The space

In accordance with the basic architectural concept, the six station spaces of the Wehrhahn Line are designed as a single continuum. Unlike the individually designed characteristics of the incisions in each station, which connect the underground stations to the aboveground urban space, the design of the actual station spaces refers to the precast concrete “segments” that constitute the tunnel space proper and interprets these in terms of generously contoured, calm and bright expansions.
The memorable and thus unmistakable visual properties of the precast concrete wall reliefs runs through all the stations and thus significantly fosters perception of the conceptual idea of the Wehrhahn Line’s spatial continuum.
Floor and ceiling surfaces emulate the colors of the walls or the body of the tunnel and contrast the volume of the station spaces clearly to the surfaces of the incisions.
All stations lie 14-18 meters below ground level, and the station platforms are between 3.3 and 4.5 meters wide. The station ceilings are 4-6 meters above the platforms. At four of the six stations the tracks, and therefore the station, are curved.
All areas of the platforms are accessible by elevators suitable for the disabled. The stairwells are fitted with escalators or fixed stairs in a variety of combinations and are partially supplemented by emergency stairwells.
The surface-level balustrades surrounding the entrances are a key characteristic of the Wehrhahn Line: Their horizontal lighting and viewing slits are accentuated by hidden built-in luminaires. The inner sides of the balustrades are covered with wall panels used for the respective station’s incision, making it clearly identifiable when standing aboveground.

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The concept

netzwerkarchitekten and Heike Klussmann designed the subway tunnel proper as a connective continuum, similar to a giant snake writing through the earth, expanding at each station before continuing on its way. In contrast to the colors of the entrance zones, it features a bright, relief-like network structure. The smallest graphic unit is a diamond. It is completed and constantly varied by the joints between the structural elements, resulting in a spatial drawing. The structure of the continuum systematically expands and contracts, creating a sense of dynamic space.
An entirely new production process was necessary to manufacture the concrete elements that allowed extreme geometries to be rolled outand achieve very high levels of precision and surface finish. The entrance zones linking the platforms to the world above the ground were designed as three-dimensional incisions. Their geometries, materials, colors and individual artistic designs contrast with the station concourses. They let plentiful natural light into the spaces below. Passengers enjoy broad sightlines leading into the spaces, a device that creates a strong sense of clarity and orientation.

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Process and Construction

Engineering, construction and manufacturing processes in the station spaces of the Continuum

The production of the concrete diamonds rhombi for the continuum by Heike Klussmann and netzwerkarchitekten would not have been possible without research into material development and production processes. In close collaboration with the manufacturer Schwab-Stein in Baden-Württemberg new production processes were developed, making it possible to produce panels with extreme geometries that do not require additional structural bracing while achieving a high degree of precision and surface quality. The emphasis was above all especially even surface, acute angles and precise edges.
Precast fair-faced concrete sections are generally produced using formwork. This means concrete is poured into prefabricated casings as a hollow form where the casing side is visible and the filled side is to the back of the finished piece. Because this method only makes sense when it is used for a large quantity of identical elements, a different approach was used for the realization of continuum.
All of the 6700 individual diamonds of continuum were cut from blank slabs and produced with a vacuum filter molding method. The mixture consisted of high-performance concrete, aggregates of Nordic white and quartz sand and, for the pigment, 130 grams of black iron oxide per 100 kg of raw mixture. It was only possible to produce the light color with complete consistency using the specially developed mixture: cement and aggregates with a precise amount of added pigment. Then, the basic mold (244 x 123 cm) was filled with the raw mixture, leveled and homogenized, so that the material was evenly spread throughout.
The homogenized raw slab then went to the concrete slab press (a 65-ton press, the largest in the world) and was pressed or rather compressed with a force of 3,000 tons per minute; in the process, up to 50% of the water was removed. This was followed by a hydraulic hardening process in optimum climatic conditions over two days. The back of the highly compacted base slabs was then calibrated and milled on the surface so that flatness tolerances in accordance with DIN V 18500 were achieved.



After another storage period to achieve the final hardness, the slabs were cut and reworked. At this point, the diamonds for the Continuum were positioned using a photo supported detection system to use as much of the plates as possible and then a 5-axis CNC saw cut the slabs into the individual diamonds. Finally, the edges were profiled and four anchor holes were drilled into the back of each diamond panel.
In order to form the edges a special scheme was designed to guarantee that inspection requirements would be met. Classification as “bottom” or “top” fold panels meant that entire area of the building shell remained accessible and each individual diamond panel could be removed. At the same time, joints required no sealants and the mounting remained invisible.
The minimal waste that was created was 100% recycled and reused as aggregate. The surface of the panels was then finished by applying a water-repellant coating and anti-graffiti protection.
However, the extremely high standards applied not only to the precast concrete sections, but also to the substructure and its installation. “Clasps” that were invisible from the front were attached to the back of the panels with an undercut drilling method. They were then inserted into mounting rails. Because of fire safety regulations and the low melting point of aluminum, it was not possible to rely here on the customary commercial solutions and a special system using stainless steel was developed.





One exceptional challenge was the installation of the concrete diamond panels in the areas where the walls curved. Here, the wall panels take the shape of a small-sectioned polygonal line following the ideal line of a circle segment along the station walls. Because the diamond pattern has no vertical joints, support posts were placed on the nodes of the regular polygonal line frame to avoid the diamond tips getting out of line. Thus, a high level of precision in the overall construction was achieved through the dimensional accuracy of the panels and joints combined with a minutely adjustable frame.




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