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Tunnel Gernsbach (B462)

1. Task definition

The B 462 federal highway, which along with the B 500 represents the most important connection between the Rhine Valley and the Black Forest, runs between Rastatt and Freudenstadt in the Murg Valley and also passes through the town of Gernsbach in its course. In the town of Gernsbach, the federal highway is divided into two directional lanes. Due to the mixture of destination, source and through traffic, and in particular also due to tourism, both roads have reached the limits of their capacity despite the separation of directions.

Initial planning for a low-level through road was already carried out in the early 1980s. Several variants were examined before the current route with the 1527 m long tunnel structure was given planning approval on June 15, 1990. With the completion of the measure, through traffic is separated from source and destination traffic and thus the inner city of Gernsbach is considerably relieved.

On the south side, the tunnel passes under the Rastatt - Freudenstadt railroad line over a length of approx. 50 m, whose operation had to be maintained during construction. For this purpose, a temporary bridge of Deutsche Bahn AG with a length of 3 x 24.5 m was used during the construction period.

The basic geometry of the structure is determined by the alignment of the line with radii between 280 m and 900 m with upstream clothoids. The gradient changes from a slope of 4% to 1% to an incline of 4%. A catch basin was to be arranged under the low point in the tunnel resulting from the change in gradient.

The realization of the tunnel and thus the removal of a large part of the traffic from the through-road offers a special opportunity for Gernsbach for an urban reorganization and development of the inner city area between the Murg and the railroad line and between the railroad station and the Eberstein bridge.

Through an attractive design of the two tunnel portals in connection with the noise protection walls of the adjacent ramps (troughs), the structure should not only blend harmoniously into the townscape, but at the same time enrich the respective location of the tunnel entrances with its appealing architecture.

2. Design of the structure

2.1 Ground and groundwater conditions

To investigate the geological and hydrological conditions, 27 boreholes were sunk along the tunnel route as part of the project preparation. Based on the drilling results, the tunnel route can be divided into two sections, both morphologically and geologically, with the change being approximately in the middle of the tunnel.

In the southern half of the tunnel, the tunnel passes through a more strongly structured slope shoulder east of the Murg valley. Here, under a thin surface layer, there is initially Quaternary unconsolidated rock up to 5.0 m thick, consisting mainly of weathered granite debris with silt admixtures. Beneath this Quaternary overburden lies slightly to moderately fractured granite.

The northern half of the tunnel crosses the almost flat, densely built-up valley floodplain of the Murg valley. Here, too, approx. 5.0 m thick Quaternary unconsolidated rock is encountered, which, however, consists of alluvial gravel. Underneath, massive to coarsely bedded fanglomerate of the Upper Rotliegende was encountered. The fanglomerates are composed of angular rock debris, of granite fragments, feldspars and quartz grains cemented by a clayey, sandy matrix.

The tunnel cuts into the permanent groundwater surface over its entire length under the Murgtalaue. As a consequence, the tunnel had to be developed as a pressurized water retaining system. In the mining tunnel section, an all-round sealed cross-section with a rounded invert was constructed for this purpose, while in the cut-and-cover method, a rectangular cross-section was built as a water-impermeable concrete structure.

2.2 Supporting structure, sealing

The entire structure with a length of 1820 m consists of the northern and southern ramps with lengths of 120 m and 173 m, respectively, and the 1527 m long Gernsbach Tunnel.

The tunnel itself was driven for a distance of 1230 m using the mining method, while the remaining 297 m could be constructed using the cut-and-cover method. The trough structures in the ramp areas had to be constructed as groundwater sumps due to the hydrological conditions.

The cross-section design of the tunnel is essentially determined by the clear space to be kept free for the road cross-section, the supply air cross-section required for the ventilation system and by the geological conditions of the rock to be penetrated.

For the mining part of the tunnel with a ridge overburden of 11 - 30 m, an approximately circular cross-section was therefore selected in order to guarantee the clearance of the standard RQ 12 t cross-section with a roadway width of 7.50 m including shoulder and 1.0 m wide emergency walkways on both sides.

The tunnel, which was constructed using the cut-and-cover method, was given a rectangular cross-section. The clearance height in the entire tunnel is a uniform 4.50m.

Taking static and geological criteria into account, a basket arch profile with inner radii of 5.45 m in the calotte, 5.05 m in the elm and 8.90 m in the invert was selected as the cross-section of the mining tunnel, with transition radii of 3.0 m and 2.50 m respectively in between. The inner lining thickness is generally 0.40 m and was increased to 0.50 m only in the cross-section widened by 2.50 m in the area of the holding bays.

Varying loads on the tunnel cross-section were taken into account by varying the degree of reinforcement in the tunnel lining.

Due to the choice of the ventilation system as semi-transverse ventilation, an intake and exhaust air duct with approx. 7.5 m2 had to be provided in the tunnel.

The air duct cross-section is located in the ridge area of the tunnel and is separated from the standard clearance space profile by a 20 cm thick reinforced concrete slab.

Since the entire tunnel lies in groundwater, a pressurized water retaining system with invert vaulting had to be used for the cross-section, thus guaranteeing that the groundwater conditions could be restored to their original state after completion of the tunnel.

After completion of the entire tunnel drive, sealing was carried out in the mined tunnel before the inner tunnel lining could be concreted. For the first time, a double-layer, chambered, vacuum-testable and repairable waterproofing system was used for an entire mined tunnel section. Compared with the conventional single-layer tunnel seal made of plastic sheeting, this technically sophisticated solution is characterized by greater durability and reliability. The two-layer sealing system, which is described in detail in [3], uses plastic pads with vacuum connections consisting of an outer and an inner layer. With the aid of the vacuum built up in the individual cushions, the tightness and condition of the seal can be checked at any time during construction and use by means of vacuum gauges.

Both the rectangular frames of the tunnel section, which was constructed using the cut-and-cover method, and the trough structures were built as waterproof concrete structures.

2.3 Operating facilities, equipment

An operations center was required to house all the equipment necessary for the electrical supply and ventilation of the tunnel, as well as recreation and operating rooms for the maintenance personnel. This center was located above ground approximately 15 m from the tunnel axis in the center of the tunnel. The operations control center, which contains rooms for the 20 kV switchgear, the emergency power generator including oil storage, the uninterruptible power supply, the low-voltage distribution, the switchgear and control system, and the room ventilation units, also integrates the tunnel's supply air shaft and the exit of the escape staircase.

The entire tunnel operation is automatically monitored and remotely controlled via a digital I&C system, with stand-alone local controls installed for all operations. All messages and measurement data are transmitted to the road maintenance department, the police and the fire department, depending on their responsibilities. If necessary, the police can intervene in automatic control processes.

Power is supplied from the medium-voltage network of Badenwerk via a 20 kV ring cable that is looped into the operations center.

In general, the safety systems, the fire alarm system, the information lights, the measuring and control systems and the night-time passage lighting are operated without interruption. Backup power is used to cover the basic ventilation/fire emergency ventilation (50 %) as well as the central control units' own requirements. The upper power level of the tunnel ventilation system and the adaptation lighting system are powered exclusively from the mains.

The tunnel was equipped with a reversible semi-transverse ventilation system. For this purpose, an intermediate ceiling was inserted above the driving space to provide an air distribution duct in which supply air/exhaust air slots are installed at 5.0 m intervals.

During regular operation, fresh air is drawn in by axial fans installed vertically in the supply air shaft integrated in the operations control center and blown into the ride room through the ceiling slots via the air distribution duct. The exhaust air escapes through the two portals. In the event of a fire, the airflow direction of the fans is reversed so that the smoke is sucked out of the driving compartment via the ceiling slots into the air duct and from there blown out via the ventilation shaft. The fresh air flows in through the two portals. In the event of a fire, the ventilation system selected ensures that the driving space remains smoke-free and can be used as an escape route.

The tunnel will be illuminated with high-pressure sodium lamps in a single-row arrangement, which for maintenance reasons will be positioned off-center to the tunnel axis. The adaptation section will receive backlighting, while the interior section will have mixed constrast lighting.

In the entrance areas of the tunnel, adaptation lighting is used to match the change in brightness between the exterior lighting and the tunnel lighting as closely as possible. With a system of luminance meters in front of the portals, this required adaptation lighting is switched fully automatically in various stages depending on the outside brightness.

A total of 15 emergency telephones are provided in niches in the tunnel. In addition, two 6 kg dry extinguishers accessible to drivers and a hydrant connection for the fire department are located in the emergency call niches. A fire is detected in the tunnel by an automatic line fire alarm system, with a fire alarm area assigned to each emergency call niche. Communication between the emergency services and with the outside is ensured by a radio system with transmitting and receiving capability. In the run-up to the tunnel and in the tunnel itself, a traffic control system with variable message signs and signal systems is installed. In the event of fire or obstruction, emergency programs are initiated fully automatically.

Surface water is discharged into the municipal sewer system via pumping systems.

To achieve optimum noise protection, the walls in the trough areas were fitted with highly absorbent soundproofing elements.

2.4 Construction method

The design of the structure specified a construction method for each section that was appropriate to the topographical, geological and hydrological conditions.

A watertight, tied-back sheet pile shoring system was selected as the excavation shoring, which was to bind on the largely watertight, unweathered rock underlying the overburden layers. Prior to driving the sheet piles, overcut pilot holes had to be drilled with a rock auger to crush the boulders present in the overburden layers. The remaining excavation pit in the poorly permeable rock could be sloped at 85° and secured with reinforced shotcrete and anchors.

The decisive factor for the construction of the portals and troughs in the ramp areas, which, like part of the tunnel (297 m), were constructed in open excavation, was to ensure groundwater circulation in the final state. Therefore, the trough section and the tunnel sections were constructed in open trench in waterproof concrete monolithically without construction joints with block lengths of 8.80 m for the tunnel cross-section and 8.65 m for the trough section. Since the tunnel cross-section and the trough section interrupt the natural groundwater flow like a barrier, a large-area culvert had to be installed to maintain the groundwater flow. For this purpose, a filter concrete package was provided between the bottom of the excavation pit and the base slab of the structure and in the lateral working spaces to allow the groundwater to flow around the structure after the sheet piles had been pulled.

Since sufficient rock cover must be available for mining, the tunnel could only be excavated from km 0 +515 or 1 + 745. The tunnel excavation was to be carried out according to the rules of shotcrete construction. As a rule, excavation was carried out by drilling and blasting, with blasting being carried out in such a way as to avoid damaging the rock surrounding the cavity and the buildings directly above the tunnel.

Directly after blasting and immediately after removal of the excavated material, the exposed rock faces were supported by a safety shoring system in accordance with the rock mechanics and tunnel construction requirements. After installation of the necessary securing means in the direct excavation area and completion of the securing in the subsequent fields, the next excavation could be tackled.

3. Construction execution

The contract for the construction work was awarded to a bidding consortium of 4 companies in a public invitation to tender on the original design of the structure; no special proposals were submitted.

The construction of the tunnel structure, which was excavated from the north using the mining method, was carried out in two stages. First, the excavated cross-section, which had been driven in sections, had to be secured and stabilized so that, in a second subsequent work step, after sealing, the final load-bearing concrete inner vault (reinforced) could be installed in blocks from south to north.

The excavation of the entire cross-section was spatially and temporally divided into 3 partial excavations - calotte, hawser and invert vault. Starting in the calotte, the rock was loosened by gentle blasting in cut-off lengths that varied between 0.65 m and 2.50 m depending on the rock conditions. Immediately after blasting and removal of the excavated masses, the exposed rock faces were secured in accordance with the requirements of rock mechanics and tunnel construction. Shotcrete, reinforcing steel mesh, steel anchors, steel spiles and steel tunnel arches in the required dimensions and combinations were used as shoring. After stabilization of the rock, excavation of the bench in cut-off lengths of 1.20 m - 5.00 m was started in the second working step approx. 150 m behind the excavation of the calotte. As a rule, the bottom excavation was carried out in advance of the concrete placement.

The excavated material, which was transported out of the tunnel by dump trucks, could be reused for the most part for road construction and as backfill or frost protection material for the tunnel measure after appropriate processing.

After completion of the entire tunnel excavation (excavation and securing), the tunnel had to be sealed all around in the invert and vault in a pressure-water retaining manner with the double-layer sealing already described in Section 2.2, which is installed with a special laying carriage. Against this waterproofing, the 40 cm thick reinforced tunnel inner lining was concreted in sections separately for invert and vault.

For the curing of the concrete, a curing carriage with affine geometry to the vault and equipped with a heat-insulating skin was used.

In the cross-section of the vault, the air duct for the tunnel's fresh air supply was subsequently created by inserting a 20 cm thick intermediate ceiling above the driving space.

The following work steps were required to construct the tunnel section using the cut-and-cover method:

  • Excavation and sheet pile shoring in the upper section
  • Excavation and slope stabilization with anchors and reinforced shotcrete in the rock area
  • Installation of a filter package between the bottom of the excavation pit and the bottom of the structure
  • Section-by-section construction of the tunnel cross-section in block lengths of 8.80 m
  • Pulling the sheet piles
  • Sealing the tunnel roof from the outside
  • backfilling of the working spaces with water-permeable filter material
  • Recultivation

The construction of the troughs in the ramp areas was carried out in the same way.

 

4. Literature

[1] Straßenbauamt Karlsruhe, Bauleitung Bühl: Broschüre B 462 Tunnel Gernsbach

[2] Regierungspräsidium Karlsruhe: Entwurfs- und Ausschreibungsunterlagen, Bauwerksbuch

[3] Maier, G., Kuhnhenn, K.: Ausführung und Erkenntnisse mit der doppellagigen Abdichtung im Tunnel Gernsbach Fachzeitschrift Tunnel 9/96

 

 

  • Country: Germany
  • Region: Baden· Württemberg
  • Tunnel utilization: Traffic
  • Type of utilization: Road tunnel
  • Client: Regierungspräsidium Karlsruhe
  • Main construction method: Trenchless
  • Type of excavation: Drill-and-blast
  • Lining: Shotcrete
  • No. of tubes: 1
  • Tunnel total length: 1527 m, incl. 297 m cut-and-cover
  • Cross-section: 90 m²
  • Contract Volume: 93 mill. DM
  • Construction start/end: 1992-1996 (approx. 4 years)
  • Opening: autumn 1996 (scheduled)