The new Zurich Thalwil tunnel lies on the same high-speed rail route south as the AlpTransit, although it is not part of the project. Eventually it will be part of the Gotthard pass TGV line, since it forms a new fast route out of Zurich’s important central station.
An old shoreside line along the lake will be used by the expanded local train service, while the new line will add to mainline train capacity. The Thalwil tunnel scheme is part of the Bahn 2000 upgrades now under way in Swtizerland to improve the entire rail network.
The tunnel has four sections, and each is formed by a different method. There is cut and cover work near the central station, being carried out by Swiss Bati Group, which is experienced in diaphragm walls.
There are also three sections of bored tunnel, all undertaken by the Tunnel Zürich-Thalwil consortium, a seven-firm group led by Zschokke Locher which includes Germany’s Wayss & Freitag. This has two contracts, one for the main length of SFr210M ($118M) and one for a link tunnel at the end for SFr70M ($39.3M).
Under the bigger contract, in the north, a Mixshield TBM is just reaching the difficult river and lake deposits that underlie the lower urban reaches of the city. It was recently switched to closed face mode, having initially driven north through good rock.
The machine will finish the last 700m of its 2.7km drive with limited cover and must pass close under buildings where underpinning preparation has been necessary. The water table is high.
A standard hard-rock TBM, another Herrenknecht, has made the second and longest drive, going 5.7km southwards from the central construction camp, where two main access shafts were built for both drives at the beginning of the contract. Like the Mixshield, it produced a 12m-diameter tunnel, suitable for double-track train operation. A flat fill floor is formed as the machine progresses, with drainage pipes and a membrane waterproofing.
Beyond the TBM’s perhaps record-speed drive, two single-track tunnels are being formed by drill and blast, to curve the line up and over to make a junction with the old line by the lake. Trains will run on the old line for the time being.
“Eventually it is planned that the Zimmerberg tunnel should continue on from here,” says Coni Scheifele, now a Herrenknecht engineer, though he has only just joined the firm. He was a contractor for 20 years and the project manager for the southern drive on this scheme until last year.
Zimmerberg will not happen until 2006 and will extend the Thalwil tunnel by another 11km, south and slightly westwards, forming the top end of the Gotthard route for AlpTransit.
Scheifele would have preferred the whole length to be made now, since the southen Thalwil drive went so well. “Geological conditions are molasse, a mixture of sandstone and marl, and is the best rock you can imagine for TBM work.”
The consortium knows it well, he says, which is why it ordered a hard-rock machine which was not over powered. It uses just 110kW motors, 14 with direct electrical drive.” And there was no continuously variable drive, just two speeds of 1.95 or 3.9 rev/min.”
The cutterhead face carried 76 cutters to work on the rock, which is medium-strength, varying from 20–80N/mm2. It used scoops mounted across, so that material does not have to be picked up after falling to the bottom of the face.
It was assembled in the southern of the two 30m-deep shafts at the central worksite, built after the contract was awarded in 1998. The site, close to a motorway, has a railhead for muck disposal and supplies.
The back-up sections of the machine were lowered into the 20m diameter northern shaft, 75m away, and were added on as an initial 325m drive was made, using only invert segments. The TBM started work in September, driving south and erecting a 300mm-thick preliminary precast segment lining as it went, for temporary support. Just five 1.7m-long segments were used “and a best erection cycle time was eight minutes”, says Scheifele.
The lining only needed grouting in the base. A pea gravel fill – a size between 8mm and 16mm – was used to fill the annulus above.
Progress was good and a maximum daily drive of 40.8m was achieved, a maximum of 170m for a 5-day week and 673m for a month. Scheifele says that the consortium thought hard about adding in a fifth segment form, because production could not keep up. It decided on two interludes, allowing the teams to take a holiday and other breaks. “There was no bonus for a faster completion.”
The client Swiss Railway’s insistence on rail-only spoil disposal proved a boon, he says. One train takes 1,000 tonnes and with two TBMs going. Some 14 were required daily at peak, and another three moved in aggregate supplies for the batching plant and the segment makers. The work camp site concentrates all these facilities in the one place.
“That would have translated into over 900 trucks on the roads here,” he says. Spoil from the tunnels was moved by conveyor, which was supplied by Svedala of Sweden via Marti Mossedorf.
A neat, clean hole is all that is left of the machine now, which was disassembled within the tunnel. “You will rarely see a sight like that,” says Sheifele, who wanted the Zimmerberg work to continue straight away.
Instead, a standard drill and blast operation is progressing on the single-track link tunnels, one curving off for a 600m length and the other taking the far line up over the route on a longer, 1.3km path. Both link up again for a short section before joining the old route.
A couple of additional headings have been added to speed up the time-critical work, says Sheifele. The consortium was unable to work the long shifts it had hoped because of restrictions on blasting vibration in a residential area.
Meanwhile, the Mixshield has been driving steadily north since being assembled in March last year. Despite being a more powerful machine, with five 400-kW motors and hydrostatic drive, progress has been less spectacular. Sheifele attributes this to a much more complex ring structure. Seven gasketed precast rings and a key have to be fitted for each push forwards and it can take a minimum of 40 minutes to do that. Like the other section, an internal lining will be added.
The Mixshield was recently switched from closed face mode, with steel plates sealing the head chamber entrances to open bentonite filled operation. Behind it, only the chain hangers for the conveyor system remain, following a changeover to a bentonite pipeline.
The ground ahead is difficult lake gravels and sands, and full of water, explains client SBB’s project engineer, Hansruedi Müller. “That is why we selected the Mixshield concept from the beginning.”
It also underlies numerous buildings. And since the tunnel ascends to the station, the cover also diminishes, dropping from 30m to no more than 3m at the end of the drive. Substantial support measures are needed.
“So, while the machine has been on the rock section we have been preparing this area,” says Müller. Support work involves creating a covering layer over the tunnel, using micro cement grout injection for the 3–4m thick blanket. It was later pressure-tested to destruction, using a 1.5m diameter borehole and hydraulic jacks.
Grouting was carried out by Italian firm Rodio, operating from a small pilot tunnel stretching some 470m from the transition point where the soft ground begins. This proved problematic to excavate: the small, second-hand Hydroshield TBM found the big 0.5m³ boulders difficult to swallow along the route.
After pushing through some 239m of the 3.72m-tunnel, progress had dropped to just 800mm a day. It was decided to take out the machine and finish the tunnel conventionally. Part of the machine had to be left in the ground because the tunnel is blind and access too difficult for a return to surface at the intermediate entry shaft made for the project.
At some points there were also grouted ground “columns” formed on either side of the tunnel line to take the loads. Buildings above could be sensitive, and some were 100 years old.
Special measures were required towards the city centre for a modern office building with a three-storey basement. Here, a series of mini-piles have been driven around one corner of the six-storey Weststrase building to support a “frame” of prestressed concrete beams, formed alongside the lower two levels of the basement.
At the far end of the drive an ‘umbrella’ has been formed for the last 130m over the tunnel line, using pipejacked concrete tubes. A total of ten 1.55-m tubes with a wall thickness of 150mm were installed by subcontractor GU Tiefbau between October 1998 and November last year.
For all these measures SBB has used expert advisers to supplement the design skills of its main consultant team BBPS, comprising Basler & Hofmann, Balestra, F Preisag and SNZ.
“We have consulted with Professor Lunardi of Italy, and Professor Kalman Kovari from the federal Institute of Technology, as well as Professor C Cava. French and German experts have also contributed,” says Müller.
One of the results is that the client wants to ensure that the machine makes a continuous 24-hour advance, without stops, in which settlement could develop.
It has been stopping in the transitional ground because the hard rock still forms part of the face. “This crushes into a fine mix in the bentonite and has been proving difficult to separate,” says Scheifele. “Bentonite has had to be thinned or changed every two days, slowing progress at present.”
However, as the machine comes fully into the gravel ground it is thought this problem will diminish. Müller is hoping to maintain about 5–8m a day.
“There may be hold-ups because we know that some grout pipes have got lost, and this could need extricating from the head. And there could be other unknown foreign bodies in the ground. But we hope progress can be maintained.”
Everyone, and a lot of instruments, are now watching to see how it goes, and how much settlement there will be. Some 40mm is allowed for as a maximum and so far, the worst seen is about half this level.
Related Files
A pipejacked umbrella
New rail tunnel
A structure of prestressed concrete beams
