Bodio and Faido, the two southern contracts for the Gotthard tunnel, were thought at the start of the project to be in the best ground and capable of the fastest progress. They were therefore the longest sections, 16.5km heading north from a portal at Bodio, and 13.5km onwards from the intermediate access gallery Faido, to join up with the Sedrun excavation.
Because of schedule revisions these second drives were eventually shortened to 11,134m in the east drive and 11,118m, with the remaining part done by the Sedrun lot which had made better progress than thought.
Faido, the second of the contracts, begun from an adit access at the narrow valley at Faido, included the excavation of all the caverns and side tunnels for the southern multifunction station as well as the running tunnels onwards. Within the 2.5km of the multifunction station some 11km length of tunnels, crosscuts, shafts and caverns were therefore to be done by conventional excavation methods before the drives onwards by TBM.
Neither section turned out to be straightforward with some of the greatest difficulties and rock problems on the whole base tunnel. Particularly the Faido MFS section had unexpected squeezing ground and faulted rock caused major headaches from almost the first days of work. The great 11.8m high cavern with a total volume of 15,000m3 for the emergency station and ventilation inflows took five and a half years to complete instead of three.
Faulting, rock falls and rock burst also slowed the TBMs on the Bodio drives and the rock conditions proved difficult on the Faido TBM drives as well, with more and heavier rock burst and faults. Greater difficulties had been anticipated for this last section however, as it is the deepest, with almost 2,300m of mountain above.
A major redesign was needed for the Faido MFS work eventually, separating the crossover tunnels from the emergency station itself and shifting them 600m southwards into better ground. The final TBM drives from Faido towards the Sedrun section were also slightly shortened, with the additional length added to the Sedrun contracts, to balance out the completion times and bring the overall breakthrough forwards.
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By GlobalData“It is the great irony of the project that the most risky and complex sections we planned for should have been less difficult than thought, though not easy, and the easiest had so many problems and challenges,” says Heinz Ehrbar, chief engineer for AlpTransit.
The Sedrun section ended up making faster progress than thought, he says. The “unfeasible” Piora, the syncline full of potentially running “sugar-rock,” which was the great risk in this southern section, was eventually found to be so solid that the drives passed through almost unnoticed.
A big ancient rock slide that covered the mountainside at the Bodio portal with giant “house-sized” boulders and mixed debris, was also dealt with as planned, using steady application of forepoling and grouting to get through its 410m in an early preparation contract, which also included a further 1,630m of drill and blast for the east bore and 720m for the west. In another preparation contract a temporary bypass tunnel of 1,200m was made around the slide, ending in a cavern, where the TBMs could be assembled to start the main drives without unnecessary delay.
If there were to be challenges on the southern lots, it was supposed these would be focused around the logistics. The concern was how such huge quantities of material and long drives could be supplied and spoil be removed, mostly through the one portal at Bodio, because Faido valley did not have the capacity to handle a huge volume of spoil, a problem made worse by a steep access with a 13 per cent gradient in the 2.7km long adit.
Eventually it was accepted that the only way to tackle it was to combine both contracts, Bodio and Faido, as one overall project with the organisation merged and coordinated.
Doing so allowed a sufficient scale of production to justify a highly sophisticated automated tunnel railway system to be installed, and to create a giant “factory-on-wheels” for all the follow-on tunnel cleaning, drainage installation and lining operations. The so-called “tunnel worm,” 650m long, built by Herrenknecht subsidiary Maschinen und Stahlbau Dresden (MSD) could run at the same time as the TBMs, speeding up operations.
Proposals for this were made by the Tunnel AlpTransit-Ticino (TAT) joint venture, which eventually won the job, a group led by Swiss contractor Implenia Bau (formerly Zschokke-Locher and Bati-Group) with the Austrian Alpine Bau, the Italian firm Impregilo with Swiss subsidiary CSC Impresa Costruzione and Germany’s Hochtief.
Olivier Bockli, the designated director of business unit underground works for Implenia and managing director of the joint venture, says that although TAT submitted four alternative bids treating the sections separately, “the logic of the sites really made no sense unless they were put together.” The group submitted two further bid proposals which merged the works and argued strongly to the client that this was the best way to proceed.
AlpTransit was convinced, even though the scale of the contract would be a new experience for Swiss construction, both in its timescale of over a decade and the cost.
In the accepted bid—worth a very large CHF 1.5bn (USD 1.5bn) and now at least CHF 800M (USD 800M) more, though contract negotiations have some distance to run yet—the plan was to use just two TBMs instead of four proposed in the outline scheme. They would do both the first drives from Bodio and the onwards drives from Faido, being renovated and adjusted after the first section to handle an increase in the cutting diameter. The contractor selected Herrenknecht.
During the first drives from the south, the multi-function station would simultaneously be excavated via a side adit from a worksite in the narrow valley where the little town of Faido is located. The big cavern and tunnels would serve as the initial reception for the machines, where they could be repaired and refitted for the more demanding drives northwards.
These first TBM bores from the Bodio portal northwards, were through the horizontally stratified pennine gneiss of the southern Leventina mountains. While these are fairly high, the cover was not going to exceed 1500m, meaning reasonable rock pressure. The rock was understood to be sound all the way.
In such a relatively problem-free zone and with little squeezing anticipated, the design called for a tight bore diameter of 8.8m to avoid any wasteful overcutting and to reduce spoil disposal. Only a minimum 250mm or 300mm thick inner lining, without reinforcement, would be required.
“The TBMs were therefore stripped down like Formula One cars with fairly light rock-support drilling equipment and so forth,” says Boeckli. The geology showed this to be a mistake.
“Almost immediately the machines ran into faulted and difficult rock,” says Ehrbar shaking his head. “And the luck was bad. There was one fairly small horizontal fault zone for example that just stayed within the tunnel section for 500m on the east side, even though in the west drive it dipped away after only 60m.”
The rock continued to be faulted or more difficult than thought. “We quickly realized that we were needing far more rock bolting and support than planned. We had expected 25 per cent bad rock and 75 per cent good. But the ratio was reversed, and for safety there was a much greater need for steel caps with 75 per cent needing,” says Boeckli. “After a while it was necessary to modify the machines.”
The TBMs were stopped in Christmas 2004 for three weeks and the L1 and L2 area of the machine trains especially were beefed-up by Rowa additions to the otherwise Herrnknecht machine train. More efficient rock drills were added. A new flexible robotised shotcrete system from Sika was added and a new transport and installation system for the ring beams.
The rock also suffered to some extent from rock falls and burst. The gneiss is fairly highly stressed horizontally, says Bockli, “because it is on the south, the side of the Alps that the African tectonic plate is pushing.”
The machine in the western drive also suffered a jam from rock convergence though this was resolved within 11 days says Bockli. “We devised a technique using hand-held rock drills, small blasts and hand shotcreting to create a small gallery over the TBM shield which took the overburden pressure away.” A few areas of the Bodio drives required some re-profiling because of convergence.
A recent 40km loose-rock jam, similar to the one in the north has since required much greater effort on the Faido drives, with a need to drive rescue tunnels from the other bore and costing three months, almost a repeat of the northern drive problem.
The difficulties and changes meant that the machines took longer to make their runs than expected, finally arriving after three and a half years at the Faido caverns in autumn 2006, where they were due to be overhauled and modified by the contractor and supplier Herrenknecht.
This later arrival was fortunate. The complex of tunnels and main cavern had been a nightmare to construct and the arrival cavern was finished “just in time” for the first TBM.
The multi-function station was excavated from the end point of a 2.7km long access tunnel that dropped from the Faido valley access above to tunnel level. “It was a very steep gradient, some 13 per cent,” recalls Simon Peggs, an AlpTransit engineer on the site at the time. “I walked up it once but never again.”
No problems had been encountered when this access adit was built in a preparatory contract between 1999 and 2001 and “it went very well,” says Ehrbar. “There were no signs of any difficulty in the rock.”
But within days of the main contractor starting excavation, the crews hit bad ground, he says. “We thought maybe it was lack of experience or a learning curve issue but it became clear that problems would continue.”
At first the contractor and the engineers resorted to heavy support. However, as the work moved on even the strongest steel arches proved to be inadequate.
“It was the beginning of many technical challenges. The ground here suffered significant rock bursts, for example,” says Ehrbar, “which could be very dangerous. It is very sudden and unpredictable. In one incident a piece of rock 20m3 came away, though such a large piece is an exception.”
Areas of the excavations had to be declared out of bounds for periods. At the excavation fronts an additional waiting period of around four hours was often needed after blasting, to allow for a period with a high risk of bursts.
In some parts of the excavation there was squeezing ground as difficult as anything on the Sedrun contracts “and with the additional problem that it was unexpected,” says Ehrbar. Early support was bent and destroyed.
The sliding arch system from Sedrun had to be applied, though without the specialised arch handling machines being available to install them. Fortunately the areas requiring the technique were sufficiently small to allow the contractor to work with normal plant to erect the arches.
“Overall the difficulties there affected everyone,” says Boeckli, “starting with the miners who had to fight every centimetre of the way, beginning with enough support to ensure safety. Three-quarters of the time was involved in that and only one-quarter in actual excavation.”
Convergences were sometimes more than a metre and meant that areas of tunnel or cavern had to be reprofiled not just once but in some places as many as four times, with new primary support.
The engineer was also challenged, he says, because eventually it was clear that changing the design was the best solution, so that the big crossover tunnels at least could be built in better rock. They were moved to the south as was the west side passenger escape concourse. The passenger walkway, which takes passengers over the tunnels to the safe side in an emergency, was lengthened.
Separate design changes were also needed to the concourse’s ventilation, as in the Sedrun MFS, increasing the number of duct passages from one to seven on each side. Vents were required by the government not simply at the end of the concourse, to exhaust any smoke, but between each of the cross passages that connect to the main running tunnel, where any burning train would be standing.
The problems were also a challenge for the client, says Boeckli, because the difficulties raised issues of increased work and costs. Both he and Ehrbar declare however that despite the fraught conditions, it was possible to keep relations going “using a tradition of Swiss compromise and realism.” Ehrbar adds that he is impressed that “not once did the work actually stop.”
Problems were to continue on the second stage of the TBM drives although the TBMs were better able to handle things after their major overhaul and rebuild. Owing to experience from the first section, additional modifications were made to the cutterhead, with more discs added for example. A spiral pattern was adopted rather than eight radial spokes. Wear plates were also strengthened.
Other changes had already been planned because the bores from Faido onwards would include the highest rock cover on the alignment, up to 2,300m. A bigger bore diameter was needed therefore to handle possible convergence and heavier support and so the cutterhead diameter was increased to 9.4m. A capacity to bore to 9.5m diameter if judged necessary was made possible with extensible cutters and changing discs from 17 inches (approx. 43cm) to 18 inches (approx. 46cm) in the calibre range of the cutterhead.
Rock burst continued in these drives, says Boeckli, which at times damaged both the machine faces and follow-on train equipment. Though no injuries were caused, it was necessary to add more rock bolting and mesh to provide protection for the miners. SuperSwellex bolts were a big help.
Due to the delays, there have been various revisions to the project to speed up work, the largest being overlapping the excavation of the two TBM drives and tunnel lining sequences of the multifunction station, originally scheduled to be executed sequentially.
The logistical systems installed at the beginning have proved their worth in coping with the numerous and extensive changes.
First is the rail system itself, which is used both for spoil removal and concrete and other material supply. For the TBM drives, everything comes from the Bodio portal where there is room for a bigger site installation than at Faido. The spoil handling systems, operated under separate contracts are there, as are the contractor’s batching plants and other materials.
That means long journey times for the trains, more than an hour in the first section and during the second drives up two and half hours to cover the 30km to the machines.
There are clearly many trains underway at a time, which meant providing a huge fleet. “There are 70 Schoma locomotives and 380 Muhlhauser wagons,” says Boeckli, “with the loading and handling areas at the portal to match, as well as workshops and maintenance.” Two tracks in each bore brought the total track length to over 300km.
With multiple trains entering, leaving and passing each other in the tunnel, the normal manual switching and control would have been impossible. Instead TAT turned to the professionals, using a control system based on Tiefenbach technology and the same software as Swiss Federal Railways. A control centre with radio and four computer displays showing train and point positions, with remote switch operation, was run for it by subcontractor VT Verkehrs- und Industrietechnik.
“They coordinated with a team of our own logistics planners who worked out precisely what deliveries were needed, when and where,” says Bockli.
Coordination was particularly important because of other works in side the tunnels. Much of this was done by the two “worms,” great multipurpose train units following on the TBMs by some hundreds of metres.
“The unit did every from cleaning and checking the tunnel and track, checking and smoothing the profile, installing waterproof membrane, drainage and ducting and then finally concreting the inner lining with two 12m long shuttering sections and a follow on curing section.
A number of other special units were built later for re-profiling work ahead of the worms as well, in order to accelerate the work and catch up the delays.
The presence of the various machines and building sites added to the logistic complexities and in the later stages of the project a number of obliquely angled cross passages were excavated to allow trains to pass from one tunnel to the other and back.
“But we have managed to keep everything running,” says Bockli.
The large number of worksites also demands significant cooling, particularly as the rock temperature under the very high cover was estimated to be capable of reaching 55 degrees C, though to date it has only reached 46 degrees. As on the other contracts, it is technically only possible to cool at the work locations.
A system for cooling began with a possibly optimistic 14MW installation but had to be beefed up to 21MW due to changed rock temperature estimation and overlap of multiple jobsites underground.
Now that drives are finished, the work will continue on finishing the tunnels. This will take until 2014, although one section has already been completed at Bodio and is the first to be fitted with permanent track and signalling.
The work camp and support facilities at the Bodio portal. Logisitcs control, spoil disposal and concrete batching was done here for the drives to Faido and beyond to Sedrun. Picture courtesy of AlpTransit Gotthard Linings, cables, and drainage for the southern portions of tunnel, where squeezing rock, among other challenges, was encountered The southern portal at Bodio Faido valley adit Heavy support was needed frequently on the southern sections Arch and mesh support for faulted ground in the Faido to Sedrun eastern TBM drive. Picture courtesy of AlpTransit Gotthard Renovating and restructuring the cutterhead for the Herrenknecht TBM at Faido before it continued the drive northwards towards the Sedrun section. Picture courtesy AlpTransit Rock drills and equipment in the rear of the cutterhead during renovation work on the TBM
