The contracting JV of Porr Asdag Tunnelbau/C Baresel/Zschokke Locher is preparing to recommence excavation of the main UX15 cavern at Point 1 of the CERN LHC Project in Geneva. The project is being designed and supervised on site by a JV of Electricite de France and Knight Piésold (UK).
In the UX15 cavern, the past nine months have seen the completion of the waterproofing and lining of the 35m span 13m high vault.
Waterproofing comprised the installation of 3,400m² of 2mm thick PVC membrane and associated 5mm thick drainage blanket. Square profile lattice arches were used to support the inner and outer layers of reinforcement cage for the 1.3m thick lining to limit the number of membrane penetrations. Some 720t of reinforcement up to 40mm diameter has been installed in the vault and 10,700 tonnes of concrete. Prior to excavation of the 30m deep bench, the vault is being suspended partly from the two access shaft linings and partly by 38 tensioned ties of 225t working load, supplied and installed by sub-contractor, Freyssinet.
Load cells on the tensioned ties and strain gauges in the concrete will be used to monitor the vault lining during bench excavation. Knight Piésold’s principal geotechnical engineer on site, Ralph Parkin, told T&TI that designing the innovative solution of suspending the vault from tensioned ties had caused severe headaches for the design team and instrumentation would be followed very closely when excavation restarts beneath. When the UX15 bench excavation reaches the level of the existing LEP tunnel, excavation switches to the out lying RR and UJ caverns before returning to finish the UX15 bench. Concreting the 5 m thick invert slab will be followed by the walls back up to the underside of the vault at which point the tensioned ties will be destressed.
The civil engineering for the LHC Project is due for completion at the beginning of 2003 with the LHC coming on line in 2006.
