The world's longest tunnel to open 15 October

such a long, large and deep tunnel has presented special challenges. “Many problems arise from depth in particular such as stresses in the rock, possible high water pressure and high temperatures,” Kovari told swissinfo.ch.

The temperature problem was, for example, solved by cooling the 45 degree Celsius air to 28 degrees using a water system, making life more bearable for workers.

Project threat

The Alps’ varied geology — rocks can range from granite hard to butter soft — has also been an issue. “We knew that we would have some very difficult zones to cross with the excavation of the tunnel,” Ehrbar said.

Some people doubted whether the tunnelers would get through the Piora Fault Zone, where initial tests had suggested “running ground”, a granular dolomite mixed with water. Work was put on hold - and the whole project threatened - while engineers debated what to do.

Extra bore holes were ordered which found that the poor quality stratum did not, as feared, extend to the deeper tunnel level. Work was able to resume.

There was also a difficult area just north of Sedrun where the rock was crumbly and subject to severe pressure. This called for thinking outside the box.

Mine technology

Experts used mining technology — a first for an underground tunnel — to solve the problem, which included using steel arches to support the excavation.

Ehrbar, however, never doubted that the tunnel was feasible. Overall, construction has gone well. “We are carrying out studies to see if we can open the tunnel a year earlier,” he said.

The final breakthrough, Ehrbar says, will be “very emotional” and a time during which people will remember both the highs and lows of this mammoth project.

For Kovari, the pioneering aspect of the Gotthard Base Tunnel has been its execution and in the openness when dealing with challenges, like at Sedrun.

Pioneering forebears

Kovari would not, however, say that the Swiss are a nation of tunnel construction experts — Britain is too — but the country’s topography has played a role. “It’s a special ability born out of necessity and continuous activities over almost 150 years,” he said.

The first pioneering tunnels were the Gotthard road tunnel and the Simplon tunnel, where tunnelers faced much more challenging conditions and deaths were commonplace.

The Swiss have honed their skills over the years. But both Kovari and Ehrbar still feel a special admiration for their 19th century forebears. “The old Gotthard railway tunnel or the existing Simplon tunnel were also the longest of the world of this time,” Ehrbar said. “I think we have to have a big respect for what our great-grandfathers did.”