In following where the strata, seams or ore bodies (reefs) lead in underground mining, great service has been brought by – and continues to come from – the long proven methods of drill and blast along with other mechanised excavation, such as using roadheaders, raise bores and also hand digging. All are able to virtually turn on a dime below ground, which is vital as the ground is read for commercial opportunity and also risk.
The expanding ranges of offerings to the mining sector include: Sandvik’s DS421 cable-bolting rig for small tunnels, its new range of UDR exploration drill rigs, and the bolter miner MB610; Atlas Copco’s recently launched range of four new Simba rigs for long hole drilling and the Boomer E1 C-DH for sites lacking water and electrical infrastructure – all of which follow the previously introducing the Boomer T1 D for narrow vein mining and the M1 L rig for low vein mining; and, from Aker Wirth, came the T1.14 roadheader which is compact and has been optimised for coal mining.
However, while not newcomers to the sector, and despite the challenge they have of typically taking much longer distances to make turns causing them not to have been seen as the ubiqitous choice for excavation in mines, there are new opportunities – and demands – for TBMs, or variations on them. TBMs are increasingly finding their place in the sector.
TBMs
A plus in the favour of TBMs is their ability to produce faster advance rates and for lower operating costs than traditional methods, although on the downside they cost more to acquire, – historically, about 1.5 times the capital spend, according to Stillwater Mining Co.
However, TBMs usually take longer to set up before production gets going and, depending on their size, can have far less local flexibility when underway compared to the other excavation methods, as noted. The shields can have the likes of a 300m turning radius compared to often 20m in conventional mining, though towards the latter scale are coming hybrid machines, such as Aker Wirth’s new mobile miner system.
But if long access tunnels – particularly those that are slight, gradual or have few deviations in alignment – are part of a mine’s development plan then the issue of the turning radius is not so much of an issue.
Mike Koski, the chief geologist at Stillwater Mine in Montana notes that the degree of manoeuvrability needed is dependent on what lies underground, such as the reef geometry, and also the access needs.
A further variation in reflecting upon the options comes with the support required for mining – physical support to help maintain structural integrity of excavations as well as all the other types of support systems and methods to ensure safety. For a given set of geological conditions the mining work, performed by different excavation methods would, most likely, call for varying degrees and types of ground support; in terms of further aspects of safety, the other support choices are around factors such as ventilation, air quality, vehicle movement, personnel evacuation from a blast area shutdown, etc, which all feed in to mine development as well as operations.
The merits of different excavation methods and typical support required depend, like with civil engineering to some degree, on the nature and scale of the objectives developing the mine and weighing the long-term economics of alternative choices. Unlike civils, however, the nature of mining would see the excavation assets working as part of the operational phase of the site; they would be there not simply as part of the set-up stage but for the longer-term, which further influences economic choices.
Robbins at Stillwater
The Stillwater mine is among the leaders in use of TBMs in mining, but the site also continues to employ a range of excavation methods, notably drill and blast (jumbos, down hole drills, and raise bores). In the third quarter of this year, the mine owner, Stillwater Mining Co, was getting underway the fourth TBM it has deployed during the life of the mine, which extracts platinum and palladium from a major reef.
The shield – a 5.5m Robbins Main Beam TBM – was not built originally for the mining task but is crossing over from civils works performed on the opposite side of the US. The shield was among the handful of TBMs on the intricate rail tunnel excavations below the heart of the Big Apple, where multiple parallel and branching tunnels were opened up as part of the complex East Side Access (ESA) project.
Following a complete refurbishment, the TBM will bore the ‘Blitz’ tunnel which will open up access on the east side of the mine. The initial plan is for a 7.1km drive and five major faults are anticipated. The cutterhead is equipped with 25 x 19in single discs plus 4 x 17in center discs, and has rated thrust of 10,898kN, rated power of 1,968kW (6 x 328kW), and torque of 2,630kNm at 7.2rpm.
While the shield gets ready to bore on the Blitz, another Robbins TBM has been extending its mining activities at Stillwater by opening up another, 2.6km long, development tunnel for the mine complex.
When it finishes its latest drive in 2013, the additional stretch of bored tunnel will be only a quarter of the total distance excavated by the machine at the mine. The shield was previously used at the Magma Copper Mine, in Arizona.
Although they are commonly viewed in the industry as ‘large and unwieldy’ machines, says Tyler Luxner, a project engineer with the mining company, TBMs have been used at Stillwater for almost quarter of a century.
Stillwater’s first TBM was a 4m diameter Kelly-type shield in the late 1980s. The mining company also acquired a 4.6m diameter CTS TBM in the late 1990s. Seli’s takes Compact south
In Chile, one of Seli’s versatile, recent-concept TBMs was made to order to help open further, underground, sections of the Los Bronces copper and molybdenum mine.
Mine owner Anglo American Chile wanted to optimise the development of the world-class Los Sulfatos copper prospect, and as customer it opted for one of the Italian tunnelling machine manufacturer’s ‘Compact DSU’ type shields – and only the second ever made.
Launched in 2009 at the mine, some 65km north east of Santiago, the 4.5m diameter TBM was used by the mine owner’s JV contractor, Dragados-Belsaco to bore the 8.1km exploratory and access tunnel. The mine until then had been an open pit and Anglo American Chile was starting to take the operation underground in the Andes mountains but space for portals was tight, which influenced the choice of TBM.
The Compact DSU is a simpler, shorter TBM meant to give the advantages of a double shield (DSU = double shield unit) while being less expensive, and it has a bolted assembly. The concept was developed to be an alternative to open shields and compete with drill and blast drives opening up headings and benches.
Another key feature of the Compact DSU system is the capability of handling a wide variety of ground conditions requiring support ranging from rock bolts, steel sets, mesh and shotcrete to steel ring erection. At Los Bronces, the cutterhead was fitted with 27 no. 19in discs, had power of 1,575kW and torque of 2,612kNm. The maximum main thrust was 15,700kN with auxiliary thrust of 14,300kN. The mine owner’s contract with Seli also included provision of backup equiment, technical assistance and specialised workers.
Very early in the drive the tunnellers met collpasing faces and foam injection helped to fill the cavity and stabilise conditions. A number of fault zones were crossed, and the mine owner shifted the tunnel alignment to have greater overburden. However, more problems were met with heavy groundwater inflows of more than 100 litres per second at the face and carrying much debris. Inflows were met at other sections, some more – up to 150 litres per second – and others less severe.
After just over two years of tunnelling, the TBM completed the tunnel in November 2011 and its average daily progress rate was approximately 16.5m, Seli says. The advance rates peaked at 46m/day earlier that year.
Not developed as a dedicated concept machine for the mining sector, the first use of a Compact DSU was in the civil engineering sector, on an Italian hydropower project.
Aker Wirth goes mobile
At another copper mine – this time in Australia, and owned jointly by Rio Tinto and Sumitomo – verification trials are getting underway with a new hybrid TBM for mines, or Mobile Tunnel Miner (MTM), made by Aker Wirth. Developed in cooperation with Rio Tinto and unveiled mid-year, tests of the MTM at Northparkes mine are the first application of the tunnel boring system.
Looking like a cross between slim, spindly fixed arms of a roadheader with the partial shield-roof and long train body of a TBM, the MTM was developed by Aker Wirth as part of Rio Tinto’s ‘Mine of the Future’ programme, launched almost five years ago. Aker Wirth began working with the mining giant under a technological agreement in 2010.
Aker Wirth notes that development the new concept machine, which ‘combines the flexibility of a roadheader with the robustness of a tunnel boring machine’, builds upon an earlier version produced in the early 1990s. The manufacturer further notes that among the points of versatility, the MTM can bore circular tunnels – like a classic TBM – but also tunnels with rectangular and horeseshoe-shape cross sections up to 6m wide. The company says this flexibility means ‘eliminating the need to backfill the lower section of the round cross section’.
Additionally, using undercutting technology, the firm says, the MTM is ‘especially efficient’ in boring through extremely hard rock, up to 300MPa. The MTM is, though, also equipped with ground support systems.
The MTM also has been designed to have a turning circle of barely 30m, which near that of conventional mining equipment but allows far, far tighter turns than the usual TBMs. A number of swivel joints are used to achieve such manoeuverability for a machine of that size and capacity.
In a statement, Aker Wirth’s chief execiutive Einar Bronland said: "We will revolutionalise safety and efficiency in underground mining with the new Mobile Tunnel Miner. With this tunnel boring system Aker Wirth will play a decisive role in shaping the future of the mining industry."
The first MTM was despatched in late June from Aker Wirth’s production facility near Dusseldorf, Germany, to be transported to Northparkes mine for verification tests in the field. Previously, the mining company noted that the trial phase had been integrated into a pre-feasibility expansion at the mine.
Contrasting drill and blast against a fresh approach with tunnel boring systems, Rio Tinto notes the MTM should double the daily advance rate to 10m-14m/day as well as cause less damage to rock mass. It adds, though, that drill and blast has long established predigree, boasting high flexibily, scalability, and wide choice of drill and blast equipment whereas tunnel boring systems, like MTM, have been prototypes only. It adds that such tunnel boring systems have ‘an immediate application in main access development and material handling system development’.
John McGagh, Rio Tinto’s head of innovation, says, "The system incorporates continuous mechanical rock excavation that will not damage new tunnel walls, while still providing the ability to mechanically install ground support in parallel with tunnel advance."
He adds: "Importantly for Rio Tinto, it provides an opportunity to introduce fundamentally safer process into the underground mining industry."
Atlas Copco, Herrenknecht
Further developments in excavation systems for mine development have been taking place under Rio Tinto’s ‘Mine of the Future’ initiative, including technological agreements with other manufacturers, such as Atlas Copco and Herrenknecht. The former has been developing a new tunnelling machine – the Modular Mining Machine (MMM) – while Herrenknecht is focued on a single shaft construction system that covers many tasks previously done separately.
Atlas Copco’s MMM builds upon its previous Robbins Mobile Miner developments and is married to long experience of other rock mining methods since the 1980s-90s. Like the earlier mobile miners, the MMM has a large diameter, forward-rotating cutting wheel but the drum is much thicker than before. The backup body is also a very different configuration.
The MMM system is getting close to trials in the field. In recent presentations, Rio Tinto has indicated that trials are proposed over 2013-14 at its Kennecott copper mine, in Utah. As required by the mining giant, the performance criteria for the MMM is akin to that of Aker Wirth’s MTM in that these two tunnel boring systems are to achieve daily rates of about double that of conventional methods, such as drill and blast.
With cross-sector adaptation and leverage of technology part of its strategy, Herrenknecht has been building upon its mechanised tunnelling pedigree to take ever longer strides into the mining sector with a particular focus on shafts. It notes that with raw material deposits being exploited at depths of down to 2,000m, the need for fast and efficient exploration methods is of increasing importance especially with conventional approaches to developing the necessary infrastructures being time-consuming.
To that end, the manufacturer provides mechanised boring with various types of equipment for varied purposes, such as constructing deep vertical shafts for hard rock or weaker ground conditions, or new or linking shafts (vertical or inclined) within mine developments. In particular, the equipment types are: shaft boring machines and roadheaders; vertical shaft sinking machines; raise boring rigs; boxhole boring machines; and, shaft sinking jumbos.
Following tests in Germany, at the Clara mine, Herrenknecht’s Boxhole Boring Machine (BBM) is now in service at Newcrest Mining Ltd’s Cadia East copper and gold mine in New South Wales, Australia. In developing the BBM, Herrenknecht took its experience in microtunnelling pipe jacking technology from the surface civils projects to bore a variety of vertical and inclined narrow shafts underground. The machine is used to prepare the draw points for ore extraction in block cave mining.
It is transported on a purpose-built small crawler to the tight location, placed upright, the jacking frame adjusted and braced, and the cutter-head begins boring with thrust pipes being added. The equipment is remote-controlled, does not require the crawler with the cable drum to be immediately adjacent. At Cadia East, working on site since late 2011, the BBM was used by Mancala Pty to bore more than 40 slot holes, of 16.5m average length, by June this year.
Separately, in its co-operation with Rio Tinto, Herrenknecht is developing a Shaft Boring Machine (SBM) that combines three separate processes – excavation, transport and structural support, says the mining group. It adds that the SBM is for sinking large diameter shafts, and combines synchronous mechanical hard rock excavation, muck transport and rock support into an innovative single system, which is to improve construction rates and safety while reducing technical risks. Rio Tinto is considering locations for trials of the SBM system, which are anticipated around 2015 or soon after.
Mine of the future
The agreements with three of the major underground equipment manufacturers are part of Rio Tinto’s push to get at more difficult deposits and improve tunnelling efficiency, safety and economics.
The mining group is pushing on two major fronts:
- To overcome barriers to more difficult or marginal minerals recovery, such as targeting ore bodies at greater depth and overcoming declining ore grades through improvements in mining as well as processing technology;
- To achieve significant improvements in safety and speed to enable the jump in excavation volumes needed for underground development infrastructure that will be key to establishing block caving for "super mines" of interlinked mineral deposits that are to exploited in parallel, such as the copper ore complexes of Resolution in Arizona and Oyu Tolgoi in Mongolia.
Like other excavation developments as part of the programme, Rio Tinto said all three new concepts are coming from application of civil engineering technologies crossed with its own experts’ mining input as well as that of contractor partners Redpath and Cementation, respectively.
McGagh says the partnerships would ‘fundamentally change the world of underground mining by further improving safety and allowing more rapid construction of new underground mines.’
He adds that the partnerships were ‘valuable in helping us to solve the challenges of developing block cave mines.’ Such a mining method involves a warren of tunnels below and around cone-shaped zones that will be blasted and collapse under gravity, then the rock extracted. The aim is to scale-out the sequential pattern and increase both productivity and output
