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Progress in rock drilling.

Progress in Rock Drilling

Early miners hammered handheld bits into rock to form drill holes into which they placed explosive. Igniting the explosive broke the rock into pieces that could be easily removed.

This principle has been the basis of many mining operations since that time. Only the power to drive the drill has changed. First air power replaced human muscle, and equipment grew in power and size. Then, because it became increasingly difficult to manually control an operating drill, the drilling jumbo was born. This mobile platform could carry one or more drills, each on a boom, each moved and powered by pneumatic cylinders. A miner could thus run more than one drill at a time, and because the miner did not carry the drill, more power could be applied. Higher power meant heavier drills, mandating heavier support and operational cylinders, and larger support carriages.

By the mid-1970s it became advantageous, in many mines, to change to hydraulic power. This change increased the power fed to the drill, improving performance. The drills were somewhat more expensive, but repaid the investment in greater capability.

Drilling through rock, however, is not the same as drilling through industrial materials. In a factory, the material has relatively consistent properties, which remain so as the drill advances. In mines, such a material is the exception. In one hole a bit may drill several layers of rock, each of different properties. These layers frequently lie at an angle to the hole axis. Thus, as the drill advances it may meet different resistance on opposing sides of the bit. Furthermore, the rock being drilled may break differently under the bit from one layer to the next. Such problems were small, however, as long as a man was manually driving the bit. They increased in size as more powerful machines replaced him.

Chips and crushed rock can jam the bit in the hole (usually several meters deep) and poor control on the thrust can cause the bit to wander with the rock layers. While a skilled driller could control the drill to overcome these problems, once he had to operate a number of drills on a jumbo it became more difficult. The answer was to put feedback controls into the control systems of the drills and begin to automate the drill. Full automation has followed, and drill jumbos are now available, which, once programmed, will advance to a face and drill the holes for a full blasting round with no one on the machine. The units are relatively large, naturally expensive, and require highly skilled technical support, but pay for themselves in increased output.

Unfortunately, while this development is logical in factories, it misses a major market need in mining and rock drilling. The United States has thousands of mines, but most are not large. Minerals are not found in regular thick layers. Gold, for example, occurs as a tiny fraction of the host rock in lodes of ore, scattered, frequently small, and often reached through small inclined tunnels. Access to the working area, or stope, may be up a ladder, or through a narrow, winding tunnel.

Large, fully automated drilling rigs cannot reach these areas, and if they could, the value gained would be low. Drilling patterns must change with conditions and the value of the rock. The capability for some continued manual control is thus a necessity. A novel drill should, therefore, be small, light, cheap, simple, and quiet. It would also be useful if it were more accurate. It takes both good practice and good equipment to achieve an accuracy of 1 percent (that is, to keep a 100-m long hole within 1-m of the required end point). This accuracy is particularly needed in mines with a large ore body. There, the distance between two adjacent tunnels may be controlled by the accuracy with which drill holes, and thus fragmenting explosive, can be placed throughout the intervening ore. The better the accuracy, and the further apart the tunnels can be placed, then the lower the tunneling cost, and the better the mine economics. These objectives are difficult to achieve with mechanical tools. Part of the reason is that, to cut faster, a mechanical bit must be pushed harder. To push harder requires stronger tools, which become larger and heavier, and at the same time more expensive, complex, and often noisy. It is time an alternative approach was found.

MERI Helps

The problem discussed above is common to many aspects of mining. Yet the sources that can propose innovative answers are becoming increasingly rare. Both major federal agencies, the departments of the Interior and Energy, under whose aegis these problems fall have significantly reduced staff and funding. Company reorganizations and takeovers have reduced the private money available for research, and even closed research departments. At universities, falling enrollment has reduced the number of academic departments and the faculty in them.

Concerned with this problem, ASME, working initially through Dr. Howard Clark, Dr. Carl Peterson of MIT and Dr. Lee Saperstein, then at Penn State University, sought a way to bring together faculty available and interested in solving the problems of the mining industry in new, effective ways. This group was expanded to include most of the major mining schools in the country and given the acronym MERI (the Mining and Excavation Research Institute). Through the help of this group, funding was found at the University of Missouri-Rolla (UMR) to examine a new method of drilling rock. This approach combines the use of high pressure waterjets, as a means of transmitting power, with the use of abrasives for precise cutting.

Waterjet Cutting

Nature erodes material in many ways. One of the most powerful is the impact of a stream of water. While natural erosion rates are often low, small increases in power can make them economical. In 1852, low pressure waterjets were used to mine gold-bearing rock. From that time until 1886, most of the gold in California and other Western states was mined by this method. Because of environmental problems, hydraulic mining fell out of favor in the U.S. It was, however, adopted in other parts of the world, and found particular use in the Soviet Union. After the Second World War, it was adapted for mining the steeply dipping coal in the Don and Kuznetsk basins. From this initial development, hydraulic coal mining was applied in mines around the world.

Because this method allowed a relatively simple and inexpensive process to evolve for remote mining, an attempt was made to adapt waterjets to the cutting of rock. Coal, however, has little tensile strength, and contains many cracks and fissures into which the jet can penetrate. In general, rock, with significant tensile strength and a lower density of smaller flaws, needs a higher jet pressure to be cut.

Early studies used water cannons to fire water pulses at pressures of up to 4,000 MPa. Such devices fragmented the rock around the impact point, but were not economical. A lower pressure alternative was more effective. The reason for this can be illustrated. If a 1-mm diameter jet of water at a pressure of 70 MPa is directed at granite without relative movement of the nozzle, little penetration of the rock occurs. If, however, the nozzle is moved over a circular path, a hole will be drilled. Although the jet does not have the power to break a single crystal, it can remove it. A traversing jet first touches the crystal at its edge. High pressure water enters this interface and is further pressurized by the following jet. The fluid wedge expands this crack to remove the crystal. The UMR "Stonehenge" shows how effective this can be. This 160-ton monument (Figure 1) was carved from granite of 180 MPa compressive strength by jets of 80 to 100 MPa pressure at a surface generation speed of 1.5 [m.sup.2]/h[1].

This approach can be used to remove concrete, which consists of aggregate in a cement matrix. While both materials must be cut by a mechanical tool, a waterjet can be directed to remove the cement matrix, letting the aggregate fall away without needing the power to cut it. This method is in commercial use throughout the world.

By itself, high pressure water is restricted as to the rocks it can effectively drill. Rocks with smaller flaws and high tensile strength require higher jet pressures for practical cutting rates. Piston pumps generate pressures to 140 MPa, but for 140 to 400 MPa, intensifiers, which need much purer water, are more common. The harsh mining environments make this difficult and expensive, so that very high pressure jets become impractical. Current waterjet drills have developed only for use in softer rocks, where they have several advantages. Because the water stream is directed through a small nozzle, the reaction force is quite small. Therefore, the drill can be small, light, relatively inexpensive, and simple. rf=0.052flp where rf = reaction force; f = flow (gpm); and p = pressure (psi).

Once the drill enters the rock, it is also quiet. Studies have measured a noise below 85 db while drilling a 3-cm diameter hole 3 m deep. However, the upper limit on pressure has reduced this use, pending a better way to cut harder rock.

This limit on pressure does not hold in industrial applications where a clean environment is much easier to achieve. The low reaction force also makes it easy to use the jet nozzle on the end of a robot arm. An easily maneuverable cutting tool that can rapidly cut through a variety of materials thus developed. Automated jet cutting is now used to cut foam, diapers, paper, carpets, shoe parts, and food products. But a way had to be found to cut harder materials.

Potential Enhancement

The first success in cutting harder rock arose serendipitously. If a cutting tool is dragged across a rock it will heat, causing rapid wear and lower performance. Hood[2] directed ahigh velocity stream of water at the tool-rock interface so that it would act as a coolant. He found that the addition of the jet improved bit penetration up to nine times. He has explained the reason for this as follows:

A drag bit crushes the rock ahead of it, and this forms a plastic zone around the cutting edge. This spreads out the force from the tool over a larger area, and a greater force must therefore be used to create a high enough pressure in the rock so that a chip will break out ahead of the bit, allowing it to move forward. The high pressure water removes the crushed rock as it is created. The force from the tool is, thus, distributed over the small area of rock along the edge of the bit. The cutting process, in turn, becomes more efficient while the water keeps the blade cool and thus sharp for a longer period.

A simple illustration of the gain this allows comes with the large tunneling machines now used. The machine weight (and cost) affects the amount of thrust which, in turn, controls the strength of the rock which can be cut. Thus a machine to cut a 125-MPa strength rock will weigh about 110 tons and cost some $750,000. A 30-ton machine costing $250,000 will only cut rock of around 95 MPa in strength. Yet when high pressure waterjets were added to the 30-ton machine, it cut 140 MPa rock[3]. This not only saves money, but the lighter machine is smaller, more maneuverable, and can be used in small mines.

This may explain how waterjets assist tools in rock cutting, but does not explain why a waterjet will similarly assist a metal cutting tool. Cutting forces are sufficiently reduced so that metal removal rates have increased tenfold, and the chips from the tool are cool enough to be caught by hand[4].

Waterjet-assisted rock drills have not been as successful. The small hole, and the limited geometries which can be used, do not allow much sophistication in the design. Furthermore, although the forces on the tool are somewhat reduced, significant mechanical force is still required, bringing back the problems of bit wear and hole deviation.

Waterjets cannot drill harder rock because of a lack of exploitable flaws. One way to create flaws has been to use those made by a mechanical tool, as described earlier. Alternately, the jet structure can be changed, so that it will form small cracks on the rock surface. One way of doing this is to induce cavitation in the jet as it leaves the nozzle. The collapse of the cavitation bubbles on the rock surface induces microfractures, which can be exploited by the main jet flow[5]. While this can be a very aggressive tool, unsurmounted problems still exist. For example, the parameters that control cavitation in a 1-mm diameter jet stream moving at 400 m/sec are not well defined. Control of the process is difficult, and cavitation attacks the drilling bit as ferociously as it attacks the rock surface.

Abrasive Cutting

In the early 1980s, a second way of inducing flaws was marketed in which an abrasive was mixed with the waterjet stream after it had been accelerated to final velocity. This jet was directed through a chamber, inducing a vacuum which drew abrasive particles into the chamber from a supply line. The water and particles mixed, and the combined flow left the chamber through a second nozzle.

The nozzle was still light enough to be used on a robot arm. But the abrasive allowed its use for cutting metals, including titanium and Inconel, and ceramics. One benefit of the tool can be seen in cutting laminated glass. A conventional glass cutter notches both panes of the glass and must run the developed crack through the intervening plastic; an abrasive jet cuts the entire sandwich in a single pass (Figure 2). The straightness of the cut through the three layers, and the ability to easily follow a contour has reduced wastage. It also illustrates that this new tool might have a use in cutting straight through the different layers of rock found underground.

The facility with which the abrasive-laden jet cuts through single layers of rock is shown by its use in carving the new Navy Memorial in Washington, D.C. The monument includes a 7-m diameter, 5-cm thick map of the world. The map was built by contour cutting inlays of white granite which were set into the equivalently shaped holes carved in a black granite "sea."

In order to accelerate the abrasive, the waterjets entering the mixing chamber are typically at pressures of 250 MPa or more. This again requires high quality water not usually found underground. A second feed line to carry dry abrasive, while not an insurmountable problem[6], is expensive. Rock drills using this technique have, therefore, not been seriously considered. Another way of adding abrasive to the jet would appear to be a necessary prerequisite.

Bureau of Mines Research

The U.S. Bureau of Mines has evaluated high pressure waterjet systems for use in mining for a number of years. Seeking to resolve the problems with conventional abrasive injection systems, a novel approach was suggested by Savanick[7]. In order for the drill to advance, two process steps are needed: the abrasive and waterjets must combine in an accelerated stream, and this stream must then rotate over the rock ahead of the drill. While conventionally this is achieved by spinning either the entire assembly, or the secondary collimating nozzle, Savanick suggested a simpler method. If the abrasive jet stream was first formed and then directed down a tube, a pair of inclined plates at the end of the tube could be used to direct it over the rock surface. If the pipe with the deflector plates were the only item rotated (Figure 3), then a much simpler and cheaper tool would result.

The concept proved very simple to manufacture using jets at a pressure of 70 MPa. The main wear item is the steel pipe that carries the deflector plates. However, this is a relatively low cost item, and replacement costs compare favorably with those of the bits which might otherwise be consumed in conventionally drilling the hard rock this tool can penetrate. This design, although now licensed, continues to be an object of investigation and improvement in Minneapolis[8].

DIAjet System

In 1986, the British Hydromechanics Research Association (BHRA) announced a method of injecting abrasive between the high pressure pump and the delivery nozzle (Figure 4), the DIAjet method[9]. By directly injecting the abrasive, a large part of the abrasive fragmentation that occurs during conventional injection is eliminated. Furthermore, tests showed that the DIAjet abrasive jets would cut through steel and glass at jet pressures below 35 MPa. This lower cutting pressure eliminated two obstacles to conventional abrasive jet drilling. Firstly the lower pressure units no longer required high pressure steel tubing to carry water to the drill, and secondly the water quality constraint was largely removed. As additional apparent advantages for the proposed method, the size and cost of pumps to operate at this lower pressure were significantly less.

Over the past year a study has been carried out to find how to apply this new tool in rock cutting. Laboratory tests have indicated the parameters which control performance, and the means to apply the method in rock drilling. The factors which control the DIAjet have been found to differ significantly from those controlling conventional waterjet drilling, and consequently gave rise to some early confusion.

In plain waterjet cutting, the depth of the cut made by the waterjet can be approximately by the relationship [Mathematical Expression Omitted] where d = depth; c = constant; p = pressure; and s = traverse speed.

Thus, when a drill is developed it gives its best performance when the waterjets used are of large diameter and where the bit spins at relatively high velocity across the surface.

With low pressure abrasive jet cutting the above relationships are no longer the critical ones. Depth of cut remains linearly related to the pressure of the jet (Figure 5), however, there is relatively little effect of nozzle diameter with the DIAjet process (Figure 6).

In review this is not surprising. The plain waterjet penetrates the rock by exploiting existing flaws in the surface, and thus the larger the jet diameter, the more flaws are likely to be exploited by the jet. Furthermore, because of the distance between the nozzle and the target, a larger diameter plain waterjet is more likely to efficiently transfer the energy to the rock, than one with a smaller diameter.

In contrast the abrasive-laden waterjet will only cut the rock through the action of the abrasive. The quantity of abrasive hitting a point on the rock surface is not significantly changed by the change in nozzle diameter, and thus this parameter is of lesser importance.

The third parameter which affects cutting ability is the speed at which the nozzle is traversed over the rock surface. With a plain waterjet, the faster that this occurs the more efficiently the jet cuts. This is because the incoming jet impacts on the surface and rebounds into the path of the ensuing jet segment. This interaction reduces the effective pressure applied on the target, and thus the jet cutting efficiency. This effect is of lower significance in abrasive jet cutting, and an optimum speed for the abrasive nozzle movement occurs at a much lower traverse speed (Figure 7).

For illustration, in cutting the UMR "Stonehenge," a 100 MPa jet with no abrasive effectively cut through the granite at a traverse speed on the order of 20 cm/sec. In contrast, the optimum speed for an abrasive jet to cut that granite, at a pressure of 35 MPa, is 30 cm/min.

The addition of the abrasive into the jet stream adds more than one additional parameter to the evaluation. Analysis also becomes more complicated because of economic considerations. With conventional abrasive injection, only some 0.7 kg/min of abrasive is used, but this is largely pulverized and discarded. With a DIAjet system, up to 10 kg/min of abrasive is used, but this is largely undamaged and can be recirculated. However, while larger abrasive particle sizes cut more effectively than smaller ones, they require larger nozzle sizes (some three times particle size) and tend to break up more during the cutting process. The relative cutting ability of the abrasive is reduced at higher concentration, although overall performance is improved.

Even at this early stage in its development it is interesting to note the ease with which this new technique meets most of the demands for a new drilling system. The 35-MPa pumps are small, inexpensive, and relatively easy to move around. The low pressure single feed line to the drill, through a flexible hose, makes it maneuverable. The low reaction force from the small diameter jets means that a very light support frame will be required to hold and advance the drill. As with the plain waterjet drill, the noise level drops significantly once the drill starts into a hole.

This method of material removal by individual abrasive impact has both advantages and disadvantages. Because the abrasives do not exploit the flaws in the rock, the DIAjet system becomes insensitive to the geology and structure of the rock ahead of it. To illustrate this, a combination rock column was prepared in which a suite of rocks was assembled and oriented to give most difficulty to the straight penetration of the drill. When the modified DIAjet system was adapted to a drill, it penetrated through the full column with no deviation. Since that time, a hole some 20 m in depth has been drilled through layers containing dolomite and chert, with no deviation over its length.

The ability of the DIAjet stream to cut through all likely rocks which it may encounter, including chert and quartzite, has been demonstrated. However, there is a disadvantage to the method of rock removal which relates to the performance of the drill. Because the jet removes material as a result of the individual impact of small abrasive particles, the resulting rock fragments are very fine. They also require relatively high levels of energy to remove. Thus, while the energy of the removal is no greater than that found when conventional abrasive jets are used to cut rock, this value is still considerably higher, for most rock types drilled, than that of conventional drilling equipment. In studying the additional parameters that control the drill performance, it has been found, much more so than with other drilling methods, that values for parameters such as abrasive hardness, size, and concentration in the stream must be optimized rather than being maximized as is the case with the parameters of most other systems.

The advantage of the method, however, is that a DIAjet drill, using low pressures and inexpensive abrasive materials, can be used to drill even those rocks which are not easily drillable by conventional means. The rotation speed at which the drill operates most efficiently is much slower than that of more conventional equipment, but the jets cut much deeper on a single pass. This requires some care in the design of the equipment to insure that the jet does not cut out large fragments of rock that cannot pass up through the annulus around the cutting nozzle assembly. It also requires careful design to insure that the jets cut clearance for the main body of the nozzle. This is important since the jet diameter is only on the order of 1.5 mm, while the nozzle assembly is 2.5 cm in diameter. Furthermore, the jets are typically cutting the rock some 3 to 5 cm ahead of the bit during operation. The most effective design to date is one in which the centrally directed, smaller jet cuts inward at 5 degrees, while an outer reaming jet, which widens the hole to allow the passage of the nozzle body, is inclined outward at an angle of 30 degrees. [Figures 3 to 7 Omitted]

PHOTO : Fig. 1. The university of Missouri-Rolla "Stonehenge," a 160-ton monument carved from

PHOTO : granite of 180 MPa compressive strength by waterjets of 80 to 100 MPa pressure.

PHOTO : Figure 2. An abrasive waterjet drill can cut through an entire sandwich of laminated glass

PHOTO : in a single pass.

References

[1]Summers, D.A., and Mazurkiewicz, M., "Technical and Technological Considerations in the Carving of Granite Prisms by High Pressure Waterjets," Proceedings, 3rd U.S. Waterjet Conf., Univ. of Pittsburgh, 1985, pp. 272-290. [2]Hood, M., "Cutting Strong Rock with a Drag Bit Assisted by High Pressure Water Jets," J. S. African Inst. Min. and Metall., Vol. 77, No. 4, Nov. 1976, pp. 79-90. [3]Koegelman, W.J., and Thimons, E.D., "Safer and More Productive Mining with a Computer Controlled Waterjet Assisted Longwall Shearer," Proceedings, 2nd Int.'l Conf. Innov. Mining Systems, Penn. State Univ., Oct. 27-29, 1986, pp. 56-68. [4]Mazurkiewicz, M., and Kubala, Z., "High Pressure Waterjet Application for Metal Machining Cooling Operations," Proceedings, 9th Int'l. Symp. Jet Cutting Tech., Sendai, Japan, 1988, paper C3, pp. 133-146. [5]Summers, D.A., "The Volume Factor in Cavitation Erosion," Proceedings, 6th Int'l. Conf. Erosion by Solid and Liquid Impact, Univ. of Cambridge, UK, 1983, paper 14. [6]Hashish, M., "The Potential of an Ultrahigh Pressure Abrasive Waterjet Rock Drill," Proceedings, 5th Am. Waterjet Conf., Toronto, Can., Aug. 27-31, 1989, pp. 321-332. [7]Savanick, G.A., and Krowza, W.G., "An Abrasive Waterjet Rock Drill," Proceedings, 4th U.S. Waterjet Tech. Conf., Berkeley, Calif., Aug. 26-28, 1987, pp. 129-132. [8]Miller, A.L., Kugel, R.W., and Savanick, G.A., "The Dynamics of Multiphase Flow in Collimated Jets," Proceedings, 5th Am. Waterjet Conf., Toronto, Can., Aug. 27-31, 1989, pp. 179-190. [9]Fairhurst, R.M., Heron, R.A., and Saunders, D.H., "DIAjet--A New Abrasive Waterjet Cutting Technique," Proceedings, 8th Int'l. Symp. on Waterjet Cutting Technology, Durham, Eng., Sep. 9-11, 1986, pp. 395-402.
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Title Annotation:American Society of Mechanical Engineers research
Author:Summers, David S.; Yazici, Sina
Publication:Mechanical Engineering-CIME
Date:Mar 1, 1990
Words:4345
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