The Robbins TBM’s breakthrough last month on the Jökulsá branch tunnel marked the end of the widely varied excavation experience on the headrace and transfer tunnels of the Kárahnjúkar hydropower project in Iceland.
Progress rates achieves with the 7.23m diameter TBM included 115.7m in 24 hours and 428.8m in the best week – both achieved early on the drive. The machine was launched in April 2007 and completed the 8.7km long bore in a year.
Geology along the route comprised basalt, pillow lava and moberg with UCS up to 300MPa. The excavation of the branch tunnel did not have the difficulties the TBM previously encountered when driving a 11.1km long stretch of the main headrace tunnel.
Two other Robbins TBMs were employed on the headrace excavation, one of which – “TBM1” – has been refurbished, enlarged from 7.6m to 9.7m, fitted with 19” cutters and has started work on the 2.4km long adit for the Ceneri Base Tunnel, in Switzerland (T&TI, March, p10). The other machine will work in China.
In total, contractor Impregilo used the three TBMs to excavate almost 50km of tunnel over four years. The power station is already being fed by the headrace tunnel, but is not at full power. Following the completion of floor concreting and walls being lined with mesh and shotcrete, the branch tunnel is due to convey additional water to the power plant by August.
A Robbins TBM breaks through on the Jokulsa branch tunnel
