TBMs raise work rate again at Karahnjukar

摘要

AFTER several weeks of slow progress due to difficult ground conditions, the three Bobbins tunnel-boring machines (TBMs) drilling the main headrace tunnel at Iceland's Karahnjukar hydro-electric project are generally making good progress again. April saw steady advances by all three machines, which had been held up previously due to the necessity to install extra support (full steel rings with mesh and shotcrete) immediately behind them. However, the work rates of TBMs 2 and 3 have slowed down since the beginning of May since they have encountered more bad ground. Drill and blast operations are now under way at both ends of the Ufsarlon headrace tunnel employing Impregilo's Sandvik Tamrock Axera drill rigs. Some 650 m have been driven, mostly at one end; drilling only began at the other end in late April. Progress on this section should improve rapidly. It is planned to drive 3,500 m by drill and blast. After this, it will be possible to use one of the TBMs drilling the main headrace tunnel to complete this section.