The BBI tunnel/station fabric construction consortium, consisting of Berger Bau GmbH (technical supervision), Schälerbau Berlin GmbH, Ingenieurbau-Gesellschaft mbH and Bleck & Söhne GmbH & Co. KG, is responsible for the punctual completion of the tunnel and ICE station, which will link Berlin’s new international airport to the capital’s rail network. Because of the repeatedly changing geometry, the tunnel, part of which passes beneath the future airport apron, consists of seven separate structures whose soffits are being poured with the aid of a total of 14 tunnel formwork carriages. In compliance with an ingenious assignment plan, the carriages will be working at times in parallel, at times in a staggered arrangement in the construction of 20 meter long concrete sections (in some cases 25 meters). After almost a year of action, the formwork carriages have completed the soffits for the multi-track station located on the second sublevel immediately beneath the new terminal – an excellent time to examine the performance of the equipment employed more closely.

Because of the tunnel geometry and other local factors, the tunnel formwork carriages in operation in Berlin have entirely different dimensions and designs. Since the station is subdivided lengthwise initially by six and thereafter by four rows of piers spanning different widths, a total of seven formwork carriages were employed for the first single structure. These included fairly small units with relatively intricate stationary shoring structures for the pouring of the only 50 cm thick station soffit. These were accompanied by wide, two-part formwork carriages with trailing elements. For larger soffit spans, these enabled the formwork carriages to be moved only a short time after pouring. To this end, the first part of the carriage is initially relieved and displaced, while the second part stays under load until the trailer is positioned as a central support under the slab. The second part of the formwork carriage can then be lowered and moved as well.

Lowerable by a meter

The biggest challenge and obstacle to the construction of the station were the existing massive joists for the subsequent staircases (linking the station directly with the terminal above). To pass the concrete beams, the tunnel formwork carriages had to be lowered by a meter – the usual retraction distance is up to 10 cm, made possible by lowering the structure on jacks. “In this case, we had to come up with a custom solution,” explains stationary shoring expert Rainer Rix, Managing Director of the RöRo Traggerüstsysteme division belonging to ThyssenKrupp Bauservice. “We designed the formwork carriages concerned so that we could shorten them by a meter as quickly as possible, i.e. first remove the materials and then refit them afterwards.” Rix doesn’t reveal the design details. But what counts is that the modification method did its intended job.

Creative width adjustment

Meanwhile, all the tunnel formwork carriages have left the station. The formwork carriages, some new and some modified, are now working their way in different directions at opposite ends of the station. The gradual narrowing of the tunnel cross section from the six-track station to a two-track and four-track tunnel at the west and east ends respectively calls for width adjustment from the tunnel formwork carriages employed. Here, again, the RöRo engineers have arrived at a creative one-off solution that saves time and money. Special inserts at the end of the normal main girders ensure centimeter-precision adaptation on both sides of the formwork unit to the changing tunnel width. A whole meter can be compensated for on both sides by fitting or removing these inserts.

In the narrower tunnel areas, it is mainly the thickness of the soffit that dictates the design of the formwork carriage. The concrete here is 0.7 to 2.3 m thick, and the loads have to be safely taken up by the formwork carriage’s steel substructure. This task is performed by a cluster of support towers on a load-bearing structure that discharges the loads from the concrete and formwork into the main and base girders and the hydraulic posts beneath.
“Although the basic principle of the tunnel formwork carriages is the same, each one is unique,” says Rainer Rix. “Rather than operating with an expensive modular system, we flexibly choose the best materials for the particular application from our huge range of stationary shoring and adapt them to the specific task. With our decades of experience, we can find a cost-effective solution, whatever the challenge,” says Rix in praise of his company.
Another advantage is the assembly and modification of the tunnel formwork carriage by the company’s own experts. “We build the formwork carriages ourselves and show the site staff precisely how they have to operate and move the equipment,” confirms RöRo project manager Uwe Baczinsky, who monitors the project locally and maintains regular contact. “Wherever possible, we also modify the formwork carriages after their first assignment and then put them to use elsewhere in the tunnel,” says Baczinsky, describing the cost-cutting overall strategy devised by RöRo. The stationary shoring experts have no doubt that the strategy will pay off and are confident about the coming two years – because this is how long the tunnel formwork carriages will be operation at the huge BBI construction site.