Delivery at depth

Deep below the snow tipped peaks of the swiss alps work is underway to construct the new 57 km twin bore Gotthard Base Tunnel which will provide the route for a new high speed rail link. One of the deepest sections – known as Sedrun – is currently being excavated through pressurised rock using blasting.

Around 2,5 km of overburden overlies the Sedrun section of the tunnel and if support is not installed soon after blasting, the pressurised rock can re-close the excavation. The Transco consortium – a joint venture of Batigroup, Frutiger, Bilfinger + Berger and Pizzarotti – is using German mining technology followed by a layer of shotcrete to provide initial support on the two 6,5 km long tunnels and cross passages.

Immediately after the blasted material is cleared, the consortium erects two interlinking sliding steel arches which lock together as the excavation 'relaxes'. This is then followed by several layers of shotcrete which is applied using a fleet of four Sika Putzmeister 500s and two Aliva 500 shotcreting machines.

The rock pressure in some sections is so great that the 6,5km long tunnels can only be advanced by a metre at a time and each cubic metre of shotcrete is being strengthened with 27 kg of 50 mm long steel fibres. The temporary support is followed later by internal formwork which can vary in thickness from 0,3 to 1,2 m. Concrete and shotcrete for the work is manufactured by a specially erected plant at the base of the 800 m access shaft and transported to the tunnel face by mixer trucks.

Shotcrete machine availability is vital as the build up of pressure in the surrounding during a breakdown could lead to a collapse. In a bid to maximise machine reliability Transco has set up a workshop for the shotcrete units to carry out maintenance and Sika has staff on site round the clock to provide specialist advice.

Construction of the CHF 1,165 billion (€ 752 million) Sedrun section started in 2003 and is expected to be completed in 2008.