Opening the world’s longest, deepest rail tunnel with precise measurement

Author: Staff, October 2016

Surveying and monitoring systems made the construction of the Gotthard Base Tunnel in the Swiss Alps possible. The tunnel was inaugurated June 1, and the breakthrough on October 15, 2010 was due to highly precise surveillance and monitoring technology. The 57-kilometre long tunnel is the longest and deepest rail tunnel in the world.

Drilling a 57-kilometre long tunnel starting from both sides of the mountain and meeting in the middle with barely any deviation was a truly challenging task. Highly precise measuring instruments were needed to successfully accomplish the project.

Measuring down to millimetre accuracy

At the tunnel site, the surveying consortium VI-GBT started measuring in 1996, when construction work began 20 years ago. The surveying engineers set up a basic network with 20 reference points. For this task, they used total stations, or optical measuring devices, and GPS, or satellite navigation solutions from Leica Geosystems, a global company with Swiss roots and a tradition of nearly 200 years.

The drilling began not only at the north portal at Erstfeld and at the south portal at Bodio, but also at three intermediary points where side channels joined the main tunnel: Amsteg, Sedrun and Faido. In Sedrun (1,405 metres), a vertical tunnel 800 m long was drilled down to the main tunnel so construction workers could push north as well as south from that point.

During tunnel construction, surveying and measuring tasks had to be repeated every 400 metres. In order to make sure the tunnel was precisely on course, the surveyors had to check coordinates of the reference points. In addition, the exact height had to be measured with levelling devices – also from Leica Geosystems.

“When we broke through in the middle of the tunnel on October 15, 2010, the two tunnel sections met with a deviation of only 8 centimetres, whereas 25 centimetres would have been tolerated,” said Ivo Schaetti, a surveying engineer at VI-GBT. He explains that surveying tunnels is very demanding due to temperature changes, humidity and dust.

“Thankfully, we could always rely on the accuracy of the surveying solutions from Leica Geosystems,” Schaetti adds. “They are very precise. You can measure a distance of 400 metres with accuracy of less than a millimetre.”

Explore next chapter: Monitoring the dams above the tunnel

Story: Opening the world’s longest, deepest rail tunnel with precise measurement
Chapter 1: Measuring down to millimetre accuracy
Chapter 2: Monitoring the dams above the tunnel
Chapter 3: A swiss tradition