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Tunnel Hochwald, BAB A71

1. Task definition

The construction of the A 71 federal freeway from Erfurt to Schweinfurt as part of the German Unity Transport Project No. 16 will significantly and sustainably improve the transport infrastructure. The A 71 motorway opens up northern Lower Franconia and southern Thuringia, connects the Franconian and Thuringian economic and tourist centers and provides an efficient transport link between southwestern Germany, northern Bavaria and Thuringia.

Due to the close succession of different engineering structures, the ridge crossing of the Thuringian Forest in the course of the A 71 is considered to be the technically most complex new construction section of all German Unity Road traffic projects. The almost 20km long section between the Geraberg junction and the Suhl interchange includes the three major bridges Schwarzbachtal (352m), Wilde Gera (552m) and Steinatal (340m) and four underground structures with a total length of 12.6km.

Starting from the Schneidersgrund depression on the northern edge of Zella-Mehlis, the Hochwald tunnel passes under the ridge of the Hochwald mountain range over a length of 1,058m and ends at the edge of the industrial area north of Suhl.

The section of the BAB A 71 is being built as a full section, i.e. a separate tube is required for each directional lane. The center distance of the directional lanes in the tunnel and in the portal areas is 25 meters. In the section adjoining on the south side, the center distance of the directional lanes is reduced to 11.00 m. In the north, the center distance is not reduced. In the north, the center distance is not reduced because the south portal of the Rennsteig Tunnel is located north of junction 16.

The two tunnel tubes, which are connected by two cross-passages at a distance of approx. 360 m, lie in a straight line along their entire length. The cross slope to the outer edges of the carriageway is 2.5% in each case.

In the cutback area at the south portal, distortion curves with R = 5,000 m are connected due to the reduction of the center distance of the directional carriageways on the free section. In the area of the north portal, the directional carriageways have a straight route.

In the area of the tunnel, the gradients of the roadway were selected to avoid a falling heading as far as possible. Therefore, with the exception of the fillets at both portals, the tunnel runs from south to north with a constant gradient of 0.501%.

The free section of the BAB A 71 has a standard cross-section of RQ 26. According to the predicted traffic volume, a standard cross section RQ 26 t (without hard shoulder) was selected for the normal section of the tunnel. This cross-section has a roadway width of 2 x 3.50 m per tunnel tube plus a shoulder of 0.25 m width on both sides as well as lateral emergency walkways of 1.00 m each. The clear height of the traffic area is 4.50 m.

In the vault area above the traffic area, the jet fans, the tunnel lighting and the traffic control systems had to be installed in accordance with the operational requirements. The area under the emergency walkways was used for laying cables. The pipes for road drainage and invert drainage as well as a hydrant line are located under the roadway.

Taking into account the requirements and the geotechnical properties of the rock mass, a vault cross-section of composite basket arches was developed for the standard profile, which closely encloses the specified constraint points of clearance and fixtures and remains constant throughout the tunnel, with the exception of the necessary widening areas in the area of the junction at the north portal. For technical reasons, it was decided not to rotate the tunnel profile with the course of the roadway. As a result, the symmetry line of the tunnel is always vertically aligned. The inclined carriageway axis is shifted by 3 cm at the top edge of the carriageway relative to the tunnel axis in order to reduce the excavation cross-section due to the transverse inclination of the clearance gauge.

For the widening cross section in the area of the north portal, the arch cross section of the standard profile was widened by 3.50 m in accordance with the additional lane width required. Due to the minimum vault cut required for structural reasons, the vault height above the carriageway increases by 0.75 m compared to the standard cross-section.

The location of the portals, the length and depth of the forebays, and the section lengths of the open and closed areas were determined by the topographical and geological boundary conditions. In order to avoid deep pre-cuts and to limit the encroachment on the forest stand, the overburden in the portal area was reduced to the minimum still justifiable for mining. The slope inclinations in the area of the preliminary cuts were determined by the natural stability of the existing subsoil and vary accordingly in a range of 30° - 50°.The maximum overburden is about 45 m in the central area of the tunnel and decreases slowly towards the portals. With the exception of the portal areas, the overburden is mostly more than 20 m. The minimum overburden at the mining stops is about 6-8 m.

2 Design of the structure

2.1 Geological conditions

The full length of the Hochwald tunnel lies in the granite of the Suhl Scholle, a northwest-southeast oriented high structure of the Thuringian Forest. The granite complex of the Hochwald is crossed by numerous gangue rocks (quartz porphyry and aplite) and is partly deeply weathered or encrusted, especially in the near-surface area. The granite or granite replacement is overlain by thin slope debris.

In both portal areas and in the northern area, the tunnel runs in sections in the weathered or weathering zone of the granite. The central section and the southern area are largely in hard granite. According to the geotechnical forecast, in the area of the irregularly crusted granite at the south portal, fractured or very fractured to slightly compressive rock was to be expected. This could also occur to a lesser extent in the Forstquellen valley basin and in the immediate area of the north portal. Otherwise, in the area of the Forstquellen valley trough and the granite floe at the north portal, post-fracture to strongly post-fracture rock was expected. In the granite floe of the high forest (middle and southern tunnel area), stable to post-fracture rock was predicted, in some cases also strongly post-fracture rock. In the area of the southern portal, the incrustation zone partially extends into the invert area. Here, the rock mass can also have the characteristic of loose material.

Rock water or groundwater is present in the granite only in the form of fissure water. Because of the strong differences in permeability between the near-surface weathering zone, the strongly fissured zone in the upper 20-40 m, and the deeper, much less fissured rock, there is a certain stockwork formation with significantly higher water flow in the aerobic zone. The level in the upper zone varies in the range of about 1-2 m, depending on seasonal precipitation conditions. The maximum water level is about 30 m above the tunnel crest in the middle tunnel section and drops to 0 m towards the portals.

2.2 Supporting structure, sealing

The tunnel is designed as a double-shell vault structure with a sealing layer of plastic sealing sheets between the inner and outer shells of the tunnel vault.

Due to the geological and hydrological conditions, an open vault cross-section is possible in the invert of both tubes over almost the entire length. Only in the area of the south portal was it necessary to install a vaulted floor over a length of approx. 60-70 m due to the unfavorable rock conditions there. Depending on the rock conditions, the outer shell of the standard cross-section consists of unreinforced shotcrete with a thickness of 5 cm or reinforced shotcrete with a thickness of 10-25 cm. The inner shell usually has a thickness of 30-35 cm, which is reinforced to at least 40 cm in the portal areas. The thickness of the invert in the area of the closed invert arch is 40-50 cm. In the case of the cross-section expansions in the area of the north portal, it was necessary to increase the cross-section thickness to 40-50 cm. The inner lining was made of unreinforced concrete B 25 (frost and de-icing salt resistant) in the central sections of the tunnel.

The block joint spacing is 10m; all block joints in the mined section were designed as compression joints. Expansion joints were arranged at the transition to the portal blocks constructed using the cut-and-cover method.

Concrete bases were formed below the emergency walkways to support the slotted channel and the required empty conduits for cables and the cable shafts. For the emergency walkways, a 20 cm thick concrete cap slab with a concrete curb or slotted channel was placed above the embedded cable conduits.

The geological conditions and the economic constraints allow the mountain water or fissure water pressure around the tunnel to be relieved with a local groundwater lowering by means of a permanently acting drainage system. For the tunnel tube, a sealing system based on the umbrella principle was selected, which is only intended to feed the inflowing mountain water to the drainage pipes. This is achieved for the open-bottom arch by means of a waterproofing system consisting of a protective fleece and the actual waterproofing skin consisting of PE-based waterproofing membranes at least 2 mm thick, which was routed up to the mountain water drainage. In the area of the cross-section with invert vault, the invert was made of waterproof concrete.

The sealing of the tunnel tubes made of plastic sheeting is kept on the portal wall by means of a clamping strip. A vertical seepage layer is arranged at the back of the portal retaining wall.

2.3 Operating facilities, equipment

Operational monitoring and control is provided by the tunnel control room at the south portal and, as part of the overall concept for the crest crossing, by the Suhl freeway maintenance department.

The tunnel tubes have lighting with adaptation sections according to the recommendations of the RABT. The lighting is provided by luminaires arranged on the tunnel ridge, which have been moved approx. 80 cm into the right-hand lane for maintenance reasons. In the area of the adaptation section, the lighting operates according to the negative contrast principle, while the interior lighting and the night lighting operate according to the mixed contrast principle.

Since natural ventilation of the tunnel is not sufficient due to its length, the fire situation that has to be taken into account and the possible two-way traffic operation when a tunnel is closed, mechanical tunnel ventilation was provided.

The tunnel ventilation in normal operation is controlled by means of CO and visual opacity measuring devices located in the tunnel. With three switching stages, the fans can be operated in pairs, adapted to the respective situation, if the adjustable limit values are exceeded. Because of the piston effect caused by the vehicles, the ventilation system generally corresponds to the direction of travel.

The exhaust air is blown out at the portals. The resulting additional immission load in the portal zones does not lead to the permissible limit values being exceeded there.

To avoid ventilation short-circuits, forward-sloping partition walls with a length of approx. 35 m are arranged at both portals between the directional lanes.

A traffic light system is located at both portals to enable the tunnel to be closed at short notice in special cases.

Since the tunnel length of 1,056 m is only slightly above the limit length of 1,050 m specified in the RABT, no breakdown bays were provided. Between the two tubes, two cross passages were arranged at a distance of 360 m which, in conjunction with the lateral emergency walkways, serve as escape and rescue routes and are marked by illuminated escape route markings.

Emergency call niches with fire alarm systems and hand-held fire extinguishers are arranged at intervals of 180 m in the tunnel. Emergency call pillars are located directly at the portals. With regard to a fire protection system that is the same in all tunnels of the A 71 motorway crest crossing, a hydrant line was also laid in this tunnel; the hydrants are arranged opposite the emergency call niches in each case. The direct supply of the pipeline from the public network is not ensured. Therefore, an intermediate storage tank for the required quantity of extinguishing water and a pressure boosting system were provided at the operating building. In addition, a television monitoring system and a radio system were installed in both tunnels.

The tunnel's power supply is provided as a ring feed at the portals via corresponding feed lines from the public power grid. For emergency power supply, a battery system (UPS system) is available in the operations building. The central operations building is located on the east side of the south portal. It has been integrated into the forebay slope in such a way that only the front façade is visible to the street side. The design corresponds to the specified architectural concept for the crest crossing.

3. Construction method

The Hochwald Tunnel was excavated over a length of 1,040 m using the blasting method. In the area of the north portal and the south portal, sections 8.5 m and 7.5 m long, respectively, were excavated in the cut-and-cover method. The embankments of the temporary pilot cuts were secured, as far as statically necessary, with shotcrete, structural steel mesh, soil nails and soil/rock anchors.

Excavation was carried out by drilling and blasting in a rock-saving method. For the respective construction stages, the incidental mountain water was collected and drained off via on-site drainage pipes or invert drainage systems.

After construction of the preliminary cuts and the air arch sections at the tunnel stopes, excavation and securing was carried out for the area of the closed construction method. For this purpose, the cavity was first excavated in sections by blasting and temporarily secured, as required, with a combined system of shotcrete, structural steel mesh, rock bolts and steel arches.

The rock conditions did not permit full excavation of the cross-section, at least in some sections. For this reason, the excavation classification was basically based on a subdivision of the cross-section into calotte, hawser and invert. In the area of the cross-section extension at the north portal and in areas with unfavorable rock conditions (e.g. stopes at the south portal), a further subdivision of the calotte cross-section into a calotte half ahead and a calotte half behind at a short distance was necessary.

Excavation and securing of the tunnel was carried out in ascending excavation from the south portal in order to keep the costs of dewatering low in view of the mountain water accumulation and to bring the excavated material to the reprocessing plant by a short route. Starting with the east tunnel, the two tunnel tubes were excavated in parallel and staggered.

Where rock conditions required, the deeper invert was excavated in a further operation at a short distance from the bench excavation and the invert vault was installed immediately afterwards to ensure a rapid ring closure.

Subsequently, the waterproofing system consisting of plastic sheeting was applied in the vault area, followed by the installation of the concrete inner shell as the load-bearing system for the final state. The concrete of the inner shell was produced in sections of 10 m each using a mobile formwork carriage. In the area of the cut-and-cover method, the tunnel lining was produced with counter formwork. In order to minimize shrinkage of the freshly concreted sections, concrete with a low tendency to crack was produced by adding fly ash to the cement. Following the concrete formulations optimized for this purpose, curing carriages were additionally used behind the formwork carriage in accordance with the requirements of ZTV-Tunnel, Part 1.

Subsequently, the remaining structural work for the interior finishing and the road construction work in the tunnel were carried out.

4. Literature

[1] BAB A 71 Erfurt - Schweinfurt Tunnel Alte Burg und Hochwald. Faltblatt der DEGES, September 1998

 

 

  • Country: Germany
  • Region: Thuringia
  • Tunnel utilization: Traffic
  • Type of utilization: Road tunnel
  • Client: DEGES Deutsche Einheit Fernstraßenplanungs- und -bau GmbH
  • Consulting Engineer: EDR GmbH
  • Contractor: Walter Bau AG, Jäger Baugesellschaft mbH
  • Main construction method: Trenchless
  • Type of excavation: Drill-and-blast
  • Lining: Concrete formwork
  • No. of tubes: 2
  • Tunnel total length: 2 x 1056 m
  • Contract Volume: 23.5 Mio. Euro
  • Construction start/end: 07/998 till 11/2001
  • Opening: 2003