In mid-October the Austrian contractor Porr, part of the Atcost 21 joint venture with Hinteregger & Söhne, Östu Stettin and Swietelsky, will stop the TBM drive on first bore of the Filder tunnel less than halfway along its 9.5km length. The Herrenknecht multimodal machine will be dismantled and brought back to the portal.
But this is not because of problems; just the opposite, the TBM has been progressing very well, up to 40m/day, through first clays and silts and then harder saturated sandstones, says Porr project manager Norbert Hörlein.
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It was planned from the beginning that the machine would stop. In fact it is to make four drives for the tunnel, two of 4km downhill from the southern portal and then two for the other end, doing two parallel sections of about 3.6km at the city centre end.
Work on the tunnel, part of the new Stuttgart 21 high speed rail system, began in November last year starting from a short 130m stub entry at the portal. In this conventionally excavated soft ground section the 10.82m diameter machine had been assembled, before setting out on its 4km drive completed in just under a year.
According to client spokesman Thomas Berner from Deutsche Bahn, progress was good for much of the drive "although there were some problems with clogging where the ground became more silty". Disc wear was good but increased in the last sandstone section and progress has slowed a little, down to 80m a week.
"The sandstone is okay but there are parts with mixed sandstone and mudstone layers," says Walter Wittke, a professor whose consultancy is specialist tunnel design adviser for several of the major tunnels on the Stuttgart 21 project. "That means both sort and hard rock in the same face, causing some clogging."
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By GlobalDataThe machine ran first in EPB closed mode for the early soft clays and silts and then in what might be called "semi-open" mode later, still using the screw conveyor for spoil removal from the face but without the need for a pressurised face.
This major part of the drive is through a complex layering of sandstones, claystones and mudstones.
The second drive, on the parallel bore of the twin tube tunnel, begins next spring once the machine is serviced and reassembled, once again heading downhill towards the city.
The reason for the odd sequencing is geology, to be precise a band of anhydrite Keuper gypsum about halfway along the tunnel. The reactive rock has been sealed between impermeable layers until now, and never exposed to water. Were it to be wetted it would hydrate much in the way that Plaster of Paris does, which is a form of gypsum.
"The tunnel runs through a layer which is about 30 per cent of gypsum anhydrite distributed through mudstone and claystone," Wittke explains. He has some 30 years of experience in the region’s rock including numerous experimental studies on its properties, results of which were used for previous tunnels.
The anhydrite form is highly reactive with water he says. "It dissolves and then recrystallises in a hydrated form in which it expands by around 60 per cent." This particular rock layer could expand some 18 per cent therefore he says, "which is enormous," and creates significant rock pressure.
Water block
To prevent this, and the danger of trapping the TBM, the central 1,500m of the tunnel is being dug with drill and blast and given a watertight lining. Additionally Wittke has devised an innovative water block system for the tunnel bore which aims to prevent any groundwater movement into this section of the tunnel alignment for its entire 100-plus years lifespan.
It works something like a hugely enlarged water stop in concrete it might be said. First, along a small length of the tunnel the bore will be enlarged and an extra thickness of concrete lining be installed as a collar. Through that, a radial pattern of drill holes for grouting will fan out into the rock around the bore which will be sealed using a fine penetrating resin mixture with a viscosity close to water.
"It should seal the finest cracks," Wittke says. The grout curtain will extend outwards to 4m additional radius, which should ensure the area of relaxation around the tunnel bore is sealed. Beyond that, he says, the rock is naturally tight and unaffected by excavation work.
The grouted water stops will be installed at three points along this southern uphill end of the central tunnel, allowing for complexities in the overlying rock strata; as well as the most saturated sandstone layers there are others which are potentially water bearing. In between the stops there will be a thickened lining of up to 1m thick of concrete along the bore.
There will also be a water stop at the far end of the gypsum section, though here only one stop is needed to seal off a single rock layer beyond.
The drill and blast drive will be extended by a side passage to the line of the second bore, which will have this 1,500m central section and its water stops excavated during the time the TBM is making the second drive, and completing before it arrives.
Once it reaches this section, rather than be dismantled, the TBM will be pushed through the central D&B section to the end from where it will re-launch for the last 3,600m-long drive to the lower, city centre, portal emerging in the central part of Stuttgart. One final drive will then remain.
For this the machine will be turned in a large cavern, currently under construction, and will now drive back uphill to complete the final part of the first bore, another 3.6km-long section. Both these drives will be done with the machine fully reconfigured in open mode and kept as dry as possible.
At present the plan is to service all these drives from a site at the southern portal explains Hörlein. It is a convenient enough location in the southern suburbs of the city just adjacent to the A8 autobahn towards Ulm and Augsburg, the same axis along which the new high speed rail line will run after it emerges from the Filder tunnel. The motorway is important for spoil disposal and site supplies including segments.
"We do not have a direct entry from the site onto the autobahn and trucks have to take a 2km route locally to get onto it but it is still relatively convenient," says Hörlein. It is just as well as the site itself is relatively small in area and has little room for storage. Quite a section of it is taken up with cement storage and a batching plant and much of the remainder is a TBM starting area. For both segment supply and spoil disposal there is a need for an efficient "just-in-time" approach says Hörlein.
Segments are quite large, with six full size pieces and a halfsize key for each ring. Each is 2m long and either 400mm thick or for some critical tunnel areas even 600mm.
"We are using separate left and right rings" he says which adds to the storage needs since both types need to be available. In the tight space available this means only about three days supply for the machine can be accommodated.
It means a complex logistical management exercise to ensure the correct rings are available for the machine as it progresses, he says "and we have an engineer dedicated to managing that".
The segments are being made in the Max Bögl plant at Neuenmarkt near Berlin, using Herrenknecht forms and are delivered using rail for the long 400km journey. They arrive every couple of days explains Atcost site engineer Katharina Hertel, coming into a larger storage area about 20km from site.
From there the rings are brought to site by truck where they are unloaded using a 45t Teichmann portal crane.
They are delivered to the TBM with a rail system using Schöma locomotives. Spoil is equally complex to manage as there is very little space to stockpile anything. Once again it is virtually a full time operation to manage the logistics says Hörlein and two more engineers have to concentrate on its just-in-time organisation. Up to 10,000t daily has to be moved. As is increasingly the norm, spoil is carried by conveyor in the tunnel, in this case a system from Swiss manufacturer Agir. Once delivered to surface the material is immediately loaded into trucks which take it to various disposal sites, mostly old quarries, up to 80km away. "We have up to 75 trucks daily," says Hörlein, "which we can run between 5am and 10pm".
Spoil will continue to be handled at this site for the second bore and the NATM excavated sections in the centre.
The centre parts will be done with drill and blast, hopefully says Hörlein with Atlas Copco boomers being used by the same contractor on a project for the Stuttgart-Ulm line which will have completed by next year. But it has not been decided yet how spoil it will be removed. The options are to use more conveyors or perhaps rail trucks. If the conveyor continues to be used for the first drive then it will be necessary to fit a second one for the next TBM drive starting in the spring. Equally the contractor is still deciding on the method for the final drive back out of the city centre and whether the logistics should be done back through the then completed second bore, with a possible 12km of conveyor, or through the central city logistics area.
The latter is an option because there is further work to complete the Filder tunnel at the city centre end, part of a complex of underground works and other tunnels that make up the Stuttgart 21 project. (see box, page 43)
"We, are doing the work but it is a different section of the contract" says Hörlein. Initial work has been to create an access tunnel underneath part of the city centre near the main from which four drives are underway with materials and spoil logistics through the access adit. The project involves several complex caverns and tunnels using observational method excavations in soft ground. The ground here is also the same Keuper formation but no longer sealed by other rock layers underground" says Wittke. "That means the gypsum has been leached out leaving a weak rock."
These caverns form the end of the Filder and also separate tunnel links into the new station. In places they must hold four tracks as they lines converge from the eight platforms in the station.
Further along there is another point where the tunnels widen out again as lines split off heading eastwards into other tunnels to Untertürkheim and Bad Cannstadt. These branching lines will also provide a link for trains to join the original main line to Munich.
"Caverns up to 20m across and 18m high are needed," says Wittke, "The height is necessary in order to achieve arching in the ground." The huge spaces are just 8m below building foundations and complex compensation grouting is being used as a measure against settlement. Extensive monitoring is being installed.
He says that the initial work has been going well although slowly, particular with complexities around a central reinforced concrete piller that separates to diverging tracks.
