1. General

1.1 Task

In the course of the new construction of the B 62n (Hüttentalstraße) in the south of the city of Siegen, the Bühltunnel was built in the district of Niederschelden to pass under the densely built-up Bühlrücken. The tunnel is located in the last construction section of the overall project between the Siegtal bridge (A 45) and the Mudersbacher traffic circle (Rhineland-Palatinate). The construction of the new tunnel will improve traffic conditions in the B 62 through road, which was previously characterized by signal systems and a large number of junctions and intersections. As a result of the inadequate traffic conditions, the downstream road network was used as an alternative route and residents were burdened with heavy emissions.

The planning originally envisaged a four-lane cross-section for this section of Hüttentalstrasse, which was to be routed around the Bühl Ridge. After cancellation of the corresponding planning approval decision in 1983 and extensive new planning, the B 62n now passes under the Bühlrücken in a two-lane two-way tunnel. From the beginning to the tunnel, the federal highway has a four-lane cross-section in the course of the B 54 and the B 62, and a two-lane cross-section until the end of the construction section in Rhineland-Palatinate.

Coming from the east, the new route runs in a radius of 500 m, which changes into a rectified radius of 400 m after a clothoid. Towards the west, a turning clothoid follows to transition to an opposite curve with a radius of 450m. In the east-west direction, the gradient initially falls at a rate of 3.5%; after about one third of the tunnel length, the longitudinal gradient decreases to 1.155%.At the western portal, the gradient rises at a rate of 3.5%. The low point of the trough in the tunnel is approx. 9 m before the western portal. The B 62n has a standard cross-section of 10.51 in the tunnel. The maximum permitted speed in the tunnel is 60 km/h.

1.2 Geology

In the tunnel area, there are predominantly bedrock layers of medium-hard, banky to massive clay shale with intercalated graywacke lenses. The overlying surface layers consist of fill, silt, gravel and rock replacement layers. In the eastern part of the tunnel is the so-called "Rudolf fault" with rocks completely pulverized to loose rock. Furthermore, the gallery of a former, widely branched ore mine crosses the tunnel route here. North of the tunnel axis runs a double-track railroad tunnel built in 1860 for the Cologne - Siegen line.

1.3 Structure design

The design of the tunnel structure is essentially determined by the integration of the eastern portal into the landscape and that of the western portal into the local area.

The eastern portal is located at the end of a 70 m long forebay, which is bordered on the north side parallel to the roadway by a natural stone-clad retaining wall. The tunnel portal was designed with a circular portal ring made of reinforced concrete and end walls perpendicular to the tunnel axis. Analogous to the retaining wall, the end walls are clad in natural stone and covered on the upper side by reinforced concrete cornices. Linear concrete elements embedded in the masonry visually soften the natural stone faces of the retaining wall and the end walls. The western portal is located at the end of a 52 m long forebay, the slopes of which are supported on both sides by space lattice walls. The circular portal crown of reinforced concrete lies in the plane of the embankment. End walls are not present. At the western portal, in the area of the tunnel section built using the cap construction method, the operations building is located above the tunnel. It stands in line with the buildings on the east side of Bühlstrasse. The building, which appears to be two stories high when viewed from the street, was designed functionally with a smooth, unobstructed facade. A low technical superstructure forms the upper floor. Its overhang protects the building accesses on the first floor. The building ties into the slope at the back, which merges into the green roof. The service building is connected to the tunnel below by a stairway and utility shaft.

2. Building design

2.1 General

The Bühltunnel consists of a tube with two lanes, each 3.50 m wide, as well as 0.25 m wide verges on both sides and 1.00 m wide emergency walkways. The clear width is 9.50 m. The tunnel has a total length of 525 meters. The maximum overburden is approx. 27 m. The distance to the railroad tunnel is between 45 and 120 m. The tunnel passes under several urban streets with residential buildings and a school. An emergency exit is located approximately in the middle of the tunnel and is connected to the operating building via a 149 m long and 2.50 m wide escape tunnel.

2.2 Construction method

The portal and the first 5 m of the tunnel on the east side were constructed using the cut-and-cover method. The 417 m long central section in hard rock was excavated by blasting using the closed shotcrete method. The 57 m long tunnel section with a low overburden below the municipal Bühlstraße on the west side was constructed using the cut-and-cover method, and the following 46 m long section up to the portal was again constructed using the cut-and-cover method due to the presence of unconsolidated rock. The tunnel floor is continuously above the ground water level, which corresponds to the water level of the neighboring Sieg River. The central section was excavated from the lower-lying western portal in order to divert the rock water produced during excavation to this portal.

2.3 Construction

The central section, which was constructed by mining and blasting, was built with an anchored shotcrete outer shell and a reinforced concrete inner shell. Depending on the geotechnical conditions, the inner shell was designed with or without a bottom arch. The standard block length of the inner shell is 10 m. Together with the portals and two shorter blocks on the east side, the inner shell consists of 55 blocks. The standard thickness of the inner shell is 35 cm at the crown and 40 cm in unfavorable rock conditions. The closed invert has a minimum thickness of 50 cm.

The floor and vault are made of concrete of strength class C 30/37. Polypropylene fibers (PP fibers) were added to the concrete as part of a pilot project to reduce the risk of spalling in the event of fire. The fibers have a diameter of 15.4 cm and a length of 6 mm. The fiber content is 1.4 kg/m3. The pilot project was used to investigate the production and installation of the PP fiber-reinforced concrete under construction site conditions. The production of tunnel liners with PP fiber concrete as structural fire protection is now the standard construction method. It was included in the ZTV-ING, Part 5.

The tunnel sealing in the area of the upper vault consists of a plastic sealing sheet between the outer and inner tunnel lining. For this purpose, a 3 cm thick sealing support, a protective fleece as a protective carpet pad and the 2 mm thick plastic sealing sheet were installed on the shotcrete lining from the outside to the inside. The sealing ends at the lateral drainage pipes. These are located 1.00 m below the gradient. In the area of the invert arch, only a separating foil was provided between the shotcrete and the reinforced concrete. The cut-and-cover tunnel sections have the same internal shape as the mined section. The crown thickness there is 60 cm. The closed invert was constructed as a water-impermeable concrete structure (WUBKO). The tunnel section constructed as a WUBKO with a cover also has the same internal geometry as the mined section. The invert is closed and the crown has a thickness of 40 cm. Above this is the vault-shaped cover with a thickness of 55 cm. This reinforced concrete cover is supported in niches in the excavated bored pile walls that serve as shoring. In the mined area, the escape tunnel consists of a temporary shotcrete outer shell, a separating sliding foil and a 30 cm thick inner shell as WUBKO. In the area adjacent to the operating building, it has a rectangular cross-section with a wall thickness of 40 cm. In this area, construction was carried out using the cut-and-cover method.

The block joints in the mined tunnel area were designed as compression joints, and the seal is reinforced at the joints. At every third joint, the external sealing is segmented by an external joint tape. The block joints in the open construction method and at the transition between the construction methods are designed as space joints with external and additional internal joint tape.

2.4 Drainage

Via the sealing and the two external drainage pipes, the mountain water is led up to in front of the west portal, where it is discharged into the main drainage pipe. Control and cleaning shafts for the drainage are located at intervals of 75 m in niches. The carriageway is drained via a lateral slotted channel connected to the DN 400 collector pipe at 50 m intervals. The roadway subgrade drains via a separate DN 200 drainage pipe.

2.5 Structural fire protection

If the concrete heats up strongly and rapidly in the event of a fire, the water vapor pressure generated in the concrete and the thermal expansion of the material can cause spalling at the surface. The heating of the then exposed reinforcement leads to an impairment of the stability of the tunnel lining. Since 2012, in order to prevent this spalling, the regulations have provided for reducing the vapor pressure in the event of fire by adding plastic fibers to the concrete instead of constructive measures. In the event of fire, the melting of the polypropylene fibers used leads to the formation of channels through which the water vapor can escape from the concrete.

The pilot project carried out on the Bühl Tunnel was intended to investigate the production and placement of the plastic-modified concrete under site conditions. The requirement from structural fire protection that the reinforcement should not heat up above 300 °C in case of fire was met with test specimens at a fiber content of 1.4 kg/m3 . The workability of the concrete was tested on a test wall at block 51. The subsequent production of the tunnel lining with factory production of the concrete in the mixing plant and installation via concreting nozzles on the tunnel formwork did not show any significant differences compared with the use of normal concrete. In particular, there was no increase in the heat of hydration. 130 New construction of the Bühltunnel Siegen, Hüttentalstraße.

2.6 Technical equipment

The new tunnel's operational and safety equipment includes:

•  Emergency walkways,
•  Central escape door in the escape tunnels (maximum escape route length < 300 m),
•  Breakdown bays in front of both portals,
•  Emergency call stations at intervals of 150 m,
•  Extinguishing water pipeline as a ring main with extinguishing water basin next to the operating building,
•  Extinguishing water extraction points at a distance of 150 m and 50 m in front of each portal,
•  ventilators (jet fans) in the tunnel,
•  tunnel lighting,
•  CO, flow and visibility turbidity measuring devices, smoke detectors,
•  loudspeakers, tunnel radio system,
•  Video cameras in the tunnel, at the portals and in the escape tunnel,
•  Escape route marking with orientation lights,
•  Fire alarm system with line fire detectors,
•  Traffic control equipment in accordance with RABT.

The control, monitoring and power supply of the technical equipment is provided from the operations building above the tunnel. The tunnel is constantly monitored in the tunnel control center at the Hamm freeway branch.

3. Construction

3.1 Tunnel excavation

In a preliminary measure, the old mining tunnels were backfilled in 2010 and a high-voltage pylon located in the area of the planned tunnel portal was relocated in 2011. Excavation of the tunnel began in October 2011 at the western portal. Tunnel breakthrough took place on December 6, 2012.

The 417 m long mining section of the tunnel was excavated from the west using shotcrete construction methods. In this method, the cavity is excavated in short sections by excavator or blasting and then immediately secured with shotcrete, reinforcement and anchors to prevent the rock from caving in. The excavation was carried out in the solid rock areas by blasting. Because of its size, the excavation cross-section was divided into calotte, bench and bottom (if a bottom arch was required). The cut-off lengths were between 1.25 m and 1.75 m in the calotte and between 2.50 m and 3.50 m in the bench. Since loose rock was encountered on the western side of the first 55 m, an advance pipe screen consisting of 27 steel pipes (d = 140 mm) with a length of 15 m and overlapping by 3.50 m was installed here. Excavation was carried out by excavator or, in the case of ascending rock, by combined excavator-blasting. The inner tunnel lining was then constructed section by section using a mobile formwork carriage in a steel formwork. The section length was 10 m.

Due to the existing overburden and the neighboring railroad tunnel, a rock-saving blasting method was used, which was adjusted to the local conditions by trial blasting. The condition of the superstructure was determined before the start of the work by means of a procedure for the preservation of evidence. An accompanying geotechnical measurement program recorded the rock deformations and forces in the excavation area, as well as the deformations and vibrations at the surface. For the vibrations, compliance with the permissible values according to DIN 4150 was checked.

3.2 Cover and open construction method

In the section with the cover construction method, the piles of the dissolved bored pile walls were first constructed from the ground surface. This was followed by excavation to the lower edge of the cover with simultaneous construction of the shotcrete infill. The cover forms a reinforced concrete arch and ties into the bored piles at its supports. During excavation below the cover, the tunnel floor was secured by shotcrete. The concreting of the inner shell in the cover area was carried out against the cover and the bored pile wall. Prior to this, filter concrete was placed on the outside from the top edge of the cover to the longitudinal drainage.

The formwork carriage for the inner lining in the area of the mining construction method was also used as inner formwork for the areas with the cover construction method and the open construction method.

3.3 Opening to traffic

After the installation of the operating technology by mid-2016, the traffic clearance for the tunnel and the overall measure of the B 62n between the Siegtalbrücke (A 45) and the Mudersbacher Kreisel took place on May 11, 2017.

4. Literature

[1] Mämpel, H.; Peter, C.; Steiner, B.; Beier, M.; Dehn, F.; Eickmeier, D.: Bühltunnel: Erfahrungen aus dem Vortrieb und Festlegung der Betonrezeptur für die Innenschale aus PP-Faserbeton; Tunnel 1/2014, S. 33-40

[2] Landesbetrieb Straßenbau NRW: Geschafft - Fertigstellung der A 4/B 54 (HTS); Dokumentation zur erkehrsfreigäbe am 11.5.2017, Verlag Vorländer, Siegen 

• Country: Germany
• Region: Siegen, Nordrhein-Westfalen
• Tunnel utilization: Road tunnel
• Client: Bundesrepublik Deutschland, Land NRW repr. by Landesbetrieb Straßenbau NRW
• Consultants: Straßen NRW; IMM, Bochum; Stredich + Partner, Mühlheim; BUNG, Heidelberg
• Contractors: Ed. Züblin AG, Stuttgart; OSMO Anlagenbau GmbH, Georgsmarienhütte
• Total length: 525 m
• Clear width: 9,50 m
• Construction cost: 26,4 Mio. Euro (Tunnel), 4,2 Mio. Euro (Betriebstechnik)
• Construction time: 9/2011 till 12/2016