The Ceneri Base Tunnel: Construction Experience with the Southern Portion of the Flat Railway Line Crossing the Swiss Alps

摘要

This paper summarizes the experience that was gained during the construction of the 15.4 km long Ceneri Base Tunnel (CBT),which is the southern part of the flat railway line crossing the Swiss Alps from north to south.The project consisted of a twin tube with a diameter of 9 m interconnected by crosspassages,each 325 m long.In the middle of the alignment and at its southern end,large caverns were excavated for logistical and operational requirements.The total excavation length amounted to approximately 40 km.The tunnel crossed Alpine rock formations comprising a variety of rock typologies and several fault zones.The maximum overburden amounted to 850 m.The excavation of the main tunnels and of the cross-passages was executed by means of drill-and-blast (D&B) excavation.The support consisted of bolts,meshes,fiber-reinforced shotcrete and,when required,steel ribs.A gripper tunnel boring machine (TBM) was used in order to excavate the access tunnel.The high overburden caused squeezing rock conditions,which are characterized by large anisotropic convergences when crossing weaker rock formations.The latter required the installation of a deformable support.At the north portal,the tunnel (with an enlarged cross-section) passed underneath the A2 Swiss highway (the major road axis connecting the north and south of Switzerland) at a small overburden and through soft ground.Vertical and subhorizontal jet grouting in combination with partial-face excavation was successfully implemented in order to limit the surface settlements.The south portal was located in a dense urban area.The excavation from the south portal included an approximately 220 m long cut-and-cover tunnel,followed by about 300 m of D&B excavation in a bad rock formation.The very low overburden,poor rock quality,and demanding crossing with an existing road tunnel (at a vertical distance of only 4 m) required special excavation methods through reduced sectors and special blasting techniques in order to limit the blast-induced vibrations.The application of a comprehensive risk management procedure,the execution of an intensive surface survey,and the adaptability of the tunnel design to the encountered geological conditions allowed the successful completion of the excavation works.