The machine broke through in late September, notching up to 658m in one month and 192m in one week. But to achieve this meant contractor Shea/Traylor JV had to overcome tough conditions as the TBM and conveyor system had to cope with overstress in the crown that resulted in significant rock fallout in seven different areas. The longest section of overstress – mostly encountered at a bifurcation of the main tunnel – was 700m long and took two-and-a-half months to get through.

The crew had to increase the prescribed rock-bolt pattern of four to six bolts at 1.5m centres to six bolts at 1m centres. This worked out to two rows per push. When that was not enough, they installed wire mesh in the crown, mine straps and channels. But it took up more time than expected to install steel support, remove loose rock and deal with rockfall. Overbreak varied from a few centimetres above the machine to 30cm or more.

Then there was the natural methane gas the crew had to contend with just before holing through. It was discovered while probing out 50m ahead of the machine – something that the crew did continuously throughout the bore, using one-, two- or four-probe holes, depending on the geology. It took two weeks to contain the gas within the cutterhead where concentration spiked at 100% LEL. Work resumed after systematic ventilation, probing and multiple grouting.

Surprisingly, the TBM was not new: it was a refurbished 1990s model comprising older components but with a brand new Robbins cutterhead and with completely rebuilt electrical and hydraulic systems. The conveyor system – also supplied by Robbins – included a 68.6m-long vertical belt which facilitated the impressive production rates.