Building a long base tunnel for the Gotthard pass was always going to be a challenge. So it has been proved.
The point of the axis is to create a ‘flat’ link through the Alps, never rising above 550m in altitude and with gradients limited to just 6.7 in 1000. Combined with very large radius limits on the alignment, this will allow high speed passenger trains like Germany’s ICE and French TGVs to run at 250km/hour or modern freight at 160km/hour.
To do that on sensible possible alignments for the tunnel meant staying deep within the mountains, under rock cover that rises as much as 2,300m above the tunnel level.
“Restrictive boundary conditions meant the alignment was found quite quickly,” says Heinz Ehrbar, AlpTransit Gotthard’s chief engineer. “The aim was to avoid high rock cover because of initial stresses and high temperatures, and at the same time to find the shortest way through the difficult zones. We also needed to find intermediate access points that would minimise the whole length of the tunnel system.”
The presence of concrete arch dam reservoirs above was a final constraint, with the route avoiding them as much as it could.
High rock cover posed the major difficulties, partly because of the stresses imposed on the rock by such high cover and because of the other dangers of tunnelling at such depth like the very high water pressures that could be expected. Inflow to the tunnels would be high and the depth would equally pose problems for safely evacuating the miners.
Most of all, the mass of rock above would trap the heat of the earth’s core, making working conditions highly unpleasant, if not impossible. Even past projects, such as the much higher level Simplon tunnel, had been obliged to use ice brought in by wagons but the new depth was unprecedented. No one quite knew how much heat would be contained there or how it would be handled, even with modern cooling technology.
As Ehrbar says, “no one had any experience of the great massifs at these levels.”
But at least there was some experience in the world to be drawn on, such as even deeper gold mines in South Africa and deep coal seams in Germany. For the geology however, the deep structure of the Alpine Massifs was unknown. The early studies for the project carried out after its initial go ahead in 1992 focused on this aspect particularly.
It was known that the tectonically tumbled and tortured masses of the great Alpine ranges present a mixed and complex geology with at least two zones that could prove on the edge of even modern engineering capacities.
One of the key challenges had been debated for decades, particularly since first proposals for a deep base tunnel had been mooted for the Gotthard Pass in 1947. It was the Piora Mulde syncline, a valley in between the great Gotthard Massif, which is the southernmost of the two huge igneous, mainly granite rock blocs that form the ‘backbone of the Alps’ and the lower mountains of the Leventina Pennine gneiss area on the Italian side of the Alps.
The basin was filled with a soft, whitish, sand-like, weathered dolomite—often described as ‘sugar rock’, with almost no coherence and which can run like the sand in an hourglass. Part of the valley followed the almost vertical rock bedding downwards and so it was suspected there would be a zone of this soft calcareous material up to 300m across that would need to be crossed on the tunnel alignment. If the valley aquifer extended this far down too, the water/sand mixture could prove not just a problem but perhaps a “killer feature” for the project.
“I think it might have meant stopping the scheme completely,” says Charly Simmen, a field engineer for AlpTransit Gotthard who oversaw much of the spoil disposal for the tunnel. “Some technical methods were available to deal with it, such as ground freezing or grouting, but at such depths against enormous pressures—remember there would be up to 1,800m head of water—it would have been either impossible or extremely expensive.”
A second major problem was a section between the Gotthard Massif and its counterpart northern bloc, the Aar Massif. Trapped between these two layers, like corn between two Gargantuan millstones, were layers of mainly gneiss and slates that had been ground and battered over millions of years. In particular, the area of the Tavetch Intermediate Massif was likely to be difficult, its softer and highly fractured rock causing significant ground squeezing.
“For the engineers in the feasibility and design stages the task was to assess and measure the risks,” says Erhard, who at that time was on one of the teams of consultants working on the design with the Zurich firm of Electrowatt. “Together they did an excellent job of risk management.”
Design work for the project was carried out primarily by two major groups of consultants. On the northern section of the base tunnel this was Gahler & Partners, Gruner, Rothpletz Lienhard and CES. For the southern part it was Amberg Engineering, Lombardi and Electrowatt Engineering (which has since become part of Poyry).
Part of the design involved some extraordinary and extended ground investigation for the Tavetch area and to some extent for another zone near it the Urseren-Garvera. “Though this was already known from the Gotthard road tunnel,” says Ehrbar.
Deep borehole investigations were carried out using directional drilling techniques borrowed from the oil industry to sample rock on the tunnel line. These bores were exceptionally long, with one measuring 1,750m, another 1,715m and others of 700-800m.
Ehrbar recalls that the squeezing ground began to show its challenges even at this stage because the drill core bits became stuck several times. The work, by ground investigation firm Foralith, was almost halted “but we tried one more time and it worked,” he says.
Sections from the warehouse full of cores were analysed at the famous ETH technical university in Zurich, which specially developed a new kind of triaxial test cell for the work. It could simulate the pressures and groundwater content to be expected at the more than 1,000m tunnel depth at this point, Ehrbar explains, “and with this we got good results for the ground parameters like cohesion, friction angle and so forth.”
With these data the designers could explore possible solutions to the anticipated ground squeezing. Technology from German coal mines, which are up to 1km deep, could be adapted it was thought. This used multiple section steel arches that have sliding friction joints to allow them to “give” initially to otherwise irresistible ground pressure, and only slowly build-up resistance to stabilise the rock.
For the Piora zone an even more extensive investigation programme was commissioned with a 5.5km long exploratory tunnel begun in 1993 by joint venture Alpi, comprising Zschokke and Locher (now together as Implenia), Manciini and Marti and Murer. Probe drilling was done by a joint venture of Italy’s Rodio and Candian firm Morisette, part of Boart Longyear. The tunnel was excavated in as far as the main tunnel line from a site at near the town of Faido, the same location for one of the later construction access points. The horizontal tunnel was 350m above the level of the base tunnel line and ended in a chamber from which boreholes could be driven.
This project produced what could have been a disastrous early result. After driving most of the tunnel, an early forward probe in 1996 hit the Piora material, which proved every bit as bad as thought. Not only that but the failure of an emergency stop valve on the drill bit meant pressurised sand and water blasted into the tunnel, rapidly filling it two-thirds deep with a ‘beach’ as one engineer recalls. At one point the material was coming in so fast that it began to spill across a local highway near the tunnel portal and evacuation of nearby houses was being considered.
“It was like drilling a hole in a submarine 1,700m down,” says professor Georgios Anagnostou at the Swiss Federal Institute of Technology, one of the experts working on the project at the time. “Fortunately the coarser particles in the material clogged the drill hole after about half an hour and the flow slowed and then stopped.”
With the tunnel cleared out and a concrete plug installed at the tunnel end, further investigation could proceed. It did not look good either, with groundwater movements of hundreds of metres daily detected in the aquifer, boding ill for attempted grouting or ground freezing work. Freezing coolant or grout would have faced water pressure of up to 130bar and moving like a river.
If the difficulty extended downwards a width of difficult ground of 230m would have to be crossed at the main tunnel level, which would be either impossible or highly expensive.
But a score of core drillings downwards from the tunnel end and seismic tests began to show better results. As Swiss engineer Felix Amberg, head of one of the major design consultancies on the project, told a meeting of the British Tunnelling Society late in 2000: “Investigation revealed that at base tunnel level, the Piora Basin consists of stable dolomite, marble and dolomite-anhydrite. No sign of high water pressure was found in the boreholes at base tunnel level.
“The geological model showed the Piora Basin to be solid Trias limestones/dolerites with occasional lenses of anhydrite or gypsum. Rock fissures are filled with gypsum, preventing major inflow of water.
“Between the exploration level and the base tunnel level, geologists have interpreted the existence of a gypsum cap formed through the transformation of anhydrite to gypsum by water percolating from nearby valleys.”
This was a major stroke of luck, which gave new confidence that a TBM drive could get through, as the “sugar-rock” is stable when dry.
All these challenges and risks were part of the huge debate and discussion within Switzerland that made up another layer of political, economic and social obstacles to be overcome when the project was first proposed. The country has a complex system of referenda that have to be passed for all major schemes. There were environmental and local community objections, too, which led to delays and scheduling problems.
Key hurdles overcome were the acceptance in 1992 of proposals for the New Rail Link through the Alps (NRLA), which provided the basis for planning. In 1998 the Swiss people agreed to a new heavy vehicle tax (HVT) and overall proposals for modernisation of the railways. The company of AlpTransit Gotthard, was set up as an offshoot of Swiss Federal Railways in the same year, which allowed the construction to begin in 1999 for access tunnels at least. Major work began at the end of 2001.
Other issues which had also been critical included the safety of such deep tunnels once in operation. Part of this was settled in 1995 after long debate on layouts, including twin-track tunnels, additional service tunnels and other configurations, with two single parallel bores to be built connected by cross passages every 325m—allowing for the opposite bore to be the rescue tunnel in the event of disaster, most likely from train fire. Cross links would be sealed by high pressure doors to keep the two tunnels separate for a complex ventilation system that would handle smoke and fumes.
Two major intermediate emergency stations would be included at tunnel level at approximately the one-third points, which would have passenger escape tunnels at 86m intervals from the train tunnel into separate additional parallel emergency tunnels, linked via a pedestrian route to the opposite side.
These so-called multi-function stations (MFS) also allowed for later maintenance work and access, and as key locations for housing important permanent tunnel equipment (most of all the powerful ventilation systems that would be used for both maintenance work and emergencies).
In the same location were to be crossover tunnels for the main train bores allowing trains to switch to the other side, though rock conditions on the southern MFS meant that this was eventually constructed at a separate point.
The large diameter chambers also provided critical spaces for the construction work and its separation into phases. To make its great length possible, the tunnel excavation was planned in five sections. One drive each would begin from south and north portals, and the other three from intermediate access points. First of these is an adit to the main tunnel at Faido, close to the Piora investigation tunnel entrance down to the main tunnel. Access here would allow advance construction of the multi-function station, the big excavation initially serving as a start chamber for a TBM drive northwards and later to receive the machines on the southernmost drive.
At Sedrun, a village high in the mountains, a shaft would descend to the tunnel line to excavate the second MFS. From this space the difficult squeezing ground could be tackled. A short tunnel of 1.1km was needed at the top to reach the shaft head.
A final drive was to begin near the village of Amsteg where a short horizontal adit, just 1.2m long was built to access the tunnel line from the narrow valley which forms the beginning of the Gotthard Pass.
The five main sections were of varying lengths, according to the anticipated difficulties for construction, with the longest at 16.5km from the southern portal at the little village of Bodio, through highly competent gneiss, and the shortest at Sedrun where the big MFS had to be built and where the squeezing ground would limit progress.
“The obvious aim was to synchronise the various parts for completion at approximately the same time,” says Charly Simmen, project manager with AlpTransit.
But the best laid plans of mice and men ‘oft gang astray’, especially when the holes they are gnawing are on such a giant scale.
As the work began the challenges and difficulties were increased. Areas of rock assumed to be sound and relatively simple to drive through proved to have some of the worst conditions of all, particularly on the southern end of the project where streamlined TBMs designed “like Formula One cars” for fast progress were stalled by the need for heavy support installation.
Meanwhile rock unexpectedly proved much more difficult for the big MFS cavern excavation to the south, which took twice as long to excavate as estimated. Squeezing ground was the culprit. Part of this section had to be redesigned, with rail track crossovers moved 600m away into sounder rock. It cost time and money.
The more difficult rock in the south, with more faulting than thought and frequent rock bursting, slowed the TBMs and eventually led to a restructuring of overlapping contracts. The contractor on the central Sedrun section of the work, who had found better than expected conditions on the southward drives, albeit still very difficult, was asked to extend these by 2.5km and the Faido drives were shortened.
Some good luck was set against all of this, however. At Sedrun the rock was not so bad as thought and the north was able to make fast progress. Anticipated levels of water in the tunnels never reached anything like the inflows that might have been in the worst case risk analyses.
Flooding in the worst possible case of a 1,000lt/sec inflow would have been most dangerous in the central section of the tunnel, which was accessed only by the 800m deep Sedrun shafts in the first stages. Pumping resources sufficient to buy time for full evacuation were needed, a major undertaking against an 800m head of pressure.
Further complications were caused to the timetabling by political and community issues, most of all on the northern portal which exits at Erstfeld. A section of new line then connects into the existing Gotthard railway link, close to the historic town of Altdorf, capital of the canton of Uri, where the famous rebel William Tell was reputed to have shot the apple from his son’s head.
Residents had been pressing for an alternative alignment for the tunnel that would have continued it onwards through the side of the valley for several kilometres. The canton’s objections held up the start of works for the northern section until 2003, following an agreement to add a stub tunnel and rail crossover chamber some 3km inside. These will provide a connection point for a possible future extension.
The delay in starting the Erstfeld section might have affected the entire programme but was partly absorbed by fast progress on the preceding Amsteg section and subsequently on the Erstfeld drives as well.
But other factors have caused the timeline to be extended anyway, most of all the very difficult rock conditions at the Faido MFS and in both the Bodio and Faido drives. On top was the impact of redesigning the MFS to get part of it into better ground.
Further design changes were made to both of the multi-function stations as well. These followed discussion with the Swiss Government, which was unhappy with the ventilation arrangements and insisted on the additions of extra vent passages, so that there would be no horizontal movement of smoke in the passenger emergency concourse.
The breakthrough on 15 October Heinz Ehrbar, AlpTransit Gotthard’s chief engineer High-speed journey times Testing the piora: a special exploratory drive driven at an angel from Faido was made to a level 300m above the main tunnel line cores were made from the end Spoil handling conveyors for the batching plants at the Erstfeld portal site. Picture courtesy of AlpTransit Gotthard Close spacing of the sliding support circles on the northern drives at Sedrun
