A not-so boring solution - tunnel boring machines
The use of tunnel boring machines (TBMs) in mining is growing. Michael Forrest talks to Lok Home, President of TBM manufacturer The Robbins Company, USA, to find out why.
For centuries, the traditional way of creating access to underground ores was by drilling and blasting. The technique is simple – holes are drilled in the desired direction, either by hand with rock bits and hammer or by machine. The holes are then filled with explosives that create a void when detonated. The amount of charge and the size, depth and length of the holes causes rock fragmentation, hopefully at a suitable size to aid removal. The blast also takes into consideration the strength of the rock, its geological inclination (including laminations and faults) and its reaction to void creation. The latter is particularly important, as bringing down the roof does not progress mining. It is the skill of the miner and the mine geologist that ensures the created structure remains intact, which may require extensive roof support from rocks, bolts and other stabilising equipment. It is also important to minimise the promulgation of shock waves that may weaken the new void.
Today, drill and blast is highly evolved and automated. Nevertheless, it still has a number of disadvantages. Foremost is speed of advancement, stemming from the iterative process of drilling, blasting, waiting for ventilation to clear fumes and removing ore and waste. In ore production, drill and blast is flexible, allowing independent grade control. In development work (building the mine infrastructure), the speed of advancement and ground control are of primary importance. Since the 1950s, the mining industry has looked at the use of tunnel boring machines (TBMs) in the civil engineering sector with some degree of envy, especially for the development of horizontal tunnels for access, haulage and ventilation.
Lok Home of US-based TBM manufacturer The Robbins Company explains, ‘The use of TBMs in mining is widening, and we expect the trend to continue as mines get larger and move underground.’
The basic design of a TBM starts with a circular rotating cutting head that determines the diameter of the tunnel. It is articulated from the drive mechanism to allow for steering and is pushed forward by hydraulic rams mounted on the machine. The sides of the machine also have hydraulically operated shields or grippers that clamp against the tunnel wall, creating a solid base from which to push forward the cutting head. Cut rock passes through the cutting head into a number of buckets and is conveyed to the rear of the TBM. Because the rock fragments are fairly uniform in size, conveying is an efficient transport mechanism that may extend to the surface.
At the outset
Early TBMs used in mining had insufficient performance to generate tunnel advancement much faster than using drill and blast. This was due to insufficient power, small cutter sizes, lack of operator experience in the mining sector and the difficulty in following a sinuous ore body. Today, however, following a move by manufacturers to produce TBMs designed for mining, their application has steadily grown around the world.
Part of this growth is a reflection of changing mining practice – namely the move to underground mining of large-scale, low-grade orebodies that was once the domain of the open pit mine. Environmental pressures favour underground mines, and many of the large-scale open pit mines have now reached maximum depth in relation to haulage and strip ratios. Low-grade mines require larger ore and ventilation passes as the amount of waste and number of mining areas increase. As a result, a number of mines have turned towards TBMs to provide access to underground orebodies. This has been facilitated by improved cutter design, advanced hydraulics, powerful electric motors and digital control of operation. Metal mining in general is in hard rock, and improvements in design have resulted in mining-specific machines. In contrast to civil engineering environments, a mining TBM will be used many times in creating access and ventilation tunnels as mining of the orebody progresses, often extending to the life of the machine.
One company to take advantage of the technology is the Magma Copper Company, at its San Manuel mine in Arizona, USA. Access to the Kalamazoo orebody for block caving required 12,800m of drifting (driving near horizontal passageways) using a 4.62m TBM comprised of a cutter head and main beam unit with 1,259kW installed power. After modification of the cutter discs for use in the relatively homogeneous rock, an average daily advance of 22.6m a day was achieved. Another successful use was at the Nye platinum deposit of Stillwater Mining Company in Montana, USA. The orebody is relatively well defined, however a 5,650m access tunnel was required. Unlike the Kalamazoo orebody, the geology at Nye comprises a number of sediments and igneous rocks with varying compressive strengths from 60–190MPa. The objective is to tailor the cutter head to the rock strength to enable efficient progress. Research has shown that correctly spaced disc cutters that maintain the correct compressive force with ample torque from the drive system, efficiently break the rock. A relatively high periphery speed of 200m/min is used for optimum advance in hard rock, while lower speeds are particularly effective in broken ground.
Counting the advantages
A major advantage in TBM tunnelling is the ability to install ground support systems – such as rock bolts, steel mesh and shotcrete – directly behind the cutting head as it moves forward. This minimises ground movement related to internal rock stress when creating a void, unlike drill and blast that promulgates shock waves, which increase the need for ground support. Tunnels cut by TBMs also provide smoother profiles that aid ventilation.
Stillwater, a pioneer in the use of TBMs in mining, has used several machines over decades on multiple projects. Advances over the years have shown that TBMs offer superior performance in speed and cost when used in tunnels several thousand metres long. New base metal mines are large scale and offer the opportunity to use TBM technology with projects in South America and Australia. Chilean mining firm Codelco has purchased a TBM for development work at Chuquicamata, where the underground mining of copper ore is planned. Also in Chile, Anglo-American is expanding its resource base at its Los Sulfatos and San Enrique-Monolito exploration projects at 4,000m elevation in the Andes mountains. The company will use a TBM to drive an eight-kilometre tunnel from which the deposit will be drilled out to allow yearround access, instead of the current December–March weather window.
Anglo will also be using first-time (ie, shipping individual components to the project site) on-site assembly of a TBM to tunnel a one-kilometre decline (grade 1:6) at its Grosvenor project at the Moranbah coal mine in the Bowen Basin, Queensland, Australia. First-time assembly offers savings of up to US$4m and can take four–five months less compared with assembly at manufacturer workshops and subsequent breakdown for transport to site. The machine will encounter lithologies varying from sand and clay to 120Mpa hard rock. Methane is expected in the tunnel, requiring the machine to be explosion-proofed.
The first TBMs were tailored to the individual mine, its rock hardness and its abrasive qualities. But with modern machines, cutting discs can now be changed from behind, reducing downtime while conveyor technology allows the rapid removal of waste. Tight turns still pose problems, but in large-scale, low-grade mines these are minimised. Above all, using TBMs in place of drill-and-blast offers a higher safety margin for those working underground, both in quality of air and rock stability. They seem to be here to stay.