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Wehrhahnlinie Duesseldorf, Concept, Konzept, Urban Context, Staedtebaulicher Kontext, Photo netzwerkarchitekten
Wehrhahnlinie Duesseldorf, Concept, Konzept

Planning the entire line: construction methods and engineering techniques

The task and its implications for the city

The new Wehrhahn Line augments and optimizes the existing subway network, and it improves access to the city center from the eastern and southern districts. The line runs beneath the central shopping district and the inner city development between Schadowstrasse, Berliner Allee, and Königsallee, and under the heritage Art Nouveau department store Kaufhof on the Kö, joining up with the existing transport hub at Heinrich Heine Allee. This mainly underground transportation system provides smooth  and barrier-free transportation for more than 50,000 passengers a day. It also underpins the interplay between private and public transport by significantly reducing the road traffic above, so the completion of the Wehrhahn Line offers passengers better, faster and more egalitarian mobility. But not only that: it also opens up new options for urban design at ground level. For instance, the Kö-Bogen project was only made possible by relocating the local transport service underground – thereby allowing the tram lines on Jan-Wellem-Platz to be removed.

Project history

In September 2000, the Ingenieurgemeinschaft Wehrhahn-Linie – IGW was commissioned as a JV to handle the planning of this major transportation infrastructure project, with Schüßler-Plan as lead managers. This was the result of a European-wide tender. The state capital Düsseldorf was the developer and the Municipal Dept. of Transportation Management coordinated all the client-side activities and assumed overall responsibility.
The scope of IGW project planning services included the civil engineering works and transportation facilities, as well as structural engineering. Based on the study of several possible solutions at the preliminary design stage, the decision was made to use a ‘shield tunneling’ method for the track, combined with a cut-and-cover method for the stations. This approach was preferred since it ensured only minimum disturbance at ground level and employs construction methods that are suitable in an urban context.

TBM (Tunnel Boring Machine) after entering the Benrather Straße target shaft, photo: Susan Feind
TBM (Tunnel Boring Machine) after entering the Benrather Straße target shaft, photo: Susan Feind


In 2001, the City of Düsseldorf held a pan-European architectural competition for the design of the Wehrhahn Line’s subway stations. A mandatory requirement for participation in the contest was that architectural practices and artists should enter as a team.
From a field of some 70 renowned applicants, the architects netzwerkarchitekten , together with the artist Heike Klussmann pre-qualified, and then emerged the winners. The competition remit and the subsequent planning contract involved designing the stations, as well as the two ramped structures leading from track level up to the surface.
Following the architecture competition, netzwerkarchitekten were commissioned to oversee the task of architectural project planning, including the special responsibility for its impact on the geometry of the shell of the civil engineering structures. In a subsequent competition, five additional artists were chosen to expand the number active on the project.
During on-site construction work, senior staff at Schüßler-Plan complemented the municipal project team for site and construction supervision. netzwerkarchitekten were responsible for supervising the artistic contributions.
Following planning approval and some preparatory work, on-site construction started in 2008 with work on pipelines and sewers, as well as special foundation works for the stations and their entrances. In 2010, tunnel construction officially commenced. Tunneling began at the southern section in Bilk, and within two years the tunneling work was all but finished. From January 2013 onwards, in parallel with the construction that was taking place in the final tunnel section between the Kaufhof an der Kö and the existing Heinrich Heine Allee station, installation of the operational facilities started in areas where the tunnel was already complete, as well as final work on the tracks.
From this point onwards, until the line was commissioned in February 2016, netzwerkarchitekten – working in collaboration with the six artists – moved ahead with the architectural work; at the same time the operational facilities were installed at the stations.

Tunneling, segment warehouse, each ring piled, length 1.50 m, Photo: Susan Feind
Tunneling, segment warehouse, each ring piled, length 1.50 m, Photo: Susan Feind


Special engineering and constructional features
Tunnel construction


IGW had already specified in the initial planning that the tunnel should be constructed mainly in a shield-drive tunneling process and thus the work essentially took place underground. In shield tunneling, a tunnel-boring machine works its way forwards underground. The tunnel-boring machine is driven forward by the cutter wheel eating into the earth ahead of it.
In Düsseldorf, the so-called ‘hydro-shield process’ was the method chosen for excavating and propping. A bentonite slurry reinforced the ground in front of the cutting wheel and then mixed with the soil before being pumped out of the tunnel using a suction line. The mix was pumped into a separation chamber, where the bentonite was removed and then supplied back to the bore head. The residual soil was used as landfill. Protected by the shield tunneling machine, the tunnel evolved  one ring at a time from single-wall pre-cast lining segments. In this case it consisted of eight ring segments, each 45cm thick. As it progressed, the shield tunneling machine levered itself on the previous ring and thrust the shield body forward by the length of one ring (on the Wehrhahn Line this was 1.5 meters), simultaneously excavating soil as it moved. The interstitial space between the outer surface of the tunnel and the surrounding soil was pumped full of mortar.
The entire 2.3 kilometer length of shield tunneling was divided into two sections – southern and eastern – and these were constructed the one after the other using the same tunneling machine between March 2010 and December 2011. After the southern section was finished, the machine was dismantled in a target shaft south of the old city center, and then transported to the eastern starting shaft, where it was reassembled.
Owing to the fact that low-floor subway cars were being used for the first time in the Wehrhahn Line tunnel, special requirements regarding the tunnel’s geometry had to be fulfilled, with respect to the continuous escape routes at the sides, so it was decided that the tunnel cross section would require a shield with an external diameter of 9.5m and and internal one of 8.3m.

Cover construction, Photo: Implenia / Susan Feind
Cover construction, Photo: Implenia / Susan Feind


Station construction work

In order to keep interventions in the inner city area to a minimum especially along Düsseldorf’s chic Königsallee and the shopping zone on Schadowstrasse all the stations were built using the cut-and-cover method. This involves first sinking vertical diaphragm walls and columns into the ground on one side of the road, then opening up the adjacent road section, into which a horizontal concrete lid is cast, supported on the diaphragm walls and columns. This lid then forms the roof of the new station. During this period, traffic continues to flow on the unaffected side of the road. As soon as the intervention is finished and the surface covered over, the same procedure is then carried out on the other side of the road, and traffic is redirected onto the newly completed surface. Only after the concrete lid is finished, can work start underground on constructing the respective station. Traffic flow is thus maintained throughout the construction period and disruption for residents and businesses kept to a minimum.
The tunnel boring machine’s cutting wheel not only removed the earth to create the track bed between stations. It also cut a 80-120cm thick circular slice out of the concrete diaphragm walls that ultimately surrounded the stations. Having thread the shield tunnel boring machine through the protective construction, excavation continued under the lid – which functioned as bracing – allowing a stabilizing concrete structure to be put in place.

View of ascended tunnel, photo: Implenia / Susan Feind
View of ascended tunnel, photo: Implenia / Susan Feind


Underground construction work below Kaufhof an der Kö

One exception to the process described above was the construction of the subway station at Heinrich Heine Allee, located under the heritage-listed Kaufhof building. A large portion of this station lies just below the foundations of this Art Nouveau department store. To prevent the occurrence of even the smallest amount of subsidence damage caused by tunnel-related construction work, a section some 70 meters long was created by a mining method that uses temporary freeze wall protection. In this process, soil below ground level is frozen by means of refrigerating lines. A 2.5cm thick body of ice stabilizes the ground, simultaneously sealing the construction work against any encroaching ground water. Within this stable frozen mass, tunnel tubes, openings, and the transverse sections of stations can effectively be carved from inside. A stabilizing concrete construction is then introduced which, as soon as the ground thaws out, absorbs the compressive forces of the earth.

Tunneling, excavation from the Benrather Straße excavation pit, photo: Implenia / Susan Feind
Tunneling, excavation from the Benrather Straße excavation pit, photo: Implenia / Susan Feind


Fire protection

In case of emergency, two smoke ventilation openings at each end of the 90-meter long platform provide natural smoke extraction to both the tunnel section as well as the train station. The smoke ventilation openings are located at the highest geometric points of the transition zones in order to prevent smoke build-up. Smoke curtains at the entrances to circulation routes leading up to concourse level shut down in an emergency to a height of 2 meters above the platform level, ensuring the safe evacuation of passengers as well as mandatory smoke-free entrances. Openings created between the incision areas and platform level are enclosed by smoke-resistant glass, and entrance areas that lie above the tracks boast F90 fire-resistant glass. These human safety measures also protect the substance of the building to a certain degree. In case of emergency, if a fire were to break out, these fire containment measures would also effectively reduce the amount of repair work required.

View of the cover construction, subterranean excavation with demolition of the tunnel, photo: Implenia / Susan Feind
View of the cover construction, subterranean excavation with demolition of the tunnel, photo: Implenia / Susan Feind


Project completion: in time and in budget

Both the planning and construction phases of the Wehrhahn Line were achieved largely in time and in budget. Over the course of the anticipated eight-year program construction work was only postponed when archaeological or culturally significant findings came to light during the special civil engineering and excavation work. This major downtown construction project was well received by the public thanks to a high degree of transparency throughout the project and the consultation of a wide swathe of the general public in each phase. Efficient project organization, together with the integrated collaboration between by the various disciplines involved in the project from planning to inception meant that, in February 2016, Düsseldorf was able to successfully launch its largest downtown transportation project to date.
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Wehrhahnlinie Duesseldorf, Concept, Konzept, Art and Architecture, Kunst und Architektur, netzwerkarchitekten, Cover method, Deckelbauweise, Photo Implenia, Susan Feind

Next stop: Art

By Gerhard Matzig

“Redline – Next Stop: Terror” is the name of a 2013 American thriller, which, justifiably, is hardly known by anyone. Essentially, it’s about two bombs that are found in the Los Angeles-based “Red Line.” One is detonated and the train derailed, speeding along the tunnel with no brakes and on fire; the other bomb has to be found and defused by the surviving passengers, something – in the darkness of the eerie tunnel –that is not exactly funny. So far, a well-known story line, or not. What is for sure: In the movies the subway serves as a vehicle of fear over and over again. Whether the film is called “The Incident” (1967) or “Subway” (1985): The tunnel, trains and stations always act as a backdrop to the thrill. Underground urban mobility in the world of fiction is, be it in cinema or in literature, usually located pretty close to hell: deeply dark and full of danger.

Schadowstraße, Ursula Damm, netzwerkarchitekten, Photo: Implemia / Susan Feind
Schadowstraße, Ursula Damm, netzwerkarchitekten, Photo: Implemia / Susan Feind


This is why there are reasons to thank the City of Düsseldorf. With the construction of the Wehrhahn Line, started in 2001, an almost heavenly reality now exists to oppose the hellish fiction. The 3.4-kilometer-long line, inaugurated in February 2016 following 15 years of planning and construction, is certainly not the definition of a gruesome non-place –rather, it illustrates an absolute utopia. The result is exemplary public spaces that serve more than just the function of movement for they are also at the service of art and architecture. The result: high-quality public spaces expanding urban space into the underground. The upshot is in the final analysis a subway line that is also a nod to the future of public transportation in urban areas. Goethe’s advice in Faust, words one probably would never have imagined applying to a subway, comes to mind here: “Stay a while.” A subway that encourages you to stay: Now that is a rarity.

Franz Kafka once had his “stranger” explore the “essence of Paris” using the metro. In Düsseldorf, the essence of the city as an ambitious art metropolis can be discovered using the new line. The bare facts of a massive infrastructure project that did not need once to “get out of hand” (as compared to the Elbe Philharmonic Hall in Hamburg or the Willy Brandt Airport in Berlin) to make the national news, but has instead gratifyingly taken the limelight for its experimental character, and thus for being visionary. On the one hand: 843.6 million euros in construction costs, tunnels with an inner diameter of 8.3 meters, 90-meter-long platforms, 13 elevators, 59 escalators … construction by means of shield tunneling … these are just a few aspects of successful transportation planning. On the other hand there are: six memorable, astonishing underground stations, which – as predicted – will be visited by more than 50,000 people a day. It’s possibly what makes Düsseldorf the most unfathomable city in Germany, namely, a city with an underground art complex. The “art city” of Düsseldorf didn’t simply build a subway. It built the most art-centered subway in Germany. If not in the entire solar system.

The fact that you can experience a spatially suggestive solar system in the Benrather Strasse station is the merit of an actually almost impossible, or at least very rarely encountered, symbiosis of transportation planning, architecture and art. Here, Swiss installation artist Thomas Stricker has realized his concept Heaven Above, Heaven Below. With the help of large wall monitors, which are distributed throughout the waiting areas in the station, located in the middle of Düsseldorf’s banking district, you get the feeling not of being a subway passenger but a supernatural passenger on a spaceship, heading for a distant galaxy.

Where one would otherwise be standing in drafty, yet stuffy, tile dungeons a real Odyssey of wandering about, waiting for the next subway to come in, normally studying miserable advertising or the ominous flickering neon lights that cast a dim light on the depressing displays of what appears to be a T-shirt shop, here, you have a view of the moving universe. You can see Mercury, Venus or Mars pass, you float towards an asteroid belt – until you have the feeling that you’re not waiting for the next subway in the direction of Pempelforter Strasse, but for the next interstellar trip through the Milky Way.

Benratherstraße, Thomas Stricker, netzwerkarchitekten, photo: Implenia / Susan Feind
Benratherstraße, Thomas Stricker, netzwerkarchitekten, photo: Implenia / Susan Feind


In this setting of ingeniously designed and sometimes futuristic-seeming spaces, one easily forgets that they’re underground. That the heavens are so very far away. And then there are always these escalators. Whereby there seems to be a rule for public transportation: for every two escalators, three are usually closed for maintenance. Not so in Düsseldorf – or that is the hope. And if they are? Then you’ll find a bit of consolation in the fact that you are standing in the kind of place where like Goethe you want to stay a while.

It is as if here, in the underground city of Düsseldorf -  where the Wehrhahn Line crosses below the city center you are not at all far from all the qualities usually associated with being above the ground. Instead the subway station is suddenly poetically similar to what Tucholsky once demanded of stations as Modernist edifices: “This is a place to stop.”

We have not only the installation by Thomas Stricker to thank for the miracle of such an intelligent and sensory reinterpretation of the history of the subway, which began in 1863 in London, at Benrather Strasse. He revisits the eternal waiting rooms of public transportation by transforming it into space per se, outer space. This genius is also the product of the architectural competition boldly concluded in August 2001. And today we can safely say: The courage back then has been rewarded. The experiment has been successful.

Schadowstraße, Ursula Damm, photo: Implenia / Susan Feind
Schadowstraße, Ursula Damm, photo: Implenia / Susan Feind


The Darmstadt architectural company that won the competition, netzwerkarchitekten  – together with Berlin-based artist Heike Klussmann  – worked a full 15 years on the Wehrhahn Line. Good things are worth waiting for. Granted, ambitiously designed subways have emerged elsewhere in recent years; in Munich, for example, the long-neglected subway was architecturally upgraded in the course of a welcome expansion to public transportation. But as a result mostly stations were created that resembled a disparate permanent exhibition of contemporary architecture. Each with a different formal language. In different materials. And with different levels of success – unfortunately, sometimes understood as pure ornamentation, as art tagged onto architecture.

If one delves deeper into the history of the subway, one finds that in the past greater advances had been made. Take, for example, the Moscow metro: the first line in 1935 ran between Sokolniki and Park Kultury. It boasted underground stations almost more extravagant than one could imagine. Lasar Kaganovich, the People’s Commissar for Transportation at the time, believed: “The metro will stimulate and illuminate our minds more than all the theaters and palaces.” The stations were designed accordingly and were rich in spatial theatricality. In the course of Modernism and its impressive engineering achievements underground transportation spaces became more technical, and also more advanced and economic in thrust, yet rarely more beautiful. Thus, decisioinmakers resorted to art to give the 20th century “transportation” spaces, which owe their existence to the calculations of technology, logistics or economy, as well as planning laws, more of a human face. Successful examples of this can be found in Brussels or in Lisbon. But also in German cities. Yet holistic solutions remained the exception.

Not so in Düsseldorf: Here, and this is something special, from the beginning engineers, urban planners and architects, as well as artists, worked together. That’s not a given. After all, worlds that could not have been more different met. The one plans for the “evacuation of platforms”, the other talks about “a spatial continuum.” And both were not supposed to cost anything. Such a relationship is almost unimaginable without couple’s therapy.

The power to launch a holistic system after much debate is innate in the design and the product of the communicative building culture of netzwerkarchitekten. Together with understanding civil engineers and an open-minded city administration they have not simply realized a tunnel system in which a subway travels underground (in Düsseldorf the subway is actually a kind of streetcar system, as the underground trains get their power from overhead catenary lines, too). The subway does not pull it at six individually designed stations that have simply been prettied with art. Rather, the architects devised the entire space as a whole, meaning the tunnels and stations, as a “continuum”, with a uniform design.

Graf Adolf Platz, Manuel Franke, netzwerkarchitekten, photo: Implenia / Susan Feind
Graf Adolf Platz, Manuel Franke, netzwerkarchitekten, photo: Implenia / Susan Feind


In other words, the stations and their zones are merely extensions, spatial openings of the tunneling project. Such that the design and materials of the latter can be experienced. What has elsewhere to be endured as a banal, technically standardized space, with mezzanine levels or zones developed according to German industrial norms, has a different shape here: We encounter light, form, cypher and space: building culture. Wherever possible the architects brought daylight all the way down to the bottom of the track beds. Wherever sensible, they created views in and out, sightlines and internal references. They enhanced confinement underground by delivering spatial quality. And first and foremost they banished all advertising and retail outlets from the Wehrhahn Line. The subway is solely the committed work of artists chosen in the second competition (2002): Ralf Brög (Heinrich Heine Allee), Ursula Damm (Schadowstrasse), Manuel Franke (Graf Adolf Platz), Enne Haehnle (Kirchplatz), Heike Klussmann (Pempelforter Strasse) and as mentioned Thomas Stricker (Benrather Strasse).

All over the world cities are growing. And in terms of transportation down into the ground, too. Düsseldorf augments this development with a unique spatial experiment: Art and architecture set the rhythm of urban and suburban mobility. Here, a ride in the subway is not an annoying, rocky ride from station to station that is best survived by checking e-mails on your cellphone. Instead, it has become a trip through space for all the senses. Driven by that creativity which succees in transforming dead time in the subway into vital art.

Heike Klussmann, who artistically interpreted the Pempelforter Strasse station and defined the entire space by running white bands through it that emulate the movement of a pinball, manages to transform a techy space into a stunning and complex volume: “Stay.” Or Ralf Brög, who uses the station at Heinrich Heine Allee as the visual and above all acoustic venue for a changing sound collage. The aural fabric he weaves challenges the presence of conventional Muzak, that is to say that bland sound that seeps out of elevators and department stores and can already be heard in the first subway stations (the preferred choice: Mozart). The public is a part of it. This, too, is a daring experiment.

Schadowstraße, Ursula Damm, netzwerkarchitekten, photo: Implenia / Susan Feind
Schadowstraße, Ursula Damm, netzwerkarchitekten, photo: Implenia / Susan Feind


In other words, it is not the architectural means that are different but the artistic interpretations of room and mobility, and is this that lends the underground architecture a marvelous appearance, while above ground it cleverly adapts to the anatomy of the city. This does not mean that the architecture is reduced to the mere production of the technically required space. On the contrary: With a keen sense of spatial proportions, with views outwards and inwards that always surprise and a well conceived choreography of shapes and materials, it very much proves itself to be both state of the art and self-confidence. It simply creates space, for something different.

Take the poetic art of Enne Haehnle, which oscillates between script and space, between sculpture and symbols, transforming the Kirchplatz station into a text you have to guess. Or the intensive color that is the result of hundreds of individual bright green panes of glass that together form a flowing line and which Manual Franke uses to foster a topography of motion from the underground layout of Graf Adolf Platz. It also holds true for the interactive installation that Ursula Damm created at the Schadowstrasse station: The world above is thus directly linked to the world below.

Pempelforter Straße, Heike Klussmann, netzwerkarchitekten, photo: Implenia / Susan Feind
Pempelforter Straße, Heike Klussmann, netzwerkarchitekten, photo: Implenia / Susan Feind


It is to be hoped that Rhine Bahn AG, as the developer of the exemplary Wehrhahn Line, is aware of how audacious and pioneering the project is. Brög has transformed a section of the Heinrich Heine Allee station by playing birdsong, modulated by a computer. It’s certainly pretty amazing that one can come to Dusseldorf to chill in the subway, surrounded by art and architecture. And a quite unique thrill. The film mentioned at the beginning “Redline – Next Stop: Terror” would have to have a very different name in Düsseldorf, where the subway is so much more friendly and very much forward-looking. Next stop: Art. It is the great art of creating the city of tomorrow.
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Wehrhahnlinie Duesseldorf, Concept, Konzept, Heinrich Heine Allee, Ralf Broeg, Photo Implenia Susan Feind

Spatial concept

Following the competition phase, the basic architectural concept drawn up by netzwerkarchitekten, working with engineers appointed from the engineering consortium, was adopted. Due to potential problems concerning the water table level, the engineers opted to build a single tunnel to house both tracks, using a shield tunnel-boring machine with a diameter of 9.5m. The six stations on the Wehrhahn Line were designed as a single connected, uninterrupted subterranean system – a »continuum.« The »incised« entrances and exits punctuating and opening up this geometric continuum at street level were positioned to fit into the surrounding urban fabric. Immediately below the entrance, passengers reach a gallery-like concourse within the station. At this point, sightlines open up towards the platforms. The open geometry of the ceiling – both at the intermediate escalator/staircase level and continuing from this level down to the platforms – creates a dynamic space that, when viewed from this perspective, offsets any sense of the space being restricted. These geometrically generated sightlines help passengers to orientate themselves and to discern the various circulation levels, ensuring both legibility and social control within the metro stations. At the Heinrich-Heine-Allee and Schadowstraße U-Bahn stations, glass skylights are employed to direct daylight deep into the stations.

netzwerkarchitekten and the artists worked closely together in the design of these transitional circulation areas. Each has its own specific theme, so passengers can easily recognize stations and find their way through them, while individual stations retain their own identity within the continuum.
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Wehrhahnlinie Duesseldorf, Concept, Konzept, Spatial Conception, Raeumliches Konzept
Wehrhahnlinie Duesseldorf, Concept, Konzept, Access Ramps, Rampenbauwerke, Pempelforter Strasse, Longitudinal section, Photo netzwerkarchitekten

Access Ramps

Along the Wehrhahn Line the geometry of the two ramps accessing Am Wehrhahn and Bilk, create a fitting termination for the Continuum theme within its urban context. These two entrances appear as rounded-off elliptical exposed white concrete surfaces, giving the construction a clear dynamic identity. An integrated stop above ground, close to the Bilk ramp, provides an interchange with the Bilk S-Bahn station.
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Urban Context

The Wehrhahn Line runs from south to north via the U-Bahn stations Kirchplatz, Graf-Adolf-Platz, Benrather Strasse and Heinrich-Heine-Allee, and further eastwards on Schadowstrasse to the Pempelforther Strasse station. At Heinrich-Heine-Allee, a hub station, the new construction links into the existing metro network and runs below Berliner Allee, Königsallee and the historically protected Kaufhof department store on the Kö. The total length of the new construction project, measured between the access ramps at Bilk and Wehrhahn, is 3.4km.
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Wehrhahnlinie Duesseldorf, Concept, Konzept, Urban Context, Staedtebaulicher Kontext, Photo netzwerkarchitekten
Wehrhahnlinie Duesseldorf, Concept, Konzept, Urban Context, Staedtebaulicher Kontext, Photo netzwerkarchitekten
Wehrhahnlinie Duesseldorf, Concept, Konzept