BETWEEN HIGH-PRECISION PLANNING AND GEOLOGICAL IMPONDERABLES.
Surprises can always occur when boring tunnels the size of the Gotthard project. It is not for nothing that tunnel constructors have great respect for their task. Their work is a tightrope walk between the highly planned and the unforeseeable. In spite of the many preparatory investigations, unexpected difficulties can always occur. For example, at the very beginning of tunnelling from the southern side of the Gotthard in February 2003 – after only 200 meters – work was interrupted by unconsolidated rock. The two TBMs that set off from Bodio – affectionately known to the tunnelling teams as ‘Sissi’ (S-210) in the eastern tube and ‘Heidi’ (S-211) in the western tube – encountered geological disturbance zones known as kakirite zones. Such geological conditions are too soft for Gripper TBMs, which are designed for very hard rock, and make rapid tunnelling progress almost impossible. Every meter of excavated tunnel must be secured in a complex process. The machines could not leave these disturbance zones behind them until August 2003, after around 400 meters of tunnelling.
The ups and downs in the mointain.
But a construction log can also include unexpected positive reports. For example, there was good news from the northern side in spring 2004. In the construction plan the geologists had predicted a tunnelling interruption of up to four months to deal with the Intschi zone. Luckily, however, this zone was around 50 % shorter than expected, and the teams operating the machines with the innocent-sounding names ‘Gabi 1’ and ‘Gabi 2’ were able to pass through the zone, albeit with reduced tunnelling speed. That said, however, the ups and downs in the mountain seemed endless. Good monthly tunnelling performances of 560 m and penetration rates of up to 12 mm per revolution were followed by difficult stretches, some with a penetration rate of only 3 mm per revolution and an advance of only 140 m per month.
There was bad news for the engineers and clients in June 2005. Suddenly and without warning, loosened rock mixed with mountain water infiltrated Gabi 2’s cutterhead in the western tube. At first, the tunnellers tried to clear the cutterhead of the fine material by hand and back up the TBM a few centimeters. Without success. Eventually, the unconsolidated area ahead of the machine’s cutterhead was solidified by injecting it with a mixture of cement and bentonite. At the same time, the tunnel builders excavated a 50-meter-long tunnel from the east tube to the west tube to free the TBM’s cutterhead from the front. Regular tunnelling was not resumed until November 2005 after a standstill of five months.
The machines also had to deal with changeable geological conditions in the south on the stretch from Bodio to Faido, which impaired tunnelling performances. Noticeable improvements were attained by adapting the two TBMs to the unforeseen circumstances. In December 2005, ‘Sissi’ achieved the best daily performance in the Gotthard Tunnel so far, with 38 meters in the east tube.
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