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A2 Second Gotthard tube
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Tunneling methods

The 16.9 kilometre-long second tube will be almost entirely excavated using two tunnel boring machines (TBM). Blasting is only necessary in the two geological fault zones and around the portals. Two further tunnelling machines were used to excavate tunnels to access the fault zones.

The tunnel boring machines

Two single-shield TBMs are in operation in the main tunnel. In shield tunnelling, precast ring-shaped lining segments (Tübbing) are installed to prop up the excavated tunnel. Hydraulic thrust cylinders advance the TBMs 10 metres at a time. The TBMs have a diameter of about 12.3 metres. At up to 4.6 revolutions per minute, they have an average drilling performance of two metres per hour.

The tunnelling machines are mounted at the respective portals and positioned in pre-excavated start tubes. After the breakthrough, the TBMs will be dismantled and removed.

Smaller TBMs with a diameter of around 7.4 metres have already been used to create the tunnels to access the two geological fault zones. A single-shield TBM was used at the southern end of the tunnel and a gripper TBM at the northern end. Unlike the shield TBM, a gripper TBM does not require Tübbing elements. Instead, the tunnel wall is secured with shotcrete (sprayed concrete), which is why a gripper TBM is only used in very stable rock.

More about the TBMs:

BLASTING IN THE FAULT ZONES

There are two fault zones to the north and south of the tunnel which affect the construction work: the approx. 270-metre long Mesozoic fault zone around 4.1 kilometres after the Göschenen portal, and the almost 300-metre long Guspis fault zone, which is around 4.9 kilometres after the Airolo portal. Pressurised rock conditions prevail in each of these fault zones, so they are being excavated using conventional means rather than using a TBM. Some areas of unconsolidated rock have to be worked on with a pick hammer and excavator shovel and immediately secured with shotcrete and anchors. Steel arches also ensure that the pressure from the rock does not reduce the size of the tunnel cross-section to any significant extent. More compact rock sections are excavated by blasting.