The Grosvenor Mine, a greenfield coal operation, was the first to utilize TBM technology for mining tunnels in Queensland. Owner Anglo American opted for an 8 m (26 ft) diameter Robbins Dual Mode “Crossover” TBM and continuous conveyor system, which was assembled on location using Onsite First Time Assembly (OFTA) with onsite support from Robbins’ experienced Field Service team. The Crossover XRE TBM was picked over the traditionally-used roadheader method for several reasons, including excavation speed and tunnel maintenance. The machine bored two decline access tunnels at grades of 1:6 and 1:8, one for conveyors and another for people and equipment.
Ground conditions varied throughout each tunnel. Both drifts contained geology consisting of mixed soil and rock conditions, with the first 300 m (984 ft) or so of each tunnel containing the majority of the mixed ground such as soft clays and soils.
The machine’s Crossover capabilities enabled it to operate in both hard rock and mixed ground. In addition, the TBM was required to operate in gaseous conditions. The unique TBM design included a cutterhead capable of interchanging hard rock and soft ground cutting tools, a two-stage center-mounted screw conveyor, a “quick removal” shield system, and flame-proof machine components due to the possibility of methane gas in the underground environment. A machine with EPB capabilities was chosen not only due to the presence of softer ground, but also to contain the methane gas where it could then be diluted or safely removed from the tunnel.
Construction on the Grosvenor project began in July 2012. The first of the decline tunnels, for conveyors, was excavated between the end of December 2013 and the beginning of May 2014, after achieving advance rates of up to 90 m (295 ft) per week. The Quick Removal System was a success, allowing the TBM inner core to be retracted back to the surface from a 160 m (525 ft) depth using specially designed transport dollies. In order to transport the machine to the next tunnel 2 km (1.2 mi) away, the TBM had to be split into two sections and required a large 600 metric ton lift.
The machine was re-commissioned for the men & materials tunnel with a new set of shields, and commenced boring in November 2014. Excavation was completed in 88 days at an average of 10.9 m (35.7 ft) per day, with a best day of 25.2 m (82.7 ft). The TBM averaged 70 m (229.7) a week, about 14 times faster than a roadheader. The bore itself was similar to the first, with few challenges encountered other than elevated methane gas levels that required several temporary stoppages in order to safely remove the gas from the tunnel. Final breakthrough was reached on February 9, 2015. Upon completion of the second tunnel, the machine shields were again left in place to provide continuous support.