TUNNELLING SAFELY THROUGH THE “ORDEAL” OF THE PIORA BASIN.
A further difficult zone awaited the tunnel builders on this stretch - the Piora Basin – a funnel-shaped formation filled with sugargrained dolomite and water, which reaches deep into the rock of the mountain range. Its existence was long known, and hardly any other part of the Gotthard had been investigated as intensely prior to the construction work. Since no one knew how far into the mountain the funnel reached, a decision was taken to make exploratory drills. A horizontal tunnel with a length of around 5.5 kilometers was driven from the cantonal road to the disturbance zone. In 1996, this zone was penetrated. The loose grains of rock turned out to be exposed to the enormous pressure of around 150 bar. A thick jet of water mixed with dolomite shot out of the mountain and flooded the road. The media called it ‘D-Day at Piora Beach’.
19 inclined drills were made from the exploratory tunnel to the vicinity of the future base tunnel, finally giving the engineers the all clear. They hit on hard rock with no water pressure. Examining drill cores, temperatures and seismic results brought the geologists to the conclusion that the bottom of the Piora Basin was sealed with gypsiferous cap-rock. And so, on October 12, 2008, Sissi reported: Piora Basin successfully crossed’, and its sister machine soon followed suit.
Tunnelling using conventional drill & blast methods between the Faido and Sedrun intermediate headings was also an extreme technical challenge. Here, the tunnellers blasted huge underground stations out of the rock. They will serve as special stopping bays for trains in emergencies. The branching tunnel will also allow trains to change from one tube to another during operation. A sophisticated system of transverse and connecting tunnels guarantees that smoke can escape and fresh air can enter the tunnel. The two ‘multifunctional stations’ are huge construction sites in themselves. Here, the rock presented tunnellers with a particular drill & blast challenge.
The mountain lets off steam.
The mountain pressure in some sections was so high that the usual method of increasing the target dimensions of the excavating cross section was of no use. Normally, this gives the mountain the opportunity to ‘let off steam’ – the more it deforms, the more mountain pressure is released. But in the critical zones of the Gotthard Base Tunnel, stability could not be achieved by just permitting deformation A counterforce was needed against the mountain to stop the cavity from reclosing completely. The solution was to use telescopically slidable rings. In this method, two telescopically slidable steel half-rings are connected to form full rings inside the tunnel cross section, excavated with an overcut of about 70 centimeters. The ring segments slide slowly together under the mountain pressure until their ends meet and they stabilize each other.
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