Edition 04/16

Expansion of the regional public transport network is under way in Karlsruhe. A new tunnel for cars will improve mobility and quality of life. This car tunnel will relieve pressure on the transport network by taking commuters and visitors to the city more quickly and more safely to their destinations. An underground tram route is available between Ettlinger Tor and Marktplatz. Due to its special geological formation, excavation work on the 300 metre long-tunnel underneath the city is quite demanding. To prevent water penetrating into the construction site, the company Pressluft-Frankfurt relies on AERZEN compressor technology.

Demanding geological conditions

When it rains on Karl-Friedrich-Straße, and you look at the asphalt, you will notice a road with fine bubbles. The reason for this:  The tunnel underneath, with its 15-metre footprint 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 would behave like a sandcastle.  On the other side, the pores are so big that groundwater would penetrate through the walls if the corresponding back pressure is not provided.
This means that the work between Ettlinger Tor and Marktplatz is literally being done under pressure. In the first construction phase, the pressure has a delta/atmosphere rate of from 0.75 to 0.85 bar. Therefore, the project is subject to German compressed-air regulations, which define the relevant rules regarding job safety. For this reason, appropriate briefings and the corresponding health certificates are mandatory to get into the pressure lock. To get out, decompression is inevitable.

More losses due to gravel and sand

The pressure lock is part of a control room, where the lock operator always keeps an eye on the operational state of all compressors on a display panel. Twelve packaged units have been installed - four machines have been installed underground and eight of them directly above (at ground level). “We have integrated the compressors with Delta Screw packages as core within container frames. This way they can be transported easily and combined on site as a space-saving unit - thanks to cartridge/multilevel design,” 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. “If we blew the hot air directly into the tunnel, it would not be possible anymore to work down there,” notes Schweitzer.

Sufficient reserves are required

For work which is not so deep below ground, the packaged units from Aerzen Rental pump a daily average amount of air of between 100 - 140 m³ per minute into the tunnel. This is also the reason for the foamy road when it rains. “The air bubbles to the top slowly and steadily. The shotcrete stabilises the walls, however, it is not as tight as the inner shell at final completion,” says Schweitzer. In order to limit the pressure experienced by the construction workers further down, the tunnel is driven into the ground in layers - from top to bottom, until the base is reached. The deeper the construction site is, the more the pressure increases from the groundwater. Schweitzer expects a pressure delta of up to 1.3 bar by the end of the structural work. Therefore, the pressure losses will increase exponentially and a higher volume flow will be required. For this reason, and according to the compressed air regulations after sufficient redundancy, the equipment supplied by Aerzen Rental covers adequate reserves. “We have to cover the complete demand by two thirds of the installed machine performance. Four of the twelve packaged units serve as spare machines,” explains the mechanical engineer of the Austrian company, specialised in tunnelling. “The specific performance of the packaged units is unrivalled,” emphasises Peter Engelke, Project Manager of Pressluft Frankfurt, describing the start of cooperation between AERZEN and Pressluft-Frankfurt more than 30 years ago. “In all these years, I have never had a broken machine - this signifies long working life and reliability.” Operational reliability is essential with tunnel projects, as breakdowns may have devastating consequences. In case of a breakdown, the construction workers would have enough time to get safely outside, although the penetrating water would cause massive damage to the tunnel. “It is mandatory to prevent this by all possible means,” advises Schweitzer. “The machines must run,” he adds. If all machines are in operation, the available volume flow is 700 cubic metres per minute - minus the stated reserve, which makes 420 cubic metres during standard operation.
As the required reserves are synonymous with undesired oversizing, the question of energy efficiency arises. After all, the compressors have an installed power output of 200 kW each. “The efficient use of electrical energy is what counts - even in terms of preliminary construction projects,” emphasises Link. Engelke has also observed that electricity costs are always an issue. Considering twelve compressors with a capacity of 200 kW each, every digit behind the decimal point is important,” he says. The single stage, oil-free screw compressors of series Delta Screw Generation 5 plus are universal tools and have been optimised to provide maximum energy efficiency. They can be controlled by means of a frequency converter as needed.
In Karlsruhe, the speed of the compressors is pressure-controlled and adjusted, so that their actual performance corresponds exactly with the pressure losses from the tunnel - these measure 15,000 cubic metres from Marktplatz to Ettlinger Tor. In addition, it is economical for BeMo Tunnelling and Pressluft-Frankfurt to rent these compressor packages in such special application cases, instead of buying them. And the independent rental service of the manufacturer of the blowers and compressors, located in Lower Saxony, takes care of all the maintenance during the time that the equipment is in operation. If all goes as planned, in about one year’s time the compressor packages will be returned for general maintenance before a second rental period is required. By that time, the tunnel in Karlsruhe should be free from groundwater, so that the actual station and line upgrading work can start.