Tunnelling complete on Grosvenor Project

Mine owner Anglo American Coal opted for an 8m-diameter Robbins dual mode rock/EPB TBM to bore two declined access tunnels at grades of 1:6 and 1:8. One of these was for conveyors and the other for people and equipment.

The TBM was optimised for hard rock and mixed ground excavation in geology ranging from soft clay and soil to sandstone and basalt. The two-stage screw conveyor worked in both ground conditions to draw muck from the tunnel face. Two sets of shields were built for the TBM's "Quick Removal" system, and were detached from the machine at the end of each blind tunnel to be left in the ground for constant support, a requirement in Australian tunnels. The inner core components of the TBM were then retracted out of the segment-lined tunnels on specially designed transport dollies.

The machine began boring the conveyor tunnel in December 2013, and after completion in May 2014 was successfully retracted and transported to the second tunnel site. The machine was then re-commissioned for the people and equipment tunnel in November 2014 with a new set of shields. Once excavation commenced, boring was completed in 88 days at an average of 10.9m per day, with a best day of 25.2m. 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.

The team is now preparing the machine for its final retraction and rollout, and it will be on the surface in less than two months. The TBM will then be stored onsite for future Anglo American mining projects.

The "Crossover" designation is new for Robbins dual mode TBMs, and according to the company will be used for all such projects going forward. The type of TBM used at Grosvenor is now known as an XRE, standing for Crossover between Rock/EPB. Other types of machines are designated the XSE (Crossover between Slurry/EPB) and the XRS (Crossover between Rock/Slurry).

The XRE TBM was picked over the traditional roadheader method for several reasons, including excavation speed and tunnel maintenance. The machine excavated at a rate approximately ten to fourteen times faster than a roadheader. The segmental lining also lowers maintenance requirements.

Robbins president Lok Home said the increasing number of projects being excavated in mixed ground would result in a swell in interest for these types of machine.