Last Breakthrough at Grouft Tunnel in LuxembourgThe entrance of the Grouft rescue tunnel was holed through on 18th June, 2007 near the Lorentzweiler-Blaschette road, thus marking the last breakthrough at the Grouft tunnel. The main downhill, 2-lane tunnel was completed on 4th December, 2006 while the uphill, 3-lane tunnel broke through on 16th April, 2007. Click here.Tunnelling and support tasks are not concluded yet. Out of the 650,000 cubic metres to excavate, 80,000 cubic metres still remain to be bored to finish the benches (advance has reached 90%) and invert. The end of tunnelling is scheduled for autumn 2007. Concreting inside the tunnel has commenced in mid-February. To date, approximately 730 metres of concrete has been poured on the crown and walls, which accounts for about 11% of the total length. Safety tunnelMidway through the tunnel, the topography made it possible to build an access from the surface. The rescue tunnel entrance is located at about 30 metres next to road CR122 that connects Lorentzweiler with Blaschette. The 2,966 m-long Grouft tunnel is therefore divided in two approx. 1.5 km sections that will substantially shorten the escape ways and improve fast access for rescue teams. The escape exit runs from the downhill 2-lane tunnel T2 though cross passage G6 to the uphill 3-lane tunnel T3, which is connected by a ventilation cavern to the rescue tunnel. Cross passage G6 has a length of about 30 metres while the rescue tunnel extending it is 45 metres long, 11.5 metres high and 10.5 metres wide. Its cross-section is identical to that of the vehicular cross passages in the Grouft tunnel. In view of its shallowness, the rescue tunnel will also be used as ventilation to extract fumes in the event of a fire. Thus, two powerful fans, installed in the side cavern, will be able to pump smoke out from each tube through large diameter ducts in the upper part of the tunnel. Civil engineeringConstruction of the Grouft rescue tunnel was not easy. Not only crews had to deal with substantial stress concentrations limited to some areas due to the complex three-dimensional design of the tunnel - which entailed sequential excavation and heavy supports - but also with poor geological conditions consisting of the Rhetian formation, which was also not easy to handle when the main tunnels were excavated.Tunnelling and support works had to be adapted to the geological conditions really met. The overburden above the rescue tunnel is less than one diameter and there is no important side cover. Considering the Rhetian stratum lying at lower level than expected and a thicker weathered mantle, engineers cautiously estimated it was better to apply heavier supports and avoid relying on the self-supporting capacity of poor quality materials. Sequential excavation was thus modified and the foundation level of the tunnel was lowered at the intersection of tunnel T3 and the cavern. This is how deformations remained under control while disrupting the least possible the massif, a strategy applied along the entire tunnel length.Digging of this real underground cathedral, which greatest headroom at the time of excavation was about 17 metres and length 28 metres, was performed in several phases: top heading, reinforcement and steel beams at the junction with the starter tunnel T3, first bench and bolting of the walls, second bench with side anchors, invert and reservations for foundation slabs. The completed invert was then filled to raise the working level to be able to dig the starter tunnel T3, which was reinforced uphill and downhill the cavern. Once supporting work was completed, construction of the chimney was carried out from the surface on a remaining depth of four metres. The rescue tunnel was built from the inside. Click lu/13. Visit www.pch.public.lu/projets/tunnel_grouft/index.html 26/07.