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Mechanical mining: new advances in machine technology and cutter design are extending applications in the mining of hard rock.

In the past 30 years there has been a gradual evolution of mechanical excavating systems in underground hard-rock mining applications. Today the main machine types in use include full-face tunnel boring machines (TBMs) of various dimensions, undercutting machines (e.g. Atlas Copco mini-fullfacer), machines that cut using a disc on a revolving wheel (Robbins Mobile Miner Type,) mini-disc machines of the type developed at Colorado School of Mines, and standard or heavy-duty roadheaders.

New information on the application and development of several of these types has recently been published (see references) and this article summarizes some of the salient points.

The ability of the modern TBM to cut any hard rock is well documented. It has become by far the most efficient and cost effective way to drive long, relatively straight underground openings for civil works, even in very hard rock. The disadvantages for mining applications are the lack of flexibility to excavate to the configuration of the mine development needed for exploration or the mining methods used on a particular orebody, the inconvenience of operating from a circular opening (or the cost of modifying it), and the high initial cost of a full-size TBM system.

Despite this, there have recently been two notable applications of full-size TBMs in mines in the United States.

At the Stillwater Mining Co. PGM mine in Montana, a TBM has been used to develop 10K ft of footwall laterals as adits in gabbros, norites, and anorthosites. The rock ranged from 12.5K up to 24K lb\[in.sup.2] in compressive strength. the lateral drifts were to parallel the orebody at a distance of about 100 feet to allow definitive drilling into the orebody. In some places the curvature of the orebody was less than the 800-ft turning radius of the TBM and this caused some problems in the diamond drilling program. However, it is reported that compared to drill and blast, there was a direct cost reduction by one third, and the roof-bolting requirement was only 10-12% of the normal usage. The biggest benefit, however, was the saving in development time and the ability to bring areas of the mine into more rapid production.

At Magma Copper co.'s San Manuel Mine in Arizona a 15.2-ft dia Atlas Copco/Robbins TBM is being used to drive development for the Kalamazoo orebody (See E&MJ, April 1994). The key factors in selecting the TBM over drill and blast were the speeded-up development time to complete 34K ft of drift and the ability to allow caving in a retreating fashion towards the shafts. This allows earlier mining of high-grade ore and minimizes maintenance of the openings.

The TBM is driving a series of continuous loops, which will become the grizzley and haulage levels for block caving the Kalamazoo orebody. The machine has been designed to negotiate many very long curves of 350-ft radius.

The ground conditions vary from severe faulted areas, which unsupported may collapse within 30 min of excavation up to stable quartz-monzonite with compressive strength up to 26K lb\[in.sup.2].

Even taking into account the initial cost of the TBM system (approximately $6.75M), the projected development cost/ft is equivalent to that of drill and blast.

Initial problems is cutting curves and in much pick-up led to the machine being modified. The modified machine is now moving towards the goals and objects set for it by Magma.

Mini-TBMs have also been tried out in hard rock. Falconbridge Ltd., Placer Dome Ltd., Redpath Ltd., and Bortec Inc. built an 8-ft dia TBM which could excavate extremely short radius turns. The machine, the Bortec CUB was only 12.5 ft long and had a turning radius of 160 ft. It bored several hundred feet under test conditions but problems arose due to inadequate space to provide ground support in the conditions encountered and in the steering/propel system. Bortec redesigned the correction to be applied to the machine, but they were not fully implemented, and the redesign of the mini-TBM waits to be tried in some other application.

Some 10 years ago an extremely flexible mini-TBM was jointly designed by Snyder Engineering/Harrison Western principally for mine application. The 7.2-ft dia machine was built by the German company Wirth for a civil engineering project where it successfully bored 4,300 ft of 25K lb\[in.sup.2] sandstone at the rate of 15 ft/hr. The machine was capable of boring 50-ft radius curves.

It seems likely that mini-TBMs could be used in underground mining to drive small exploration drifts that would minimize the number of exploration shafts needed, and to drive secondary access escape ways, ventilation drifts, conveyor drifts, and water drainage drifts below the orebody.

Smaller still is the Atlas Copco/Robbins BorPak. This is a remote-controlled micro-TBM with an in-hole cutter drive which bores a hole of approx. 4-ft dia. In recent tests at INCO mines in Canada it has drilled very hard rock of 37K lb\[in.sup.2] at an average rate of 2.7 ft/hr.

INCO will use a BorPak to drill blind slot raises at the Coleman mine in ore and waste with strengths from 25K to 45K lb\[in.sup.2].

The application of bored raises is obvious, but one can now also consider small bored horizontal openings for ventilation, drainage, and other applications. If the orebody is uniform or massive enough for the machine to stay in ore, mechanical mining of the orebody is a possibility. This has been tried on South African gold reefs by boring overlapping holes in the reef, but reef undulation was a problem.

Recently the Cigar Lake high-grade uranium orebody in Canada was being mined by blind-hole boring. Holes of 5-ft dia are drilled through the hard basement gneiss and the reamer expands to 8-ft dia to mine the ore. The hole is then backfilled before the adjacent hole was mined.

Atlas Copco pioneered the undercutting mini-fullfacer machine some 20 years ago using a rotating disc for cutting only part of the face at a time. In a recent development HDRK Mining Research (Falconbridge, INCO, and Noranda) have joined with Wirth to build was is called the Continuous Mining Machine. This machine has four programmed and circulating ranging arms, which have an undercutting disc attached. The CMM can cut circular, semi-square, or horseshoe shaped openings up to 18-ft dia and down to an 52-ft radius curve.

The machine was first tested in an underground sandstone quarry in Germany where it produced 32 [yd.sup.3]/hr of very coarse product from rock with a strength of from 17K to 20K lb\[in.sup.2]. It is now being tested in INCO's Creighton in Canada, primarily for driving development headings. It is a massive machine, 150-mt, but its productivity could make it of interest for actual production mining too, e.g. in cut-and-fill stopes.

The development of the Robbins Mobile has been closely followed by the mining industry throughout its development period or more than 10 years. Two machines have been tried out in mining environments. While day to day performance to date has fallen short of the levels anticipated, instantaneous performance has been impressive for given rock conditions and continues to encourage further development.

The first mobile miner was ordered by Mt. Isa Mines in 1983. The machine experienced high cutter costs and failed to perform reliably. Despite this, it had successfully bored some of the hardest rock in Australia. Pasminco Ltd recognized the potential of the technology and decided to work with Robbins to develop and improved machine.

Robbins and Pasminco spent six years producing the new machine. A primary objective was to correct the structural deficiencies which had plagued the prototype mobile miner. After more than 500 hours of operation in a wide range of rock conditions, that objective appears to have been met.

The Pasminco Model 130 mobile miner was taken underground at Pasminco's southern operations in Australia in May 1992. Mobile Miner trials were undertaken on the 5 level of the South mine in a complex of interlayered, vertically oriented, variable strength, and highly abrasive schistose country rocks.

The so-called face mapping system is well suited for Pasminco Level 5 conditions where rock strength is normally highly variable across the cutting face. The principle of the face mapping system is to have the machine sense how the cutter wheel drive system is responding to the rock over various segments of each swing. The control system endeavors to apply maximum available power to within prearranged safe limits by widening or narrowing the spacing (speeding up or slowing down the swing rate) as cutting becomes easier or more difficult. the program 'remembers' the adjustments of the previous face pass and applies them to the new pass while at the same time testing the current pass to sense if conditions are changing. This cutting optimization program increases cutter efficiency by 20-30% in highly variable ground when compared to a non-interactive system.

The MM 130 would usually be operated concurrently with conventional drill-and-blast production stoping and development operations. The machine's control systems provided for the continuation of boring and mucking without operator attendance during the twice-per-shift evacuation of the mine for coordinated firing of explosives in the drill-and-blast operations. The practicality of this kind of automated operation was demonstrated on many occasions.

The advance rate doubled from what had been attained at Mt. Isa. The cutter life of the Model 120 had been poor. The high garnet and quartz content of the Pasminco rock dulled cutters rapidly. A wide variety of cutter ring materials and shapes were tested in this rock. To combat the extremely hard and abrasive ground harder ring material, wider tip cutters, and extended cutting edges were developed. The new cutter rings produced from 30 to 40 lineal meters of drift from the six-cutter set.

The machine was redeployed to bore a drill drive on the nine level in October 1993. The nine level ground was a massive, hard, abrasive mix of ore and extremely tough country rock. The increased homogeneity of the rock reduced penetration rate, but the miner ran more smoothly in the more homogeneous conditions and it was also possible to run at somewhat higher power levels.

The MM130 has now been assigned to drive an extension of the main Pasminco South decline from the 10 level 1.8 km to the 16 level. The work began at the end of 1994. The persistence of pasminco to bring the mobile miner into production is a strong indication of the company's confidence in the potential of this equipment. The Pasminco decline extension will be a major test of the machine viability for mainline development work.

M.I.M. Holdings, parent company of Mt. Isa Mines, owner of the first mobile miner, are considering a new machine for the McArthur River project. McArthur River is located in the Northern Territory of Australia, approximately 1,000 km south and east of Darwin. The mine is comprised of a series of tabular orebodies of varying thickness and dipping at approximately 20[degrees]. This dip would challenge the efficiency of conventional room-and-pillar mining methods. Ore grade is critical, but the need for flat floors and modest grades for drill jumbos and haulage equipment will result in some dilution. M.I.M. suspected that a mechanical mining system might do better, at least in the thinner orebodies.

M.I.M. approached Robbins to propose a mechanical mining system that would reduce the dilution unavoidable using the conventional approach, while at the same time increase yield per man/hour and reduce cost/ton. The mining company felt that the modest strength of the ore, a slightly altered shale, would provide a cutting environment ideal for mechanical mining. It also saw this as an opportunity to apply the mechanical mining technology that it had invested in 15 years earlier at Mr. Isa with the first Robbins mobile miner.

In July 1994 Robbins was commissioned to carry out a feasibility study on the application at McArthur River. M.I.M. established a production goal of 1M mt/yr ore for the mechanical mining system. M.I.M. felt that the shale host rock offered an excellent cutting environment, whereas previous applications of the mobile miner had been in extremely hard rock both at Mt. Isa and Pasminco. The low strength and low abrasivity should result in longer cutter life and low cutter costs. The thin, horizontal orebody was felt to lend itself to advantageous stoping block layout to suit the mining machine.

One hoped-for advantage of mechanical mining is the possibility to work strictly in ore on the 20[degrees] dip.

The mineralized zones range from 2.5 to 6 m thick. The mobile miner needed to be able to follow these height variations by some means, taking minimum waste in the thinnest seams and maximum ore in the thickest. A 3-m minimum cut was eventually selected.

To accommodate thicker seams several alternatives were considered. These included multiple benching and also a system whereby the machine would cut an initial bottom pass at cutter wheel height and then return to the starting point using 'fill' from the back cut to continuously create an elevated floor ahead of the miner of the desired height for completing the back cut.

Finally a ranging wheel concept was studied. The single ranging wheel system shared the common drawback of double pass cutting due to cutting only part of the face, but it had the very strong advantage of requiring only one trip along the stope to take the ore. In computer simulation models this system proved significantly more efficient than either of the other systems.

Several room-and-pillar stopping designs were also modelled including full parallel pass with the wide pillar and heavy ground support, one full and one partial parallel pass with medium ground support, and single pass with narrow pillar and reduced ground support. The latter proved most efficient.

To minimize waste excavation in transitioning from the near-horizontal access drive into the stope the mining machine needed a very short turning radius in both the horizontal and vertical planes. However, the desire for a wide single-pass stope dictated a relatively long boom and consequently similarly long machine platform. The ability to swing the boom into a turn effectively shortens the machine length in a horizontal plan. The vertical ranging feature offers the same shortening effect for vertical turns.

Employment of a second tracked vehicle with a powered hitch for severe tram steering, used successfully on the Pasminco machine, was also adopted for the McArthur River machine. To further augment vehicle control, each track unit on the machine is independently powered and can be individually controlled. Horizontal turns radius is 14 m and vertical is 20 m.

The feasibility study finally centered on a 3-m dia single wheel machine which would have to go well beyond the cutting capacity of any of the earlier mobile miners if the necessary productivity were to be realized.

As on all mechanical mining machines, cutter performance is vital. McArthur River Mining is supplying large block samples of ore from the mine for testing on a large disc cutter test machine at the Australian Centre for mining Technology and Equipment (CMTE). M.I.M. will underwrite a comprehensive series of test on these samples utilizing disc cutters provided by Robbins. Results of these tests will be used to evaluate preliminary performance estimates and to identify the best cutter regime for the McArthur River ore.

Eventually CMTE tests are expected to shed light on such subject as the practical limits of cutter rolling speed on a mobile miner, the effect of cutter tip heat generation on ring wear, and the effect of a skewed cutter track on cutter life and the machine's overall performance.

Pasminco sees the mobile miner as economically viable for along main access decline. M.I.M. sees that mechanical mining of the McArthur River thin-bed zinc deposits could have significant economic benefits. CMTE sees the future survival of many underground mines in the realization of a successful hard-rock continuous miner.

The single disc cutter was the key innovation influencing the successful development of the modern TBM. Early on the cutter action was not well understood, but in 1956 the astounding tunnel advance record of 105 ft in one 24 hr period in a civil tunneling job demonstrated the potential for the tunnel boring industry. The project set the stage for the next nearly 40 years of evolutionary development of both cutter and boring machine design.

Development got off to a slow start. For the next 20 yr TBM manufacturers installed cutters in such a way that they cut concentric grooves about 3 in. apart. It was noted, however, that the harder a cutter was pushed, the further it sunk into the rock, and the faster the TBM advanced. The capacity of the disc cutters went from a humble 20K lb for a 12 in. dia cutter to 40K lb for the new 15.5 in. dia cutter introduced in 1974 during the early phases of a metro project in Washington DC, and on today's 75K lb, 18-20 in. cutters. It was not until the mid to late 1970s that a research program at the Colorado School of Mines (CSM) was successful in developing a formula which would describe the relationship between the force on a cutter and its penetration performance.

The recognized variables at the time included compressive and shear strength of the rock, diameter of the cutter, angle of the then triangular ring cross section, and the effect of spacing (distance between cuts).

In the mid 1980s a rare opportunity for research in the field occurred as a U.S. Air Force missile program developed a deep base defense system. Millions of dollars in funding became available to study not only how to make a TBM go fast, but also to study how to make it go through fractured rock and rubble and to give it the ability to turn from the horizontal to the vertical. These efforts contributed to the upgrading of the performance estimating program into a quite accurate science.

TBMs became bigger and more powerful and more versatile machines using up to 20 in. dia cutters routinely running at loads of over 65K lb thrust each. Thus the TBM designs evolved, which had the size and structure to utilize the over 400-lb disc cutter and its large saddle mount. Fine for the big equipment, but for small bore sizes, technology nearly stood still. For a cutter head less than 6 or 7 ft dia it became physically impossible to install a large high-capacity cutter of the current design.

The principal tools for smaller cutter heads are multi-row carbide insert cutters, button row cutters, cone-shared cutters, strawberry cutters, and even some with random spaced buttons. Many of the applications for small cutter heads, or bits as they may be called, derive their power and thrust through a drill string. Torque and power are more limited therefore than on the huge direct-drive TBMs. When supplied with limited torque and thrust, the cutters will obviously indent the rock less and spacing must be reduced to insure that chips will form. Some cutter types have reduced the spacing to the extreme where they virtually pound the rock to dust.

A large penalty is paid for making small chips or powder. A ton of rock can be excavated with less energy if the cuttings are brought out in large particles. In an instrumented test, an off-the-shelf 9 in. tricone bit required 80 hp-hr/mt in concrete and 120 hp-hr/mt in basalt, compared to the 3-7 hp-hr/mt that disc cutters achieve on large-diameter cutter heads. Yet the single rolling disc cutter has not found common application on small diameter excavating tools. There are perhaps two principal reasons for this:

* Those who have the technology of the disc cutter are focused primarily on the large-bore end of the industry. The importance of high thrust, maximum spacing (fewest cutters), and cutter head balance is a closely held science.

* The smaller higher-production single discs are 14 in. dia, while the smallest special order discs are 12 in. dia. Even the 12-in. cutters with their bulky saddle mounts occupy too much cutter head space to use effectively on small diameter cutter heads. It has also been commonly believed by traditional manufacturers and users of single-disc cutters that a cutter of significantly smaller diameter cannot be made robust enough to survive the high forces imposed by excavating hard rock.

With the energy curve showing TBMs excavating a ton of rock of 3-7 hp-hr, raise borers with multi-row close spaced cutters in the 20-30 hp-hr range, and tricone and strawberry arrangements as high as 80-120 hp-hr, there appeared to be much room for improvement. A mini-disc cutter of 5 in. dia has been developed at the CSM Earth Mechanics Institute in cooperation with Excavation Engineering Associates of Seattle. Prototype small-disc cutters built in tungsten carbide and hardened all-steel versions were tested in the CSM Earth Mechanics Institute laboratory to determine the performance potential. A very hard 43K lb\[in.sup.2] rock was chosen to shake out any weaknesses as quickly as possible.

The results were beyond expectation. At 2 in. spacing, the 5 in. mini-disc cutters achieved a penetration of 0.125 in. with only 11.7K lb thrust. To put this into perspective, a standard 17 in. TBM cutter requires over 60K lb to achieve this penetration in the same rock. The specific energy was also measured and was only around 7 hp-hr/mt, thus far superior to the best multi-row or button type cutters ever tested.

The first commercial application of the mini-disc cutters on a hard-rock machine has been on the Iseki Discmole small-diameter full-face tunneling machine designed to cut rock up to 50K lb\[in.sup.2] strength.

Hard-rock miners have for a long time envied their counterparts in soft-rock mining for the advantages of using highly mechanized, highly productive machines such as roadheaders for rapid cost-effective development. In hard rock, however, the capabilities of roadheaders have always been undermined by two limitations: First, the stiffness of the system which is directly related to the machine mass, and second, the ability of the existing drag-type cutting tools to transfer the forces and power to effectively cut rock.

In recent years a new generation of heavy-duty machines have been developed and introduced into the market to overcome the limitation of the lack of stiffness of the system - with its adverse effects on machine life, productivity, and cutter life. These heavy duty machines have a higher mass and power together with special provisions such as telescopic booms and multi-speed gear-box arrangements with stiff electric drives intended to extend their applicability to cut harder materials than was possible before.

The problem of cutting tools remained, but now the development of the mini-disc cutter appears to offer a real solution which will permit the heavy-duty roadheader to become a true hard-rock mining machine.

An extensive study was performed to investigate the technical and economic feasibility of using mini-disc cutters on heavy-duty roadheaders to excavate hard rock. The results show that roadheaders fitted with mini-discs can excavate hard-rock formations at acceptable production rates and cutter costs. Field testing of a roadheader with mini-disc cutters is being undertaken in a hard-rock formation. The test include sumping and shear down methods of cutting while closely monitoring the productivity as well as all machine parameters during the cutting process. The tests also include evaluations of cutter wear and overall excavation costs. If successful, it is believed that mini-disc equipped roadheader technology will be available to the hard-rock mining industry within the next year or so.

E&MJ will publish a follow-up article focusing on the development and application of this extremely promising new cutter technology for hard-rock mining shortly.


R.L. Bullock, the Gradual Evolution of Mechanized Hard Rock Mining, SME Annual Meeting, Denver, Colo, 1995.

N. Dahmen, Recent Mobile Miner Developments, SME Annual Meeting, Denver, Colo, 1995.

J.E. Friant, L. Ozdemir, E. Ronnkvist, Mini-Cutter Technology, the Answer to a Truly Mobile Excavator, North California Tunneling 94 Conference, Denver, Colo, 1994.

L. Ozdemir, J. Rostami, D. Neil, Roadheader Development for Hard Rock Mining, SME Annual Meeting, Denver, Colo, 1995.
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Publication:E&MJ - Engineering & Mining Journal
Date:Jul 1, 1995
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