Record-breaking Robbins TBM demolishes rock at Kárahnjúkar

Jul 02, 2007

On June 25, 2007, a Robbins 7.23 m (23.7 ft) Main Beam TBM set its second world record in the 7 to 8 m (23 to 26 ft) size range while boring at Kárahnjúkar. The TBM blew away its previous world record of 92 m (302 ft) by excavating 106.1 m (348.2 ft) in just 24 hours!

The machine previously excavated a 10.3 km (6.4 m) long section of the main headrace tunnel for the Kárahnjúkar Hydroelectric Project in Iceland, breaking through in October 2006. This machine, TBM # 2, was one of the three Robbins TBMs working on the project for owner Landsvirkjun. Based on the success at Kárahnjúkar thus far, Italian contractor Impregilo SpA elected to reuse this machine, as well as Robbins cutters and field service personnel for an additional leg of the project. In November 2006 the TBM was dismantled and transported to the new tunnel site. The Robbins HP (High Performance) TBM was then reassembled and underwent routine maintenance before its launch on the Jökulsá Diversion Tunnel.

The Jökulsá Diversion Tunnel will be 8.7 km (5.4 m) in length and will provide additional water for the project by drawing from nearby Ufsarlón Pond. The tunnel will connect with the main headrace tunnel and the combined waters will then flow to an intake at the powerhouse. Since its second launch in April 2007, the TBM has excavated about 25% of the bored tunnel length, in the direction of Ufsarlón Pond. The entire tunnel will span 9.5 km (5.9 m) and is being drill and blasted at the opposite end by Icelandic contractor Arnarfell.

Geology in the Jökulsá Diversion Tunnel is much the same as that encountered in the main headrace tunnel, a mixture of basalt, moberg, and pillow lava geology up to 300 MPa (44,000 psi) UCS. The TBM has turned in record excavation rates despite hitting a very hard section of basalt rock early on. The machine continues to utilize back-loading 19-inch disc cutters, which offer increased cutter loading and longer cutter life in hard rock. Kárahnjúkar was the first project in the world to successfully utilize back-loading 19-inch cutters.

Water inflows have matched the expectations of contractor Impregilo SpA, and have not posed a problem on the drive. The rock so far encountered has been quite dry and hard, reducing the need for concrete reinforcements and shotcrete. Muck is being removed via continuous conveyors, which will extend as the TBM advances. From the conveyor, muck is then shoveled into trucks for transport to the muck storage site.

Pond and intake structures are currently being built for the Ufsarlón water resource. Water from the Jökulsá í Fljótsdal River as well as three tributary rivers will flow into the pond. Once the project is operational, the combined headrace tunnels will provide a maximum flow of 144 m3 (38,000 gallons) per second and an annual generating capacity of 4,600 GWh for the Alcoa aluminum smelter.

Further information:
The Robbins Company
Brianna Home (Marketing Director)
Solon, OH 44139
Tel: +1 253 872 4484

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The Robbins Company

44139 Solon, OH. USA


+1 440 248 3303


+1 440 248 1702



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