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Eching tunnel, BAB A96

1. Task definition

After its completion, the A 96 Lindau - Munich federal freeway will connect the Munich metropolitan area with the Allgäu/Lake Constance region over a total length of 173 km. It not only connects the south and southwest German highway network with each other and connects in Lindau to the continuing highway network in the direction of Austria and Switzerland, but also relieves the federal roads B 12 and B 18 and thus also the cities and communities affected by the through traffic in the local thoroughfares.

Kilometer-long traffic jams and alarming accident figures in the Greifenberg - Inning section north of the Ammersee lake made it necessary to upgrade the completely overloaded B 12 federal highway to the A 96 there as well. An important part of this expansion is the Eching tunnel, which is located between the Amper and Windach rivers in the northern siltation zone of the Ammersee basin, which is sensitive in terms of water management and landscape.

The B 12 runs through the municipal area of Eching and separates the village center from the district of Kaaganger to the south. In order to restore the unity of the village and to better integrate the extension line into the landscape, but above all to relieve the residents from noise and exhaust emissions, the route had to be laid low over a length of 684 meters. A tunnel structure with subsequent trough structures in the ramp areas therefore had to be built to cover a 400 m section of the route. The tunnel concept was essentially determined by the alignment of the route, which, as a result of the regional planning procedure, is adapted to the existing B 12 and thus runs in an S-shape with alternating cross slopes, and by the location of the entire structure in the groundwater. In the interest of harmonious integration of the lowering section into the local development, the design of the structure, in particular the ramps and tunnel portals, and the landscaping were subjected to a design competition.

2. Structural design

2.1 Ground and groundwater conditions

Since the geological and hydrological conditions had a major influence on the design and construction, extensive exploratory boreholes (53) and groundwater investigations with regard to the flow behavior were already carried out during the preparation of the construction project.

In the eastern section, the tunnel passes through an alluvial cone of sandy, slightly silty gravel with a thickness of 7 m in general. The geological conditions on the west side, however, are relatively difficult due to the valley fill found there, consisting of washed-out moraine material and redeposited Tertiary sediments with a soft to mushy consistency. In addition, the boreholes revealed that silty sands, sandy-clayey silts and silty-sandy, gravelly clays merge fluently in horizontal and vertical direction for about 300 m in the area of the structure.

In addition to the inhomogeneous subsoil conditions, the complicated groundwater flows that change over the length of the tunnel required measures to be taken in the construction and final stages.

In the western part of the structure, the groundwater flows from west to east. In the eastern part, the gradient flattens and the flow direction swings southeast toward the Ammersee. Without technical precautions, the tunnel would cause groundwater to back up. For this reason, the design of the structure includes appropriate water crossings. These consist of two culverts each in the west and east on both sides of the structure. The groundwater is collected in the west, diverted about 200 m to the east and then infiltrated again. A complete blockage of the groundwater flow was not acceptable from a water management point of view, even in the state of construction, so the structure had to be built in sections.

In order to keep the interference with the groundwater system as low as possible, the tunnel structure was built as close to the surface as possible. Nevertheless, the structure dips into the groundwater along almost its entire length, up to 4.0 m at the highest groundwater level.

2.2 Supporting structure, waterproofing

The choice of the supporting structure for the 684 m long structure was primarily determined by the cross-section of the highway, the construction method and the groundwater present.

The tunnel, which will be created by covering the section over a length of 400 m, consists of a two-row closed rectangular frame with clear widths of 11 m each and a clear height of 4.80 m. The tunnel is designed to be built over the entire length of the motorway. The clear height is available across the entire width of the lanes, as the required luminaires are located above the emergency walkways. In the ramp areas, too, where rectangular trough structures adjoin the tunnel portals, the standard cross-section of 26 Tr is continued with two separate directional lanes including emergency lanes and emergency walkways on both sides. The total width is thus 24.60 m with a maximum height of 8.0 m. The standard cross-section is almost identical to that of the tunnel.

The standard cross-section is used unchanged in almost the entire tunnel area. Only in the first 35 m behind each portal are there minor changes, as noise protection cladding is installed on the walls in these areas. Walls and covers are also reinforced in the first block in the portal area.

The tunnel was statically designed and built as a two-row closed rectangular frame. The entire structure is made of concrete B 25. The outer walls, the ceiling and the invert are constructed according to the principle of "waterproof concrete construction" without external waterproofing.

Since the portals of the tunnel are already below ground level, the design of the structure called for the slabs in the portal areas to be semicircular. In front of each of the tunnel portals, a center wall was to be erected over a length of 75 m to provide noise protection and to separate the incoming and outgoing air flows of the directional lanes.

Noise protection cladding was installed on the walls in the entire ramp area and in the first 35 m of the tunnel tube. In the case of the outer tunnel walls, the space for the 15 cm thick noise protection cladding was gained by reducing the wall thickness. This was possible due to the relatively low earth cover of these construction sections and the resulting lower wall load.

Adjacent to the western 134 m long ramp are retaining walls with a length of 32 m in the north and 66 m in the south, which serve as backfill for noise barriers. The eastern trough walls and the western retaining wall are terminated by wing walls, the upper edge of which extends into the future terrain. The trough-shaped tunnel ramps form a water-impermeable concrete structure with the double tube.

As a result of the design competition, reference was to be made in the building design to the nearby Ammersee. For this reason, the so-called "Ammersee A" was erected as the structural end of the tunnel center wall - in the area of the eastern and western portals. The precast elements in bush-hammered B 25 are approx. 3.5 m wide and 8 m high, overhanging the end of the tunnel center wall by 3.5 m in height.

2.3 Operating facilities, equipment

 For the operation and monitoring of the tunnel's technical equipment, a three-story operations building, located approximately in the center of the tunnel, with an integrated, low-lying rainwater retention basin was required. Surface water from the roadways and emergency walkways is channelled into this 85 m3 rainwater retention basin, which consists of a smaller wastewater basin and a rainwater basin, via reinforced concrete slot channels located at the lower roadway edges. The tunnel wash water as well as the contaminated precipitation water at the beginning of a rain event, the so-called first flush, reaches the wastewater basin via manhole drains with sludge trap and baffle via the DN 500 drainage pipe. This wastewater is pumped via a lifting device to a lightweight separator, from where it flows in free fall to the public sewer. Stormwater from the larger basin is also lifted and sent to the highway drainage channel for infiltration.

Due to the shortness of the tunnel and the separation of the directional lanes, aeration and de-aeration facilities were not required. Likewise, no stopping and parking bays had to be arranged, especially since the continuous hard shoulders provide space for maintenance, inspections and in case of accidents. The tunnel's interior lighting, with emergency and safety lighting, is adapted to the exterior lighting via adaptation sections. The power supply cables run in cable protection tubes located under the emergency walkway. Emergency call facilities are located at the tunnel portals, and in the center of each tube there is an emergency call station with fire extinguisher separated from the roadway by a door. Line fire detectors have been installed above the lanes. For emergencies, there is a connecting opening in the center of the tunnel for passing from one tunnel tube to the other.

Tunnel operation is largely automated. Emergency and fault messages are transmitted to the permanently manned Munich-Freimann traffic control center. In the event of fire, exceeding of the carbon monoxide limit or traffic disruptions (congestion, accident), the tunnel entrance is closed automatically.

2.4 Construction method

The structure was built using the cut-and-cover method. In order to ensure the maintenance of traffic on the B 12 and because of the water and sewage pipes crossing the construction area, the structure could not be built in one go, but had to be divided into three construction sections.

For dewatering purposes, wells had to be sunk and groundwater lowered for each of seven individual excavation pits. After completely enclosing each individual excavation pit with sheet pile walls, the earth was excavated to 50 cm below the anchor layers. Only after the anchors had been tensioned and the groundwater had been lowered was it possible to carry out the full excavation. Due to its location in the groundwater along its entire length, the structure causes a groundwater back-up, which necessitated appropriate measures for water transfer. For this purpose, two culverts connected by a longitudinal pipeline had to be installed under the western and eastern halves of the structure. The groundwater is collected in the west and led about 200 m to the east, where it seeps away again.

The tunnel as well as the trough structures were to be built as a water-impermeable reinforced concrete structure without external waterproofing. By using waterproof concrete in conjunction with a large number of concrete-technological, constructional and executional measures with the aim of reducing the constraint in the still young concrete and thus avoiding cracks, it was possible to produce a water-impermeable concrete trough or frame and to dispense with the relatively costly external sealing.

For lighting reasons, concrete carriageways were provided in the tunnel and, taking noise protection criteria into account, drain asphalt pavements were provided outside the tunnel.

3. Construction

The structural work for the tunnel and the operations building was put out to open tender within the European Community. The contract for the main bid was awarded to a Munich subsidiary of an Austrian company.

The construction project was carried out according to the tendered construction design, only the originally planned semi-circular ceiling shape of the portals was replaced by a parabolic design with haunches.

After the B 12 had been relocated to the south in the western part of the construction area, construction of the tunnel, including the ramps, could begin in mid-September 1993.

The tunnel was constructed in three stages with open excavations ranging in length from 50 to 145 meters. The construction of the excavation pits was carried out in the following work steps:

  • Removal of topsoil
  • Bunging of the excavation pit to a maximum depth of 15 m
  • Excavation of the earth to create the grouted anchors
  • Production of the grouted anchors
  • Construction of the wells for lowering the groundwater
  • Pumping of groundwater from the excavation pit with subsequent filtering and discharge into the Windach river
  • Earth excavation up to the foundation base

Within the individual excavation pits, the tunnel, which is 24.60 m wide in total, was constructed in 10 m long blocks. The construction joints were located above the highest groundwater level at the upper edges of the outer walls. The invert and the outer walls were concreted in one go. The center wall and slab could be constructed later in separate operations.

Elastomeric waterstops with vulcanized sheet metal strips and injection hoses are located in the joints between the individual blocks. All joints were additionally secured by external waterstops. Thus, the entire tunnel structure is impermeable to water and, due to its own weight, has a buoyancy-proof foundation.

After completion of the structure in sections, the excavation pits were backfilled and the sheet pile walls were pulled.

Prior to construction of the middle and final section, Kaagangerstrasse and the access road to the B 12 were relocated to the eastern tunnel section, which had been completed in the meantime. In order to be able to cross the B 12 without any intersections, a temporary bridge for traffic and a provisional crosswalk had to be built.

The construction work was successfully completed after a construction period of 28 months without any special incidents or accidents.

4. Literature

[1] Autobahndirektion Südbayern, München: Prospekt BAB A 96 Lindau - München Tunnel Eching am Ammersee

[2] Autobahndirektion Südbayern, München: Entwurfs- und Ausschreibungsunterlagen

[3] Krämer, J: Tunnel Eching am Ammersee Beitrag zum Schutze der Anwohner, Landschaft und Umwelt Zeitschrift Die Bauverwaltung + Bauamt & Gemeindebau 6/95


  • Region: Freistaat Bayern
  • Tunnel use  Road
  • Client Autobahndirektion Südbayern
  • Consultants: PSP Beratende Ingenieure, Kling Consult
  • Contractors: Universale-Bau GmbH, München
  • Total length: 134m + 400m + 150m
  • Cross section: 110,4 m²
  • Contract value: Rohbau: 34,7 Mio. DM, BTA: 3 Mio. DM
  • Construction time: 1993 till December 1995