The 7.2m diameter Robbins hard rock TBM boring the Adit 3 scope of the headrace tunnel has been re-launched on the Kárahnjúkar hydropower project (T&TI, October 2005, p16) in Iceland for the 4.5km long drive towards its sister machine that is boring a 11.1km long section from Adit 2 of the headrace tunnel.
The project is being constructed by Iceland’s major electricity supplier, Landsvirkjun, The National Power Company. Impregilo was awarded the contract for the headrace tunnel in 2003 with a tendered price of US$341M. Contract supervision has been delegated to the VIJV comprising Mott MacDonald, Línuhönnun, Hnit, Fjarhitun, Sweco, Norconsult and Coyne et Bellier.
Three Robbins TBMs are at work on the headrace tunnel, through Icelandic basalt and glacial waters. A 7.6m diameter machine, TBM 1 is boring the 13.3km long section of the downstream headrace tunnel. TBM 3 is the recently launched machine that had run into difficulties at the end of 2004 on a 6.3km long drive from Adit 3 towards the Power Intake at Hálslón reservoir.
The ground was highly fractured and large inflows of water were encountered. T&TI was told peak flows at the portal were as high as 1000l/s, with 200l/s issuing at the face. Various measures such as pre-grouting were attempted to enable the machine to struggle on, but in July last year after completing 5,198m of its drive, the decision was made to abort the TBM method and opt instead for drill and blast for the balance of the drive as it more easily allowed grouting of the entire face. TBM 3 was pulled back and turned 180 degrees to start the bore towards TBM 2. After being re-launched, it managed to excavate 92m in the final days of 2005.
TBM 2 is the 7.2m diameter sister of TBM 3, it is boring a 11.1km long section of the headrace tunnel. Robbins told T&TI it was restarted in November last year after being stopped for nearly three months for pre-treatment of an extensive fault zone. In August last year, a collapse almost caused the TBM cutterhead to lose rotation due to the horizontal pressure of the collapsing muckpile. In January, Robbins said the ground conditions “continue to be difficult and high water inflows continue to plague the effort”. Notwithstanding this, TBM 2 excavated 360m in November and more than 200m in December.
Of the three, TBM 1, the 7.6m diameter machine, has had the easiest ride so far. In the last quarter of 2005 its average monthly advances were 722m, 854m and 626m. Robbins told T&TI that in January that for the past 15 months TBM 1 has achieved an average monthly advance rate of 629m per month, regardless of ground conditions and water inflows.
