Squeezing Ground Obliges to Innovate at Gotthard Base TunnelThe 800 m Sedrun shaft enables to descend to the heart of the Gotthard base tunnel in 67 seconds, at a speed of 12 m/sec. The elevator has been developed to transport as much as 6,000 tons of excavated rock per day - this is the equivalent of a 1.4 km train with 108 cars - and 2,000 tons of equipment. The 6.2 km Sedrun section is undoubtedly the most difficult from the geological point of view of the entire 57 km twin-tube Gotthard base tunnel. If TBMs are capable of driving tens of metres a day, their progress is easily slowed down or even stopped when the geology becomes tough. At Sedrun, the drill/blast method has been selected. When the geology is favourable (firm rock), the excavation diameter does not exceed 9 m. When the geology is tricky (squeezing rock), the excavated diameter can be as wide as 13 m and additional measures to reinforce the resistance to pressure must be implemented. Progress is sometimes very slow, only 50 cm or one metre a day. And the constraint to take the spoil back to the surface through the central shaft does not help any better. It is anyway useless to want to speed up the construction because all phases require extreme precautions. The next section to be excavated, north of the Tavesch massif, is believed to be particularly difficult. Core drills have revealed the presence of squeezing rock. The pressures exercised is such that, if nothing is done, they tend to plug the excavated cavities unless heavy support is installed.To tackle the problem, engineers and miners have designed a strategy which will be implemented from the beginning of the summer. First, they decided to dig a 13 m-diameter tunnel, which is bigger than the 9 m-diameter in favourable ground. This is to give the loose ground space to occupy. However, since the tunnel must have a constant diameter and the excavated cross-section must remain open, a special system of securing the excavated space with deformable steel arches will be used. This concept has never been used before in tunnelling with such dimensions. After excavation, two concentric steel arches are installed. Every arch consists of eight segments which are joined together by slightly yielding connectors to form two concentric rings. Under the pressure of the rock, the rings are slowly pushed together until the maximum supporting force is reached.Because there was no practical experience of such large dimensions, AlpTransit Gotthard and the Transco consortium decided to perform full-scale on-site tests of the entire system. The tests were concluded by a major trial of two complete steel rings with a diameter of 13 m. The steel arches were loaded until they failed. The theoretical calculations and the knowledge gained in earlier small-scale tests regarding the compression behaviour and supportable load were confirmed. The test series revealed that the selected concept is suitable for enabling the difficult excavation in the Tavetsch massif in the coming months.Because of the lack of space in single-track tunnels, new methods are also being used to achieve the best possible progress. For the first time in tunnelling, a 50-ton versatile installation machine is suspended to the vault for installation of the steel arches, plates and mesh, bolting and shotcreting. The robot moves along two rails suspended to chains. Click ch/22.At the start of May 2004, progress had reached 45.6 km out of the complete 153.3 km tunnel system (main tunnels, shafts, access adits, cross passages, etc.), which is 30% of the entire Gotthard base tunnel. The world's longest railway tunnel is being excavated from five drives in Ersfeld, Amsteg, Sedrun, Faido and Bodio. To date, 1,550 employees native of 13 countries work at different jobsites. Visit www.alptransit.ch and www.transco-sedrun.ch/seiten/transco-sedrun.htm 24/04.