TUNNELLING is about to start on a key part of the SFr7.4bn (£3bn) Bahn 2000 high-speed railway programme in Switzerland.
The Bahn 2000 plan aims to separate high-speed intercity services from slower local and heavy freight traffic. Large sections of the new line will run through tunnels because of the mountainous terrain, eliminating steep gradients and tight curves that restrict speeds on the existing lines.
One of the largest and most important links is the 45km long Mattstetten to Rothrist link on the intercity line between Zurich and Bern. At present, the 100km journey between the two cities takes 72 minutes with train speeds of about 120km/h.A third of the new line will run through eight tunnels, allowing trains to travel at up to 200km/h, cutting about 15 minutes off the journey.
The 4.7km Murgenthal tunnel through the Aarburg is the longest. This is a single tube, double track hard rock tunnel which will be excavated using an Herrenknecht TBM.
The Onzberg tunnel is also a single tube, double track tunnel and, at 3.1km, is the second longest on the route. It will be excavated using a 12.35m diameter Herrenknecht Mixshield TBM, which has been working on the soft ground section of the Zurich-Thalwil rail tunnel, also part of the Bahn 2000 programme.
The TBM is due to arrive on site this month and will begin tunnelling in September.
The machine will excavate most of the tunnel in open mode through good quality dry Molasse (conglomerate and sandstone) at an anticipated rate of about 400m a month. It will work two eight-hour shifts a day, five days a week, and erect the precast concrete segmental lining as it proceeds.
However, the last 300m of the tunnel is through soft, saturated moraine deposits beneath a small village, with only 7m cover.Here the TBM will revert to the closed pressurised bentonite slurry mode and progress at about 174m a month.
Work at Onzberg has been concentrating on completing the shotcrete-supported open cut approaches to the tunnel portal and the 65m long TBM starter tunnel through weaker material. The TBM will be launched at a point where there is at least 1m of rock outside its 12.35m diameter profile.
A high strength steel fibre wet mix shotcrete is being used, applied with a new Aliva AL-500 mobile machine which carries 7m 3of shotcrete for one cycle.The shotcrete is designed to reach a specified toughness of 800J, a required early strength of 4N/cm 2within six hours and a minimum 28-day strength of 44N/cm 2.At the eastern end of the project, two spur tunnels will connect the track to a new branch line for the town of Solothurn.These single-track tunnels are 590m and 400m long with 65m 2cross sections.They are also shotcrete supported and will be excavated in a bench sequence using an Eickhoff roadheader.The longer of the two tunnels passes up and over the main line tunnel.
Tunnelling is due for completion by the end on 2003, with trains running from 2004.