Construction of the world’s longest railway tunnel has reached the half way mark, Martina Booth reports on the challenges overcome so far and those still facing Gotthard Base Tunnel contractors.
When the Gotthard rail tunnel opened in 1882 it was one of the marvels of nineteenth century engineering. Now, 128 years later, Swiss engineers are halfway through construction of another sensation − the Gotthard Base Tunnel.
This will accommodate a 57km twin bore flat rail link through the Alps, making it the world’s longest railway tunnel − reclaiming the title originally held by its 15km-long little brother.
The rail link will add to the growing European high-speed network, allowing high speed rail and heavy freight trains to cross the Alps without resorting to winding mountain routes.
With portals in Erstfeld in Canton Uri and Bodio in Canton Ticini, the tunnel will cut journey times between Zurich and Milan by an hour when it opens in 2017.
Trans-alpine times will decrease further when the Ceneri Base Tunnel − under construction to the south of Gotthard − opens in 2019. Contractor Alp Transit Gotthard is building both tunnels for the Swiss Federal Government. A third tunnel, the Zimmerberg Base Tunnel, has been postponed indefinitely.
“Tunnelling means construction in not well-known material. We should always be prepared for surprises”
Heinz Ehrbar, Alp Transit Gotthard
The Gotthard tunnel comprises two single-track tunnels, linked by connecting galleries every 180m. These serve as an escape route from one bore to the other. To optimise time and costs, construction has been split into five sections, which are being driven simultaneously.
They are Erstfeld (length 7.4km), Amsteg (11.4km), Sedrun (6.8km), Faido (14.6km) and Bodio (16.6km). Tunnel boring machines (TBMs) are being used to excavate most of the tunnel, but the Sedrun section, cross passages and the access tunnel − are being excavated using the drill and blast method.
Daily advance rates are at the behest of rock conditions. Good classes of rock allow an advance rate of 20m/day, but poor classes of rock drop the rate to below 1m/day, because every metre blasted or bored has to be supported immediately with steel-arches.
If the rock is particularly brittle, a thicker layer of sprayed concete has to be applied than elsewhere in the tunnel, adding to an already time-consuming process.
“Generally speaking, we are happy with the rate of tunnelling advance,” says chief construction officer Heinz Ehrbar. “But tunnelling means construction in not well-known material. We should always be prepared for surprises.”
Innovative construction solutions have been deployed to resolve such problems. In the squeezing rock conditions of the Tavetsch Massif North near Sedrun, a particularly difficult 1.2km area had to be crossed.
Squeezing takes place when a particular combination of induced stresses and material properties pushes some zones around the tunnel beyond the limiting shear stress at which creep takes place.
To counter the effects of this, new methods of tunnel construction had to be used to ensure excavated cavities did not reclose.
“To provide support for keeping the excavated crosssection open, deformable steel rings were inserted”
“To provide support for keeping the excavated crosssection open, deformable steel rings were inserted. These gradually close under the pressure of the rock as it settles until their maximum supporting force is attained,” says a project spokesman.
“Although this technology was already known from German coal mines, it had never before been applied in these dimensions in tunnel construction.”
“The deformable steel inserts more than fulfilled expectations,” he adds. “Despite sometimes very large deformations, driving and securing work in the difficult geological conditions of the north drive progressed so well that breakthrough took place nine months ahead of schedule.”
Last month brought further challenges when a rock fall in the west tunnel between Faido and Sedrun −about 600m behind the drive in the east tunnel − brought work to a standstill.
For a tunnel of such length, safety is paramount.
The Gotthard Base Tunnel is two single-track tunnels 40m apart, joined every 325m by cross passages. Emergency stations at Sedrun and Faido, contain crossovers and railway operations and ventilation equipment. The crossovers allow trains to change from one tunnel to another in an emergency. In a fire, smoke will be sucked from the affected tunnel and fresh air blown into the emergency station through the side tunnels and cross passages.
Test bores revealed that the hollow space subsequently formed was 3,000m3, extending 40m above the tunnel invert, over a 6m to 10m length.
To resume TBM driving, the loose area in front of the cutting head will be solidified with cement injected through a grouting tunnel from the east tunnel. This will take about three months.
While this has been a frustrating setback, the Gotthard Base Tunnel is on target for completion in 2017.
In March 1.1km of the Gotthard Base Tunnel was driven, meaning that to date, 144.8km of tunnels, galleries and passages − or 95% of the total 151.8km − has been excavated. Over at the much smaller Ceneri Base Tunnel, 7.7km − or 19.4% of the total 39.8km − has been excavated.
The first running tunnel breakthrough in the Gotthard Base Tunnel is scheduled to take place in the east tunnel between Faido and Sedrun in autumn 2010.
(Costs are at 1998 prices, excluding inflation, VAT and interest)
Gotthard 2002 - 2017
Cener 2010 - 2019
Faido 9.4m - 9.5m
Bodio 8.83m - 9.13m
Client: AlpTransit Gotthard AG
Contractors − Gotthard: AGN Strabag Transco Sedrun (Implenia Bau, Frutiger, Pizzaroti, Bilfinger Berger)
Consorzio Tunnel AlpTransit- Ticino (CSC Impresa Construzioni, Hochtief, Impregilo, Implenia Bau, Alpine Bau) Transtec Gotthard (Alpiq, Alcatel- Lucent/Thales, Alpine-Bau, Balfour Beatty Rail)
Contractors − Ceneri: Consorzio Condotte Cossi