2019-12-17   -   Article Under pressure: tunnelling in groundwater

Anyone who stands on Karl-Fried-rich-Straße in Karlsruhe during a downpour and takes a look at the asphalt will notice a fine-pearled foaming road.  The reason: the tunnel underneath, with its base 15 metres below street level, loses air. “Here, we have to deal with a lot of sand, gravel and loose rock. These are materials which are not useful in tunnel construction,” says Robert Schweitzer, Construction Manager, describing the challenges he faces in the centre of Karlsruhe.  On one side, due to the geological conditions, BeMo Tunnelling GmbH is forced to stabilise the walls by propelling shotcrete at them, as otherwise the ground could behave unpredictably. On the other side, the pores are so large that the ground water would run directly out of the walls if the appropriate counterpressure is not provided on site. This leads to the conclusion that the work is literally under pressure. In the first construction phase, this is at a delta of 0.75 to 0.85 bar to the atmosphere. Therefore, the project is subject to German compressed-air regulations which define the relevant rules regarding job safety. Against this background, nobody is permitted to enter the pressure lock without a special briefing - and must not leave it without decompression taking place. 

The pressure sluice supplies data to a control room where the sluice monitor always has an overview of the operating status of all compressors on a display.  Twelve assemblies are installed - four underground and eight on the road directly above. The spatial division is due to the fact that soil analyses undertaken at the beginning of the tunnel project suggested a higher degree of impermeability. This assessment subsequently proved incorrect and thus the flow capacity which had been calculated for the compressors did not apply either. “Making forecasts for old riverbeds is almost impossible.” We simply do not know what to expect”, says Peter Engelke, project manager of Pressluft Frankfurt. The company received an order from BeMo to design the compressed air according to the initial soil analysis in the Rhine plain.

As it soon became clear that the calculated performance would not be sufficient, Pressluft Frankfurt and Aerzen Rental started looking for an efficient solution that would be quickly available. “The cooperation between all three companies is quite flexible and very professional”, emphasises Engelke.  

“We have integrated the compressors with Delta Screw packages (VM 60, 1.140 to 4.550 m³/h, 45 to 250 kW) as core within container frames. These can be easily transported, combined to form space-saving units on site and, thanks to the cartridge design, can also be stacked on top of each other,” explains Peter Link who is responsible for the German rental business. The headquarters of Aerzen Rental is at Duiven in the Netherlands. In Karlsruhe, the compressors type CVO4400 are combined with water coolers which cool down the air outlet temperature of the compressor from 120°C to 20°C.