Successful excavation by TBM of the first of two coal seam access tunnels in Queensland paves the way for mechanised shield tunnelling methods to be more widely used in the booming Australian coal industry.
Completion of the first 1,100m concrete-lined access tunnel by an 8m diameter EPB/rock dual mode Robbins TBM fulfils the project owner’s (Anglo American) requirement for a tunnel of 40-year design life that will last for the entirety of Grosvenor Mine’s working life.
Completion of the drift access tunnel in nine months – despite excavation challenges associated with expected encounters with methane gas – is three times faster than the traditional roadheader method.
“This is the better methodology,” said Adam Foulstone, Underground Construction Manager at Anglo American, when asked if he thought more TBMs would be used on mines in the future. “Use of TBMs opens up a new chapter not just with Anglo American, but with the whole coal industry in Australia. Now we can draw up a new coal mine in less than a year, compared with two to three years if we use roadheaders.” Foulstone also noted that there are few limitations for TBMs in mines. “Anywhere we need to get men and materials into an underground environment is an opportunity to use a TBM,” he said.
The 8m Robbins machine and continuous conveyor system were chosen by Anglo American for the Grosvenor Decline Tunnel Project, on which its contractor Redpath began excavation in December last year (2013) for two tunnels through an expected geology of sedimentary hard rock up to 120 MPa UCS, mixed ground of mainly sand and clay, and coal seams.
The TBM was chosen over the traditional roadheader method for several reasons including speed of excavation. Another reason was maintenance: “The final tunnels need to remain intact for the life of the Grosvenor Mine [about 40 years], and be maintenance-free with cement linings,” said Foulstone. “This was the biggest factor when determining our tunnelling method.”
The machine is optimised for hard rock single shield excavation because only the first 300m of each tunnel is through mixed ground. A two-stage, centre-mounted screw conveyor works in both hard and mixed ground conditions, and the cutterhead can be fitted with back-loading cutters in hard rock mode, as well as knife bits and scrapers in EPB mode.
Additionally, the machine uses its EPB technology to deal with methane gas, a standard feature in coal mines. To ensure worker safety and to avoid the risk of explosion, gas levels must be kept under 2% at all times. If any methane leakage is detected, a snuffing box evacuation system draws the methane out of the screw conveyor and directly into the ventilation system. In the first tunnel, methane was detected within the first 300m. “The snuffing box was very useful,” said Foulstone. “It allowed us to monitor the tunnel face, plus a boundary sensor ensured we didn’t go above 0.5% methane content.”
The first tunnel, excavated at an incline of 16.5%, is for men and materials; the second tunnel, at a 12.5% incline, will be for conveyor systems. The approximate lengths of the tunnels are 1,100m and 950m respectively; these are approximate and based upon the depth at which the coal seam is hit. “As soon as we see coal coming out of the conveyor we conduct a face inspection to verify. Once we have a coal seam taking up approximately 50% of the tunnel diameter, we know we’ve gone far enough,” explained Foulstone.
Australian tunnels require constant ground support, and the TBM was customised with a Robbins-designed “Quick Removal System” which allowed the machine to be removed from the initial tunnel and retracted from its outer shield bodies, leaving them behind to support the ground at the end of the drive.
For excavation of the second tunnel a new set of shields will be assembled onto the machine; these also will be left as support at the end of that drive. In order to transport the machine to the next tunnel, which is 2km away, the TBM had to be split into two sections and moved by low-loader. It is expected to begin boring the second tunnel in November (2014), following reassembly, with second completion scheduled for March next year (2015).