Excavation of the last main bore at the Kárahnjúkar hydro scheme in Iceland is underway with a re-assembled Robbins TBM having been re-launched to drive just over 8km of the Jokulsa diversion tunnel.

The 7.2m diameter machine – ‘TBM2’ – had previously helped to bore the 39.7km long headrace tunnel, experiencing difficulties with fault zones on the way. Having been launched in 2004, the machine finally completed 10.33km of tunnel by late September last year. However, during the drive the TBM achieved a 24-hour advance of 92m.

Originally, the 10km long Jokulsa diversion tunnel was to have seen more TBM excavation but scheduling opportunities have allowed local contractor Arnarfell to undertake slightly more drill and blast work. The 15-month TBM drive is due to complete in mid-2008.

Geology in the area is basalt, pillow lava and moberg, but there are a number of fracture zones some with heavy water ingress that tunnellers discovered when driving the headrace. Rock along the headrace had UCS up to 300MPa. The diversion tunnel will run from the Ufsarlon pond to join the headrace near the mid-point, not too far from where TBM2 had experienced its own difficult ground conditions.

TBM2 was one of three Robbins machines used to excavate the headrace tunnel. The two other machines – 7.6m diameter TBM1 and 7.2m diameter TBM3 – bored 14.65km and 14.56km, respectively, with hole through last September and December. The latter machine worked with TBM2 on the upper headrace, delivering programme benefits when tough ground was encountered having itself had to change plans and re-launch early, leaving the remainder of a troublesome stretch to drill and blast (T&TI, January 2007, p4).

The two machines are now lined up for other projects, Robbins tells T&TI. Already on its way to the centre of Europe, TBM1 is being supplied to the Ceneri Base Tunnel project, in Switzerland. The other machine, TBM3, is to be refurbished and delivered to the Jinping scheme in China to bore a drainage tunnel, separate to the recent Robbins awards for machines to drive two of four parallel headrace tunnels.

Meanwhile, Impregilo, the contractor at Kárahnjúkar, is due at the end of this month to bring first flows down the headrace, reports the client Landsvirkjun. Additional workers were added to help complete the tasks, which included lining and building three rock traps that are 100m long by 4m deep.