After five years of drill+blast excavations from the central cavern at the end of the TBM bored adit, first breakthrough of the southbound base tunnel into the portal at Vezia is achieved for the 15.4km Ceneri base tunnel.
The final blast at noon on 17 March marked completion through the southern end of the west bore and into the 400m long drill+bast opposite heading north from the Vezia portal. More than 600 tunnel workers and other project participants gathered for the celebrations in the presence of Claudio Zali, Director of Construction of the Swiss Canton of Ticino. At breakthrough a deviation of only 20mm horizontally and 10mm vertically was recorded by the contractor, the joint venture team of Condotte and Cossi that was awarded main construction in 2009 for a contract price of CHF987 million (€944 million).
“This first breakthrough in the Ceneri base tunnel is a further important milestone in the construction of the new rail link through the Alps,” said Renzo Simoni, CEO of AlpTransit Gotthard, the wholly owned subsidiary of Swiss Federal Railways which is responsible for delivering both the Gottard and Ceneri trans-Alpine base tunnels. He added: “Only with the 15.4 km base tunnel under Monte Ceneri will the continuous flat route from Altdorf to Lugano become a reality.”The total length of the main drill+blast headings, the adit and exploratory tunnels, and the cross passages, is 39.8km, with excavations now reported as 91% complete.
AlpTransit further reports that final breakthrough of the parallel southbound tunnel (east bore) is expected next month (April 2015), with breakthroughs of the opposite northbound headings towards Vigana portal expected at the beginning of 2016. Currently there are 1,500m of drill+blast excavations required to complete the northbound west tube, and 2,000m to complete the parallel east tube.
Once complete the Ceneri and Gotthard tunnels will be integrated into the existing Gotthard Railway route between Switzerland and Italy, significantly reducing journey times through the Swiss Alps by bypassing existing high altitude routes where speed is restricted by steep gradients, tighter curves and overcapacity. Excavations on the 57km long Gotthard Base Tunnel (to the north of the Ceneri Tunnel) were completed in 2011, with the railway line expected to become operational in 2017. The Ceneri Tunnel is on target to be operational two years later, in 2019.
Construction of the Ceneri tunnel officially began in June 2006 with the laying of the foundation stone. Geological data at base tunnel level was gathered during excavation of an exploratory tunnel near Sigirino between 1999–2003. At this point it was concluded that TBM operations would be restricted to excavation of the access/adit tunnel, with the base tunnel subsequently split into two headings, outwards from a cavern located half way between the two portals at either end of the main base tunnel headings.
In April 2007 the Consorzio Monte Ceneri (CMC) JV – a consortium of CSC, Lugano, Frutiger SA, Thun, Rothpletz, Lienhard + Cie, and Aarau – procured a 9.7m Robbins Main Beam TBM to bore the 2.4 km adit tunnel. The TBM was completely refurbished near Milan, Italy, and the cutterhead diameter increased from 7.6m to 9.7m.
This was the first machine on the AlpTransit project to utilise 19in cutters, which were designed to offer a higher cutter load, longer cutter life, and fewer cutter changes. The refurbished machine had previously excavated the main headrace tunnel of the Kárahnjúkar Hydropower Project in Iceland.
Geological conditions for the drive consisted of schist, Swiss molasse, and Ceneri orthogneiss; with a UCS in the range of 30–130 MPa (4,300–18,800 psi). Much of the tunnel was excavated under high cover of 600m, but no squeezing ground or large water inflows were encountered during the drive. New probe drills, designed by Robbins in the US, were used to verify ground conditions ahead of the face. Temporary tunnel support included rock bolts, ring beams and shotcrete, depending on geology.
On 6 November 2008 excavation of the adit tunnel was completed on schedule after only ten months. Only 30 cutter rings were changed during the last kilometre of excavation, with the cutters excavating a combined 160,000m3 of hard rock. Daily advance rates averaged 18.5m, approximately 61% higher than the averages achieved by similar machines procured to excavate sections of the Gotthard Base Tunnel using 17in cutters.