Mechanized mining using TBMs is a burgeoning trend worldwide, but for Stillwater Mining Company (SMC), it’s an integral part of their operation. The mine, located in Nye, Montana, began using TBMs to access platinum and palladium reefs in the late 1980s. For Tyler Luxner, project engineer at SMC, TBMs offer many advantages: "The common perception in mining is that TBMs are large and unwieldy machines, but we have used them successfully for 24 years."
The mine’s use of TBMs may be different compared with civil works tunnels — they are typically used for the life of the mine, on thousands of meters of tunnel — but their success is setting the standard worldwide, particularly for mines in China, Chile, and Australia, which are considering TBM usage of their own.
Unique geography
The Stillwater Mine is the largest producer of platinum group metals in North America, responsible for nine per cent of the global palladium and four per cent of the global platinum supply. The mine’s metals are located in the J-M Reef, a 45km-long ore body tucked between layers of mafic to ultramafic rock.
While the mine uses several methods of excavation including blasting with muck removal by load haul dump machines, longer development bores were needed to access new parts of the reef and extend the life of the mine. When considering TBMs to bore their access tunnels, also known as footwall laterals, mine engineers considered both the benefits and drawbacks. TBMs can produce faster advance rates than traditional mining methods. Historically, the capital cost for TBMs has been approximately 1.5 times that of conventional mining fleets, but with only 33 per cent of the operating costs. The size of a TBM can be an issue for mining, however, because of the ability to make turns. The turn radius in conventional mining is often 20m, while TBMs are capable of a 300m turn radius.
The maneuverability is not necessarily a problem though, if long and straight tunnels are the goal. "It depends on the geometry of the ore body (reef) and the access," says Stillwater Mine’s chief geologist Mike Koski. "In our mine the ore body runs parallel to a river valley that runs east to west for 45km. That lends itself well to TBMs, because you can develop large reserve blocks that are easier to ventilate and don’t require as much ground support as a drill and blast tunnel." The reef at Stillwater Mine, as a seam-like ore body, is well-suited for the longer excavations that TBMs make efficient excavators.
Stillwater’s TBM pioneers
The platinum and palladium mine first purchased a 4m diameter Kelly-type TBM in 1988 to excavate a mine access tunnel through the area’s characteristically layered gabbro, norite, and anorthosite rock. Koski says, "We were faced with trying to further develop the mine and open up its reserves, but one of the challenges was the ventilation. We wanted to reduce the cost of driving foot wall laterals, but didn’t want to have vertical ventilation shafts drilled, like those that are necessary for drill and blast. We also determined that ground support for circular tunnels would be less."
The mine wound up using that first machine for several tunnels in the late 1980s and early 1990s. "I think our early estimations were spot on — the ground doesn’t need to be supported nearly as much. For long chunks of footwall lateral, it’s a good way to go," says Koski.
In 1999, Stillwater Mine purchased a refurbished Robbins Main Beam TBM from Magma Copper Mine in Arizona, using it along with another TBM to excavate access tunnels in the East Boulder section. "We drove twin tunnels, and then mined between them to create the main access for the East Boulder part of the mine," said Justus Deen, manager of technical services at Stillwater’s East Boulder Mine.
According to Deen, the Robbins machine is one of the oldest working TBMs, having been in operation for more than 20 years, and it has required no refurbishment after 5.6km of excavation. "It’s been boring another 2.6km mine development tunnel since September 2011, so by the time it’s done it will have bored 10km total. After that, maybe we will do some refurbishment on it." The TBM is currently slated to finish up its latest bore, known as the Graham Creek Expansion, in the second quarter of 2013 next year.
Boring the Blitz Tunnel
Stillwater Mine’s latest endeavor is the Blitz Tunnel, a development tunnel project of at least 7.1km that will provide information about the reef along the eastern portion of the mine.
A 5.5m diameter Robbins Main Beam TBM, previously used on New York City’s East Side Access Project, was completely refurbished in Robbins’ Solon, Ohio, facility for the project. The TBM is slated to begin tunnel excavation later this month, detecting the location of the reef in relation to the TBM while simultaneously creating a rail haulage tunnel that will be used for the life of the mine.
Detecting the reef in relation to the TBM will require careful analysis using two core drills mounted on the machine. Deen says: "Steering the TBM will be an interesting process because we have very complex geology here. We will do diamond (core) drilling ahead of the machine, above the machine, and to the side after every 150m. We will drill and log the core right there while we are drilling it, then interpret the results, so it will be concurrent with boring."
The operators will then make adjustments to the TBM bore path based on the perceived distance to the reef. Koshi explains: "We don’t want to get too far away from or too close to the ore body. The ore zone is located in a very distinct layer of igneous rock, so if we penetrate the right rock types then we know we are in the right place. Platinum and palladium is hidden in base metal sulfides, so we will look for the presence of the ore, but those metals are only present in core samples about 50 per cent of the time, so the rock types will guide us."
Geotechnical probe drilling will also be a regular occurrence on the TBM, in addition to core drilling to identify the reef position. "At the same time that we are diamond drilling, we also want to identify rough ground conditions that are ahead, such as weak rock, water, and dykes," says Koski. "We will probe ahead every 550ft [167.6m] and look at the results, and in that way we will know we are placing footwall laterals in the best possible ground."
The TBM will excavate Norite rock and will utilize a variety of ground support depending on the conditions identified. Support will range from ring beams, shotcrete, and rock bolts to McNally crown support consisting of continuous steel slats. "We’re expecting mainly competent rock with claylike material in fault zones. Our standard ground support will be bolts with wire mesh; however, if we encounter very poor or blocky ground, we will switch to the McNally support system," said Luxner. The system has been employed successfully in very deep tunnels including the 2,000m deep Olmos tunnel in Peru.
The method utilizes steel slats that are extruded from pockets in the machine’s roof shield as the TBM advances. The slats are bolted in place using rock bolts and steel straps, creating a network of support that prevents fractured ground from shifting and collapsing.
TBM launch and excavation
TBM assembly and launch are being carefully executed due to the small size of the launch portal, just 4.1m high and 4.4m wide. The main beam and cutterhead support were assembled outside the portal, and then moved through the portal into a 350ft (107m)-long launch chamber using specially designed transport dollies. Final assembly of the cutterhead, roof and vertical supports, gripper shoes, and bridge is currently being completed in the launch chamber. Lifting of components inside the launch chamber is done using lifting eyes supported by rock bolts. A shortened startup configuration consisting of four back-up decks and a four-car muck train will be used until the machine mines ahead 45m, allowing the remaining six back-up decks to be installed and a longer muck train to be used.
Excavation is planned to take about three years. Stillwater Mine says it will likely continue to use the machine after its initial project, and may extend the Blitz tunnel if it proves successful. Ultimately, the benefits of TBMs for Stillwater are clear, says Deen: "TBMs offer faster advance rates for us than drill and blast, and the drives are easier to ventilate. It’s also easier to train our TBM miners, as the required skill set is not as complex."
