?The first of the pair of TBMs to bore the largest remaining stretch of twin running tunnels at the Gotthard Base Tunnel has begun excavation, and its sister machine is due to start work next month.
Driving the east tunnel of the Faido-Sedrun section, the 9.43m diameter Herrenknecht Gripper TBM (S-210) will bore through complex geology, noted client AlpTransit Gotthard, and it expects final breakthrough in 2011. The drive length is just over 14km.
With the second TBM (S-211) due to set off on the parallel west tunnel drive in October, some three months after the first, the machines are following a similar lead to that employed on their previous drives on the Bodio-Faido section.
They started boring in 2002-03 and finished the Bodio-Faido section last year. The TBM drives were each 13.5km and the machines experienced difficult geology along their alignments.
In early 2006 they were caught by squeezing ground, and once past the zones overburden deformed the tunnels by 500mm along a length of 300m. The stretches of tunnel were reprofiled, slightly delaying some concrete lining works.
Since finishing the Bodio-Faido drives, the TBMs have been overhauled – including increasing the cutterhead diameters by 600mm to deal with squeezing ground from the overburden – and transported through the Faido multi-function station for re-launch.
Ahead of them lies the Sedrun multi-function station, excavation of which was completed earlier this year. Before then are partially bored tunnels awaiting the arrival of the TBMs. Grouting of the water bearing rock mass of the west tunnel bore, which is some 1300m below the region of the Nalps reservoir, was also completed after some months of work. The ingress rate was reduced by three-quarters from 12 l/s to 3 l/s.
Beyond Sedrun, to the north, is the Amsteg section, which consists of completed 11.35km twin bores, most of which were driven by another two Herrenknecht TBMs (S-229, S-230). These machines were removed and are being overhauled for re-use (to be renamed S-421, S-422) on the final section of twin running tunnel to be excavated, just beyond the Amsteg – Erstfeld stretch, which will have main tubes just over 7.7km long.
After disputes over the contract award of the Erstfeld section were finally settled, the AGN JV (consisting of Murer–Strabag, and Strabag) was confirmed as contractor earlier this year. Work began on site preparation in the second quarter of the year. The JV had built the Amsteg section using the same TBMs.
Gotthard Base Tunnel is almost 70% excavated, and it will be the world’s longest tunnel on its opening, scheduled for late 2017. However, AlpTransit noted that an appeal has been lodged against the award of the rail infrastructure contract, which needs to be awarded shortly to keep to programme.
First TBM re-launch for Faido-Sedrun drive at Gotthard
