Tunnelling Ends on Metro Project in LausanneThe final milestone has been reached on the M2 metro line in Lausanne, Switzerland when two teams of tunnellers made the junction at a 136 m-long tunnel under Caroline and Langallerie streets, thus connecting Bessieres and Ours stations on 28th July, 2006. This final breakthrough marked the official end of tunnelling. However, short does not mean easy and fast. The Langallerie tunnel has been the most complex underground section of all the M2 line tunnels since it required sequential excavation, a gradual and slow process carried out in four phases. First, two little sideline tunnels, 3 m wide and 4 m high, have been drilled from June 2005 on either side of the base of the future tunnel. On completion of these side drifts, crews concreted the foundation sole of the tunnel on its entire length. Excavation of the top heading could then be achieved in a second stage. The final stage consisted in building the invert, i.e. the space between the two sideline tunnels. Tunnelling took place in poorly cohesive sand and silt, using roadheaders. Click here for more. Full-section tunnelling would have been too dangerous, with risks of subsidence and collapse of the face. But to speed up the advance, tunnelling took place from two drives since last winter. Junction at the Langallerie tunnel on the M2 line in Lausanne on 28th July, 2006 The day before that final breakthrough, the 314 m-long Saint-Laurent tunnel between Flon and Riponne stations was completed. Further to the collapse in February 2005, crews built a curtain of concrete piles around the collapsed area to create a plug to prevent further soil to fall down into the tunnel (read E-News Weekly 16/2005 & 10/2005). Then, reinforcements were implemented prior to excavation using forepoling (canopies of steel pipes around the vault) and fibre-glass nails anchored into the face. Excavation progressed metre after metre through the plug using a simple excavator, followed by the placement of steel beams and shotcreting.The M2 line is composed of eight short tunnel sections from station to station, spanning a total 2.8 kilometre. They have been excavated in just two years since June 2004. Opening of the line is programmed for the second semester of 2008. The Lausanne metro will climb a 12% incline and will be the world's steepest metro system. Click ch/36. Read E-News Weekly 25/2006. Visit www.t-l.ch/m2 and http://snom2.crfpower.ch 33-34/06.