1. General

The Losenberg Tunnel is part of the expansion of the B 480 federal highway in North Rhine-Westphalia. The B 480 is a supra-regional north-south connection that runs from the A 44 in Wünnenberg-Haaren via Brilon, Olsberg, Winterberg, Bad Berieburg and ends south at the B 62. Due to the high volume of traffic, the existing Olsberg through road was heavily over-zoned and was to be replaced by a bypass (B 480n). As a result of the variant investigation, the new construction variant with an under-tunneling of the Losenberg was found to be the most favorable solution for topographical, traffic, ecological and economic reasons. The tunnel had to pass under the K 15 county road and a service road, and the effects on the residential area not far from the tunnel route had to be taken into account. A traffic load of 9,000 vehicles per day is forecast for the tunnel.

2. Construction design

2.1 Geology

At the tunnel site, clay with rock debris is initially present under a thin layer of topsoil. Underneath, clay shales and claystones of the Devonian period are found, which were encountered almost unweathered from a depth of approx. 2.0 m. The excavations were carried out at the site of the tunnel. The ground investigations showed that the rock near the surface can only be loosened by loosening blasting or by using heavy chisels. There is no aquifer at the level of the tunnel. The access of fissure and seepage water was to be expected.

2.2 Structure design

The Losenberg Tunnel is designed as a two-way tunnel with one directional lane each way. Its covered length is 123.5 m. In relation to the base points of the portals, it has a length of 153.5m. The two lanes have a width of 3.50 m each, plus a shoulder of 0.25 m each. Emergency walkways of 1.00 m width are provided on both sides. In total, the clearance gauge is 9.50 m x 4.50 m. However, a height of 4.70 m must be maintained above the roadway. The longitudinal gradient in the tunnel is approx. 2% at the north portal and increases to 5% up to the south portal. The roadway has a constant cross slope of 4% from the north portal to about the middle of the tunnel. Up to the south portal, the cross slope decreases continuously to about 2.5%. According to the planning approval, the tunnel structure was planned as a closed frame using the cut-and-cover method. However, a feasibility study commissioned during the design planning phase revealed considerable problems with regard to noise protection for residents in the nearby built-up area. Excavation of the excavation pit would have required loosening blasting, combined with an extremely large number of perforation holes, the execution of which would have made it virtually impossible to comply with noise immission values and vibrations. In addition, the construction of the open excavation pit would have required the relocation of the K 15 district road during construction and the closure of a service road. For these reasons, preference was ultimately given to the construction of the tunnel by means of mining. The tunnel was built in a closed construction method as a double-shell vault construction with an outer shell of shotcrete and an inner shell of water-impermeable reinforced concrete. The thickness of the inner shell is 40 cm in the standard cross-section. In the area of the underpass of the K 15 and at the transitions to the portals, it will be increased to 50 cm. The standard cross section has a clear width of 10.80 m. The ridge height from the upper edge of the carriageway to the lower edge of the inner lining is 6.10m. The construction was carried out using the mining method in 13 blocks of 9.5 m standard length. The block joints were designed as compression joints. The tunnel has a circumferential sealing made of 3 mm thick plastic sealing sheets, which ends at the block joints to the portals. The maximum cover over the tunnel is about 10 m. It decreases towards the portals. It decreases towards the portals. The portals are constructed using the cut-and-cover method and are made of waterproof concrete as so-called collar portals.

2.3 Tunnel equipment

Lighting

The tunnel is equipped with counter-beam lighting.

Emergency call facilities

Emergency call facilities are available at both portals.

Extinguishing water supply

A DN 150 extinguishing water pipeline is installed in the tunnel as a dry line. The extinguishing water supply comes from the public mains at a pressure of 5 bar. There are underground hydrants about 20 m in front of each portal. The lack of pressure is compensated by the fire department's equipment.

Drainage systems

The drainage system consists of a slotted channel through which the water passes via cross pipes and inspection shafts to the longitudinal tunnel drainage (DN 300) under the carriageway. From there, it leads to a separate emergency basin. The size of the emergency basin is designed for an emergency with approx. 72 m3 of extinguishing water and 30 m3 tank content of a truck. The DN 500 roadway drainage system is routed through the tunnel in a closed system without an inlet and ends in an adjacent light liquid separator.

Noise protection

For noise protection reasons, the tunnel is clad with highly absorbent aluminum panels along its entire length.

An operations building with a lay-by is located at the south portal.

3 Construction

3.1 General

Construction work on the tunnel began in the summer of 2008. The tunnel was driven in a downward direction from south to north, which is rather unusual from a geotechnical and construction point of view. But in this way it was possible to install the excavated material from the tunnel in the embankment in front of the south portal without having to transport the material via the public road network. Two special features had to be taken into account during the mining operation. Firstly, the K 15 district road had to be driven under at a distance of only 1.50 m from the tunnel crest to the upper edge of the road. Secondly, the nearby residential buildings were not to be damaged during blasting.

3.2 Undercutting the K 15 county road

The K 15 road crosses the tunnel route not far from the south portal. Since it was not possible to designate a detour route for buses and heavy goods traffic, the undercutting of the K 15 had to be mastered while traffic was still moving. The extremely small distance of 1.50 m from the tunnel ridge to the upper edge of the road required special measures. To secure the road and the tunnel cross-section, a pipe screen consisting of 46 pipes with drill pipe spacings of ^ 30 cm and a length of 20 m was constructed from the southern stop wall. At this point, the stop wall of the tunnel was only 2 m away from the edge of the K 15 carriageway. After all the pipes had been drilled, a force-locking bond had to be created between the pipes and the surrounding rock. For this purpose, a cement suspension was carefully injected into the approximately 2 - 3 mm wide annular gap between the pipe and the borehole wall. During the subsequent excavation, no wide advance of the calotte base was permitted, since the tunnel cross-section was to be secured as quickly as possible by annular closure of the 30 cm thick shotcrete lining. After 75 cm of cut-off length of the calotte, the tunnel face was secured with 7 cm of shotcrete and 12 m long tunnel face anchors. During this phase, the K 15 was only accessible on one side at a maximum speed of 30 km/h, with the left or right lane being open depending on the progress of the tunneling operation. In order to realistically represent this critical phase of driving, a 3-D finite element simulation was carried out. On the basis of this, a realistic FE calculation was possible, which also covered the interactions between the pipe screen, excavation and securing and made it possible to predict the maximum settlements to be expected. The measurements carried out during execution confirmed the calculation results.

3.3 Blasting in the case of nearby development

The Losenberg tunnel runs close to residential areas. Approximately in the middle of the tunnel, the closest approach is reached with a distance of only 12m. The tunnel tubes were driven mainly by drilling and blasting. In order to avoid damage to the existing buildings, the client was required to prove compliance with the reference values according to DIN 4150, Part 3, by means of vibration measurements. For this purpose, geophones with 3-component transducers were installed in all affected houses to measure the blasting vibrations and blasting tests were carried out. The blast vibration measurements were continuously recorded and evaluated. Accordingly, the loading quantities per ignition time stage were then determined. As a result of these measures, the goal of avoiding damage to buildings was almost achieved.

The tunnel was cut through in December 2008. Traffic was opened in 2010 together with the inauguration of the B480n bypass.

4. Literature

[1] Buddenkotte, G.; Werfling, J.; Wittke, W.: Bau des Tunnels Losenberg im Zuge der Ortsumfahrung Olsberg (B 480n), geotechnik 32(2009) Nr. 4

[2] Spang, Christian; Schiller, Mathias: Losenbergtunnel: 3-D Simulation eines Rohrschirmvortriebs

 

• Region: Olsberg, Nordrhein-Westfalen
• Tunnel use: Road
• Client: Land NRW repr. by Landesbetrieb Straßenbau NRW
• Consultants: WBI Beratende Ingenieure GmbH, Dr. Spang Ingenieurgesellschaft für das Bauwesen, Geologie und Umwelttechnik mbH
• Contractors: Alfred Kunz Untertagebau, München
• Total length: 123,5 m
• Clear width: 9,5 m
• Contract value: 5,8 Mio. Euro
• Construction time: 08/2008 to 11/2009 (16 month)