1. Task definition

The federal road B 2 Munich - Garmisch-Partenkirchen represents an important connection between the conurbation of Munich and the leisure and recreation center of Garmisch-Partenkirchen. In addition, it also serves as the European road E 533 via the federal roads continuing to the south in the direction of the Brenner Pass and the Reschen Pass for supra-regional and cross-border traffic. With the construction of the A 95 motorway between 1962 and 1972, the through roads through Starnberg, Weilheim and Murnau were relieved of through traffic. Since 1982, the A 95 has ended south of Eschenlohe. From there, traffic still flows the approximately 14 km to Garmisch-Partenkirchen on the old B2 with the Oberau and Farchant through-roads and on the B23 with the Burgrain through-road.

The Farchant through road is extremely congested. At peak times, more than 30,000 vehicles pass through the village daily. Although the road is marked with three lanes, the existing cross-section of the road could no longer cope with this high traffic load without disruptions.

The further construction of the BAB A 95 from Eschenlohe to Garmisch-Partenkirchen was already pursued at the beginning of the 1970s. However, the ideas at the time were no longer compatible with the changed importance of environmental protection and the discussion of standards in road construction, so that in 1986, when the requirements plan for the federal trunk roads was updated, the planning was included as a new construction/expansion of the B2 to/with four lanes.

On this basis, the Farchant/Burgrain bypass was approved in 1994.

The core of the Farchant bypass is the tunnel structure with its two parallel, approx. 2,390 m long tunnel tubes, which start in the north directly behind the Loisach bridge on the eastern high bank of the Loisach, pass under the western foothills of the Wank in a radius of 1,300 m and connect to the existing federal highways B 2 and B 23 at the northern end of Garmisch- Partenkirchen.

The route, the location of the tunnel portals and the tunnel length were determined by the greatest possible consideration for the ecologically sensitive areas and residential development. The northern tunnel portal could be inserted into the Loisach high bank in the area of a favorably located, approx. 80 m wide construction gap between the Föhrenheide settlement and a sports ground, thus achieving optimum noise protection for the residents.

As a result of the cramped conditions and the low overburden in the Föhrenheide area, the two tunnel tubes had to be designed as a double rectangular cross-section, which increasingly spreads out into separate rectangular cross-sections over a length of 90 m until the transition to the vault profile. From here on, the route runs in two tubes with vault profile to the south portals.

The location of the south portals, offset by about 110 m, was mainly determined by the preservation of an important nature recreation area of the municipality of Garmisch- Partenkirchen at Philosophenweg and by the oblique cut of the Schweinsbichl slope.

The maximum distance between the tunnel tubes is about 28 m and is reached approximately in the middle of the tunnel. It gradually decreases to 15 m up to the south portals.

The course of the tunnel gradient depended mainly on the constraints of the Loisach bridge in front of the north portal and the Garmisch-Partenkirchen north junction. In the area of the Loisach bridge, the gradients had to be raised so far that the superstructures were free of flood water, and at the north portal they had to be lowered again so far that the tunnel roof was below ground level.

The gradients therefore fall into the tunnel at a rate of 0.7% over a length of 80 m (west tube) or 100 m (east tube) and rise at a rate of 0.5% to 0.7% in the further course to the apex. The last 224 m or 252 m to the southern end of the tunnel descend at 0.7%.

2. Structural design

2.1 Geological conditions

The tunnel, which runs east of the Loisach Valley, is located in the area of the western foothills of the Hohe Fricken and Wank massifs, which are overlain by thick debris surfaces, mainly from the Kuhflucht and Markgraben.

In the northern section, the terrain is flat to slightly hilly-bumpy. In the central and southern sections, the tunnel crosses sloping terrain sloping westward toward the Loisach River. In an easterly direction, the terrain rapidly becomes steep until steeply rising main dolomite indicates the bedrock.

The mountain range to be penetrated consists of an outer shell of Quaternary unconsolidated rock and an inner core of rock, which can be assigned to the main dolomite of the Alpine Triassic. It is predominantly unweathered, but has an irregular parting plane structure due to tectonic stresses during the formation of the Alps.

The southern loose rock is composed of main debris and moraine material. In the northern tunnel area, the main dolomite dips steeply. Thereafter, the tunnel crosses two Quaternary unconsolidated areas. The upper area again consists of slope debris and moraine material. However, large deposits of mudflow debris are also present here.

This mudflow debris, a cohesive material of gravel, sand, clay and rubble, has a large number of stones and boulders (boulders up to max. 40 m3).

Under the slope debris there are river and still water deposits. The loose rock is groundwater-bearing, and water-bearing fractures were encountered in the main dolomite.

Level measurements showed that a water pressure of max. 65 m above the tunnel floor is to be expected.

2.2 Supporting structure, sealing

Due to the different geological conditions and rock cover, open and mined construction methods were used.

The carriageway consists of two lanes, each 3.75 m wide, and two emergency walkways, each 1.0 m wide.

The tunnel begins in the north with a double rectangular cross-section that spreads out into two single rectangular cross-sections, which then merge into the vault cross-sections. The tunnel cross-section has a clear width of 9.50m and a clear height of 4.50m. Since the tunnel is largely below the groundwater level, the entire vault cross-section was designed as a closed cross-section with a continuous floor. The maximum width of the rectangular cross-section is approx. 24.80 m. In the tunnel, there are two accessible cross-sections in the outer areas and one accessible cross-section in the middle.

All vault cross-sections of the tunnel were provided with a 3 mm thick plastic sealing membrane made of PE, which is protected from damage by a fleece on the uphill side.

The block joints were sealed by external post-injectable waterstops. In the event of damage to the plastic sealing membrane, this provides a barrier against water flow in the longitudinal direction of the tunnel. The use of waterproof concrete for the inner lining also provides increased protection against leakage. To limit the high water pressures in the rock, a pressure-regulated drainage system was used for the first time in the Farchant Tunnel. With this system, the high rock water pressures prevailing in the central area of the tunnel section were reduced to a minimum pressure of 2.5 bar, which is also the maximum pressure. This was achieved by placing a riser equipped with an overflow in the ventilation shaft of the tunnel. The overflow is located at a height of 25 m, so that an overpressure of 2.5 bar above the tunnel floor is always maintained in the system. By reducing the pressure loads, it was possible to dispense with a reinforcement of the inner shell in the central area and with the formation of a structurally complex sealing system.

The system is maintenance-free. There is no risk of sintering, since no air supply is possible and no turbulence can occur in the sufficiently large flow cross-section at the low water volumes.

2.3 Operating facilities, equipment

The subject of safety in the tunnel was given the utmost attention at the planning stage.

The safety equipment includes the following facilities:

• operating and traffic control systems
• automatically controlled tunnel ventilation system
• automatic fire alarm system and pushbutton detectors in all emergency booths
• continuous video surveillance in the tunnel, in the escape crossings and in the tunnel's preliminary zones
• Extinguishing water connections at all emergency call points in the tunnel
• 30 emergency call booths in the tunnel, equipped with emergency call detectors and two dry fire extinguishers, as well as 4 additional emergency call pillars in the tunnel's preliminary zones
• Emergency call and escape route signs
• escape route marking by flashing beacons
• automatically controlled lighting system with emergency fire lighting
• radio equipment for police, fire department, rescue service and company radio
• Loudspeaker system and radio feed with RDS including emergency intercom option
• Feeding of all common mobile radio providers
• Measuring equipment for recording pollutant concentration, visual turbidity and wind speed
• Altitude control equipment

The tunnel ventilation system is designed for normal operation in such a way that all the exhaust air in the tunnel is extracted by fans in the center of the tunnel and at the portals and discharged via an exhaust air stack with the addition of fresh air. This ensures that no increased exhaust gas concentrations occur at the portals. In the event of a fire, the automatic fire alarm system detects the source of the fire in an area of 50 meters. Subsequently, a total of seven fire extraction flaps open in the intermediate tunnel ceiling above the detected source of the fire. The smoke gases are extracted directly above the fire site and led down the chimney. This is to prevent the smoke from spreading to the entire tunnel area. People in the tunnel are thus given the opportunity to leave the tunnel safely via the escape routes (cross tunnels) or the portals.

In addition to the portals, the three cross tunnels into the neighboring tube serve as escape routes. The maximum distance to an escape route is approx. 600 m. The tunnel is supplied with electrical power directly from the medium-voltage grid via two independent lines. In the event of a total power failure, an emergency power supply ensures the power supply or emergency lighting. Emergency call facilities (emergency call detectors in cabins with doors) are located at intervals of approx. 150 m in both tunnel tubes. Accidents can be reported via these emergency call detectors by pressing a button or by voice connection. Access for emergency services is provided against the direction of travel. For this purpose, separate access roads have been newly constructed onto the B 2 where necessary.

Control and monitoring of the tunnel normally run fully automatically. In the event of a fire alarm, for example, both tunnel tubes are automatically closed, or special traffic jam programs are used to prevent the risk of rear-end collisions when traffic is backed up. In addition, the tunnel is also monitored around the clock by video, so that in the event of an incident, immediate and targeted intervention is possible. A loudspeaker system provides up-to-date announcements in dangerous situations.

3. Construction methods and execution

For each section of the tunnel, a construction method was chosen that was appropriate to the topographical, geological and hydrological conditions.

In the northern section, the tunnel was constructed over a length of approx. 300 m as a rectangular cross-section using the cut-and-cover method. In order to use as little land as possible, the excavation pits were constructed with Berlin shoring or steep slopes with shotcrete protection. The rectangular tunnel was founded flat on the existing ground. The tunnel was constructed with 10 m long blocks using the invert-wall method, i.e. the invert and walls of a block were concreted in one operation. The ceiling of the tunnel then followed in a second working operation. After the waterproofing and protective concrete had been applied to the tunnel roof, the working areas were decayed and the tunnel was backfilled.

At the southern end of the tunnel, sections of different lengths were constructed using the cut-and-cover method, for the east tube over a length of 200 m and for the west tube over a length of 145 m. The tunnel was then excavated using the cut-and-cover method. For both roadways to be constructed using the cut-and-cover method, the vault profile of the closed construction method was planned as the lining.

The mining operation was divided into a northern section to be excavated in unconsolidated rock and a southern section to be excavated in rock.

In accordance with the different geological formations, the northern loose rock section was excavated with different excavation sections and shotcrete support, i.e. the calotte was excavated in a first working pass and then the bench and invert were excavated in a second working pass.

In the breakdown bays, where the cross-section to be excavated was again considerably enlarged, another vertical division was required in addition to the horizontal division during the excavation of the calotte.

In the southern area, the tunnel tubes could be constructed in the rock. Due to the fissuring of the rock, the excavation was divided into a calotte excavation on the one hand and a bench and invert on the other. The excavation of the calotte could be carried out for the most part without advance safety measures. The support was provided by a reinforced shotcrete lining with support arches, i.e. immediately after excavation of the rock, the resulting cavity was sealed with shotcrete and provisionally secured.

Here, too, the hawser and invert were rebuilt in a separate operation. In the entire rock area, fissure water was to be expected, which was collected and drained off to the Loisach. However, the amount of water was low.

The final load-bearing system, the concrete inner shell, was subsequently drawn. In the rock area, blasting was carried out in the calotte with cut-off lengths of 1.0-2.5m. With an average of four excavations per working day of 24 hours, an advance rate of approx. 6-10 m per working day could be achieved.

In the area of the unconsolidated rock, the rock had to be secured ahead of excavation due to its low stability. This securing was very costly due to the large boulders embedded in the rock. Along the entire circumference of the calotte, 7 m long spiles were installed as advance support. This was followed by a 1 m cut-off, which was secured with reinforced shotcrete before 7 m long spiles were installed again, and so on. As a result, up to six spiles were placed on top of each other. In the protection of this spike screen, even large boulders could be held safely and the rock could be excavated safely with the excavator. Boulders protruding into the cross section were removed or blasted off with the blasting chisel. The multi-layered spike protection also virtually prevented the loose material in the ridge from breaking away. The construction of the "Farchant spile shield" proved to be a very economical and practicable method. The advance rates achieved averaged 3 m per 24-hour working day.

The central operations building, with its connection to the central breakdown bay, is located on a slope. To prevent unilateral shear forces acting on the structure in the final state, the slope was secured with permanent anchors.

 The north breakdown bay was constructed using the cut-and-cover method. In order to minimize the impact on the environment, local residents, etc., the so-called cover construction method was used. The vertical excavation pit wall consisted of overlapping drilled piles. The curved cover supported on the piles also served to brace the piles. After construction of the cover, backfilling and restoration of the ground took place.

Excavation and construction of the tunnel vaults took place underground under the protection of the cover.

The approx. 650,000 m3 of rock and loose rock extracted during the excavation of the tunnel were used as valuable road construction material in the subsequent sections, above all for the Burgrain bypass and the Farchant-Süd junction. Accordingly, the tunnel was mainly excavated from the south. In this way, the material could be transported within the construction site over a short distance and thus without major disruptions to the road network and the neighborhood.

4. Literature

[1] Schikora, K.; von Soos, P.; Jedelhauser, B.; Heimbrecher, F.; Thomee, B.: Tunnel Farchant-Abdichtungs- und Entwässerungssystem

[2] Autobahndirektion Südbayern: Faltblatt B 2 neu/B 23, Eröffnung Umfahrung Farchant/Burgrain

[3] Autobahndirektion Südbayern; ARGE Ortsumgehung Farchant: Faltblatt Tunnel Farchant

 

 

• Country: Germany
• Region: Bavaria
• Tunnel utilization: Traffic
• Type of utilization: Road tunnel
• Client: Freistaat Bayern, Autobahndirektion München
• Consulting Engineer: Ingenieurbüro PSP
• Contractor: Bilfinger + Berger Bau AG, Hochtief AG
• Main construction method: Trenchless
• Type of excavation: Excavator/Drill-and-blast
• Lining: in-situ concrete
• No. of tubes: 2
• Tunnel total length: 2275 m (western tube), 2390 (eastern tube)
• Cross-section: 87-111 m2
• Contract Volume: 119 mill. DM
• Construction start/end: 1995-2000
• Opening: May 2000