1. General

1.1 Task definition

The federal highway 312 connects the A 7 near Memmingen with the A 8 near Stuttgart. It represents an important regional traffic axis between Stuttgart, Reutlingen, the Swabian Alb and the Upper Swabian region. The B 312 is an important Alb access road; a road was already built in this area at the beginning of the 19th century. In Reutlingen the federal road led through the inner city, the high traffic volume of up to 65,000 motor vehicles/day meant a considerable noise and pollutant load for the inhabitants. Already in the 1960s, there were first considerations for the construction of a bypass for Reutlingen. In the 1990s, the building rights for the "Scheibengipfeltunnel bypass" were created within the framework of a development plan procedure. After completion of a supplementary planning approval procedure, which became necessary due to a technically required extension of the tunnel construction measure, the construction of the Reutlingen bypass could be started in 2009. The core structure of the 3.1 km long eastern bypass of Reutlingen is the more than 1.9 km long Scheibengipfeltunnel. It provides a north-south connection between the B 28 and the B 312n. The tunnel passes under the Scheibengipfel ridge next to Reutlingen's local mountain, the Achalm. The tunnel cover varies between 20m- 40m in the area of the residential area at the Achalm and a maximum cover of approx. 100m.

1.2 Road design

In plan, the tunnel axis follows an alignment of circular arcs with various radii, clothoids and straight sections. Starting at the south portal, the gradient rises at a rate of 0.8% towards the north, followed by a change in gradient after the tunnel. The gradient high point is located in the tunnel. In the transverse direction, the roadway slopes at 2.5% toward the inside of the arch. The standard cross-section chosen for the tunnel is RQ 10T with two 3.50 m wide lanes, two 0.25 cm wide shoulder strips and 1.00 m wide emergency walkways on both sides. In the three sections with the stopping bays on both sides, the cross-section is widened by 2 x 2.50 m in each case. The clearance gauge is limited to 4.50 m in height. The tunnel is closed to pedestrians and cyclists.

1.3 Structure design

The Scheibengipfel Tunnel is 1,910 m long. It consists of a 1,620 m long section, which was excavated using the mining method, and a section using the cut-and-cover method. This division results from the topographical conditions and the adjacent buildings. The length of the cut-and-cover method is 240 m at the south portal and 50 m at the north portal. It was determined taking into account the following aspects:

•  Avoidance of serious impacts from noise and pollutant emissions,
•  Reduction of operational impairments of the biotope and the local climate,
•  Avoidance of impairment of cold air runoff.

The rescue concept required the construction of an additional escape and rescue gallery. This adit was excavated by mining over a length of 1,620 meters. The adjacent open cut sections on both sides are 200 m long in the south and 120 m long in the north. The axis of the rescue gallery runs parallel to the axis of the main tunnel at a distance of 21.00 m between the mining stops. The gradient is 0.40 m higher than in the main tunnel. The main tunnel is connected to the rescue tunnel via seven cross passages. The clearance is 2.80 m x 3.10 m.

The tunnel and its portals were integrated into the landscape in such a way that the natural course of the terrain was largely preserved. The entrances and exits are designed as horizontal portals with concrete collars. Newly created embankment areas were landscaped, planted and designed in accordance with the accompanying landscape conservation planning.

2. construction design

2.1 Geology and construction method

The Scheibengipfeltunnel is located in the area of the Swabian strata. In the subsoil, thick claystone series merge into an alternating sequence of sandy limestone and fibrous marlstone. In the fore-cuts and portal areas, Quaternary weathering horizons up to 12 m thick cover the mudstone sequences. The tunnel route crosses two fault zones: the Achalm fault zone in the northern half of the tunnel and a fault in the area of the Betzenried valley at the southern portal. With the exception of these fault zones, the rock mass is stable, resulting in favorable tunnel construction conditions.

2.2 Construction

The tunnel is designed as a double-shell structure with a reinforced shotcrete outer shell and a water-impermeable reinforced concrete inner shell (WUB-KO). Between the two shells, there is a separating layer of PE foil in the invert area to allow deformations of the inner shell without constraints. In the vault area, an air cushion foil takes over this function. The outer shell is 15 to 30 cm thick and reinforced by steel arches. The standard thickness of the inner shell is 40 cm. At the portals, the thickness increases to 60 cm. The material used was concrete of strength class C 30/37. A multi-part cage arch was selected as the cross-section, which is constant along the entire length of the tunnel except for the areas of the holding bays. Rescue tunnels and cross passages are also made as double-shell structures. The thickness of the reinforced shotcrete outer shell is 15 to 20 cm. Since in the mining areas of the rescue gallery the required inner shell thickness is only 30 cm, their design as WUB-KO was not possible. Instead, these areas were provided with an umbrella seal made of plastic waterproofing membranes. The arch cross-section of the rescue gallery also describes a composite basket arch. For practical reasons, a rectangular profile was selected as the WUB-KO for the open construction method. Due to the existing geological boundary conditions, statically contributing closed inverts were required along the entire length of the tunnel and the rescue gallery. With the exception of the portal and pass blocks, the main tunnel was constructed in block lengths of 10.00 m. The tunnel consists of a total of 196 blocks. It consists of a total of 196 blocks. Of these, 181 blocks have the standard cross-section and 15 blocks have special cross-sections for the stopping bays. In the mining area, pressure joints with internal elastomer waterstops and metal waterstops were formed between the blocks. In the open cut area, space joints were made with a 3 cm thick mineral fiber board insert. The rescue gallery consists of a total of 175 blocks with block lengths of 10.00 m in the open cut and 11.54 m in the mining method. In the entrance and exit areas, the structure was provided with a 3.00 m high sound-absorbing lining over a length of 35.00 m in each case. The Scheibengipfel Tunnel has a separate drainage system for discharging the surface water from the roadways and the mountain water drainage.

2.3 Technical equipment

The operational equipment of the tunnel was designed according to the principles of the "Guideline for the Equipment and Operation of Road Tunnels (RA.BT)". A total of six jet fans are located above the traffic area in the portal areas. In the central tunnel area, an intermediate ceiling with controllable smoke extraction flaps was installed over a length of 1,593 m for near-event fire gas extraction. This device limits the spread of smoke in the tunnel. The fire gases are sucked out of the tunnel via a separate fan building and blown out via the smoke extraction shaft at the south service building. Empty pipe routes for the technical equipment run under the emergency walkways and in the rescue gallery. Collecting pipes of the road drainage system, the mountain water drainage system, the extinguishing water line and the seepage line are located under the roadway. A lighting system with adaptation sections at the portals is installed in the tunnel. The adaptation lighting adapts to the external lighting conditions. In darkness, it is completely switched off and the passage lighting is on at minimum level. In order to be able to close the tunnel at short notice, variable message signs are installed in the tunnel aprons and traffic light systems with barriers at both portals. Self-luminous marking elements at intervals of 25 m at the edge of the carriageway as active guidance devices improve visual guidance in the tunnel.

The tunnel's safety facilities also include:

•  emergency walkways on both sides with orientation lighting and escape route signs,
•  seven emergency exits in the rescue tunnels at intervals of 240 m,
•  three breakdown bays at intervals of 480 m,
•  15 emergency stations with hand-held fire extinguishers at intervals of 120 m,
•  extinguishing water pipeline with extinguishing water basin and pressure boosting system,
•  16 fire water tapping points with above-ground hydrants,
•  manual and automatic fire alarm system,
•  video surveillance,
•  visual turbidity, CO and air velocity measuring devices,
•  loudspeakers, traffic radio/radio,
•  tunnel radio.

The control, monitoring and energy supply of the operational systems is carried out in two operating buildings. The tunnel can be controlled autonomously from both control centers. A joint fan and operations building is located in the southern section of the open structure. The two-story structure is integrated into the landscape in such a way that only the entrance and the fire gas stack are visible above ground level. The second auxiliary plant building with fire water basin and booster station is located near the north portal.

3. Construction

3.1 General

On August 18, 2009, the groundbreaking ceremony for the overall construction of the Reutlingen bypass took place. In January 2011, work began on the preliminary cut and slope stabilization at both tunnel portals.

Construction work on the Scheibengipfel Tunnel itself started on May21,2012. May2012. Initially, excavation and securing was carried out along the entire length of the tunnel. Only after breakthrough was the inner lining installed. Since the strata found in the subsoil have only low water permeability, water ingress during the construction work was low.

3.2 Danger from methane gas during excavation of the tunnel

A special feature of this construction project occurred shortly before the start of the mining excavation of the tunnel cross-section: Methane gas intrusions were detected. This finding was not known from the subsoil investigations and sampling carried out up to that time. Far below the tunnel alignment, there is a layer of Posidonia shale in the Lias that is about 10 m thick. This Posidonia shale may represent a source rock for oil and gas. Natural gas, with methane as its main component, can rise through the existing fracture systems or fault zones. Explosive atmospheres can develop when larger fractures are approached or when methane gas accumulates in the tunnel cross-section. To counter this risk during tunnel driving, several safety measures were taken. During construction, methane gas occurred at several locations. A major leak occurred on April 23, 2013, as a result of which tunnel driving had to be shut down for five days so that the gas could be discharged without causing damage. The high safety-related effort proved its worth.

3.3 Tunnel driving

Tunnel driving took place on October 12, 2012. The mining of the main tunnel and the rescue tunnel was carried out using the shotcrete construction method. Both tunnels were driven simultaneously from the north in a downward direction. For excavation classes 4 to 6, the excavation was carried out by blasting, and in excavation class 7 as mechanical excavation with loosening blasting. The cross-section of the main tunnel, divided into calotte, bench and invert, was excavated in three sections for stability reasons. The calotte was excavated in January 2014, after which the bench and invert were excavated together from south to north. In contrast to the main tunnel, the smaller cross-section of the rescue tunnel could be excavated as a full excavation. Immediate securing of the excavated cross-section was provided by the shotcrete outer lining with single- or double-layer reinforcing steel mesh reinforcement. Further securing means were: 

•  Rock bolts,
•  support arches,
•  spiles as advance securing,
•  pipe shields (two at the north portal and one at the south stop wall),
•  face protection,
•  Calotte hollow vaults as a ring closure during construction.

The effectiveness of the securing means and the entire excavation were monitored with a geotechnical measurement program accompanying construction. Two local landfills received the excavated material.

After completion of the excavation and subsequent profiling work, concreting of the invert vaults began from south to north in April 2013. Concreting of the vaults followed from August 2014. The vault reinforcement was self-supporting with supporting arches. Since a vault block was produced in 24 h, 5 blocks could be realized per week. Three curing trucks were available for curing the concrete.

3.4 Open cut construction

Since the tunnel cross-section in the open cut method corresponded to that of the closed cut method, the same formwork carriage, supplemented by a polygonal counter formwork, could be used for the production of the open cut sections. After completion of the concreting of the inner shell, the formwork thickness was checked using the impact echo method and the concrete cover was checked using an electro-magnetic test method (Ferroscan). The required shell thicknesses were confirmed. After completion of the structural work and construction of the carriageways, work on the operational equipment began in October 2016. Prior to the opening to traffic, the functional test of the fire ventilation system was carried out in a large fire test on September 28 and 29, 2017.

3.5 Traffic release

The ceremonial opening of the Reutlingen bypass took place on October 27, 2017. 

• Country: Germany
• Region: Reutlingen, Baden-Würtemberg
• Tunnel utalization: Road
• Client: Bundesrepublik Deutschland, Land Baden-Würtemberg repr. by RP Tübingen
• Consultants: Regierungspräsidium Tübingen; Lahmeyer International; GBI Gackstatter; K+S Ingenieur-Consult; Prof. Kirschke
• Contractors: Max Bögl, München; Strabag Infrastructure & Safety Solutions, Köln
• Total length: 1.910 m (290 m open cut); 1.940 m (rescue tunnel)
• Clear width: 9,5 m
• Construction cost: approx. 85 Mio. Euro (shell structure), ca. 15 Mio. Euro (equipment)
• Construction time: 8/2009 till 10/2017