Tunnel Alte Burg

1. Task definition

The German Unity Transport Project (VDE) No. 16 connects the Thuringian state capital Erfurt via a fork near Suhl with the Franconian centers Schweinfurt/Würzburg (A 71) and Coburg/Bamberg(A73). The Thuringian Forest is a natural barrier that has to be overcome. The new construction of the A 71/ 73 forms an important bridge to Bavaria for the development of the Free State of Thuringia and also considerably improves traffic relations within the state. Locational disadvantages for local companies will be compensated, economic centers will be strengthened and at the same time the downstream road network will be relieved. In conjunction with the reduction in noise and pollution, I the good infrastructural connection also has a positive effect on the tourism industry.

In the core area of the Thuringian Forest, a protected landscape area, the Rennsteig, the famous ridge trail of the low mountain range, has to be crossed. The length of the crossing is about 20 km. With its dense series of viaducts and tunnels, the section between the Gräfenroda junction and the Suhl interchange is of outstanding importance in the course of the construction of the A 71/73. During the planning and construction of the freeways, great importance was and is attached to ecological concerns. This objective is particularly taken into account by the fact that a large part of the crest crossing is underground. The four tunnels Alte Burg, Rennsteig, Hochwald and Berg Bock with a total length of approx. 12.6 km are responsible for this. The Rennsteig tunnel, with a length of around 7.9 km, is the longest road tunnel in Germany. The crest crossing is probably one of the most demanding tasks in terms of engineering and logistics that DEGES (Deutsche Einheit Fernstraßenplanungs- und -bau GmbH) has been entrusted with to date.

The sequence of the tunnel descriptions follows the course of the A 71 from north to south. In addition to the descriptions of the individual tunnels, general topics related to the construction projects, such as the environment, logistics and geology, are highlighted.

2 Construction design

2.1 Challenge: Mass balancing

During the construction of the four tunnels with a total length of about 12.6 km, an excavation volume of 2.4 million m3 had to be handled. The task was to minimize the impact of dirt and increased traffic on federal and state roads caused by construction vehicles. In order to meet the set requirements of environmental protection, one of the main objectives of the logistics concept had to be to reintegrate the excavated material into the route of the A 71 motorway as far as possible. All the earth materials produced were processed. For this purpose, three sites had to be selected as intermediate landfills and processing sites. In order to be able to transport the earth materials along the route, the valley bridges had to be completed at an early stage. As early measures, the viaducts were removed from the planning approval procedure and early planning approval procedures were initiated instead.

2.2 Geological conditions

The Thuringian Forest is an attractive part of the German low mountain range landscape. Coming from the northwest from the area around Eisenach, it runs for almost 70 km to the foothills of the Thuringian-Franconian Slate Mountains in the southeast. Already in prehistoric times, trade routes led over the 15-20 km wide and up to almost 1,000 m high natural barrier between the two early agriculturally and industrially developed areas. As a modern traffic route, the A 71 now bundles the heavy goods traffic previously distributed over various federal highways and thus relieves the environment.

During the Carboniferous period, powerful internal forces created the Variscan Mountains, a folded mountain range comparable to the extent of the Alps. The course of this folding lay across today's Thuringian Forest. The mountain ranges were characterized by highly consolidated rocks, whose origin is to be sought in marine deposits. The incipient erosion process leveled the mountain body again except for a few remnants. In the course of this process, the characteristic rock complexes of the Thuringian Forest were formed 300 to 270 million years ago by accumulation of sedimentary debris. Parallel to this, volcanic eruptions typical of the Lower Permian period brought to light deposited ashes and cinders, so-called tuffs. These were also eroded and mixed with the existing rock debris. The rock sequences cut in the course of the A 71 are called the Oberhof-Goldlauter and Georgenthal formations.

The rock in the Alte Burg tunnel and in the central and northern sections of the Rennsteig tunnel consists of volcanic rocks and tuffs that are assigned to the Oberhof Formation. It is the youngest of the formations cut by the tunnels described here in terms of chronological history and is characterized by violent volcanic activities during its formation phase.

In the southern part of the Rennsteig tunnel gravelly-sandy and silty-clay sedimentary rocks are crossed. This sedimentary sequence, which is more than 1000 m thick, was formed by deposition in lake and marshland areas that were temporarily populated by animals and plants. These spread out in depressions at the edges of the mountain massifs. From these edges, during the formation of the Goldlauter Formation, mighty alluvial fans pushed into the depressions of the Thuringian Forest.

The rock sequence, called the Georgenthal Formation by geologists, is composed of gravels, sands, clays, volcanic rocks, so-called latites, and tuffs. It is exposed in the central part of the Berg Bock tunnel.

The beginning of the formation of the Thuringian Forest, as we perceive it today, lies about 100 million years ago. At that time, the African continental plate and the Alpine mountain front pushed against the European continental plate in the south. The European continental plate could not withstand the enormous pressure. It broke into individual fracture clods, which were partly lifted up at the fracture lines. This is how our low mountain ranges were formed and thus also the narrow mountain ridge of the Thuringian Forest. An important fault zone, the Franconian line, is cut in the southwest by the tunnel Berg Bock. This uplift process is chronologically located in the course of the Upper Cretaceous and the Tertiary.

The witnesses of the maritime character of the landscape during the Zechstein and Triad periods (i.e. before the uplift) have long since been eroded on the heights of the Thuringian Forest by the renewed attack of weathering. The present valley and river system was formed mainly during the glacial period, the Pleistocene. Different compositions of the rocks exposed by erosion of the upper overburden layers caused their varying degrees of weathering. This led to the formation of the present relief of the Thuringian Forest. Thus, the relatively high susceptibility of the granite to weathering in the valley basin of Suhl/ Zella-Mehlis has led to predominantly flat slope forms. In contrast, in the ridge region of the Thuringian Forest we find predominantly rugged, steep mountain slopes and rock cliffs (Goldlauter Formation and Oberhofer Rhyolite Complex).

2.3 Preliminary explorations

The exploration of the rock down to the depth at which the tunnel tubes will later pass through the rock formed the beginning of the activities. For this purpose, test borings were made at short intervals along the planned tunnel axes. These provided information on the type and condition of the rock, faults, fractures and fissures, rock series changes, and groundwater and mountain water. The risk of cavities, such as old mining tunnels, could also be narrowed down in this way.

Seismic investigations supported these statements. The facts that rock boundaries reflect seismic waves and that seismic waves propagate at different speeds in different rock layers were exploited.

Geoelectrical investigations, in which the different electrical resistivities of the individual rocks were measured, completed the picture of the preliminary explorations.

The boreholes were also used to investigate the location and properties of groundwater and mountain water. Water squeeze tests were used to determine the water permeability of the rock. Some boreholes were converted into groundwater monitoring wells. These provided information on groundwater and mountain water levels throughout the planning and construction period. These are naturally very low, which precluded any adverse effect on the vegetation on the mountain slopes due to local groundwater drawdown. Surface runoff was also not disturbed by the drawdown measures.

After careful documentation and evaluation of all measurement results, it was possible to make a scientifically based prediction of the nature of the bedrock along the route.

2.4 Ongoing investigations

Ongoing examination of the actual situation on site is essential in every phase of driving a tunnel. In this respect, the geologists had to work hand in hand with the miners. Where possible, daily face recordings were made. Among other things, these provide information on rock types encountered, interfaces, water ingress and conspicuous features.

In addition, weathering classes, water ingress, installed securing means, profile accuracy of the excavation, driving performance and deformations of the tunnel cross-section were continuously documented.

2.5 Information feedback

A constant comparison of the measured values with the previously given forecast was carried out. In this way, the engineers were provided with the decisive information by the geologists in order to be able to draw the necessary conclusions for further action in very specific areas.

At the Rennsteig tunnel, for example, the excavation of the supply air gallery for the later air exchange center Flößgraben encountered the heavily water-bearing old mining gallery of the "Reinhilde" mine. There were no indications of this old mine either in the literature or in the course of the preliminary investigations. Geologists and miners had to adjust spontaneously to the new situation and expect further old mining tunnels in the course of driving.

The sum of all geological findings, in addition to their importance for the unhindered progress of construction, also represents an indispensable basis for the planning of future conversion and repair measures.

2.6 Operational and safety equipment

"The safest tunnel in Europe is the Rennsteig tunnel". This was the result of a test conducted by the ADAC in 2004.

Operational safety has been a top priority for all tunnels in the course of the A 71 from the very beginning. They have separate tubes for each directional lane. Cross tunnels, some of which are passable, provide escape routes at regular intervals. Shelters, breakdown bays and emergency call niches in sufficient number round off the safety concept.

Other operational and safety facilities include: Lighting, traffic guidance systems, radio systems for radios and cell phones, fire detectors, fire extinguishing systems, video surveillance and sensors for heat and pollutant emissions. Around the clock, the data received from these facilities is bundled and monitored in a central operations control center. The removal of any fire extinguisher, for example, is immediately registered in the central operations control center and results in a reduction of the maximum permitted speed of the following vehicles to 60 km/h and an increase of the tunnel lighting to 100%. A fire is indicated either automatically by the fire alarm system or by manual call points. In the event of a fire, orientation lights have been installed in addition to the normal escape route identification drawing to ensure safe orientation in the tunnel even in the event of heavy smoke.

During normal operation, ventilation is controlled by opacity measuring points and CO measuring points. Furthermore, the ventilation program is coupled with the fire alarm system.

The traffic engineering concept with systems for advance warning of congestion and tunnel closure as well as variable message signs extends beyond the actual tunnel tubes. It includes the highway access roads in the area of the crest crossings.

A radio system ensures communication for emergency services in the tunnel tubes. Contact with tunnel users is possible via some radio stations that can be received in the tunnel. Instructions on the radio and on the announcement system are available in three languages.

Between the Rennsteig and Hochwald tunnels, the central operations control center for the federal highways of the Free State of Thuringia has been set up. Here, not only the data from the Thuringian tunnels that have already been completed, but also that of the tunnel structures and traffic control systems planned for the future are brought together.

3. Construction

3.1 Construction method

For all tunnels in the course of the A 71, driving and support were carried out according to the principles of shotcrete construction. The aim of this construction method is to keep rock loosening as low as possible and to establish a frictional connection with the rock as quickly as possible. This results in a reduction of the loads on the final structure.

The basic principle of the construction method is based on an alternating sequence of excavation (rock-preserving blasting or mechanical excavation with excavators) and securing of the cavities. Depending on the rock conditions encountered, mining can be carried out by full excavation or partial excavation.

If excavation is by blasting, about 100 holes are drilled in the rock per cut and filled with safety explosives. The excavation length is determined by the rock behavior and varies between 1 m for friable rock and 4 m for stable rock. Immediately after excavation, the reveal is secured by a combination of shotcrete, support arches, reinforcing steel mesh, anchors and spiles. The choice of securing method depends on the rock conditions encountered. The effectiveness of the securing must be continuously checked and documented with a geotechnical measurement program.

Sealing is achieved by means of a loosely laid single-layer plastic sealing sheet on geotextile between the outer and inner shells. Via this so-called "umbrella sealing", incidental mountain water is collected in two laterally arranged drainage pipes in the area of the lower elms and discharged with the tunnel gradient. For cleaning the drainage pipes, flushing shafts are arranged at intervals of approx. 50 m. The drainage system is divided by transverse pipes. The drainage system is designed by means of cross pipes and additional inspection shafts in such a way that the flushing effort can be adapted in sections to the local occurrence of calcite-separating water. In this way, sintering (calcium deposits) can be prevented and maintenance can still be carried out economically.

After the rock deformations have subsided, the final lining takes place. This consists of a minimum 35 cm thick reinforced in-situ concrete inner shell.

The inner shell was produced in blocks of 12 m length using vault formwork carriages. These formwork carriages can be moved, lowered and folded and are thus very well suited for use in tunnel construction. Five such formwork carriages were used in the Rennsteig Tunnel. Concreting was carried out in daily cycles, i.e. around 12 hours after completion of concreting, the formwork carriage was moved and work on the next block prepared. Behind the formwork carriage, up to four curing carriages connected in series ensured that the concrete was protected from cooling too quickly. For this purpose, heat-insulating and waterproof mats were placed on steel structures. The temperature and humidity in the gap between the mats and the freshly concreted inner shell were constantly measured and regulated by water cooling. In this way, each block could be gradually adapted to the climate in the tunnel.

Supervised by the Federal Highway Research Institute, a single-shell test section was installed in the supply air gallery for the Kehltal air exchange center (Rennsteig Tunnel). Experience is to be gained on this section as part of the research project "Investigations into the stressing of single-shell composite structures in mining tunnel construction". In combination with the simple shell, a system for free rock drainage is used. Five different shell variants were investigated over a total length of 80 m. The shell thickness varied between approx. 47 cm (double-shell standard construction) and 32 cm. The production was carried out in two layers. In the first step, an outer shell was produced in a conventional reinforced shotcrete design. In the second step, either a layer of reinforced concrete (reinforced/unreinforced) or a shotcrete layer (mesh reinforcement/steel fiber reinforcement) was applied.

3.2 Old Castle Tunnel

The former Federal Minister for Family Affairs, Claudia Nolte, was the godfather of the shortest tunnel, which is 874 m long.

Starting southwest of Geschwenda, it penetrates the Alte Burg ridge to before the Schwarzbach valley. The tunnel gradient is a constant 2.5% towards the east portal. Both tubes were driven by mining, starting at the eastern portal. Only the portal blocks were constructed using the cut-and-cover method. The design of the vertical portals with natural stone cladding was oriented to the landscape and the Schwarzbachtal bridge adjoining to the west.

The tunnel crosses an area of medium to strongly fissured quartz porphyry, conglomerates, sandstone, siltstone and mudstone of various ages. These conditions proved favorable for tunneling from a geological point of view. But the mountain held a surprise in store for the miners. In a fault zone of the mountain, manganese was probably mined in earlier times. After mining activities had ceased, the adit had been covered with briar. When this old adit was now approached in the course of tunnel construction, around 2,000 m3 of slurry and grus poured into the tunnel. The vent was immediately sealed with concrete. It was then possible to drive through the fault zone of almost fully deconsolidated material under the protection of a pipe screen.

The pipes of three independent drainage systems run in the tubes. On the one hand, the mountain water and, on the other hand, the drag and cleaning water are drained off. Furthermore, the drainage pipe of the Schwarzbachtal bridge is led through the southern tube.

The tunnel stop on 19.9.1998 marks the beginning of the tunnelling work. An interruption in the rapid work occurred 130 m before breakthrough due to the sludge collapse on June 30, 1999. The tunnel breakthrough at the west portals took place on November 9 and 18, 1999, respectively. After the tubes were used for earth transport, the interior work was completed in August 2002.

3.3 Rennsteig Tunnel

With a length of 7,916 m of the western tube, the Rennsteig Tunnel is the longest road tunnel in Germany. The tunnel's patron was Mrs. Christiane Herzog, the wife of former German President Roman Herzog.

The tunnel penetrates the mountains from the western slope of the Wilde Gera and ends with the south portal just before the Oberhof junction. In the penetrated mountains, mainly Permian rocks are found, which can be assigned to the Oberhof porphyry plate. This plate represents a main distribution area of the volcanic rocks in the Thuringian Forest.

In the tunnel, the highest point of the A 71 is about 670 m above sea level. Nevertheless, the maximum overburden of the tunnel is still 205 m. The maximum longitudinal inclination is 2%. The maximum longitudinal gradient is 2%. To adapt to the terrain conditions, the end walls of the tubes were offset by 5 m at the north portal and by 10 m at the south portal.

The topography varies considerably along the tunnel axis. The altitudes range from 865 m above sea level (Tragberg) to 600 m above sea level. (in the intermediate valleys). Constraining factors for the elevation were the intermediate valleys of the mountain range, which were driven under with an overburden of about 6-20 m, as well as the Brandleite tunnel already existing in the mountain range. This railroad tunnel, built between 1881 and 1884, is 3 km long. In the run-up to the project, the options of driving under or over the Brandleite tunnel were investigated. The route above the tunnel in the existing structure was preferred, as it was expected to have less impact on the existing structure.

Four variants for crossing the main ridge of the Thuringian Forest were investigated:

Variant 1: two tunnels (2,700 m and 2,200 m) combined with three bridges (one between the tunnels and two in their connection); in the south, another tunnel of 1,050 m (route of the regional planning procedure).

Variant 2: a 5,310 m long tunnel followed by two bridges, in the further course analogous to variant 1

Option 3: one long tunnel, with air exchange centers in the Kehltal and in the Flößgraben; gradient routing in such a way as to ensure attack possibilities for mining in three intermediate valleys (Kehltal, Flößgraben and Bäckerbachtal); crossing of the Brandleite tunnel at intervals of approx. 7 m

Option 4: two tunnels (2,620 m and 4,370 m), with only one bridge in between across the Kehltal.

After weighing the criteria of traffic, construction technology, environment and costs, the decision was made in favor of variant 3. The complete tunneling represents the best protection and the minimum possible disturbance for the landscape including flora and fauna. In addition, this solution offers better traffic quality and more traffic safety, as frequent changes between tunnel and open road (hei l/dark effect for the road user) as well as disturbing weather conditions, such as black ice and fog, are avoided. The costs of the long tunnel are in line with those of the other solutions.

As structural safety facilities, 12 breakdown bays were installed in both tubes at a maximum distance of 700 m apart. At least every 350 m, both tubes are connected with transverse tunnels, every second of which, combined with a breakdown bay, is passable. Emergency walkways on both sides of the standard cross-section round off the structural safety concept.

One challenge with such a long tunnel is the ventilation problem. The maximum possible tunnel length for a longitudinal ventilation solution with jet fans is 3 km. The mountain range offered natural help in solving the ventilation problem due to its topography, more precisely due to the two crossing valleys Kehltal and Flößgraben. Air exchange centers were located in the crossing valleys to provide the additional ventilation required. This created three ventilation sections of approximately equal length, 2.5 km in length. The supply air tunnels have a diameter of 6 m and can thus be used as additional access for rescue and maintenance vehicles.

The mountain water levels are above the gradient in some areas and below it in others. The maximum groundwater level above the gradient is around 85 m. In the intersection area with the Brandleite tunnel, it is 20 to 30 m below the grade.

The tunnel was driven using the shotcrete construction method. The standard cross-section had to be divided in sections into calotte, bench and invert. In favorable rock classes, excavation could be carried out in full excavation. The stable rock mass largely permitted an unreinforced inner shell with umbrella sealing. It was constructed with a total of 5 formwork carriages (block length 12 m). The invert area was constructed as an open invert in the stable rock and as an invert slab in soils susceptible to weathering. Invert vaults were installed where the rock exerts pressure on the invert.

The crossing of the Brandleite railroad tunnel presented a challenge to all involved. Only 6 to 7 m remained as rock stabilization between the lining of the two tunnels. Both the rock mass and the lining of the Brandleite tunnel, consisting of a quarry stone ridge with friction-locked back packing, were in good condition. It was therefore possible to limit the closure of rail traffic to the short phase of blasting the Rennsteig tunnel. During driving of the highway tunnel, work was started on the west tube in good rock. In the east tunnel, advance heading was carried out with reduced heading height as a quasi pilot tunnel. The remaining rock mass towards the rail tunnel was secured by nailing and grouting. Only then were the bench and invert excavated.

Construction work on the Rennsteig Tunnel was initiated on August 26, 1998, with the groundbreaking ceremony. Subsequently, both tubes were excavated simultaneously from both portals. On November 22, 2000, with the last breakthrough, both tubes were open to traffic. Concreting of the inner shells was completed in October 2002. After completion of the expansion work, the tunnel and with it the entire section of the ridge crossing in the course of the BAB A 71 was opened to traffic on July 5, 2003.

3.4 Tunnel Hochwald

The tunnel passes under the ridge of the Hochwald mountain range. Starting at the Schneidersgrund depression on the northern edge of Zella-Mehlis, it penetrates the mountains over a length of 1056 m and ends at the edge of the industrial area north of Suhl. Gisela Huber, the wife of DEGES Supervisory Board Chairman Ministerialdirektor Dr.-lng. E.h. Jürgen Huber, was the godmother of this tunnel.

The tunnel was constructed over a length of approx. 1,040 m using the mining method. In the area of the portals, the cut-and-cover method was used over a length of 7.50 m and 10.40 m respectively. The Oberhof junction follows directly behind the north portal in the course of the A 71. In this area, the tunnel cross-section had to be widened by 3.50 m to accommodate the entrance and exit lanes.

The two tubes are connected by transverse tunnels at a distance of approx. 360 m from each other.

All roadway water is discharged via slotted channels or pipelines into a collection basin for treatment. The mountain water flows separately and untreated into the receiving watercourse.

With the tunnel cut on September 15, 1998, driving of both tubes began from the south portal. By October 1999, both tubes had been driven through the mountain. On November 5, 2001, the section between the Oberhof and Suhl/ Zella-Mehlis junctions was opened to traffic.

3.5 Berg Bock tunnel

The Berg Bock tunnel forms the southern end of the "Thuringian tunnel chain". It begins directly behind the Steinatal bridge, crosses Berg Bock and ends in the south between the Linsenhof settlement and Suhl-Albrechts. The sports shooter Anke Schumann had taken over the sponsorship of the 2,738 m long tunnel.

The rock to be cut through consisted of very different rock complexes. The Suhl granite in the north is followed by alternating layers of clay, silt and fine sand interspersed with conglomerates. This is followed by the latites of the Georgenthal sequence and in the south by variegated sandstone.

At intervals of 680 m, the pipe cross-sections were widened to create breakdown bays. Nine transverse tunnels connect the tubes. They are about 300 m apart. Three of them are passable.

An unforeseeable geological situation forced the miners to construct a drill pipe shield as an advance safety measure. For this purpose, 40 drill pipes of 15 m length were arranged at intervals of 35 cm and slurried with cement-bentonite slurry. Three overlapping pipe umbrellas were produced for each tube in both the calotte and the rung tunnels.

The Berg Bock tunnel was the last to be excavated on March 17, 2000. Here, too, the shotcrete construction method was used. Excavation for both tubes was carried out simultaneously from both portals. This procedure made it possible to limit the time required for driving to one year. The entire structural work, including the construction of the waterproofing and the inner lining, took two and a half years. On December 20, 2002, the section between the Suhl/Zella-Mehlis junction and Meiningen was opened to traffic.

4. Literature

[1] Bundesministerium für Verkehr, Bau- und

Wohnungswesen

Herausgegeben durch:

DEGES, Deutsche Einheit

Fernstraßenplanungs- und -bau GmbH:

Verkehrsprojekte Deutsche Einheit Nr. 16

A 71 Erfurt-Schweinfurt/A73 Suhl-Lichtenfels

6 Tunnel der A 71 in Thüringen

 

 

• Country: Germany
• Region: Thuringia
• Tunnel utilization: Traffic
• Type of utilization: Road tunnel
• Client: DEGES Deutsche Einheit Fernstraßenplanungs- und -bau GmbH
• Consulting Engineer: VBM/Scetauroute
• Contractor: Schachtbau Nordhausen/Leonhard Weiß
• Main construction method: Trenchless
• Type of excavation: Drill-and-blast
• Lining: Concrete formwork
• No. of tubes: 2
• Tunnel total length: 2 x 874 m
• Contract Volume: 43 Mio. DM
• Construction start/end: August 1998 till approximately 2003
• Opening: 2003