Fri Sep 8th, 2006 at 10:25:22 AM EST
When opened around 2015, the Gotthard Base Tunnel (GBT) in Switzerland will be the world's longest with 57,051 m (also see my tunnel diary). The twin-tube rail tunnel is excavated from five points: from both ends and from three intermediate access shafts. On Wednesday, 6 September, miners celebrated the first breakthrough: after almost four years, the tunnel boring machine (TBM) carving out the southernmost section of the east tube reached the next section.
Some data: the TBM was the Herrenknecht S-210, 8.83 m wide, with trailers 410 m long. It "ate" 13,425.77 m of rock – with blasted/dug sections, getting 15,969.05 m from the south portal. Yet the deviation upon holing through was only 5 centimetres horizontally and 2 centimetres vertically. Its daily record was 38 meters over a 17-hour working day.
The actual holing-through was a rather dusty affair:
You can look at a lot of other photos of the event at the builders' page. I'll show one more, looking at a giant cavern, with spectators walking towards the breakthrough site:
Why the two exit tunnels at the other end? The section the TBM arrived to is one of the two complexes where there will be an emergency stop with shelters, and connecting tunnels to change tracks. (Left on the picture is the southward west-tube-to-east-tube connector.)
How do works stand on the entire tunnel? I'll explain using the start of September overall progress chart (note: the complicated parts, e.g. the access shaft areas are not to scale):
From north to south (left to right):
- Erstfeld (north portal): this section was long delayed due to locals' disputes about how the second stage of the new north–south line (of which the GBT will only be a part) should continue from here. The final decision was a long extension of the GBT (to 75 km total!), with stubs of the branching-off tubes to be built already in the first stage. This year only the work site and the portal area will be finished, the TBMs may start only next year due to a legal challenge from the company that lost the construction tender... but as this is the shortest section, there may be no overall delay.
- Amsteg (northernmost intermediate access): two TBMs headed south from here more than three years ago. Going faster and meeting less difficulties than expected, one finished in June, the other has 500 meters (a month's work) left (out of 10.7 km). But the most difficult 700 metres are still ahead, and may take up to two years: the end of a geological transition with shear stresses and bad rock, to be excavated with explosives, baggers and special reinforcements.
- Sedrun (middle intermediate access): cutting through a whole series of geological transitions, this section was expected beforehand to be the most difficult, especially the last one kilometre to the north, traversing rock that is compressed under pressure and expands when relieved. But the special method invented just for these killer 1200 metres worked smoothly (more than 60% done, though rest may take up to three years), while the south drives faced less difficult conditions than expected, so the latter section will even be extended.
The access point of this section is also the northern track-changing/emergency-station complex. We discussed on ET the idea and final decision to convert this into a 'normal' passenger station "Porta Alpina", in which skiers can board lifts to ascend a world record 800 m in the present vertical access shafts.
- Faido (southernmost intermediate access): a second brutal problem region was expected in the middle when the tunnel was first proposed: a water-bearing gravel zone, pressurized by the weight of two kilometres of mountain, known methods to pass it: none. But an exploration tunnel found that miraculously, only a few dozen metres above the tubes of the future tunnel, there is a phase transition into water-excluding rock of sugary composition - an easy section for TBMs! Yet problems further south will still make the Faido–Sedrun holing-through the last one (in 2009–2010).
One spot unexpected geology struck was right at the end of the access shaft, where the second track-changing/emergency-station complex (see photos at start) was to be: a horizontal layer broken into grainy material by shear forces, right across the centre of the complex, very difficult to excavate and prone to wall collapses. Thus TBMs towards Sedrun haven't been started (the two from the south will continue from here next year), and even the excavation of the complex itself nears finishing only now:
You see two main tunnels with the cross-connections, the emergency exit tunnels connecting the two emergency stops, and the ventilation tunnels for the latter.
Green is "raw" excavated tunnel, red is tunnel whose wall was already stabilised, blue is what the TBM of the next section excavated, the purple arrow points at the point of holing-through. The striped black is concrete on which the TBMs will be pushed through the complex. Black arrows show where there was active work last week.
Weekly updates here
- Bodio (southern portal): Trouble with gravel zones just beyond the portal was expected, but all the rest was to be for TBMs (installed via a small bypass tunnel). Here, too, an unexpected problem zone very similar to that at Faido was met right at the start (of the TBM section). It took a year to pass it, but then the machines munched the remaining 13 kilometres at record speed. The second TBM has only some 650 metres left (2–3 months).
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