tunnel engineers have a choice to make before they start excavating: use traditional techniques like drilling and blasting or use expensive – but faster – TBMs. TBMs handled 75% of the Gotthard tunnel excavation, and were a good choice, according to Walton.

“TBMs take a lot of time and money to build, but can move quickly once they start boring, so they make the most sense for projects where a large amount of tunneling is required. TBMs were a great fit for the Gotthard project for this reason.

“Drilling and blasting is more common for smaller projects, but it also has the advantage of being more flexible, both with respect to excavating openings with complex geometry and with respect to the ease with which unforeseen conditions can be managed. Accordingly, the drill and blast sections for the Gotthard project corresponded to areas with more complex excavation geometries (i.e., stations) and more unfavorable geological conditions (that is, fault zones),” Walton says.

Engineers knew they would need to traverse challenging terrain, as the geology ranged from various types of gneisses and granites of the Gotthard and Aare massifs to finer-grained sediments in between. The preliminary investigations and exploratory bores showed that the most “constructionally difficult zones” would be the Piora Syncline and the Tavetsch Intermediate Massif North, says Renzo Simoni, CEO of AlpTransit Gotthard Limited, the railway’s owner.

(Click to enlarge) The Gotthard Tunnel is expected to speed passengers and freight between southern and northern Europe. Image source: AlpTransitInitially, geologists expected “’floating rock’ conditions, i.e., water-saturated sugary dolomite under high pressure in the Piora Syncline,” says Simoni. The 150-300-foot-long zone was forecast to contain “loose fine-grained sand under a water head of about 5,600 feet,” Anagnostou says. These types of sediments are inherently unstable and threatened the feasibility of the project.

Yet the test bores suggested the layer would probably be the more constructionally favorable dolomite, according to Simoni. The test bores turned out to be accurate: In 2008, engineers were able to use a TBM to carve through the dolomite in the east tube.

The most challenging part of the project turned out to be tunneling through the 3,280-foot section of the Tavetsch Massif. The squeezing rock conditions made tunneling difficult, so the engineers created innovative supporting measures.

“By means of deformable steel rings and anchors, steadily increasing forces to counteract the deformations could be created,” Simoni says. “This novel concept allowed the large initial deformations that were necessary for a technically and economically viable lining process.”

Drainage an Ever-Present Risk

Mitigating the risk of a tunnel collapse wasn’t the Gotthard tunnel engineer’s only concern. The tunnel is constantly exposed to “large volumes” of ground water entry, according to Simoni. The solution was to use so-called “umbrella” waterproofing.

Here’s how it works. When “soiled” water enters the tunnel, it is routed using a specially developed sealing foil to the vault base and its drainage pipeline. The vault drainage pipeline, which runs under the railway track, connects to the main drainage pipeline at 100-meter intervals. “The soiled water drains into a collection basin outside the tunnel, where it is analyzed,” Simoni says. “Depending on its composition, the water is processed and then discharged into natural watercourses.”

Simoni says the tunnel’s lining and support structures were designed for a “life span of 100 years without major maintenance.” The foil was developed because commercially available seals wouldn’t fulfill the unique requirements of the Gotthard tunnel, Simoni says. “This foil must withstand high temperatures, ground water and rock pressure throughout the 100-year service life of the tunnel.”

Now that the lengthy and arduous construction process is complete, the opening of the Gotthard Base Tunnel promises a more efficient way to transport people and goods in the heart of Europe.