The name Verbindungsbahn (German for connection line) is used in Stuttgart to describe the railway line between the subterranean S-Bahn Stuttgart station at Stuttgart Hauptbahnhof and the tunnel exit at the station in Stuttgart-Österfeld, which connects, via tunnel, the Stuttgart valley and the Filder plateau. The term originates from the planning stages in the 1960s, when similar projects for the S-Bahn München and S-Bahn Rhein-Main were referred to with the same term.
With a length of 8.788 km, the tunnel is the longest S-Bahn tunnel in Germany, and was the longest railway tunnel of any kind in Germany from 1985 until 1988, when the Landrückentunnel was opened for service. The tunnel is made up of two sections: the 2.6 km long section from Stuttgart Hbf to the station at Schwabstrasse, and the 5.5 km long Hasenberg tunnel, which ascends to the Filder plateau. As part of the project Stuttgart 21, the tunnel is scheduled to be extended by new underground construction, such as the new Rosenstein tunnel.
The first section of the tunnel was constructed between 1971 and 1978, mostly utilizing the cut-and-cover method of construction; only at the terminal loop and a short piece between the Hauptbahnhof and city center was the mining/boring technique of construction used. The second section, constructed between 1981 and 1985, utilized the mining method, with the exception of the station at the University of Stuttgart.
History
Planning phase
Stuttgart mayor Arnulf Klett, in a letter to Deutsche Reichsbahn dated 11 July 1949, proposed the construction of a tunnel, roughly 1.2 kilometres in length, to run between Central station and Alter Postplatz, just south of Rotebühlplatz; he also mentioned the willingness to undertake financial participation in the project by the city of Stuttgart. This tunnel was meant to absorb the suburban service utilizing electric multiple unit trains, which had started in 1933. In response, Deutsche Bundesbahn underwrote a study, which was published in 1956 in the journal Die Bundesbahn. The initial plans envisioned an extension of that line to the Stuttgart–Horb railway, which would also offer faster connections to commuters in the area to the south and southwest of Stuttgart. To achieve this, a tunnel with a grade of 50 ‰ was planned, which would have connected to the Stuttgart–Horb railway just before the former station at Wildpark, and units of DB Class ET 30 were slated for service. A new, four-track subterranean station, called Stadtmitte (city centre), was planned to be built underneath the then-under-construction Theodor-Heuss-Strasse, where half of the trains originating at Hauptbahnhof would have turned around. The station underneath Hauptbahnhof was also meant to be expanded to four tracks to enable a concept to utilize the highest possible operational efficiency; this concept was later put into operation in the S-Bahn station under the Frankfurt Hauptbahnhof. Another subterranean station, with the name Schwabstraße, was planned for the west of Stuttgart.
Initially, no progress was made in terms of the financing of the project. In 1964, the Bundesbahn published an updated planning document, which featured these differences to the study of 1956:
the stations at Hauptbahnhof and Stadtmitte would have only three tracks the tunnel to connect the line to the Stuttgart–Horb railway was to be lengthened, reducing the grade to 35 ‰
In 1967, the city of Stuttgart decided not to go forward with existing plans for an underground railway line (by then labelled “U-Bahn”) parallel to the Verbindungsbahn. As a result, a new station with the name Feuersee, between Stadtmitte and Schwabstraße, was added to the plans for the Verbindungsbahn. Also, all trains originating from Hauptbahnhof were to stop at all stations to Schwabstraße, in the name of better service along the line. These changes now enabled the Bundesbahn to develop the plan for the Verbindungsbahn as it was later realized:
Stations featuring two tracks, with an island platform between these tracks, at the stations Hauptbahnhof, Stadtmitte, Feuersee, and Schwabstraße. The Bundesbahn favored the clear arrangement and ability of passengers to change trains on the same platform. A terminal loop, with the ability for passing, connected to the station at Schwabstrasse. Even though this meant higher construction costs, the Bundesbahn saw this method as economically more efficient than an underground terminating track, especially in light of the many trains that were scheduled to turn around at this station. The reduction to two tracks made possible the relocation of the station at the Hauptbahnhof closer to the city center, underneath the narrow Lautenschlagerstrasse. This change made changing service to the Stuttgart Stadtbahn easier, shortened the walking distance to the city centre, and meant no change in getting to trains in the above-ground main line station.
Construction of the inner city section Hauptbahnhof–Schwabstraße
In May 1971, construction began to build the three subterranean levels for the Klett-Passage for pedestrians, the Stadtbahn, and the S-Bahn. On 5 July 1972, work started on the construction of new a raised track in the Hauptbahnhof apron. In April 1972, the first phase of construction on the actual Verbindungsbahn began with the construction of a tunnel underneath the tracks of the main line station, parallel to the underground passages under the main hall.
Hauptbahnhof
During the construction of the Verbindungsbahn section underneath tracks 1 through 3 of the Hauptbahnhof, no more than one main line track was scheduled to be taken out of service at any time. For this reason, a makeshift platform was constructed at the north-westerly track 1a, and the excavation pit was split lengthwise into two lateral pieces. Construction was done utilizing the cut-and-cover top-down method, where the outer shell was made up of a 28-metre-deep bored pile wall, on top of which the roof structure of the tunnel was built. The tracks and platforms where then reconstructed on this tunnel roof by May 1974, which meant the complete resumption of main line traffic. Underneath the roof structure, space was then excavated from the top down, and the inner shell was constructed, consisting of a floor and inner walls, which carried the roofs of up to three internal levels. On the topmost internal level, an underground parking garage with 120 spaces was built, which doubled as a civilian shelter, secure against airborne agents, such as poison gas, and nuclear attack, with a capacity for 4,500 persons. The inner shell was completed at the end of 1975.
The technically most challenging aspect of the construction of the Verbindungsbahn was thought to be the subterranean section, 74.6 metres long and about 25 meters wide, underneath the main building of the Hauptbahnhof, which is a protected cultural monument, in the area under the Kleine Schalterhalle (small counter hall). In deciding whether to use the mining/boring construction method, or dismantling and reconstructing parts of the building, here too the decision was made to instead utilize the top-down cut-and-cover method. To be able to move the weight of the building onto newly constructed outer support pillars, the foundation walls were surrounded by concrete sleeves, or pairs of concrete beams, which braced each other. Underneath the sleeves, horizontal beams, which rested on the new support pillars, were constructed. Once both the sleeves and beams were in place, hydraulic presses were used to put pressure onto this supporting structure, until the pressure generated equaled the weight of the building to be put on top. To complete the process, the presses were then replaced by wedges, and the resulting settling of the Kleine Schalterhalle was limited to 1 millimetre. The old building foundation was removed, the site underneath the building was excavated, and in September 1973, construction of the inner shell of the tunnel was begun.
Management of the construction process underneath the main line station forecourt, with the Klett-Passage, Stadtbahn station, and S-Bahn station, was the responsibility of the City of Stuttgart.
The section between the Hauptbahnhof and the station Stadtmitte was to be built utilizing open construction, according to the plans, where roads above the construction site would be demolished, the tunnel built, and then the road and other structures would be replaced. However, a bidding construction company suggested that a 322-metre-long section between Thouretstrasse und Fürstenstrasse should be constructed using a mining method, in a depth of 8 to 10 metres. As the most economical solution, this suggested method was used, but as it resulted in settling of over 50 centimeters in places along the section, instead of the expected settling of 5 centimetres, the sewer system and roads above the site had to be reconstructed. Utilizing the open method of construction after that point, an emergency exit was also added near the Kanzleistrasse. In June 1973, this section was the first shell section of the Verbindungsbahn to be completed.
Stadtmitte
Starting in November 1972, the station Stadtmitte, 22.5 meters wide, with a platform 12.8 metres in length, was constructed under the Theodor-Heuss-Straße, which is near the Rotebühlplatz and is 45 metres wide. The platform area was also built to be able to serve as a civilian shelter with a capacity for 4,500 people, which would be usable in case of an emergency if the S-Bahn traffic was also stopped.
In addition, a crossing tunnel was constructed in advance for the Stadtbahn Stuttgart, which was taken into service on 31 October 1983. Lines U2, U4 and U14 operate there, but due to Stuttgart 21 project, lines are redirected in 2017 (now U1, U14 and U21). The western entrance to a pedestrian tunnel under the Rotebühlplatz was constructed in the inner courtyard of a new building for the Allgemeine Rentenanstalt, a public pensions institution.
Feuersee
The Verbindungsbahn continues on its way under the 23-metre-wide Rotebühlstrasse, and this section was also built using the open construction method, starting in November 1973. The existing tram line was rerouted to run on the Gutenbergstrasse, which runs parallel to the northwest of the Rotebühlstrasse, while the road traffic was rerouted to the Augustenstrasse, parallel southwest of the Rotebühlstrasse. This section of the tunnel has a lengthwise grade of 37.5 per mille, and the station at the Feuersee has a grade of 20 per mille. The Feuersee, a lake which had been built in 1701, and borders both the excavation zone as well as the Johanneskirche, was drained of all but half a metre of water during the construction phase.
Schwabstrasse
The Schwabstrasse station (German spelling: Schwabstraße) was constructed at a depth of 11 metres to 27 metres under the street surface, underneath the ground water table, which is at a depth of 6 to 8 metres. Lengthwise, this section has a grade of 1.6 per mille, and was only completed in December 1977, due to complications in the construction of the ascending terminal loop, which utilized a mining construction method.
Terminal loop