Hybrid tunnel boring machine takes on Australian decline tunnel
On December 20, 2013, an 8 m (26.2 ft) diameter Robbins hybrid single shield/earth pressure balance tunnel boring machine (EPB TBM) was launched to bore the Grosvenor Decline Tunnel for Australia’s Anglo-American coal mine. The machine was built using onsite first time assembly (OFTA) in order to fit within a tight project schedule. Assembly at the remote jobsite near Moranbah, Australia took about 4.5 months, and then the machine was walked down into a launch tunnel.
Two decline tunnels, at grades of 1:6 and 1:8, will be used for mine access to new coal seams. The hybrid machine is tackling mixed ground conditions ranging from sand and clay to varying types of sedimentary hard rock up to 120 MPa UCS, as well as coal seams. Methane gas is expected to be present throughout the tunnel, so the machine has been designed as explosion proof compliant to ERZ-1. “Grosvenor is the first underground coal mine in Queensland to use a TBM for drift construction,” said Glenn Tonkin, Grosvenor project director. “We are proud to be pioneering this technology.”
Only about 300 m (984 ft) of ground in each decline tunnel are expected to require EPB mode, while the rest of the tunnel will be bored in hard rock mode. Thus, the design was optimized toward hard rock excavation.
In EPB mode, the machine uses a two-stage, center-mounted screw to deal with watery muck. One screw conveyor will run faster than the other, creating a muck plug that will push water out of the screw conveyor. The machine’s mixed ground cutterhead is fitted with interchangeable knife bits and disc cutters. In addition, the cutterhead is clad with extensive wear plates for abrasion protection. The mixing chamber can be filled with water, foam and other additives to keep it spark-safe in the presence of methane.
“The machine essentially uses its EPB technology to deal with methane gas safely,” said Doug Harding, Robbins vice president. If any methane leakage is detected, an evacuation system called a “snuffing box” will draw methane out of the end of the screw conveyor and directly into the ventilation system. “The TBM tunneling method will deliver advances in safety, higher quality drifts, and faster project development,” said Tonkin.
To convert to hard rock mode, a hydraulically-operated muck chute is deployed around the screw. The muck is picked up by paddles in the muck chamber to load the screw. Interchangeable EPB knife bits will be replaced with disc cutters on the cutterhead, and the EPB scrapers on the cutterhead must be replaced with hard rock bucket lips. A skew ring offsets or reacts to the torque of the machine in hard rock, allowing for more efficient single-direction cutterhead excavation and muck pickup.
A final unique aspect of the project is a specially-designed “quick removal system.” As no ground in Australia can be left unsupported and the machine is boring a blind tunnel, it will be able to retract in one piece from its shield, leaving the shield in place. “The core of the machine is a bolted design that separates from the shield, in a process that does not require a cutting torch,” said Harding. The TBM core and back-up will then be walked up the decline tunnel on a set of custom transport dollies and moved to the second decline tunnel, where another set of shields will be waiting for machine assembly prior to launch.
“We commenced tunneling last month and the machine has advanced more than 100 m (328 ft) so far. We are on track to reach pit bottom in the next couple of months. The first few weeks were largely spent commissioning the machine in compliance with the mine standards. We expect the machine will perform well and we will achieve the planned cutting rates,” said Tonkin. The machine is expected to complete the first decline tunnel in April 2014.