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Engelberg Base Tunnel, BAB A81

1. Task definition

The A 81 Heilbronn-Leonberg federal freeway joins the A 8 Karlsruhe-Munich federal freeway at the Leonberg interchange. The A 81 was widened to six lanes up to about 4 km before the Leonberg interchange. The remaining section was dominated by the existing 300 m long Engelbergscheiteltunnel, the oldest highway tunnel in Germany built in 1934-38. Long approach ramps to the tunnel, each with a 6% incline, combined with only two lanes in each of the two existing tunnel tubes caused traffic jams almost daily with peak traffic loads of up to 120,000 vehicles in 24 h and a very high proportion of trucks.

As early as 1970, plans were therefore initiated with the aim of fundamentally improving the traffic flow, which initially envisaged both a widening of the existing tubes and the construction of an additional third tube. However, since this would have meant that the large inclines to the tunnel and the severely detrimental condition of Leonberg's urban fabric caused by a dissecting traffic ribbon would still have existed, these solutions were not pursued further.

The aim of more recent planning was therefore to replace the existing tunnel tubes with two new tubes about 60 m lower down.

The new tunnel length is around 2,500 m, of which 700 m is open cut and 1,800 m is closed cut. Each of the new tunnel tubes will have 3 lanes with a width of 3.50 m, 2 x 0.50 m shoulder strips, a continuous emergency stop strip with a reduced width of 2.00 m and 2 x 1.00 m emergency walkways. This results in a total width of the clearance area of 15.50 m. Due to the continuous emergency stop lane, it was possible to dispense with the arrangement of breakdown bays in the tunnel. For reasons of driving dynamics, the maximum gradient (from north to south) is only 0.9%. The two tunnel axes are routed separately and have a center-to-center distance of up to 36 m.

In addition to the tunnel construction, the fundamental reconstruction of the Leonberg freeway interchange was necessary in order to achieve an efficient and traffic-safe interlinking of the A 81 and A 8 freeways. For this purpose, 8.5 km of freeway had to be widened or rebuilt.

A total of 8 new bridges had to be built and a large number of temporary traffic facilities installed.

Between the eastern and western tunnel tubes, 7 cross tunnels were provided as escape routes or for the passage of cars, ambulances and fire departments.

2. Structural design

2.1 Geological conditions

The Engelberg mountain spur lies at the edge of the Stuttgart Keuper landscape. The tunnels cross mainly layers of the middle gypsum keuper. The mostly marly rocks with gypsum and anhydrite inclusions are offset by about 80 m from each other by a tectonic thrust zone striking across the highway route, the so-called Engelberg fault. Here, mountain water inflows of several liters per second were already encountered in the test tunnel.

A very irregular leaching front with locally stronger water flow separates unleached, dry rock carrying gypsum and anhydrite from leached, slab-slumped and differently plasticized rock. Residual cavities with mushy silt are present with minor overburden in the southern section. Seepage occurs irregularly and in small quantities in the leached rock. It cannot be discharged in a controlled manner. Where it encountered anhydrite with corrensite intercalations in the north section of the structure, strong swelling processes occurred.

In 1977 and 1978, an exploratory tunnel of about 1,000 m length with two test sections of 50 m each was excavated in the northern tunnel section of the present east tunnel. While test section I was constructed in the low cohesion leached rock, test section II was located in the swellable unleached rock.

When passing through the unleached rock, water inflow from an insufficiently sealed core borehole from above ground resulted in severe wetting of the rock at the bottom of the exploratory gallery. As a result, 36.7 cm of invert heave was measured within 5 months, corresponding to a daily heave rate of 2.5 mm. In general, the heave was greatest in the middle of the invert; however, the lateral areas of the elm footings also heave significantly due to shear fractures in the elm shotcrete and buckling of reinforcing steel mesh.

Another unpleasant experience resulted from the sinking of a water level borehole down to the level of the test section bottom. About three years after the test section II was driven, a borehole was sunk at a distance of about 60 m from the exploratory gallery. Here, water entered the test section II via fine fissures in the unleached rock, which was considered dry and dense. Within a few days, such bottom pressures developed that 6 m long slopes cracked and the reinforced shotcrete support in the immediate catchment area of the wet spots was destroyed.

The following conclusions were drawn from the experience gained during the excavation of the exploratory gallery and the test sections, as well as in tunnel structures of the city of Stuttgart, and from the results of laboratory tests, and were used as a basis for further project execution:

- The swelling process cannot be prevented in the long term.

- It is not the swelling pressures measured in the laboratory on selected specimens that are decisive for the design, but the resistance of a superimposed rock wedge.

- The swelling process will continue as long as water is present and the swelling pressure is not overpressed by the existing external stress state.

2.2 Supporting structure, sealing

In the entire area to be excavated, a double-shell tunnel in shotcrete construction was used. The outer shotcrete lining was used for temporary support and as a sealing girder. In the final state, the reinforced inner shell made of concrete B 35 had to bear all loads occurring. Due to the presence of concrete-aggressive mining water, a 3 mm thick PE foil seal was placed between the outer and inner shells. The inner lining was constructed at block intervals of 10 m, the individual tunnel blocks being completely separated from each other by means of continuous space joints. In the area of the joints, an additional external PE bulkhead joint tape was arranged, which was welded to the PE foil sealing.

The shape of the rectangular blocks of the open construction method is replaced at the south service building by the standard profile of the tunnel. The vault of the Maulquer section spans approximately 18 m. The high earth cover in the last section of the open cut south can be accommodated by this load-bearing system; rectangular cross-sections with very thick components would be extremely uneconomical here. In addition, the tube provides sufficient space for the aeration and ventilation ducts located under the roadway, which have to connect the two operational buildings.

In the areas of the mouth cross-section (underground and open construction), the traffic is guided onto a roadway bridge consisting of a support wall (= separating wall between supply and exhaust air) and a roadway slab.

The standard cross-section of the closed construction method was designed with a 70 cm thick reinforced inner lining, and the excavation area is 200 m2 . In the tunnel sections located in the anhydrite area, the design had to be based on the resistance principle.

The tunnel has to absorb possible anhydrite swelling pressures without danger. The bottom rehabilitation of the swollen anhydrite area, which was carried out in the as-built condition, had a favorable effect. Due to the elliptical shape, the lateral transfer of the occurring loads via the additional concrete layers installed, and the load application point of the anhydrite swelling pressure moving further and further into the rock as the swelling process progressed, it was possible to reduce the original design approach for the tunnel inner lining from 2 to 1 MN/m2 . The required inner lining thickness is 3 m in the tunnel floor, 2 m in the lateral elm areas and 1 m in the tunnel roof; the excavation area of this cross-section is 265 m2 .

The areas with particularly strong swelling behavior were spatially precisely delimited by further exploratory drilling. In the sections defined in this way, the invert was lowered by about 3.50 m after the bottom of the shotcrete lining had been closed, and a deformation layer was installed in order to be able to absorb the deformations without damage during construction, which increased the total excavation area to 332 m2 .

Of the total tunnel length of around 2500 m, 56 blocks were constructed in the south and 16 blocks in the north using the cut-and-cover method. The block length is generally a uniform 10m. Multi-level service and utility structures in these areas connect the two tubes. Little is visible of these structures in their final state, as they are backfilled up to 15 m in some areas, planted and thus largely adapted to the natural terrain.

2.3 Tunnel portals

Coming from the direction of Heilbronn, two conical exhaust air stacks are visible from a distance in the north, protruding approx. 6.50 m from a cone of debris. To the left and right of them are intake shafts. After a trough section, one approaches the portal blocks, which show the predominant appearance of the Engelberg Tunnel with its vaulted structure. On-set portal collars collect surface water and any dislodging rock material and accentuate the tunnel entrance. In the tunnel, the inner wall is broken up into individual columns in the area of the exhaust stacks over a distance of about 30 meters.

From the Leonberg triangle, one enters the tunnel in the south in the direction of Heilbronn through a rectangular portal with a span of about 21 m, a cantilevered flying roof and sloping wing walls. As in the north, a supply air shaft is located at the side of each tube at the level of the operations building, but the exhaust air stacks are missing here.

2.4 Operating facilities, equipment

Passable transverse tunnels are arranged at intervals of about 300 meters. The ventilation of the tunnel tubes operated in the directional traffic is carried out as half cross ventilation. In the area of the north and south portals, a multi-story service building was erected in each case, which is arranged between the two tunnel tubes and is covered. Because of the nearby buildings in the area of the south portal, exhaust air emission is not possible here. Venting in the middle of the tunnel is also ruled out because of the buildings.

The exhaust air from the west tunnel is extracted from the traffic area in the south operations building, fed 50% each into the exhaust air ducts of the east and west tubes, and blown out by fans to the north via an exhaust air stack.

The exhaust air from the east tube is extracted directly from the traffic area in the north operations building and emitted via another exhaust stack.

The fresh air is sucked in at both operating buildings, transported longitudinally in the supply air duct under the tunnel carriageway and blown laterally into the traffic area at intervals of 10 m (half cross ventilation).

A total of 9 fans were installed in the north operations building and 2 in the south operations building. The outputs are up to 630 kW/fan, the max. air volume 240 m3/sec./fan.

In addition to ventilation, other equipment important for the safety of road users was installed:

  • Extinguishing water supply
  • power supply
  • tunnel lighting
  • television monitoring system
  • fire alarm system
  • emergency call facilities
  • Traffic control equipment
  • tunnel radio system
  • tunnel control system

2.5 Construction method

The tunnel, which was to be constructed predominantly using the closed construction method, was built as a double-shell structure using the shotcrete method. It was secured immediately after excavation by means of a reinforced shotcrete lining with a thickness of 20 - 40 cm, produced by the wet-spraying method.

The thickness depended on the geology. Additional securing means used were support arches, grout anchors, injection pipe anchors, steel spiles, etc.

Due to the size of the tunnel cross-section and the geological conditions, it was not possible to carry out a full excavation.

For this reason, a subdivision of the tunnel drive into excavation sections was specified in the invitation to tender. Two elm tunnels were driven in parallel in advance, and the remaining calotte section was excavated and secured at a distance depending on the rock class. Subsequently, the bench and floor were each excavated and secured on half sides.

In the area of the depleted gypsum-keuper, the rock could be loosened to the exact profile by means of tunnel excavators. In the anhydrite area, the method was changed to locomotive blasting.

The open cut method was predominantly shallow founded. In areas with highly variable ground conditions, pile foundations and partial soil replacement were also carried out, depending on the ground conditions encountered.

The excavation pits for the open construction method in the north and south were constructed as tie-backed girder pile walls - in some areas provided with shotcrete infill for stiffening - with up to 8 anchor layers. Since in many areas the outer walls of the tunnel tubes were concreted directly against the shoring walls, their deformations had to be kept low so as not to reduce the concrete thicknesses.

The start of underground construction (in the north) was marked by a 26 m high and 60 m wide shoring wall, against which the tunnel was slammed. In the south, a similar picture emerged, except that here two separate excavation pits (for the east and west tubes) and the associated exit walls were constructed.

3. Construction

The contract was awarded on July 24, 1995. Construction work began just one day later on July 25, 1995. The two tunnel tubes to be excavated by mining were driven in parallel from north to south, so that the north excavation pit had to be completely finished by the time the tunnel was cut in November 1995.

In order to be able to complete the tunnel excavation within the specified construction time, high-performance equipment configurations were used in continuous operation. Tunnel excavation and securing were carried out in 24-hour operation on 7 weekdays.

The breakthrough of the east tube of the Engelberg Base Tunnel was celebrated as early as July 1997, followed by the west tube in September 1997. The logistical achievement required to reach this construction time is remarkable. Within the 20-month (east tunnel) and 23-month (west tunnel) driving period, a total of approx. 1,000,000 m3 of excavated material had to be excavated and approx. 140,000 m3 of shotcrete and 150,000 Ifdm of TH 36 steel sections installed. In addition, approx. 90,000 anchors of various types and 25,000 pipe spiles were used.

In order to meet the extremely short construction time of only four and a half years, it was necessary to start installing the inner lining while the tunnel was still being driven. The cross passages or connecting tunnels between the two tubes are spaced at intervals of around 350 meters. They were widened in such a way that the Schutter vehicles could pass through without any problems. This meant that driving could continue almost undisturbed and the inner lining could already be installed coming from the north.

Due to the geometry of the tunnel cross-section, the high reinforcement contents and the tight schedule for the structural work on the inner lining, new manufacturing methods had to be developed for the production of the invert and the vault.

First, the invert vault was concreted, followed by the carriageway bridge at intervals of 3 blocks, and finally the vault at intervals of at least 60m.

In the north rough cut and on the south side, the cut-and-cover construction method was used in parallel with the heading work. Then, first for the east tunnel, the lining elements and the bituminous roadway consisting of sealing, mastic asphalt, stone mastic asphalt and a surface course of asphalt concrete were installed. On September 11, 1998, the east tunnel was opened to traffic in a 4+0 traffic pattern. In the remaining construction months until fall 1999, the entire construction project was completed.

4. Literature

[1] ARGE Engelberg in Zusammenarbeit mit dem Landesamt für Straßenwesen Baden-Württemberg: Engelberg-Basistunnel und Autobahndreieck Leonberg, Neubau und Modernisierung eines Verkehrsknotenpunktes

[2] ARGE Engelberg: Sonderausgabe zum Tunneldurchschlag BAB A 81, Engelberg- Basistunnel, Durchschlag 4. Juli 1997

[3] DGGT - Deutsche Gesellschaft für Geotechnik, Taschenbuch für den Tunnelbau 1999: Engelbergbasistunnel und die dabei gewonnenen Erfahrungen beim Bau eines großen Tunnelquerschnitts im schwellenden Gebirge

[4] Züblin-Rundschau 30: Ohne Stau durch Europas größten Straßentunnel

[5] Dietz, W.; Lorscheider, W.: Der Engelbergbasistunnel bei Leonberg Baden-Württemberg, ein Autobahntunnel mit großem Querschnitt in wechselhaftem Gebirge. [6] Der Engelbergbasistunnel und der Umbau des Autobahndreiecks Leonberg, Tiefbau 10/1998


  • Country: Germany
  • Region: Baden-Württemberg
  • Tunnel utilization: Traffic
  • Type of utilization: Road tunnel
  • Client: Bundesrepublik Deutschland (Land Baden-Württemberg, Landesamt für Straßenwesen Autobahnbetriebsamt Heilbronn, Bauleitung Stuttgart)
  • Consulting Engineer: Ingenieurbüro Müller+ Hereth, Ingenieurgruppe Bauen, Autobahnbetriebsamt Heilbronn, Bauleitung Stuttgart mit der Ingenieurgemeinschaft Bung und Weidleplan
  • Contractor: Ed. Züblin AG, Bilfinger + Berger Bau AG, Hochtief AG, c. Baresel AG, Wayss & Freytag AG, Wolff & Müller GmbH & Co. KG
  • Main construction method: Trenchless
  • Type of excavation: Excavator/Drill-and-blast
  • Lining: In-situ concrete
  • No. of tubes: 2
  • Tunnel total length: 2 x 2,530 m including a 1,780 m mined section
  • Cross-section: 200 m² in soft rock resp. 265 m² in anhydrite
  • Contract Volume: 604 mill. DM for the complete project
  • Construction start/end: 7/95 till11/99