Just over 106m has been excavated in 24 hours by a 7.23m diameter main beam TBM driving the Jökulsá branch tunnel of the Kárahnjúkar hydro project in Iceland, which Robbins said was a world record for the 7m-8m machine class.
In the latest drive, ‘TBM2’ has benefited from ‘quite dry and hard’ rock conditions, reducing the need for concrete reinforcement and shotcrete along the 25% of the 8.7km machine-driven length of the tunnel so far bored, Robbins noted. The TBM is fitted with 19” back-loading disc cutters for the drive through basalt, pillow lava and moberg with UCS up to 300MPa.
The previous record for the class, also held by the same machine on the hydro project, was an advance of 92m in 24 hours during its 10.33km drive on the headrace tunnel. Geological conditions encountered by the TBM in the upper part of the headrace were tougher than those encountered so far on the Jökulsá bore (T&TI, May, p10).
TBM2 was relaunched in April after completing its drive on the headrace by late September 2006, having been working at Kárahnjúkar since 2004. In November, the machine was dismantled and transported to be rebuilt for the Jökulsá drive, most of which is TBM-driven except for a drill and blast section of about 800m at the top end by local contractor Arnafell. Mucking out is by continuous conveyor, being extended as the face advances.
Two other Robbins TBMs worked on the Kárahnjúkar project and completed their drives on the headrace last year, and are now earmarked for the Ceneri Base Tunnel in Switzerland and the Jinping project in China, respectively. Together, the three machines bored most of the 39.7km long headrace tunnel, some in very difficult conditions, and the balance being completed by drill and blast.
Contractor on the project is Impregilo and the owner of the hydro scheme is Landsvirkjun, which is developing Kárahnjúkar specifically to supply electricity to the Alcoa aluminium smelter. The project is expected to produce 4600GWh per year from a maximum flow of water down coming the upper headrace and the Jökulsá branch, which will tap the Ufsarlon pond, of 144m³/s.
Robbins’ 7.23m 0 ‘TBM2’ before working at Karahnjukar, Iceland; and after its headrace drive Robbins It is now driving a branch tunnel Branch Tunnel Good ground is healping ‘TBM2’ to beat its previous record TBM
