Creating the first underground train system in the German city of Karlsruhe is not straightforward. The high groundwater level, the sand and gravel strata, the shallow depth of tunnelling and the requirement to keep the existing metro running above, meant that one solution did not fit all on this particular scheme. “I think we are using almost all tunnelling technologies that exist in Karlsruhe,” remarks Harald Burgstaller, project manager for the tunnelling contractor BeMo, under the EUR 295M (USD 331M) contract awarded to ARGE Stadtbahntunnel Karlsruhe. “We are building seven stations using the cut and cover method, the main tunnel is a hydroshield, and we are building a conventional tunnel under compressed air,” he says.
Starting construction in October 2010 the project began with station construction at Europaplatz before bringing in the 9.3m diameter Herrenknecht Mixshield machine in July 2014. With groundwater at just 4m below ground level the machine was the most appropriate choice for supporting the tunnel face during construction. In this hydroshield model the excavation chamber is divided by a submerged wall which at the front is filled with a suspension to give better face support.
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The rear section is filled with suspension to just above the machine axis and counter pressure from the face comes from a compressible air cushion in this upper area. As a result, the machine can withstand heterogeneous geologies and high water pressures of more than 15 bar. The 90m-long machine was set up at the easternmost station Durlacher Tor and space was constrained. “We needed a huge set up. To mix the bentonite, to treat the wastewater, different storage areas–all this was on the surface of the station,” says Burgstaller.
There was only space to store 30 of the steel reinforced tunnel lining segments at any one time making the logistics crucial and trucks carrying the segments from Glass Gmbh in southwest Germany arrived daily.
By November 2014 the machine was ready to begin its 2.05km bore placing the 2m-long segments in a 6+0 arrangement creating a final inner lining of 8.2m. Face support pressure varied between 0.5 and 1.1bar and thanks to the shallow depth risk of blowout was a key challenge. Running beneath the busy city centre of Karlsruhe the overburden ranged from just 4.9m at the east to 6.9m as it progressed westwards. “We are really shallow. We have extensive monitoring,” says Burgstaller.
In charge of this extensive monitoring was BeMo chief of monitoring Ulf Tetzlaff. “We had levelling bowls on many houses, all houses we mounted prisms for automatic total station monitoring. We have points in the ground automatically monitored by total stations, we have special vehicles for the tracks,” he explains, noting that the car would drive over the existing tramway every three hours during the TBM drive.
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By GlobalData“Then we have a lot of extensometers at different cross sections all over the city to see if there is any deformation in the ground based on the tunnel excavation. We have a lot of inclinometer points in the ground also for the tunnel areas but even more around all the pits or sheet piled walls.”
These inclinometers came from Germany’s Gloetzl whereas the total station monitoring was supplied by Leica. All this data, including some manual monitoring information, was fed into an IRIS tunnel software package from VMT/ ITC Engineering. This then collated the information and sounded alarms if the movement hit certain trigger levels calculated using finite element modelling. Manual data entry came from areas such as building cellars adjacent to the tunnel.
“We observed if there was any damage or differences to the day before,” says Burgstaller. “The pre-calculated worst case scenario movement was 16cm. This would be catastrophic. But the maximum movement that we had was just 5mm along the axis of the tunnel. In reality we were much better than the calculation.”
As a precaution the team undertook grout and gel stabilisation to support the buildings identified by client KASIG as being the most vulnerable. “We had five buildings which were really sensitive we did the grouting and on the rest just the monitoring. I would guess around 450m length of buildings, all next to the hydroshield, were protected in this way,” says Burgstaller.
Although the tunnelling was completed on time by September 2015, achieving a maximum rate of 32m in one day, the bore did have some setbacks. An incident at the Lammstraße station held up tunnelling for three months. “The station is so small as we advanced the groundwater level increased and we had a change of geology, an old unexpected channel, so in total this was too much for the balance at the front of the shield and the face collapsed,” says Burgstaller. Fortunately, the concrete station roof had already been constructed protecting the buildings above so there were no adverse effects at the surface. To rescue the situation the team – after getting access under the concrete lap – had to, fill up the weld puddle, repair some pipes in the digging chamber and unblock cutting wheel.
Just when tunnelling was back on track a second incident caused another problem. “At the end of the station we had a diaphragm wall and on the outside of the station we had a high cement injection block as additional measure to ensure that when the machine comes from the station out to the normal ground, nothing happened. This block had a leak. So, we had a blow out,” says Burgstaller.
Fortunately, the site team had noticed some ground movement on the road above and closed it to traffic just in the moment the blow out erupted. This time the delay was only two weeks. The work recommenced after the team unblocked the machine with a maximum pressure water jet, using the same method as before.
UNDER PRESSURE
Less eventful has been the compressed air tunnelling which is taking place along the north-south line running from the Marktplatz in the north where the lines meet, down to Ettlinger Tor station. Known as the Karl-Friedrich Strasse, this section consists of a 200m regular cross section but then widens over a 50m length. “Of course this is much slower advance. The distance is too short to use the hydroshield machine and the geometry of the tunnels is changing. We start with 80m2 cross section and we end with 180m2,” says Burgstaller.
Once again the low overburden presented the risk of blowout and the team also wanted to limit the potential for air loss so this section of tunnelling ground reinforcement was carried out around a 2m circumference with cement in the first phase and gel in the second. “We had a maximum loss of compressed air at 500m3/minute. But we expected just 140m3/minutes so we had to employ more compressors to commit the amount of compressed air that we needed. The ratio would have been much worse if we had not done this injection,” says Burgstaller.
Using traditional NATM for the majority of the works the tunnel required a 300mm shotcrete lining reinforced with 50 to 60, 4m long, 32mm diameter spiles with 150mm spacings. A lattice girder is also placed every metre. For face support 10m long 12stk face bolts were inserted. The permanent lining to create the 8.2m diameter tunnel varied from 450mm to 1300mm of reinforced concrete lining using concrete of strength C30/37 At the enlarged section the tunnel runs beneath a historic sewer with an overburden of just 1m an alternative methodology was required to protect this piece of vital and historic infrastructure. “Under the sewer we did the pipe arch umbrella with steel pipes injected with gel and cement to stabilise the ground,” explains Burgstaller. “And then because of the thick cross section we excavated in 4 main phases with 8 sections in each phase. Each phase took several weeks,” he says (see diagram page 25).
Each phase meant excavating a section of around 40m in length. The primary lining included 400mm of reinforced shotcrete with face bolts. The final lining here is between 500mm and 2500mm reinforced C30/37 concrete. “In this section at the moment we have about 200 people because we are doing the advance of the invert and the inner lining,” says Burgstaller. “We also have to consider that under compressed air we have reduced working time because of the heavy work.”
With just 60m to go construction is almost complete on this section now. Looking back on this highly complex scheme Burgstaller says that the biggest challenges were not technical but about balancing the needs of all of the stakeholders affected by this inner city project. “There are many parties, organisations and companies that have interests here and to find solutions which are OK for everybody is the most challenging thing”.
