Final finish for Gotthard Baseline excavation Mar 2011
- Some 15 years after excavations began, creation of the underground space that is the 57km long twin-tube railway passage through the base of the Gotthard Massif of the Swiss Alps is complete. On Wednesday last week (March 24, 2011), the last of more than 28 million tonne of rock was transported out from deep inside the mountain to mark the end of mammoth journey for men and machinery; for managers and politicians.
Era of excavation ends for Gotthard tunnellers
- In a much more understated event than the reports of the first tube final breakthrough on October 15 last year, that sent echoes resounding around the world, this final breakthrough in the western tube was for the workers and those who have dedicated much of their careers to seeing this extraordinary project realised. As with the previous first tunnel final breakthrough, the finish of the parallel tube was also marked with the last of four Herrenknecht TBMs that worked on the project completed its 11.1km drive from the Faido access adit on the southern attack of the project pushed through into the drill+blast effort advanced from the interim 800m deep Sedrun Shaft access point.
- Counting all 152km of excavation completed on the project, about 44% was drill+blast and about 56% by TBM. Counting only the 114km of the two main tunnels, more than 85km or about 75% was bored by the four Herrenknecht gripper TBMs.
- During the course of reaching the current status of the project, many different contractors and engineering firms, many thousands of workers, and several different suppliers and equipment manufacturers have been involved, many from overseas. Of these, three main contracts were let for excavation of the main tunnels and their passages and chambers. At Sedrun, the consortium of Implenia/Frutiget/Bilfinger Berger/Pizzarotti completed 11km of drill+blast headings at the bottom of the two 800m deep access shafts. At the south end TAT, the Tunnel AlpTransit Ticino JV comprising Implenia/Alpine Bau/CSC Impresa/Hochtief/Impregilo was awarded two lots and operated two Herrenknecht TBMs running parallel for about 14km from the Bodio portal initially and then carrying on into the 11km long drives from the Faido access adit to breakthrough into the Sedrun drill+blast. From the north the AGN, Arbeitsgemeinschaft Gotthard-Basistunnel Nord JV of STRABAG AG of Switzerland and STRABAG AG of Austria, managed the two other Herrenknecht TBMs on the project for the initial 10.7km of tunnel from the Amsteg adit to breakthrough with the Sedrun drill+blast and then the 7.1km parallel lengths from the Ersteld portal to Amsteg.
- In each section immensely difficult conditions were encountered and conquered.
Deep shaft access
From the bottom of the 800m deep access shafts at Sedrun, sunk from within a cavern deep inside the mountain, not only was the ambient temperature of the underground environment up to 45°C, but, under the maximum overburden of 2,300m, squeezing behaviour and tremendous stress relief deformation was encountered in the headings. To cope with these conditions, the now famous sliding arch supports that provided support while yielding under excessive movement, were developed and deployed with great success.
- The support logistics that maintained a bearable as well as safe working environment demanded impressive ventilation, shaft access and backup installations.
Boring from Bodio
Tunnelling in the south with the two Herrenknecht TBMs from the Bodio portal by TAT started in January and February 2003. Almost immediately, after only 200m, Sissi and Heidi, the two 8.83m diameter TBMs, were slowed down by geological fault zones featuring the so-called brittle rock kakirites. The unstable conditions made it practically impossible for the gripper machines to achieve good performance rates. Every meter driven had to be secured in a complex manner. It was not until August 2003 that the machines finally completed the difficult 400m and left the fault zones behind them. Three years later, in September and October 2006, they broke into the multifunctional station at the intersection with the Faido access adit after tunnelling 13.4km in the eastern tube and 14.1km in the western tube.
- Following extensive revision work of the TBMs that included fitting new larger 9.43m diameter cutterheads, the two rock machines set off again towards Sedrun in July and October 2007. En route, the TBMs worked through the much-feared Piora Basin geological feature. in 2008 and 2009. During the planning stages, great effort and resource was spent investigating this feature until it was discovered that a cap of gypsiferous rock protected the tunnel alignment by sealed the pressurized sugar-grained dolomite at the bottom end. From March to July 2010 however, the western tube TBM was halted by collapsing rock conditions that required substantial stabilization measures to allow advance to resume. Meanwhile Sissi carried on in the parallel bore to achieve the first 57km long eastern tube's final breakthrough on 15 October 2010 to great fanfare and jubilation.
- Breakthrough celebrations last week in the western tube saw TBM Heidi emerge into the undergound chamber and mark an emotional end of the "Gotthard era" of excavation for the tunnelling teams, as Renzo Simoni, CEO of AlpTransit Gotthard AG put it. Over the coming few weeks, the TBM, like her twin in the eastern tube last year, will be dismantled and transported back out through the now completed tunnel to the Bodio portal.
North TBM headings
The two TBMs on the northern side of the mountains were launched on their journeys in late 2003 and early 2004. Despite encountering difficult geology such as the Intschi Zone, that had one of the TBMs trapped for more than six months, the machine drives from the Amsteg adit toward Sedrun achieved top speeds of up to 40m a day. In June and October 2006, the two 9.58m diameter Gabi 1 and Gabi 2 Herrenknecht gripper TBMs reached their section boundary, nine and six months earlier than planned, to close more than 10.5km of tunnelling each.
- After being retracted from their dead end headings, the TBMs were transported to the northernmost Erstfeld portals where they were launched in Spring 2008 on their second drives toward Amsteg. On these drives the TBMs turned out to be true mountain sprinters, boring and supporting up to 56m of tunnel in a single day, and reaching their destinations near Amsteg in June and September 2009 - yet again six months earlier than planned. On all the their drives, the TBMs were guided to maximum precision. After 7.1km of tunnelling, the northern TBMs hit their final breakthroughs within 4mm and 8mm of horizontal and vertical deviations from tunnel axis respectively. The deviation from the target on the project's final breakthrough last week was a mere 3mm on the vertical axis and absolute precision or 0mm of deviation horizontally. In total, the four Herrenknecht TBMs excavated about 10.5 million m3 of rock that was hauled by continuous conveyors to huge muck piles at portals, a large percentage of which was recycled back into the tunnels in their primary shotcrete and final in-situ concrete linings.
Parallel headings of the 57km long mountain pass through
- Once the high-production final lining operations are completed and the tracks and infrastructure systems installed, the first high-speed trains are scheduled to start speeding across - or rather through - the New Railway Alp Crossing (NEAT) at 200km to 250km/hr in 2016. The final breakthrough in Sedrun last week represents a decisive milestone for this mammoth project.
- Epoch-making Gotthard Baseline breakthrough - TunnelTalk, Oct 2010
Concrete contribution to Gotthard undertaking - TunnelTalk, Oct 2010
Great celebrations for Gotthard TBM arrival - TunnelTalk, July 2009
Gotthard TBM safely across the Piora Mulda - TunnelTalk, Nov 2008
- AlpTransit Gotthard AG
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