The Global Magazine of Leica Geosystems

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4 | Reporter Gotthard Base Tunnel Length: 57 km Scheduled completion: End of 2017 Construction costs (2008): 6.43 billion euro (8.62 billion US$) Construction costs (2008) including Ceneri Base Tunnel: 7.76 billion euro (10.41 billion US$) Surveying: Consortium VI-GBT c/o Grünenfelder und Partner AG and ARGE LOS349 c/o BSF Swissphoto AG, www.bsf-swissphoto.ch This would have been way outside the accuracy of 10 cm horizontally and 5 cm vertically required of the engineers working for the surveying consortium VI- GBT today, because the new Gotthard Base Tunnel is designed for a modern low-level, high-speed railway. The highest point on the route is only 550 m above sea level and the maximum gradient is limited to 0.8 percent. This means that from the end of 2017 (scheduled date), high-speed trains with people, goods, and vehicles on board will be carried safely through the Alps at speeds of up to 250 km/h. Perfect alignment of the track is essential for this to happen. The hole in the right place Ensuring that the alignment complies with these requirements is the task of Ivo Schätti and his team from Grünenfelder & Partner AG, the lead engineering consultancy in the surveying consortium. As the client's surveying engineers, they are responsible for maintaining the correct position, height, and direction as the mega-tunnel heading advances. “You could say we have to make sure the hole is in the right place,” grins Schätti during our visit to his office in Domat-Ems, a small town near the Gotthard section of the works. A map on the wall showing the whole project gives rise to the suspicion that it might be somewhat more difficult than that. “The special aspects of this project naturally include its length, Instruments used for tunnel surveying: Precision total station Leica TCA2003 Digital level Leica DNA03 Nadirlot NL Instruments used for monitoring : Precision total station Leica TCA2003 Monitoring software Leica GeoMoS Leica GPS System 500 Monitoring the Nalps dam wall the five entry points out of which we work, and of course the required accuracy,” he adds. Great effort for control surveys The mega-tunnel is being built from five places at once – the two portals at Erstfeld (north) and Bodio (south), and intermediate headings at Amsteg, Sedrun, and Faido. The basic primary network was created in 1995 and was completely resurveyed ten years later. “In 1995 the use of GPS was still relatively new, but our measurements confirmed that the points were stable,” declared Schätti. One of the principal tasks of Grünenfelder & Partner AG at the moment is tunnel control. As the client's surveyors, the team can take more time for its measurements, but also work more intensely, using special instruments such as a gyrotheodolite and enjoying better working conditions than the contractor's survey teams, who continuously monitor and control the heading. Their measurements are usually taken when the site is shut down. “This entails coming into work over Christmas and on other public holidays,” says Schätti. The checks are necessary as errors can creep in through many factors. One source of errors is temperature fluctuations: ventilation equipment distributes the relatively high temperatures uniformly
under the mountain specifically for the surveying operations and the survey concept was designed to suit these conditions. “When you are underground you do not know how gravity is behaving in response to different rock densities – these errors would be transferred immediately into our measurements. Therefore we refer back to survey models based on measurements we have taken on the surface,” explains Ivo Schätti. Dam wall monitoring on the surface While the tunnel boring machines are continuously eating their way through the base of the Gotthard massif, other measurements are being taken on the surface. Some parts of the tunnel pass directly under three water storage reservoirs, and although the tunnel lies very far underground, the effects on the surface should not be underestimated. “A tunnel affects the normal flows and pressures of ground water. The pressure loss due to the removal of the water in the rock could cause the mountain literally to cave in,” explains Ivo Schätti. A change of pressure like this could have disastrous consequences for the dam walls of the three reservoirs at Curnera, Nalps, and Santa Maria in the Upper Rhine Valley – and the tunnel driving would have to be stopped immediately. For this reason BSF Swissphoto AG, the lead surveying consultant of the ARGE Los349 consortium, is monitoring the valley walls near the dam walls and the area in front of them. Measuring points were fixed directly on the rock or were set on up to 3metre-high concrete pillars which the team constructed themselves. No easy task, given that all the materials had to be flown in by helicopter and the prisms fixed on to vertical rock faces. Highly precise Leica TCA2003 total stations, protected against the weather in small enclosures and controlled using Leica GeoMoS, measure the movements of the prisms and transmit the data to monitoring software developed in-house. “The instruments have been operating since 2000 and continue to work perfectly,” he adds on behalf of a satisfied BSF Swissphoto. Spot level surveys were carried out at particularly critical areas along the route using a Leica GPS System 500 to determine the settlement of individual points. The long, hard winters in the Swiss mountains make the team's job even more difficult: “In places the snow depths are huge, some points in avalanche areas are off limits for the whole of the winter, and often ice forms on the prisms and can remain there all day. In spite of all this we have been able to reach 90 percent of our points, even in the winter,” concludes Ivo Schätti. The Global Magazine of Leica Geosystems | 5
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