Gotthard Base Tunnel: Tunnelling works in the eastern tube of the Piora Basin finished

Dec 23, 2008

The Herrenknecht Gripper-TBM S-210 passed the Piora Basin – a critical zone of the Gotthard Massif - without problems. The tunnel boring machine S-211, driving the western tube, will also arrive in the Piora Basin at the beginning of 2009. North of the basin, both machines will have to tackle another seven kilometers of hard rock - at an overburden of 2,000 meters and with rock temperatures of up to 50 degrees centigrade.

What the constructor AlpTransit Gotthard AG predicted during a press conference on October 8, 2008 in Faido came true on Sunday, October 12, 2008: The Herrenknecht tunnel boring machine S-210 successfully excavated the 150 meter long Piora Basin. Before entering the Piora Basin on September 29, 2008, the tunnel boring machine – a Hard Rock Gripper TBM – underwent major modifications, which helped to reduce the risk of TBM-related downtimes. Another measure to compensate rock deformations and prevent TBM jamming was to skip the regular maintenance shift. However, this option did not have to be used, since tunnelling went smoothly.

Steel ribs (TH profiles) were installed directly behind the cutter head at one-meter intervals for rock support. The rock surface was sealed with a shotcrete layer and an inner lining was then applied in the form of a 30 centimeterer thick shotcrete segment ring. All this measures proved themselves in practice. “We could not measure any deformations,” explained Heinz Ehrbar, Senior Tunnel and Rail Line Manager of AlpTransit Gotthard AG. The tunnel boring machine driving the western tunnel has to excavate another 1,400 meters of tunnel until it reaches the Piora Basin, probably in the first three months of 2009.

Tunnelling north of the Piora Basin.

Once the Piora Basin is passed, another approx. 6.9 kilometers of tunnel have to be mechanically excavated until the Sedrun section is reached. This is the zone with overburdens of up to 2,000 meters and the highest rock temperatures of 50 degree centigrade encountered within the world’s longest railway tunnel.

Tunnelling progress in the Gotthard Base Tunnel.

In the north, two Herrenknecht Gripper TBMs – the S-421 and S-422 – already drove 3.2 and 1.9 kilometers of the two tunnel sections, each measuring 7.178 kilometers in length, along the Erstfeld-Amsteg section. According to the constructor the tunnelling works move forward as planned. Until October 1, 2008, the tunnellers excavated 118.4 kilometers of tunnel, shafts and adits, equalling 77 percent of the planned 153.5 kilometers.

Previous history: Piora – the tunnellers’ nightmare.

In the run-up to the construction of the Gotthard Base Tunnel, a major controversy broke out among geological experts. Some geologists even doubted the construction of the tunnel due to almost unmanageable fault zones. They expected a sugar-grained dolomite and high water pressures in the Piora Basin. Geological investigations were therefore carried out in the Piora Basin during 1993 and 1998. A 5.5 kilometer long exploratory adit was driven 350 meters above the planned base tunnel alignment from Faido using a small tunnel boring machine. During a core drilling on March 30, 1996, the Piora Basin and the sugar-grained dolomite, subjected to a water pressure of 150 bar, were reached for the first time. Within three hours, 1,400 cubic meters of soft rock mixed with water poured out of the drill hole which was only 10 centimeters in diameter. The TBM was disassembled and recovered and the adit was closed with an 8 meter thick concrete plug.

After that, 19 inclined probe drillings were carried out from the exploratory adit toward the planned base tunnel alignment. Laboratory analyses of the drill cores revealed carbonate-sulfate trias, i.e. a rock with relatively good rock mechanical properties. Probe drillings, carried out at the tunnel face this September, confirmed the results from the laboratory analyses: The dolomite rock of the Piora Basin is hard, non-aquiferous and therefore suited for mechanized tunnelling.

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