AU2006202936A1 - Method and apparatus for slotting a blast hole - Google Patents

Description

-1-

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name of Applicant/s: Actual Inventor/s: CMTE Development Limited Mehmet Doktan Address for Service is: SHELSTON IP Margaret Street SYDNEY NSW 2000 CCN: 3710000352 Attorney Code: SW Telephone No: Facsimile No.

(02) 9777 1111 (02) 9241 4666 Invention Title: METHOD AND APPARATUS FOR SLOTTING A BLAST HOLE Details of Associated Provisional Application No. 2006900688 dated 13 Feb 2006 The following statement is a full description of this invention, including the best method of performing it known to me/us:- File: 46103AUP00 500910082 1.DOC/5844 -2- FIELD OF THE INVENTION The present invention relates generally to mining and more particularly to blasting during mining operations.

BACKGROUND OF THE INVENTION The invention has been developed primarily for use in relation to reducing blast damage to a mineral seam during open cut mining, and will be described with reference to this application. It will be appreciated, however, that the invention is not limited to this particular use and may be also used in other blasting operations such as to improve fragmentation and to control fracturing.

Coal loss and dilution in open cut coal mines is a significant problem. One major factor identified as responsible for reduced recovery is that of blast damage to the coal seam. Blast damage is caused by blast energy and its associated shock front, softening and preconditioning the coal seam. Fig. 1 a shows a mineral seam 1, a borehole 2 through overburden 3 and blast energy 4 radiating from the toe 5 of the borehole to damage the seam at 6. The weakening and excess fragmentation of the coal seam results in coal loss during cleanup or mining operations. Furthermore, coal loss may occur by shearing and dragging of coal chunks into the pit to be buried under the muckpile. Some of the mechanisms of coal loss resulting from blast damage are shown in Figs. lb to ld where Fig. lb shows shearing, and Figs. lc and ld show dragging and tensile splitting of the seam, respectively.

Ideally, the right amount of energy is needed to fracture the overburden rock above the coal seam while leaving the coal itself intact. Of course a critical factor determining damage to the coal seam is the distance between the charge, located at the borehole toe, and the seam. This is known as the stand-off distance. If the charge is located too close to the seam for a given charge size, either by incorrect calculation of the stand-off distance or by drilling the borehole too deeply, there will be damage to the seam.

Correct stand-off distances are not easy to achieve in practice. They require accurate information regarding the depth of the top of the seam, the type of rock above the seam, correct calculations as to the charges required given the overburden and then accurate drilling and positioning of charges. If any of these factors are inaccurate, the result may well be excess blast energy and confinement at the toe.

Blast design controls include backfilling, modifying stand-off distances, and baby decking. The most widely adopted industry practice involves drilling every fifth or sixth hole down into the coal seam to gauge distance. However this method is far from ideal in terms of damage control because it relies on the assumption that the seam is uniform between holes.

Backfilling, whereby overly deep boreholes are filled with backfill, also presents a problem in that the explosive energy and resulting gas can push the backfill into the coal seam resulting in heavy damage.

Baby decking, whereby a smaller precharge is detonated before the main charge, may, if done incorrectly, result in dead pressing (compression of ANFO beyond its critical density), pre-compression (charge damaged by shock wave of previous deck) or sympathetic detonation (sensitive explosives detonating due to shock waves of others).

In addition, misfires can and do occur. If a mine is having difficulty implementing design drill collar, drill depth and stand-off distances, developing and implementing a correct baby decking practice may be a difficult task for everyday applications.

Buffering the entire length of the coal seam with waste material is another solution.

However, while this method may be effective and simple to implement, there are major drawbacks that are not directly apparent. For instance, buffering the face provides external confinement to the seam. The energy/stresses created by the explosion can still act on the coal seam and inflict significant damage even if block movement is prevented. Damage may then manifest itself as coal loss during subsequent processes.

Also, the effectiveness of the buffer is a function of compaction, floor conditions, and material properties. Additionally it is a high-cost process and only a partial solution to the problem.

It is an object of the present invention to overcome or substantially ameliorate one or more of these disadvantages of the prior art, or at least to provide a useful alternative.

DISCLOSURE OF THE INVENTION According to a first aspect, the invention provides a method of modifying a blasting borehole in mining operations, the method including the steps of: providing a borehole having a borehole toe, the toe spaced from a deposit by a stand-off distance; and -4cutting a slot in a sidewall of the borehole, the slot configured to direct blast energy away from the deposit thereby reducing blasting damage to the deposit.

Advantageously, at least in a preferred form the invention reduces confinement at the toe of a blast hole by creating artificially slots. More advantageously, these slots would divert part of the blast energy away from coal seam where it would induce damage into the rock for further breakage.

Preferably, the slot extends circumferentially around the borehole adjacent the borehole toe.

Alternatively, the slot extends laterally, longitudinally with the borehole axis.

Preferably, the slot is cut by passing a high pressure fluid jet slotting tool over a predetermined cutting path.

Preferably, the slotting tool is adapted for attachment to a drill string.

Preferably, the slotting tool is combined with a borehole drilling tool and the slot is cut without removal of the drilling tool.

Preferably, the method includes the further steps of: placing an explosive charge in the borehole adjacent the slot; and detonating the charge.

Preferably, the slot is cut to a depth of at least 0.3 times the borehole diameter measured from the borehole sidewall.

More preferably, the slot is cut to a depth of at least 0.5 times the borehole diameter measured from the borehole sidewall.

Most preferably, the slot is cut to a depth greater than the borehole diameter measured from the borehole sidewall.

Preferably, the depth of cut is selected according to a set of explosive variables including, the mechanical properties of the rock, the magnitude of the required explosive, the depth of the overburden, the magnitude of the charge.

According to a second aspect, the invention provides a method of modifying a blasting borehole in mining operations, the method including the steps of: drilling a blasting borehole toward a deposit through an overburden, the borehole having a borehole toe spaced from the deposit by a stand-off distance; and cutting a slot in a sidewall of the borehole adjacent the toe, the slot extending laterally outwardly from the borehole and configured to direct blast energy away from the deposit thereby reducing blasting damage to the deposit.

According to another aspect, the invention provides a slotting tool for cutting a slot in a wall of a borehole, the tool including: a high pressure fluid cutting nozzle for directing a high pressure fluid cutting jet substantially outwardly such that with the tool in a borehole, the nozzle is positioned in cutting proximity with the wall.

Preferably, the jet is a cavitating water jet.

Preferably, the nozzle is configured to pass the jet over a predetermined cutting path.

Preferably, the tool includes an arm, the nozzle being located on a distal portion of the arm.

Preferably, the arm is extendable generally radially outwardly from the tool axis from a retracted position to an extended position thereby maintaining the nozzle in cutting proximity as the slot is cut.

Preferably, the arm is hingedley mounted to the tool about a pivot axis, the pivot axis spaced from the longitudinal axis such that the nozzle extends and retracts relatively radially from the tool axis in response to rotation of the arm about the pivot axis.

Preferably, the tool includes bias means to bias the arm toward the extended position.

Preferably, the bias means is provided by reaction force of the jet.

Preferably, the arm is resiliently biased toward the retracted position.

Preferably, the tool is configured for multiple passes over the cutting path.

Preferably, the tool is configured for rotation about its longitudinal axis.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figs. 1 a to 1 d are schematic representations of widely observed mechanisms of coal loss; Figs. 2 and 3 are cross sectional representations of a coal seam, blasting borehole and slots in accordance with the invention; Figs. 4a and 4b are cross sectional representations of a borehole, slotted, in accordance with the invention; Fig. 5a is a pictorial view of a slotting tool in accordance with the invention; Fig. 5b is a detailed view of the slotting tool shown in Fig. 4; Figs. 6a and 6b are additional views of a slotting tool in accordance with the invention; Figs. 7a and 7b are schematic representations of a fixed nozzle slotting tool and self activating arm slotting tool in accordance with the invention, respectively; Fig. 8 is a pictorial view of a nozzle arm in accordance with the invention; Fig. 9 is a top view of the nozzle arm shown in Fig. 8; Fig. 10 is a pictorial view of another nozzle arm in accordance with the invention; Fig. 11 is a top view of the nozzle arm shown in Fig. Figs. 12a to 12d are schematic top views of the self activating slotting tool in various states of extension; Figs. 13a and 13b are schematic side views displaying the slotter with the arm retracted and extended positions respectively; Figs. 14 and 15 are alternative extendable arm configurations in accordance with the invention; Fig. 16 is a photograph of another slotting tool having a stabiliser device in accordance with the invention; Fig. 17 is a schematic side view of another slotting tool having a stabiliser device in accordance with the invention; Figs. 18 and 19 are pictorial representations of an independent, truck mounted slotting rig in accordance with the invention; and Fig. 20 is a schematic representation of an independent slotting rig in accordance with the invention.

PREFERRED EMBODIMENT OF THE INVENTION It has long been known that the existence of natural weak zones (discontinuities) in rock cause the rock, when subjected to a large external load, to fail in particular ways.

For instance, under the forces of blasting, natural flaws in the rock structure and changes -7of rock composition or properties will heavily influence the way blasting energy is transferred to the rock and ultimately, how the rock fractures.

More specifically, fracture is a failure mechanism that involves the stable or unstable propagation of a crack within a rock. Inherent microscale flaws in rock such as grain boundaries and pores predilect failure. However, once the crack starts to extend due to external stress from one of those microscale flaws, crack propagation may occur extremely rapidly. The mechanism by which cracks propagate is understood to be dependant on a combination of applied stress, crack dimension and the material's fracture toughness at the crack tip. Particularly, if the crack driving force (expressed as the applied stress intensity KI) is greater or equal than the fracture toughness of the rock Kmax, fracture will occur. The crack extension may continue until crack tip stress reduces or the crack is arrested by an existing fracture or discontinuity.

The conventional understanding of blasting process is divided into two distinct events "shock" and "gas" events. The shock phase is held responsible for breakage and the gas phase of material movement and heave. The splitting rock mass with slots may be more related to the gas phase because, essentially, gas is seen to be the driver of crack propagation. Gas expansion will drive the crack until the crack is arrested or the stress becomes less than the fracture toughness of the rock.

The role of slots is analogous to the role of natural defects and weakness planes during blasting. Slots will act as "pre-existing" cracks or flaws encouraging crack propagation in a specific predetermined direction. Furthermore, the development of a crack in a particular direction tends to reduce pressure on cracks in other directions.

Thus by forcing the development of a crack away from the mineral deposit, blasting energy can be diverted as required.

One of the most significant threats to the success of the blasthole slotting technique, particularly in soft rock, is considered to be crushing around the blast hole annulus. Excess crushing may collapse the slots and is thought to be caused by the "shock" event of detonation of an explosive charge. Slot collapse will prevent gases being guided in the required direction.

One solution is to create slots deeper than the crush zone so that even if a small part of the slot is buried in the crush zone, the remaining part will still be functional.

-8- Another solution may be to decouple the charge. Decoupling charges refers to the practice of spacing the explosive charge from the rock to reduce the "shock" event.

Decoupled charges create less crushing and so permit the explosive gases to enter into the slot extending them over long distances. The drawback of this however is the need for additional work.

Decoupling may be either vertical or lateral or a combination. While lateral decoupling is more effective because the charge is fully decoupled along its entire length, it has limitation because of the minimal hole diameter and length of hole that preprepared charges, in the form of sausages can be lowered up to. On the other hand, vertical decoupling may be achieved by decking the hole such that individual decks are fully coupled but separated in the hole by "spacers". The total weight of explosive is reduced but local pulverization still occurs on the walls. The air deck is the most effective but local concentration of shock and gas damage where the deck is not situated still occurs.

However, the two approaches of slotting and decoupling charges may be convergent. This is because slots created at certain distances along the borehole will introduce decoupling and therefore will effectively reduce the blasthole pressure.

During blasting a certain thickness of the borehole wall is crushed. The size of the crush zone will vary depending on the rock and explosive as explained above. This crush zone needs to be taken into account as part of the borehole and the part of the slot that is outside the crush zone needs to be taken as the effective slot depth. Previous studies have developed the stress intensity factor as a function of a circular cavity slot depth parameter. It is indicated that when the slot depth parameter reaches 1.3 1.4 then the stress intensity ratio tapers off (stabilizes). It can be inferred from this that if the depth of slot is equal to half the radius then the maximum stress concentration is reached in the hole.

Nevertheless, the rock structure may emerge as the main controlling factor in splitting with slots. If dominant joint orientation and spacing are particularly favorable many combinations of hole diameter, spacing and decoupling will work effectively.

Structures running parallel to the required split plane may help splitting. However if they are sub parallel they may hinder successful splitting by diverting the split away from the required line. A 900 difference in orientation is not the worst case as it may be possible to drive the split across the planes. Dealing with coal measure rocks, in most cases it is expected that the bedding will assist the splitting initiated with slots.

The invention proposes a method and apparatus for modifying a borehole utilising the above understanding of slotting to control damage to a mineral deposit during blasting operations.

Figs. 2 and 3 show an open cut mine blasting borehole modified in accordance with the invention. In the figure, the deposit is a mineral seam in the form of a coal seam 1. The seam is covered by overburden rock 3. The blasting borehole 2 is drilled through the rock to a depth spaced from the mineral seam 1. As can be seen, the toe 5 of the borehole is spaced from the seam by the stand-off distance 7. The required stand-off distance will depend upon many variables including the type and thickness of rock however, as will be seen, the exact depth is not as critical as would be the case with conventional blasting operations.

Before a charge is laid in the borehole, a laterally extending slot 8 is cut in the rock to direct blast energy 4 away from the seam 1. This may be achieved because the slot 8 will predilect blast energy outwardly, substantially in the plane of the slot. Thus, by carefully selecting the orientation of the slot plane, a measure of control of the blast energy may be achieved.

For instance, in the embodiment shown in Figs. 2 and 3, the slot/s 8 are cut as a circumferential, radially extending channel defining a slot plane. As discussed above, the slot provides a weak point in the rock to initiate crack propagation in the direction of the slot plane. The blast energy and gas used to drive this preferential crack, reduces the tendency for cracks to form in other directions. Accordingly, as indicated by Figs. 2 and 3, blast energy 4 is directed somewhat outwardly in the plane of the slot rather that downwardly toward the seam 1. The reduction of blast energy directed at the seam tend to reduce damage and pulverisation of the coal.

Other benefits of slotting may include a better distribution of blast energy in the borehole and a related reduction in the amount of explosive required, and a decoupling effect resulting in reduced shock and borehole pressure. In addition, because the blast energy is directed substantially outwardly, the otherwise critical stand-off distance 7 to the coal seam 1 becomes less critical.

In alternative embodiments, the slot 8 may be cut in the borehole wall in other configurations. For instance, as shown in Fig. 4a, additional circumferentially radiating slots 8a may be used to enhance the effects of decoupling and blast energy control. Slots may also be cut at any oblique angle to the borehole depending on the required direction blast energy is to be directed. Furthermore, semi or part circumferential slots may be used. In addition, the one or more longitudinally extending slots 8b, as shown in Fig. 4b, may be used in isolation or in combination with circumferential slots.

As discussed above, the slot should be deeper than the expected crush zone.

Although the extent of the crush zone is dependant on a number of variables including the hardness of the rock, and the size and density of the charge, it is preferred that the slot is deeper than about 0.3 times the diameter of the borehole. For instance, for a borehole of 180 mm diameter, the slot should extend about 54 mm into the sidewall of the hole. In a more preferred embodiment the slot is deeper than about 0.5 times the diameter of the borehole and in a particular preferred embodiment of the invention, the slot extends deeper than the borehole diameter. For instance, it is particularly preferred that for a borehole of 180 mm diameter, the depth of the slot is greater than 180 mm.

While the invention is not limited to a particular method of cutting the slot, the cutting of deep slots can prove particularly problematic particularly down the end of a borehole. To this end as shown in Figs. 5a and 5b, and 6a and 6b, the invention provides a slotting tool 10 for cutting deep slots in a borehole.

In this embodiment the tool 10 is configured for attachment to the rotating plate end of a drill string, replacing the drill bit. However, in alternative embodiments, as will be discussed later, the tool may be attached to a specialist independent insertion rig and stem. The stem may be a relatively ridged design or a flexible hose line.

In either case, flange 11 of the tool 10 may be bolted to a similar flange on the drill string/insertion rig stem. As such the tool 10 may be rotated or moved longitudinally in the borehole by the string/stem. Any supply of power, water or the like may be provided via the string/stem from the surface, through the flange.

The tool 10 includes a high pressure fluid, cutting nozzle 12 disposed such that with the tool located appropriately in the borehole, the nozzle is positioned in cutting proximity with the borehole sidewall, and may direct a high pressure fluid cutting jet outwardly. Thus by aiming the jet to pass over a predetermined cutting path on the -11borehole sidewall, a slot may be cut in the sidewall. Repeated passes of the jet over the cutting path will increase the depth of the slot cut. For instance, by rotating the jet around the longitudinal axis of the borehole, a circumferential, radially extending slot, such as those shown as 8 and 8a in Fig. 4a, may be cut. Alternatively, by raising and lowering the jet within the borehole, a longitudinal radially extending slot, such as shown as 8b in Fig. 4b, may be cut.

It will be appreciated that the depth of the slot cut will be influenced by a range of factors including the type, configuration and number of cutting nozzles, the pressure and flow rate of the jet/s, the hardness of the rock, the number and speed of passes of the jet over the cutting path and the proximity of the jet to the rock.

In the embodiments shown in Fig. 5a and 5b, and 6a and 6b, the drill string is used not only for general positioning of the tool in the borehole, but also for moving the entire tool and consequently the cutting nozzle over the cutting path. Generally, the tool depicted in Figs. 5 and 6, is designed to be spun around its longitudinal axis by the drill string, thereby rotating the nozzle over a circumferential cutting path to form a slot as shown in Fig. 4a.

However, in alternative embodiments, the cutting tool may include a separate bearing and motor, so that the nozzle may be rotated within the borehole independently of the drill string. A similar configuration may be applied to an independent insertion rig.

In this embodiment, the nozzle 12 is disposed on a nozzle arm 13 and uses high pressure water as a cutting fluid. The nozzle is positioned at a distal end 14 of the arm. The nozzle arm 13 is extendable and configured to move generally relatively radially between a retracted position and an extended position. As can be seen in Figs. 7a and 7b, in comparison to a fixed nozzle tool 15 shown in Fig. 7a, the radially extendable arm 13, shown in Fig. 7b, allows the nozzle to advance into the slot as it is cut into the rock.

Since the nozzle has a limited effective cutting range, advancement into the slot maintains the nozzle in cutting proximity with the rock, extending the cutting reach of the nozzle and allowing for cutting of the slot to a greater depth.

The particular embodiment of the tool shown in Fig. 5a and 5b, includes an elongate support shaft 16 extending from the flange 11. The support shaft has a longitudinal axis coaxial with the longitudinal axis of the drill string. An auxiliary shaft -12- 17 is attached to the support shaft by means of brackets 18 and 19. The auxiliary shaft is parallel to, but axially offset from, the support shaft.

The support shaft and/or the auxiliary shaft may act as supply and/or return conduits from the surface. For instance, in the embodiment shown in Fig. 5a, highpressure water is pumped down a hose within the drill string through the flange 11, to the nozzle via a conduit inside the auxiliary shaft 17. To extract excess water pooling in the borehole, and return it to the surface, the support shaft also includes a conduit connected to a return line in the drill string. This advantageously eliminates the need for the entire borehole to be wetted.

The arm 13 is hingedly attached to the tool 10 at its proximal end 21 around pivot axis 22. The axis 22 is parallel to, but offset from, the central longitudinal axis of the tool 10. The pivot axis 22 is set as close to the edge of the tool in order to provide for maximum reach of the arm when in the extended position. In this embodiment the auxiliary shaft functions as an axel for the pivot axis. It will be appreciated that this means that the tool does not extend directly radially outwardly. Instead it follows an arcuate path as shown in Fig. 7. The benefits of such a path will be explained later.

A detailed view of the nozzle arm is shown in Figs. 5b, 8 and 9. The proximal end 21 of the arm has an attachment boss 23 including a fluid inlet 24. The distal end 14 of the arm includes three nozzle attachment bores 25, 26 and 27 which, as shown in Fig 9, are fluidly connected to the inlet 24 by conduits 28. Each bore includes threadably engageable attachment means to engage corresponding threads on the respective nozzles to releasably retain the nozzles within the bores. This allows the nozzles to be removed and replaced to provide different nozzle selection. In addition, closer inspection of Fig.

8 shows that two of the bores 25 and 26 are slightly offset from horizontal to widen the cut.

The nozzle arm also includes leading and trailing edges 29 and 30, each edge extending between the proximal and distal ends. The leading edge 29 includes a curved deflection portion 31 at the distal end. Another feature of the arm which can clearly be seen in Fig. 8, is the nose portion 32. This portion extends past the nozzles so that they are effectively recessed and protected from damage due to the impact with the rock or obstructions in the slot.

13- The exact arm configuration, including shape, length, number of nozzle bores and the like may be altered depending on the application. For instance, in certain types of rock the alternative arm shown in Figs. 10 and 11 and having bores for only two nozzles may be preferred.

The nozzles are designed to cut rock with a high-pressure water jet. More particularly, the nozzles are designed to produce cavitating water jets in the submerged operating environment encountered during slotting. Cavitating water jets can be extremely effective in submerged conditions, utilising the erosive power of cavitation bubble collapse to enhance material excavation. Moreover, they have no moving parts and do not rely on direct impact with the rock to cut.

As foreshadowed, the nozzles may be selected according to particular requirements. For instance the nozzles may be selected to match particular rock types or to match the operational performance of the other tool components and complementary equipment. For instance, larger diameter nozzles may provide higher power but also consume more water. This may require a water delivery system with a higher flow rate.

In this embodiment, the nozzles are commercially available 2.9 mm and 1.2 mm attack type nozzles, selected to maximise the hydraulic power of the high-pressure water. The attack type nozzles are claimed to be long lasting and effective in cutting due to their design.

In other embodiments, nozzles may be used which provide jets include pulsating, abrasive and sheathed jets. These jet systems may give superior cutting rates to conventional water jets. The abrasive jet requires sand fine garnet particles), which complicates the bailing of materials in holes as deep as 50m, and also requires a sand separation/recovery process. Sheathed jets on the other hand, work with the addition of high-pressure air. The main jet stream is covered with compressed air on exit and this prolongs the jet's cutting capacity. A pulsating jet system works by pulsating the pressure of the water jets.

The operation of the extendable arm 13 is schematically illustrated in Figs. 12a to 12d and Fig. 13a and 13b. The diagrams show the tool 10, the slot outline 2 and an indicative effective cutting range 36 of the nozzle. This effective cutting range of the jets is dependant on a number of variables including water pressure and flow rate, rock hardness, jet number and type.

14- In Figs. 12a and 13a the arm is shown in the retracted position with borehole outline shown as 2. In this position the arm is configured to fit into the borehole and within the extreme diameter of the tool represented here by line 10. This protects the nozzles from contact with and possible damage from the borehole walls during insertion and extraction of the tool into and out of the borehole. A biasing means applies a retraction force to keep the arm in the retracted position when not operating. In its simplest form the biasing means may be a resilient spring, however in heavy-duty equipment a powered retraction mechanism such as a hydraulic motor may be used.

Figs. 12b and 12c show the arm in intermediated positions of extension, and Figs.

12d and 13b, show the arm fully extended. In all the arm may travel through an arc of around 1800 from fully retracted to fully extended. The arm is prevented from rotating past the fully extended position by stop means.

Given that the arm is biased to the closed position, an extension force opposing and greater than the retraction bias must be applied to the arm in order to extend it.

While such a force may be provided by hydraulic, pneumatic or mechanical means, in this embodiment, the arm is configured to utilise the reaction force of the jets.

In operation, the present embodiment utilises water reaction forces from the jet to extend arm 13. Referring to Figs. 9 and 11 it can be seen that the three jets 24, 25 and 26 share generally parallel longitudinal axes. Thus the jets are disposed in operation, to produce a resultant reaction thrust force indicated by arrow A. It will be appreciated that the force is generally parallel to these axes, and approximately aligned to pass to the side of the axis of rotation of the arm. This force and its displacement from the pivot axis 39 creates a torque causing the arm to rotate in a clockwise direction about the pivot as viewed on the page. As will be appreciated, the arm is configured on the tool so that the rotation results in the arm extending radially outwardly.

Conveniently, the reaction force is only present when the nozzles are in operation. That is, when pressure to the nozzles is reduced, the extending thrust force is correspondingly reduced. At some point the arm will retract under the bias force. Thus, during cutting operations the arm is forced to extend outwardly to maintain cutting proximity of the nozzles with the rock. When cutting is completed, the arm will automatically retract from the slot to allow for repositioning or extraction of the tool.

It is also important that some separation between the nozzle and rock is maintained otherwise the nozzles may be damaged. To this end the nozzles are, as discussed above, somewhat recessed and shielded by the nose portion 32.

As previously mentioned, the nozzle arm follows an arced extension path with respect to the rest of the tool. This results in the distal end of the arm following the proximal end as the arm is rotated by the tool. Accordingly, should the arm encounter an obstacle in the slot such as aggregate or a harder section of rock, it will be deflected toward the retracted position. As the tool continues to rotate, the arm will clear the obstruction and self extend into the slot under influence of the extending thrust force from the jet. This simple design feature eliminates the need for an expensive and complex obstacle detection system in hash operating conditions. The deflection portion of the leading edge is curved to aid deflection. The edge may include a replaceable wear strip of a low friction material.

As discussed above, one of the nozzles is directed to project a cutting jet with a few degrees positive elevation from the horizontal plane and another with a few degrees negative elevation. By directing the cutting jets in this way, the slot created is wider than the arm. This tends to further reduce the likelihood of the arm fouling on obstructions.

While this embodiment uses an arm mounted to pivot on an axis parallel to but offset from the tool axis to provide arcuate radial extension, in other embodiments, as shown in Fig. 14, the arm may be configured to extend directly radially in a linear fashion. In this embodiment, water pressure extends the telescopic arm outwardly.

Another embodiment is shown in Fig. 15 whereby the nozzle arm is a flexible hose which extends radially outwardly due to water pressure.

In an attempt to reduce vibrations, the tool may be provided with a stabiliser. In one form the stabiliser includes a circumferential spaced array of rollers 40 as shown in Fig. 16. The rollers are designed to brace against the borehole wall and prevent excessive movement of the tool. In another form the slotting arm assembly is contained between two expandable packers 41, 42 as shown in Fig. 17. The hydraulic packer system holds the slotter arm tightly in place eliminating movement vibrations. The packer system is also needed for water recovery and cuttings transfer purposes. The bottom packer is be removable when slotting at the very toe of the hole.

16- In this form the tool requires a high-pressure water reticulation system, pump, water recycling plant and all the necessary facilities. Advantageously, moving the tool on an existing drill string eliminates the need for a separate rotation motor and cuttings transfer system as well as reduced operation cost. This option is also favorable in terms of reduced vibration and increased rotation control of the slotting arm.

As previously mentioned the slotting tool may be configured as a slotter attachment to be attached to the drill stem of a drill rig or, it may be self-contained and an independent down hole device. One embodiment of a self-contained independent blasthole slotter is shown in Figs. 18, 19 and The self-contained slotter system includes all the equipment required to facilitate moving, slotting and recycling the water. Despite the fact that the slotter needs to have an additional rotation motor and in-hole movement mechanisms, the main superiority of this option is that the whole system is independent of drilling operations.

The slotter shown in Figs. 18, 19 and 20 includes a transport vehicle 50, a slotting tool 10, to be filled to a high pressure down hole string, a fluid pressure and management system and control equipment. In this embodiment, the slotting tool is mounted on a down hole string in the form of a high-pressure hose 51. The hose may be wound and stored on a hose reel 52 for convenient extension and retraction.

A variable speed down hole hydraulic motor may be included on the tool. Or optionally down the hole motors rotating with the supplied water) can be used. This motor may be used to rotate the arm and jets over the cutting path. In this respect, in contrast to the tool shown in Figs. 5a and 5b, only part of the tool is configured to rotate when cutting.

The same hydraulic motor may activate a down hole pump to bail out the cuttings and water. An air jet pump working with compressed air supplied from a compressor at the surface can be used. In fact this offers many advantages over off the self deep borehole pumps. This include elimination of pump bum-outs, ability to lift drill cuttings and water to surface. Alternatively, as displayed in Fig. 20, the pump may be housed in a high-pressure water unit 55. The system may also include a water recovery plant and tank. In this embodiment, the slotter uses approximately 230 1/min of water and for multiple slots in multiple holes it will be appreciated that a significant amount of water is required. As a result, the water recovery plant and tank separate the cuttings and recycle -17the water. Transport of the cuttings and water to surface is through a return line in the hose. This prevents the need for wetting the entire hole.

The embodiment of the independent slotter also includes a control system 56 providing an operator display for indicating the location and orientation of the slotting arm in the hole. The control system provides information on the system status as well as means controls for raising and lowering the slotting arm, inflating deflating the packers, adjusting the rotation speed and the ability to change the mode of slotting along the hole (vertical) or perpendicular to the hole (horizontal)). The system also includes a sensor and display for indicating the extension position of the arm during the operation.

A diesel fuel tank, hydraulic oil reservoir with filters and oil cooling units must also be accommodated.

It will be appreciated that the invention provides a method for reducing blasting damage to a mineral deposit during mining operations. This is achieved by the deep slotting of boreholes to provide preferential crack propagation and deflection of blast energy away from the seam. Even if the shock waves create a zone of crushed material, by slotting the rock to a depth beyond the expected crush zone, part of the slot will still be present to function as a crack initiating point.

In order to cut deep slots, a downhole slotting tool has been developed, the tool is practical, simple and has low construction cost. Under normal circumstances, no part is in contact with the rock thus reducing wear and tear. In terms of construction, the slotter is made out of mild steel, high-pressure steel pipes and nozzles that are commercially available in the market. In all these respects, the invention represents a commercially significant improvement over the prior art.

Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims (24)

1. A method of modifying a blasting borehole in mining operations, the method including the steps of: providing a borehole having a borehole toe, the toe spaced from a deposit by a stand-off distance; and cutting a slot in a sidewall of the borehole, the slot configured to direct blast energy away from the deposit thereby reducing blasting damage to the deposit.

2. A method according to claim 1 wherein the slot extends circumferentially around the borehole adjacent the borehole toe.

3. A method according to claim 1 wherein the slot extends laterally, longitudinally with the borehole axis.

4. A method according to any one of the preceding claims wherein the slot is cut by passing a high pressure fluid jet slotting tool over a predetermined cutting path.

A method according to any one of the preceding claims wherein the slotting tool is adapted for attachment to a drill string.

6. A method according to any one of the preceding claims wherein the slotting tool is combined with a borehole drilling tool and the slot is cut without removal of the drilling tool.

7. A method according to any one of the preceding claims wherein the method includes the further steps of: placing an explosive charge in the borehole adjacent the slot; and detonating the charge.

8. A method according to any one of the preceding claims wherein the slot is cut to a depth of at least 0.3 times the borehole diameter measured from the borehole sidewall.

9. A method according to any one of claims 1 to 7 wherein the slot is cut to a depth of at least 0.5 times the borehole diameter measured from the borehole sidewall.

A method according to any one of claims 1 to 7 wherein the slot is cut to a depth greater than the borehole diameter measured from the borehole sidewall.

11. A method according to any one of the preceding claims wherein, the depth of cut is selected according to a set of explosive variables including, the mechanical properties of the rock, the magnitude of the required explosive, the depth of the overburden, the magnitude of the charge. -19

12. A method of modifying a blasting borehole in mining operations, the method including the steps of: drilling a blasting borehole toward a deposit through an overburden, the borehole having a borehole toe spaced from the deposit by a stand-off distance; and cutting a slot in a sidewall of the borehole adjacent the toe, the slot extending laterally outwardly from the borehole and configured to direct blast energy away from the deposit thereby reducing blasting damage to the deposit.

13. A slotting tool for cutting a slot in a wall of a borehole, the tool including: a high pressure fluid cutting nozzle for directing a high pressure fluid cutting jet substantially outwardly such that with the tool in a borehole, the nozzle is positioned in cutting proximity with the wall.

14. A tool according to claim 13 wherein the jet is a cavitating water jet.

A tool according to claim 13 or 14 wherein the nozzle is configured to pass the jet over a predetermined cutting path.

16. A tool according to any one of claims 13 to 15 wherein, the tool includes an arm, the nozzle being located on a distal portion of the arm.

17. A tool according to claim 16 wherein the arm is extendable generally radially outwardly from the tool axis from a retracted position to an extended position thereby maintaining the nozzle in cutting proximity as the slot is cut.

18. A tool according to claim 17 wherein the arm is hingedley mounted to the tool about a pivot axis, the pivot axis spaced from the longitudinal axis such that the nozzle extends and retracts relatively radially from the tool axis in response to rotation of the arm about the pivot axis.

19. A tool according to claim 18 wherein the tool includes bias means to bias the arm toward the extended position.

A tool according to claim 19 wherein the bias means is provided by reaction force of the jet.

21. A tool according to any one of claims 16 to 20 wherein the arm is resiliently biased toward the retracted position.

22. A tool according to any one of claims 13 to 21 wherein the tool is configured for multiple passes over the cutting path.

23. A tool according to any one of claims 13 to 22 wherein the tool is configured for rotation about its longitudinal axis.

24. A method of modifying a blasting borehole in mining operations, said method being substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples. A slotting tool for cutting a slot in a wall of a borehole, said tool being substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples. DATED this 3 0 th Day of July, 2006 Shelston IP Attorneys for: CMTE Development Limited