TBMs may be the Way of the Future
Unconventional technology must be adopted to ensure that safe, efficient and responsible access to minerals is possible. Competitive mining operations will become increasingly dependent on Tunnel Boring Machines (TBMs) for mine development and expansion.
TUNNEL BORING machines may well be the next method to bring worldwide mining operations to the next level, but getting there requires both acceptance and the right combination of variables.
– Miners can use TBMs in the same way as civil infrastructure projects—the only difference is that in most tunnelling, the hole in the ground is the product. In mining, the tunnel is the means to an end. Mining companies need to familiarize themselves with the technology and become more comfortable in using TBMs with their own crews, said Dr. Jamal Rostami, Centennial Mining Engineering Chair at Pennsylvania State University.
Rostami cites several factors--from manoeuvrability to lack of skilled workers-- that are currently the limiting factors in many mines. But despite these issues, some mines have fully embraced TBMs and have reaped significant benefits.
– Stillwater Mine is the best at this, because of the fact that the company is comfortable with TBMs and has used them over and over, said Rostami. The U.S. mine launched its fourth TBM, a 5.5 m diameter Robbins Main Beam machine, in autumn 2012. Its TBM legacy is a model for recent efforts by mines in Chile, Australia, and China that may well bring on the next wave of mechanized mining.
TBM Use in Mines
Current trends in mining for ore bodies include a global reduction of surface deposits and continued increased awareness of environmental impacts from mining. As mining operations are driven deeper underground, the need for access tunnels grows.
Capital cost to purchase a TBM is higher than a drill and blast operation— an obvious barrier to early adoption. However, Rostami emphasizes that overall costs during the project can be much lower. In fact, mines such as Stillwater have reported that their TBM operations run at about 33% of the operating cost of similar D&B operations.
– TBMs offer a cheaper cost per meter of tunnel once the mine is familiar with the method and is able to run the machine in-house, he continued. This result correlates with field data collected over decades: in tunnels over 2 km in length, TBMs are the most effective tunnelling method and their increased advance rates more than make up for initial costs (Tarkoy & Byram, 1991).
More About TBM Advantages
In comparison with D&B methods, TBMs are a more automated form of construction, requiring fewer workers. It has been shown that less ground support is needed in comparison with drill and blast due to the round excavation profile. The type of ground support is also more versatile for TBMs - from wire mesh to ring beams, rock bolts, and steel slats using the McNally Support System. Installation of these types of ground support from within the machine shield, paired with the absence of explosive materials for excavation, also makes TBM tunnelling safer in general than Drill and Blast.
Time is both the main advantage and disadvantage of TBMs. The advantage comes in the form of advance rate whereas the disadvantage is due to delivery/ setup time. TBMs average speeds of 20 m per day, and are able to complete an access tunnel of several kilometers in 30% of the time needed for D&B. However delivery and setup for a new, custom TBM takes about 1 year. This means that the TBM will start six months after the D&B operations would. Despite the six-month latency, use of a TBM will still beat D&B to the finish by nearly a year. Furthermore, a TBM can be reused, so if a mining operation were to own one then the lead time for startup could be reduced from one year to a couple of months (Gratias et al, 2014).
TBMs have been used successfully in mines around the world a Robbins machine still in use at the Stillwater Mine was previously used at the Magma Copper Mine in Arizona, USA (seen here).
A 5.5 m diameter Robbins Main Beam TBM began excavating the Blitz mine development tunnel in Autumn 2012.
Operations at Stillwater Mine in Montana, USA include the use of TBMs to bore footwall lateral to access the ore body. Raise bores are then constructed for level-to-level access.
The Blitz tunnel drive will be at least 7.1 km in length and crosses several fault lines in mafic norite rock.
The Robbins TBM at Stillwater is using both probe drills and core drills to determine the ground ahead of the machine and the distance from the ore body.
Possible Disadvantages
Managers at Stillwater Mine added some limiting factors for the machinery in addition to the higher initial investment.
– The size of a TBM can be an issue for mining, because of the ability to make turns. With conventional mining you have a 20 m turn radius. TBMs have a 300 m turn radius, said Tyler Luxner, Project Engineer at Stillwater Mining Company. For that reason, TBMs are being developed with a shorter machine body and customized back-up systems to take smaller radii curves.
The manoeuvrability 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, explained Mike Koski, Chief Geologist at Stillwater Mining Company.
– In our mine the ore body runs parallel to a river valley that runs east to west for 45 km. 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.
Stillwater Mine’s TBM Legacy
At Stillwater Mine, located in Nye, Montana, USA, TBMs have been in use for years. The platinum and palladium mine first purchased a 4.0 m diameter open-type rock TBM in 1988 to excavate a mine access tunnel through the area’s characteristically layered gabbro, norite, and anorthosite rock.
– 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 footwall laterals, but didn’t want to have vertical ventilation shafts drilled, like those that are necessary for D&B. We also determined that ground support for circular tunnels would be less, said Koski.
– I think our early estimations were spot on - the ground doesn’t need to be supported nearly as much.
In 1999, Stillwater Mine purchased a refurbished Robbins Main Beam TBM, using it to excavate access tunnels in the East Boulder section. The Robbins machine has been in operation for more than 20 years, and has bored more than 10.3 km since its commissioning. The machine recently finished up a 2.6 km long extension at the East Boulder Mine, known as the Graham Creek expansion.
Blitz Tunnel
Stillwater Mine’s latest endeavour is the Blitz Tunnel, a development tunnel of at least 7.1 km that will provide information about the reef along the eastern portion of the mine.
A 5.5 m diameter Robbins TBM is determining 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 requires careful analysis using two core drills mounted on the machine.
– Steering the TBM is an interesting process because we have very complex geology here. We do core drilling ahead of the machine, above the machine, and to the side after every 150 m. We drill and log the core right there while we are drilling it, then interpret the results, so it is concurrent with boring, said Deen.
The operators make adjustments to the TBM bore path based on the perceived distance to the reef.
– 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, and if we see some ore we know exactly where we are, said Koski.
Decline Tunnels
Decline tunnels for access to deep ore bodies may be the latest way of ushering TBM use in mining. These unique tunnels often require customized TBM and back-up setups.
An explosion-proof hybrid TBM was launched at Australia’s Grosvenor Decline tunnel for the Anglo American Coal Mine in December 2013.
GROSVENOR DECLINE, AUSTRALIA.
A unique tunnel has just begun excavation near Moranbah, Australia at the Anglo American Coal Mine. An access tunnel is required for deep coal drifts. Two decline tunnels, at grades of 1:6 and 1:8, will be used for the mine access to new coal seams.
An 8.0 m hybrid EPB/rock machine was supplied for mixed ground conditions ranging from sand and clay to varying grades of 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. The TBM was launched in December 2013.
The machine has been specially designed with a “Quick Removal System”. As no ground in Australia can be left unsupported and the machine is boring a blind tunnel, it is designed to be retracted in one piece from its shield, leaving the shield in place. The core of the machine is a bolted design, allowing for separation without requiring a cutting torch.
The machine will then be walked up the decline tunnel on a set of specially designed transport dollies and sent by rail to the second decline tunnel, where another shield will be waiting.
CARRAPATEENA DECLINE TUNNEL.
Another decline tunnel, yet to begin excavation, is located at the OZ Minerals copper and gold mine in southern Australia. A high grade, cylindrical ore deposit has been identified 500 to 1,500 m below the ground. To excavate the ore body, a TBM access tunnel 1,000 m deep is required. A 5.8 m diameter Main Beam TBM was procured to excavate a 7 km access tunnel at 15.4% grade.
The angle of decline requires the TBM and continuous conveyor to be uniquely designed to maintain an acceptable angle for conveyor muck removal. The tunnel conveyor is brought directly up to the rear of the TBM, rather than using a transfer conveyor. The TBM is currently being built at a manufacturing facility in Shanghai, China.
The Next Level
Given the various aspects that these projects have demonstrated—boring longitudinal ore bodies, steep declines, and in gaseous conditions - modern TBMs have what it takes to make mine development rapid, efficient and economical. For deep ore bodies requiring drives over 2 km in length, TBMs should be seriously considered for their higher advance rates, improved range of ground support, and safety.
With the global demand for minerals increasing, mines can only be pushed in one direction - deeper. As the location of deposits changes, excavation must necessarily evolve with it. Those mines embracing mechanized tunnelling, and more specifically TBMs, will experience a paradigm shift in their mining operations. Ore bodies, which were once considered inaccessible, will finally be within reach. Early adopters of the TBM method will be able to better meet the increased demand and possibly extend the life of the mine – a result every miner hopes for.
