Another milestone in tunnelling in the tough conditions at Kárahnjúkar hydro scheme, Iceland, was reached in December with hole through by the third of three Robbins TBMs driving the 39.7km headrace tunnel.

The 7.2m diameter Robbins TBM3 broke through to complete its 11.5km drive on 5 December, completing bored excavation of the headrace, which also required some drill and blast work.

Robbins said TBM3 performed well, averaging 520m per month with 1,193m driven in the best period, early in the difficult bore. Tunnellers have faced basalt, pillow lava and moberg in the headrace. In late 2004, TBM3’s upstream advance slowed as the ground became more fractured with heavy water ingress.

The contractor, Impregilo, planned to use TBM3 in split drives, boring upstream and then downstream from Adit 3. However, the adverse ground conditions saw the plan overhauled and the TBM taken out early from the upstream drive, which was turned over to drill and blast (T&TI, October 2005, p16).

Instead, in a re-scheduling move with programming benefits, TBM3 was started early on the downstream drive towards its sister, 7.2m diameter machine, TBM2. The change of plan helped the tunnelling works when TBM2 soon met fault zones up to 8m wide and progress slowed whilst closing the gap between the machines.

TBM 3 bored 14.56km in total, including adit opening, Robbins told T&TI, and it achieved the best monthly advance of the trio of the company’s machines. The others, TBM2 and TBM1 (a refurbished 7.6m diameter TBM), managed 1,183m and 865m, respectively.

In terms of average monthly progress, though, TBM3 was behind TBM1 (a main beam machine) which delivered 586m but was ahead of TBM2’s advance of 383m. Rock along the headrace had UCS up to 300MPa.

Such performances were no doubt a relief to Robbins given the difficulties faced by TBM3 early in its journey. Ferruccio Borroni of Robbins Europa, said in a statement: “There were many unexpected problems at the outset. At Adit 3, the TBM experienced a dramatic quantity of water inflows, but the problem was overcome because of diligence and good planning from Impregilo and the quality of the TBM and cutters.”

TBM3 may next head for Switzerland. Either it or TBM1, which holed through in early September 2006 after driving a total of 14.65km, will be transported and rebuilt for the Ceneri Base Tunnel scheme, in Switzerland.

Lok Home, Robbins’ president, said: “The project was quite large, and in terms of risk and problematic tunnelling, like the AlpTransit Project. The risk was quite high, but the project turned out extremely well.’ He added: “There were no major mechanical failures during the project. Considering the length, diameter and difficult conditions of the tunnels, that’s a satisfying result.”

TBM2 holed through late September 2006 after slower going in the faulted stretches, said Robbins, driving 10.33km in total. However, during the drive it advanced 92m in 24 hours, which Robbins said is a record tying with its TBM in the same class on the Epping to Chatswood rail drive in Australia, in 2005.

TBM2 is being re-assembled to drive a branch tunnel from near the mid-point of the headrace. Again working out of Adit 2, the TBM will bore most (8km) of the Jokulsa diversion tunnel, slightly less than earlier planned as scheduling allows drill & blast to also be used. Excavation will start this quarter for completion in 2008.