In the German town of Fürth, the underground rail network was expanded from the Stadthalle to Klinikum stations. In the course of tunnel driving, some 2,500 m³ of ground was frozen directly under a row of listed houses in Vacher Straße. In order to underpin these historic buildings at the eastern tunnel entry, Quaternary sand and gravel had to be brinefrozen over a length of 60 metres. These "frozen caps" were generated by carrying out 60 metre controlled horizontal drillings though the slope of pit against pressing groundwater.
A saline solution cooled down to -38°C was pumped through the 1.4 kilometre long, closed pipe system in the soil. Within five weeks, the soil around the freeze pipes froze to become a 1.5 metre thick frost body, thus forming temporary protective sheath for driving the tunnel. The ground freezing project of the Fürth underground is unequalled worldwide because of the use of controlled horizontal drilling to insert the freeze pipes.



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