The new A71/A73 highway is one of seven projects commissioned by the German Federal Government in 1991 as part of the “Verkehrsprojekte Deutsche Einheit” (Traffic Projects planned to improve the infrastructure in the new German Länder). Project No 16 (the A71 and A73) will complete a North-South connection between the Thuringian capital Erfurt and the industrial centres in Northern Franconia, Schweinfurt/ Wurzburg and Coburg/Bamberg.
The crossing of the Thuringian Forest is one of the most demanding and difficult sections of all the “Verkehrsprojekte Deutsche Einheit”. In the centre of the Thuringian Forest, which is a protected area, a mountain range that varies in height from 600 to 800m asl had to be crossed for a length of more than 20km. The area’s strict ecological demands required that the highest possible percentage of the alignment be placed underground. Four tunnel structures with an overall length of approximately 12.6km are currently under construction. This includes the 7.9km Rennsteig tunnel which, when completed in spring 2003, will be Germany’s longest road tunnel. The four tunnels are:
The Rennsteig tunnel contract, worth around US$215M, was awarded to the Arge Rennsteigtunnel JV, a consortium made up of Ed Züblin AG/Wayss & Freytag AG/Bickhart-Bau. The JV began construction in August 1998.
Route and alignment
Originally, during the planning phase, the Rennsteig tunnel was designed as a chain of three tunnels separated by two valley bridges. Later, in the preliminary design phase further routes were investigated and evaluated under the following criteria:
During the investigations, it was found that the originally envisaged solution, with three tunnels and two bridges, would have had a strong negative effect on traffic safety, especially during the area’s harsh winter months (ice and fog etc). A deeper, one-tunnel solution was considered as more favourable and suitable, not only from a safety point of view, but also by the resulting reduction in gradient from 3.8% to 2%. This also had a positive effect on the project’s environmental impact by having nearly all the construction activity along the alignment underground.
A key technical point could also be solved. The single tunnel would provide a solution to the ventilation and air conditioning over the 8km distance. Studies showed that by subdividing the tunnel into three roughly equal lengths and incorporating two air changing plants, longitudinal ventilation was possible.
The cost estimations, which also included the life-cycle operating costs, showed that the one-tunnel solution was no more expensive than the three tunnel/two bridges solution. These factors resulted in the one-tunnel structure getting the green light.
Geological conditions
The central part of the Thuringian Forest is characterised by strong relief with the highest points of the tunnel alignment being up to 885m asl (Tragberg) and the lowest points in the valleys between 600 and 670m asl. The area is mostly covered by dense forest. Geologically the Thuringian Forest is a northwest-southeast elongated granitic and volcanic rock massive, which is bordered on both sides by large faults and cut by a network of secondary faults that separate the underlying strata. The Rennsteig tunnel crosses the Oberhof and the Beerberg blocks, situated in the middle of the forest.
The rock in this area belongs to the Oberhof and Goldlauter formations and is of Permian age. In the Oberhof formation a porphyritic rock type prevails, mostly coarse or fine-grained rhyolite. The underlying Goldlauter formation is a sedimentary rock consisting of conglomerates, sandstones and claystones of varying composition. The most important mechanical properties of the rock types are seen in Table 1.
The groundwater level is mostly above the tunnel crown. The water level in the north is about 20m above the tunnel and in Kehltal between 10 and 20m. At the tunnel midpoint the water horizon dips to 20m below the invert, however, at the southern end the water table rises to as much as 45-80m above the tunnel. The water inflow during excavation was estimated to range between 1 to 6 l/s per 100m in normal conditions and up to 100 l/s per 100m in highly fractured zones. These figures were confirmed after excavation had been completed.
Tunnel design
The twin tubes of the Rennsteig tunnel have a length of 7,916m with an overburden thickness reaching up to 20m. The distance between the two tunnel axis is 25m and each tube has a cross section of about 90m2. The cross section corresponds to ‘RQ26t’ according to the German standard for construction and operation of road tunnels (RABT). RQ26t means the tunnel is equipped with two 3.75m traffic lanes and two 1m wide pavements. The tunnel features 12 emergency bays located at 700m intervals for broken down vehicles. In an emergency evacuation, cars can change tubes using the tunnel’s 13 cross passages situated every 350m.
Construction details
The tunnelling was carried out simultaneously from both portals and from the central ventilation shaft using NATM (or the shotcrete method). Excavation and support were adapted to each of the geological conditions, which are defined by different excavation classes. In the bidding specifications the tunnel cross section was subdivided into top heading, bench and invert. But due to the unexpectedly favourable conditions in some parts of the tunnel it was possible to excavate the whole heading in one step.
The blasting of the rock mass was mostly carried out using the conventional method of drilling, loading, blasting, mucking followed by support. In difficult sections additional support was applied using injection grouting ahead of the tunnel face. With a typical excavation round of 1.7m in depth and four to five blasts at each face per day, a best performance of up to 50m in a day was achieved.
The tubes were lined with a 100-300mm shotcrete primary layer and a 300-400mm cast in-situ concrete final layer. The construction of the cast in-situ concrete lining was carried out using five mobile formwork units, each with a length of 12m.
Between the sprayed and cast concrete a waterproof membrane was installed to prevent water ingress. The sealing was carried out using a 2mm thick PE-plastic layer, protected from the rough primary shotcrete by a fleece geotextile.
Depending on the geological conditions, three different types of invert structures were constructed:
The groundwater ingress is collected in side wall pipes and drained outside the tunnel.
Crossing the Brandleite tunnel
Near the Oberhof station the Rennsteig tunnel crosses the 112 year old Brandleite railway tunnel. This 2.84km twin track tunnel is still in service and forms a critical connection for railway links between the regions of Thuringia and northern Frankonia.
The crossing was going to be particularly difficult as only a thin rock mass of 6m to 7m would remain between the floor of the highway tunnel and the crown of the railway tunnel. It would be a long crossing of nearly 124m, because of the 37 degree angle of attack.
After passing through the tunnel and making an extensive inspection of the existing construction and rock conditions in the crossing zone, it was established that the stone lining of the old tunnel could be made secure with careful back grouting.
No loosening of the original rock treatment was detected, however, it must be said that the rock in the crossing was of variable quality. Therefore, it was decided to maintain a true NATM survey watch on the tunnel cross section during the blasting operations and to carry out strengthening of the rock between the tunnel floor and the rail tunnel as the drive proceeded (figure 3). In this way it was ensured that the Brandleite tunnel was not damaged and the water flow capacity of the drainage tunnels remained unchanged.
In co-operation with Deutsche Bahn and the railway authorities the tunnels were well instrumented, with measuring points and a monitoring programme established around the crossing. This included:
During the excavation of the highway tunnels the following measures were taken:
The Brandleite tunnel was watched around the clock during the most sensitive 116m stretch of tunnel. Only during the actual blasting was there a brief closure, which was lifted again following measurement analysis.
For safety, a maintenance train for any necessary repairs was kept on standby. The work was completed without incident.
Air exchange caverns
The two air exchange caverns were designed to be 150m long x 16m high, one in the Kehltal and one in the Flossgraben section. These sections of the tunnels had to be enlarged to around 200m2. The heading of the cavern was executed from the already constructed Kehltal and the Flossgraben access tunnels. The construction sequence started with the excavation of the cavern roof including the extensions for the enlarged walls. Secondly, the step by step excavation of the upper part of the caverns was carried out including support by additional anchoring followed by excavation and support of the cavern benches. The final stage was the excavation and support of the floor sections. The process was carried out according to models and extensive stability calculations. An essential influence on the calculations was the tension concentration in the rock mass pillars, discontinuity planes in the rock mass and the efficacy of the anchors. Due to the difficulty of the calculation the results were checked by comparative calculation methods and tests.
Logistics and operations
The complexity of the different construction works created an array of technical challenges, there were also special logistical problems. First and foremost was bulk spoil transport; the four tunnels, with a total length of 12.6km, produced a spoil volume of 2.4Mm3. A large amount of this spoil was used on the road project’s embankments. It was also used to create a frost protection layer and crushed stone subbase for the road on adjacent construction sections over a total 40km length. Bulk transport took place along the embankments avoiding the need to use heavy spoil disposal trucks on the local public road network.
Construction rate and conclusion of works
The 7.9km long twin tunnels have been completely excavated in just under two years. The lining of the first tube is already complete and the second will be ready at the end of September this year. Fitting out work has started meaning that the Rennsteig tunnel will go into service in the spring of 2003.
Problems were expected crossing the Brandleite tunnel, with a clearance of 6m to 7m. However, due to the experienced tunnelling personnel involved, the section was passed without difficulty.
Related Files
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