The Hirschhagen Highway Tunnel (BAB 44) in Germany: Pre-Support in extremely difficult and inhomogeneous ground conditions

VOLKMANN Günther M. DSI – DYWIDAG-Systems International GmbH, Pasching, Austria.


The construction of the BAB 44 motorway between Kassel and Herleshausen (both Germany) is an important part of the trans-European road network. After its completion, the A 44 will permit an efficient connection of the Benelux States in the West and Poland in the East. In the section between Eschenstruth and Hessisch Lichtenau the Hirschhagen Tunnel is constructed with a length of 4.1km. Most sections of the tunnel are situated in weak ground conditions. Hence a combination of face bolts with spiles (short forepoling) or pipe umbrellas (long forepoling) additionally support the tunnel heading on a regular basis This article will focus on the application of a newly developed method to connect the pipe umbrella support, which is mostly installed by conventional drilling machines nowadays. The installation method limits the outer diameter to a maximum of 168mm and a pipe wall thickness of 12.5mm because workers must be able to handle pipes and its drill steel. Typically these pipes are connected with threads that create a weak link in the support system so during the last decade alternative connection systems were developed to overcome this disadvantage. One of these alternative coupling systems is the so called Squeezed Connection, which will be explained in this article followed by a discussion of its advantages and disadvantages regarding technical specifications, construction time, and safety issues.

The Hirschhagen Highway Tunnel

The German highway A 44 directs from Aachen through the Ruhrgebiet to Kassel (all Germany) and it is extended to Herleshausen (Germany). The new section connects the A 7 to the A 4 after completion. This upgrades the east-west highway connection starting at the Benelux Union and heading to Poland so it is an important part of the trans-European road network. The newly constructed section has a total length of approximately 63 km and comprises 13 tunnels. All tunnels are constructed as double tube tunnels with 2 lanes and emergency sidewalks on both sides and 14 emergency cross passages. The Tunnel Hirschhagen starts in the north of Eschenstruth and ends near Hessisch Lichtenau (Figure 1). The length of the northern tube is 4,147 m and that one of the southern tube is 4,204 m. So it is the longest tunnel of the A 44 and the second longest highway tunnel in Germany when it is put into operation. On its alignment it crosses underground the Losse valley with the state road 7, the streetcar Kassel-Hessisch Lichtenau and the Losse stream. The cover above crown varies from 10 m to 95m and it is the lowest when crossing the Losse stream.

The Hirschhagen Highway Tunnel (BAB 44) in Germany: Pre-Support in extremely difficult and inhomogeneous ground conditions

Figure 1. Site plan of the Tunnel Hirschhagen (Rehbein 2015)

Excavation and support method

The excavation of both tubes takes place simultaneously from all 4 portals. The tunnel support is installed in 2 liners; a primary lining consisting of ground dependent measures as well as a shotctrete lining and a reinforced inner lining of in-situ concrete, which is installed when all ground displacements have stopped. Excavation and support are following the concept of the New Austrian Tunneling Method (NATM) and the cross section is divided into top heading, bench, and invert during construction.

Geological conditions

The tunnel alignment is mainly situated in colored sandstones but when crossing Losse valley the tunnel advances through tertiary ground and quaternary granular soil as well as silty gravel are close to the tunnel crown. Additionally quaternary slope loam and layers of debris are located at the portals.

Designed pipe umbrella support

The heading was additionally supported by a pipe umbrella pre-support at those sections where the ground investigations identified the weakest and most unstable ground conditions. The application of pipe umbrellas was planned for crossing under the Losse valley. Additionally the western portals were supported by pipe umbrellas though a deep cut was constructed that was supported by bolted shotcrete and tied-back bored pile walls (Figure 2). In both sections the pipe umbrella was applied in the top heading. The pipe umbrella pipes were 15 m long with an overlap of 3m defined in the drawings so the net excavation length was less than 12 m for one pipe umbrella field (Figure 3). The outer diameter of the pipes was 139.7 mm with a wall thickness of 8 mm (steel grade S355) and the axial distance from pipe to pipe was not longer than 350 mm. The coupling points of the pipes should be staggered to decrease the influence of the weaker standard thread connections. Due to the fact that the pipe umbrellas were installed recurrently the excavation profile was performed in a so-called saw-tooth profile to create the necessary space for the next installation sequence.

The Hirschhagen Highway Tunnel (BAB 44) in Germany: Pre-Support in extremely difficult and inhomogeneous ground conditions

Figure 2. Western portal of tunnel Hirschhagen (south tube) with the necessary support to stabilize the portal cut.

The Hirschhagen Highway Tunnel (BAB 44) in Germany: Pre-Support in extremely difficult and inhomogeneous ground conditions

Figure 3. Typical support pattern for a pipe umbrella supported section.

AT – Pipe Umbrella System

The AT – Pipe Umbrella System was originally developed as pre-support in sequential tunneling to create a supporting umbrella in difficult ground conditions (UCS < 1 MPa). Here, the pipe umbrella support is applied due to the risk of ground shear failures around the heading and to reduce ground subsidence due to the excavation induced stress transfer processes. These points gain in importance because of existing housing and infrastructure on the surface. One precondition in the development of the AT – Pipe Umbrella system was that installation should be possible by normal tunnel crew with machinery available on site. This feature has enabled pipe umbrellas, which at that time were regarded as special measures, to be used more often as a regular support measure (Volkmann et al. 2012).

The Hirschhagen Highway Tunnel (BAB 44) in Germany: Pre-Support in extremely difficult and inhomogeneous ground conditions

Figure 4. System components of the AT – Pipe Umbrella System (Volkmann et al. 2012)

The AT – Pipe Umbrella System essentially consist of the following components (Figure 4):

  • starter unit with a single-use drill bit,
  • extension tubes,
  • end tube (without injection holes and valves).

The drill bit adapter and the drilling rods, which are necessary for force transfer during installation, can be withdrawn after drilling and be reused. The casing pipes remain in the ground after drilling and are ready for use after installation. The usual drilling depths with this system are up to 18 m, although the drilling of holes up to 30 m is possible. The maximum achievable depth does, however, always depend on the properties of the surrounding rock and the requested precision of the pipe alignment.

Installation procedure

The pipes are installed prior to excavation and should be located at the outer perimeter of the following excavation profile. Its installation position must consider emerging pre-settlements due to the excavation process. Thus, it is important that the starting point of the drilling as well as its direction is as exact as possible. Therefore, the starting point as well as the orientation of the drill boom is usually determined by a theodolite before installation.

The Hirschhagen Highway Tunnel (BAB 44) in Germany: Pre-Support in extremely difficult and inhomogeneous ground conditions

Figure 5. AT – Pipe Umbrella System ready for the installation

When using the AT – Pipe Umbrella System, one umbrella pipe consists of single pipe pieces that have a common length of 3 m. While rigging the boom, the first pipe piece (referred to as “starting unit” including the drill bit), and the drill rod are already placed on the boom. Consequently, the boom is brought into the correct drilling position, and the first pipe piece is installed (Figure 5). After installing this first pipe piece, the boom remains in position. The shank adapter is disconnected from the drill rod and moved to a backward position.

The next pipe piece including another drill rod can be placed either manually or automatically on the drill arm. A basket is usually used to carry the pipes when the drill arm is at a high position. As soon as pipe and drill rod are placed on the drill arm, the drill rod as well as the extension tube are connected to the already installed parts as well as to the drifter so the next pipe piece is ready for the drilling process. This process is repeated until the installed pipe has reached its designed length. If the drill jumbo is equipped with two booms, both can of course be used parallel to save time.

Connection types

Different connection types are available on the market these days. Very common is a standard threaded connection. Standard threaded connections are generally not well suited for connecting pipe umbrella pipes. By mechanically removing a certain portion of the steel tube for a thread (Figure 6a), the effective cross-section is reduced. This fact drastically decreases the load-bearing capacity and stiffness in the connection area (Table 1). At most standard pipe umbrella dimensions the maximum elastic moment (design value) of the regular pipe is at the level of the ultimate moment (failure) of the standard threaded connection at laboratory conditions. Hence, to achieve certain given pipe umbrella design parameters, an over-dimensioning of the un-weakened pipe section is a practical way – though highly inefficient – to overcome this limitation.

As a result of dangerous problems when using standard threaded connections during construction, the so called nipple coupling was developed. Nipple connections consist of threaded connection fittings, which are pressed and welded into the ends of standard pipes (figure 6b). This connection type provides an elastic design load as well as stiffness properties equal to an un-weakened pipe (Table 1). By using this connection type, default design parameters are constant over the entire length of installed, connected pipe umbrella drills.

The latest development in the field of pipe connections is the squeezed connection, this connection type results from the attempt to provide a tough and easy-to-connect alternative to conventional threaded systems (figure 7a). By means of the squeezed connection, non-threaded pipe ends are mechanically connected in terms of force-fitted squeezing using a boom-mounted press (figure 7b).

The Hirschhagen Highway Tunnel (BAB 44) in Germany: Pre-Support in extremely difficult and inhomogeneous ground conditions

Figure 6. a) standard threaded connection and b) nipple coupling

The Hirschhagen Highway Tunnel (BAB 44) in Germany: Pre-Support in extremely difficult and inhomogeneous ground conditions

Table 1. Exemplary elastic design values for 114.3 x 6.3 pipes with different connection types. (Volkmann, 2014)

In detail the pipes are delivered to the side with one reduced end that fits into the other unedited end. So after feeding an extension tube with its drill steel to the boom the drill steel is conventionally screwed together and the smaller pipe end is simply moved into the regular pipe end at the squeezing console position. Then the squeezing unit is activated by remote control and the pipes are squeezed together after a 2 seconds long squeezing process. This squeezing process creates a force-fit connection. So there are on one hand less rotating parts, which increases safety and simplifies handling and on the other hand there is no thread, which increases the system security and decreases the coupling time for the installation process.

This connection type features a higher elastic design load in the connection area compared to standard threaded connections (table 1) as well as a slight reduction in stiffness compared to the regular pipe. The ultimate moment in the connection area is at least 50 percent higher than the maximum elastic moment of regular pipes so at correctly designed support cases a failure can be excluded at the coupling area. Besides simple design and handling, this connection type provides operational benefits due to decreased time intervals required for pipe connection. Similar to the nipple connections the squeezed connections also reduce the inner cross-section of the pipe umbrella pipes so an installation in combination with sacrificial, single-face drill bits is required.

The Hirschhagen Highway Tunnel (BAB 44) in Germany: Pre-Support in extremely difficult and inhomogeneous ground conditions

Figure 7. a) squeezed connection and b) squeezing console mounted on a drill boom

AT – Pipe Umbrella System at Tunnel Hirschhagen

Applied pipe umbrella system As mentioned above the tender documents specified pipe umbrella pipes with the dimensions 139.7×8.0 mm in S355 without any definition for the connection itself so a standard thread connection is allowed but the position of the connections must be staggered after installation. This additional definition does not increase the overall bearing capacity significantly because back calculations of in-situ measurements show that the internal moments increase to the peak and decrease to zero over a length of about 4 m (Volkmann and Krenn 2009) and staggered connections are only 1.5 m away from each other. So with this additional definition only the peak values cannot act on all connections at the same time and place respectively but the main portion of the moment acts on all connections in the critical area at the same time.

The Hirschhagen Highway Tunnel (BAB 44) in Germany: Pre-Support in extremely difficult and inhomogeneous ground conditions

Table 2. Comparison of technical values for tendered and executed AT – 139.7 system

When Hessen Mobil – Road and Traffic Mangement awarded the tunnel construction to the Joint Venture Tunnel Hirschhagen (Baresel GmbH and Köster GmbH), it was looked for a cost effective alternative . Ground conditions, which are typical for pipe umbrella applications, are always critical regarding stability and mostly sensitive on settlements so each offered alternative must at least be technically adequate and discussed and agreed with the designer or an owners representative. In the presented case 2 optimization possibilities are possible:

  • Increase of pipe length to 18m
  • Change to another coupling type

An extension of the drilling length results in less installed pipe support per tunnel meter without the necessity to change any other design specification. Additionally less installation interruptions result in a smoother overall construction process. But, it is not possible to install 18m long pipes in all ground conditions so this point must be verified on site beforehand.

A change of the coupling system increases the strength and stiffness of the pipe umbrella support. As a result of this optimization it may be possible to decrease specified outer diameters or wall thicknesses of pipes without losing bearing capacity. This point safes steel weight so it spares power of labor because the pipes are lighter and easier to handle in long working shifts. Last but not least this also has an influence on the safety on site.

In Table 2 a comparison of relevant technical values are presented for two different 139.7 pipes including the influence of its coupling type. In both cases the coupling is the weakest link in the support system but the 139.7×6.3 pipe with the squeezed coupling is not only lighter it is also stiffer and stronger compared to the heavier system. The owner’s representative approved the system after this technical comparison and the newly developed coupling system was successfully applied.

The Hirschhagen Highway Tunnel (BAB 44) in Germany: Pre-Support in extremely difficult and inhomogeneous ground conditions

Figure 8. Installation of pipe umbrella support

Performance of installation

At this project the ground conditions were in some sections much worse than predicted which was countered by using side drifts during top heading excavation (Figure 8). This unexpected ground conditions (Figure 9) also resulted in unexpected problems during installation. The very harsh ground conditions at this project lead to unusual long drilling times and sudden stops of the drilling process. The reason for both issues was the ground; on one hand it was hard to release the material during drilling decreasing the drilling speed and on the other hand the ground was so weak and deformable that the small gap between the ground and the pipe tended to close during installation. So the time frame for a successful installation was very short – in the observed cases about 1.5 hours. For all that the pipe umbrellas could be installed properly and support the following excavation process.

The Hirschhagen Highway Tunnel (BAB 44) in Germany: Pre-Support in extremely difficult and inhomogeneous ground conditions

Figure 9. Examples of unexpected ground conditions during the excavation process

The Hirschhagen Highway Tunnel (BAB 44) in Germany: Pre-Support in extremely difficult and inhomogeneous ground conditions

Figure 10. Detailed documentation of a pipe umbrella installation

In January 2015 the installation process was documented in detail for one pipe umbrella installation. At that pipe umbrella 46 pieces of 18 m long pipes were installed so in total a length of 828 m of pipes. The installation was performed with an Atlas Copco L2E jumbo and the drill arms were equipped with an Atlas Copco 1838ME drifter and a Danfoss 272E rotational motor –common drilling machinery for tunneling – and the AT – Automation Unit for Squeezed Couplings.

A detail of this documentation is presented in Figure 10. Here we can see that the left drill arm installed pipe N° 17 in less than 1:20 hrs and the right drill arm installed parallel to this process pipe N° 6 in less than 1:10 hrs. These times include the cleaning due to the sticky material, positioning of the drill rig before the installation and also the uninstallation of the drill steel and the adapter, which can be reused so the entire installation process for a pipe is shown. Depending on its drilling depth and the ground ahead of the face the 3 m pieces could be installed in 2:45 min to 10:00 min. This variation is much higher than normal confirming the harsh ground conditions. The necessary times to disconnect the drill steel and elongate the drill string varied between 1:30 min and 6:50 min. The higher elongation times only appeared when there was a conflict with elongation on the second drill arm so these times stay very small.

The entire pipe umbrella was installed after nearly 67 working hours of drill arms (excluding nonprocess related times) and it was drilled with 2 drill arms so this pipe umbrella was theoretically installed in 33 hrs (25.1 m pipe installation per hrs including all related times). In reality one drill arm was faster so the installation was finished after 35 hrs and 10 min. This result in an average installation speed of 0.4 m per minute including all manipulation times related to the installation process.

Main advantages of the squeezed connection

  • Installation with conventional drill jumbos equipped with a squeezing console
  • Execution of pipe umbrella drilling with on-site personnel
  • Simple and robust system components
  • Higher maximum elastic moment compared to standard threaded connections
  • Clearly higher maximum ultimate load compared to the design load of normal pipes
  • Less moving / rotating parts during elongation
  • Easy handling
  • No risk for damaging threads due to handling on site
  • No risk for jamming threads during coupling processes
  • Dirt insensitive
  • Quick coupling times
  • Faster construction of a pipe umbrella support system


The 2nd longest German Tunnel Hischhagen is under construction since 2013. Since the beginning more than 50,000 meters of AT – Pipe Umbrella pipes with Squeezed Connection were installed and the following tunnel excavation safely executed. This successful application demonstrates that the newly developed Squeezed Connection in combination with the AT – Automation Unit improve both the structural behavior as well as the construction performance of pipe umbrella support. It ensures quicker overall installation times by a higher grade of mechanization, higher safety for the involved workforce by less rotating parts and less heavier pieces to handle and last but not least a high cost efficiency of the installed pipes as support element.

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