The two halves of the tunnel, worked from opposite sides, were joined together with incredible precision. The greatest deviation in the width of the tunnel was only 12 millimetres, which even the experts would be hard pressed to notice.
A champagne celebration
Councillor Radovan Šteiner, who is responsible for transport in Prague, even carried out the detonation himself.
The location of the blast was about 300m from the point where the two halves connect. Immediately after the rumble of the explosion echoed through the tunnel a special digger went to work, removing the earth that separated the two parts of the tunnel. Finally, as the tunnel workers shook hands, there was one more small explosion and a champagne cork flew through the air. “It’s absolutely alcohol-free,” the tunnel workers reassured the safety officer.
With the help of GPS and other technology
Nowadays the accuracy of the tunnelling work is ensured by technology. On the surface a network is created with the help of GPS. This does not work underground, so a network is formed on the basis of measurement to determine further progress. Over a distance of 2.5km the tunnellers were only 12mm off the mark, and only 7mm in terms of height. According to standards there can be a tolerance of up to 8cm. Before the use of modern technology it depended on experience. Even the Austrians, who have had a lot of experience in tunnelling, didn’t meet up on several occasions,” noted Metrostav spokesman, František Polák.
There’s even gold down there
During the digging of the tunnel many different minerals were encountered. It can now be said that there is even gold in the Blanka tunnel. It was discovered at a transition of rock layers somewhere between Stromovka and Letná. Nevertheless, there were no nuggets of gold, only grains, such as you can find in gold-panning competitions.
A metre further each day, but
Work on the Blanka tunnel is continuing at a rate of about 120cm per day. Under Stromovka, where the tunnel caved in twice, it was only 60cm per day. The rate of progress depends on the material encountered. The hard slate of Letenská pláň is safe, but the soft layers beneath Stromovka or the crumbling slate of Troja slow down the work.
More champagne in a month’s time
Work on the Blanka tunnel is now focusing on the south tunnel tube, which should break through in a month’s time. About 200m remains to be dug. Work will then shift to the side-tunnels, which will provide ventilation. The newly-dug tunnels will then be fitted with the very latest technology, especially in relation to safety-systems.
