Tunnelling is a critical part of Switzerland’s SFr7.4bn ($4.26bn) Bahn 2000 investment aimed at improving intercity rail services, to establish an ‘on-the-hour’ schedule at interconnecting stations and provide smooth transfer on to other intercity and international European routes – without long connection delays.
The major tunnelling projects are on dedicated high-speed lines designed to separate intercity services from slower, local and heavy freight train traffic, as well as eliminate the steep gradients or tight curves that impede train speeds on existing intercity lines.
Fast track
One of the largest of these projects is the 45km long Mattstetten-Rothrist link on the intercity line Zurich-Olten-Bern. At present, the 100km journey between Zurich and Bern takes 72 minutes with train speeds of about 120km/h. Once completed, the new line with some 30% running in eight tunnels will allow intercity traffic to maintain speeds of up to 200km/h, cutting about 15 minutes off the journey.
The longest tunnel, the 4.7km Murgenthal tunnel through the Aarburg, is a single-tube, double-track, hard rock tunnel excavated using the same Herrenknecht TBM that worked originally on the Bözberg highway tunnel in Switzerland.
Another single-tube double-track facility, a tunnel 3.1km long through the Önzberg, will be excavated using the 12.35m diameter Herrenknecht Mixshield that is currently completing the soft ground section of the Zurich-Thalwil rail tunnel, also part of the Bahn 2000 programme.
The machine was expected to arrive at Önzberg in March 2001 but is delayed by a large glacial boulder in the tunnel face. It is now scheduled to arrive this month and start boring in September. It will complete most of the tunnel in the open mode through good quality dry molasse and at an anticipated average of about 400m/month, erecting precast concrete segmental lining as it goes. The last 300m is through soft wet moraine beneath a small village and with a limited cover of only 7m. For this section, the Mixshield will revert to the closed pressurised bentonite slurry mode and progress at about 174m/month.
Current work at Önzberg, however, concentrates on completing the shotcrete supported excavations. The slopes of an open cut excavation leading to the main tunnel portal are shotcrete supported, as is the 65m long TBM starter tunnel through weaker portal material. The machine will be launched at a point where there is at least 1m of rock outside the 12m diameter TBM profile.
Branch line
In addition, there are two spur tunnels at the east end of the project that will connect the new line to a new branch line to the town of Solothurn. These two single-track tunnels, 590m and 400m long, are about 65m² in cross section and are also shotcrete supported. They will be excavated on a top-heading and bench sequence using an Eickhoff roadheader. The longer of the two tunnels passes up and over the main line tunnel.
The shotcrete used at Önzberg is a specified steel-fibre reinforced wet-mix shotcrete designed to reach high early strengths.
Equipment includes a new AL-500 mobile shotcreting unit from shotcrete specialists Aliva. The fully equipped AL-500 unit is fitted with an Aliva AL-302 boom, an AL-403 dosing unit for accurate PLC-controlled dispensing of the specified alkali-free liquid accelerator, and a protective cage for the on-board accelerator container. The smaller AL-302 nozzle boom, with a reach of some 8m, manages the faces of the TBM starter tunnel and is compact and effective in the top heading of the 7.5m wide x 9.5m high branch line spur tunnels.
To apply the wet mix shotcrete, the joint venture has chosen an Aliva Duplo AL-285 rotor pump. “We selected the Duplo because the volume of shotcrete in each shotcreting cycle is relatively small at about 7m³,” explained Ing Frey, the JV’s project manager for the shotcreting works. “With the Duplo, we avoid the long cleaning process that is needed at the end of shotcreting with large dense-stream concrete piston pumps. The compressed air function of the Duplo rotor pump clears the line of concrete completely and at the end of each shotcreting cycle, and there is also no need for priming the system before starting the cycle.”
In the mix
The concrete mix for the high early strength wet-mix shotcrete required at Önzberg is based on a cement volume of 425kg/m³, a water:cement ratio of 0.48, a 1.2% dose of superplasticiser Sikament B-10, and an injection of liquid alkali-free accelerator Sigunit L60, fed accurately into the thin stream shotcrete by the AL-403 dosing unit at the nozzle. The 40kg/m³ of Dramix steel fibre reinforcement is added to the wet mix shotcrete at the on-site batching plant, as is the superplasticiser. Routine tests of the shotcrete confirm the specified toughness of 800 joules, the required early strength of 4N/cm² within six hours, and the minimum 28-day strength of 44N/cm².
“Arrival of the TBM from Thalwil is now unfortunately delayed for about two months but despite this we are bound to complete this project by the end of 2003,” said Ing Böckli, project manager for the construction joint venture, led by Zschokke Locher and including Prader, Murer, CSC, and Wayss & Freytag. “The segment fabrication facilities for the TBM drive have already been transferred from Thalwil,” said Böckli, “and segment production is scheduled to start in March. The TBM is now expected to arrive in the spring and start boring by September 2001.”
Completion of the tunnel by the end of 2003 is essential to allow electrical and mechanical equipment to be installed during 2004, and to have this new line, and all new lines in the Bahn 2000 programme, ready to be incorporated in the new pan-European high-speed railway timetable scheduled for introduction at the start of 2004.
