WO1995028551A1 - Controlled fragmentation of hard rock by pressurization of the bottom of a drill hole - Google Patents

Abstract

A controlled-fracturing process is accomplished by pressurizing the bottom of a drill hole (6) in such a way as to initiate and propagate a controlled fracture (11) or propagate any pre-existing fractures. A cartridge (5) containing a non-detonating propellant charge (4) is inserted at the bottom of a short hole drilled in the rock. The cartridge (5) is held in place or stemmed by a massive stemming bar (3). A firing pin (2) in the stemming bar (3) strikes a primer which then ignites the propellant (4) in the cartridge (5). The primary method by which the high gas pressures are contained is by the massive inertial stemming bar (3) which blocks the flow of gas up the drill hole (6). A preferred embodiment incorporates an indexing mechanism (37) to allow both a drill (39) and a stemming bar (3) to be used on the same boom (36) for drilling and subsequent charge initiation and firing operations.

Description

CONTROLLED FRAGMENTATION OF HARD ROCK BY PRESSURIZATION OF THE BOTTOM OF A DRILL HOLE

INTRODUCTION The following describes a method and means to break rock efficiently and with low-velocity fly-rock such that drilling, mucking, haulage and ground support equipment can remain at the working face during rock breaking operations.

This method is a small-charge blasting process as opposed to a mechanical method or drill and blast type method for breaking rock. A small charge blasting method implies that the rock is broken out in small amounts (typically on the order of h to 3 cubic meters per shot) as opposed to episodic conventional drill and blast operations which involve drilling, blasting, ventilating and mucking cycles.

The present invention involves breaking rock or other hard material, such as concrete, by drilling a short hole, placing a cartridge containing a propellant charge in the drill hole, positioning a massive stemming bar in the drill hole in contact with the cartridge, and igniting the propellant. The stemming bar, which can be made of a high- density, high-strength material such as steel, confines the pressure in the hole bottom by inertially controlling and minimizing recoil of the cartridge during the rock-breaking process. Ignition and burning of the propellant effectively pressurizes the bottom of the drill hole and the method is thus referred to as Hole-Bottom Pressurization or HBP. The HBP method induces a controlled fracturing of the rock which is considerably more energy efficient than the drill and blast method or mechanical rock excavation methods.

If the rock is of high strength and massive without extensive jointing, this controlled fracturing will be manifested by a type of fracture in the rock that is referred to as Penetrating Cone fracture (PCF) . The basic features of PCF rock breakage by the HBP method are illustrated in Figure 1. PCF breakage is based on the initiation and propagation of an axi-symmetric fracture from the bottom corner of a short, rapidly pressurized borehole. Such a fracture initially propagates downward into the rock, and then turns towards the free surface as surface effects become important, resulting in the removal of a large volume of rock. The residual cone left on the rock face by the initial penetration of the fracture into the rock provides the basis for the name (Penetrating Cone Fracture, or PCF) given to this type of fracturing.

If the rock contains joints or other pre-existing fractures, the controlled fracturing will be manifested by the opening and extension of these fractures. In either case, the rock breakage is characterized by a fracturing controlled by properly pressurizing and only pressurizing the very bottom of the drill hole. Because of the high pressures, in the range of 200 MPa to 1,000 MPa, required to properly effect the controlled fracturing of hard rock, or comparable materials, several innovative design and application concepts had to be realized and are the subject of the present invention. The pressures developed within a HBP propellant cartridge and applied to the hole bottom are comparable to those occurring in the breech of a high- performance gun. The primary method by which the gas- pressures are contained at the hole bottom until relieved by the opening up of controlled fractures, is by the massive inertial stemming bar which blocks the flow of gas up the drill hole except for a small leak path between the stemming bar and the drill hole walls. This small leakage can be further reduced by design features of the propellant containing cartridge and of the stemming bar. The cartridge may be designed with a tapered wall, which is thicker nearer the stemming bar, and a similarly tapered base plug which can slide within the cartridge walls as the stemming bar recoils. This type of sealing mechanism can reduce the possibility for premature cartridge rupture and leakage of propellant generated gases. A sealing mechanism on the stemming bar may also be used to obtain better or complete sealing near the hole bottom.

A key feature of the hole-bottom, controlled fracturing method is the benign nature of the flyrock which allows drilling, mucking, ground support and haulage equipment to remain at the working face during rock breaking operations. A second key feature of the method and apparatus is that they may be used in either dry or water filled holes.

BACKGROUND "Borehole Blasting Device" U.S. Patent No. 3,307,445, Stadler et al., 7 March 1967.

This patent relates to a device for breaking relative soft rocks, such as coal, with an improved blast cartridge to be used in water-injection blasting operations. The particular features of the device include:

■ a "probe means operable to be inserted into a borehole".

■ a "means radially expandable against the side walls of the borehole to form a forward sealed chamber."

■ a means for "injecting a liquid under pressure into the forward sealed chamber."

■ "a cartridge containing propellant charge means and ignition means," such that the rapid burning of the propellant will assist in the pressurization of the forward chamber of the borehole and the breaking of the material.

The key features of the device are that the burning of the propellant provides additional pressure to a water injection blasting process for breaking coal or comparably soft materials. Sealing of the device or probe in the borehole is accomplished by expanding a soft rubber hose or hydraulic packer against the borehole walls with moderately low pressure liquid. Such hydraulic packer devices are operable only to pressures less than 100 MPa. Stadler et al. clearly envision applying the propellant augmented pressure to the entire forward (deeper) portion of the borehole. Stadler et al do not anticipate the benefits that might be obtained by applying a gas pressure to the hole bottom only so that a controlled fracturing from the hole bottom might be realized. Neither the propellant cartridge nor the hydraulic packer seal described by Stadler et al would support the very high pressures of 200 MPa to 1,000 MPa required to break hard competent rock or comparable materials. The soft rubber hose section serving to connect the forward propellant-charge portion of the probe to the rearward portion of the probe would preclude the probe from providing a significant inertial contribution to hole-bottom sealing. Stadler only envisions and anticipates pressurizing a major section of a borehole with propellant augmented liquid pressures adequate only to break a soft coal like material by conventional fracturing.

"Controlled Fracture Method and Apparatus for Breaking Hard Compact Rock and Concrete Materials" U.S. Patent No. 5,098,163, 24 March 1992. This patent relates to breaking rock by inducing a characteristic type of fracture called Penetrating Cone Fracture (PCF) by:

■ drilling a short hole, with a length-to-diameter ratio in the range of 3 to 15, using a percussive drill. Practical hole diameters can range from

1 inch to 20 inches.

■ using a gun-like device or gas-injector to burn propellant in a combustion chamber. The burning and burnt propellant then expands down a short barrel and into the bottom of the hole where it pressurizes the bottom of the hole to induce PCF.

The barrel diameter of the gas-injector is smaller than the drill-hole diameter to provide adequate clearance.

One of the key features of this device is that the muzzle of the gas-injector forms a dynamic seal near the hole bottom so that only the hole bottom is pressurized. This allows the technique to operate effectively even in the presence of considerable natural fracturing in the rock mass. This method is referred to as the Gas-Injector method or Injector method.

While the Injector method has proven practical and successful, it has the potential disadvantage of damaging the muzzle end of the gas-injector with repeated usage.

The reason for this is that, under some operating conditions, the propellant load is never completely burned inside the gas-injector because it does not have the confinement of a projectile to allow the burning to go to completion. A significant fraction of the initial propellant load (15 to 30 percent) can be driven out of the gas-injector into the bottom of the hole in a partially burned condition, where it compacts and burns to completion extremely rapidly. If not controlled, this rapid burning can cause a very high-pressure pulse to develop which can damage the muzzle of the gas-injector. A similar condition occurs in wet drill holes (typical of shots drilled downward which are filled with water from the drilling process or from water influx from the ground) . The water fills the hole bottom and causes the initial pulse of propellant gases, which expands down the barrel of the gas-injector, to shock-up in or near the tip of the injector barrel which can damage the muzzle of the gas- injector.

The Injector method is illustrated schematically in Figure 2. Figures 3 and 4 illustrate pressure histories in the gas-injector combustion chamber and in the hole bottom as calculated with an explicit-finite difference technique. Figure 3 illustrates a normal operation case where the majority of the propellant is burned in the chamber of the gas injector. In normal operation, pressures of 200 to 500 MPa are achieved in the hole bottom. Such pressures are necessary and sufficient to initiate and drive the desired penetrating-cone fracture. Figure 4 illustrates an abnormal case where the gas-injector hardware could be damaged. In the abnormal case, a significant mass of incompletely burned propellant is blown down the injector barrel to the bottom of the drill hole. When the incompletely burned propellant and burnt propellant gases impact the hole bottom, the pressure rises abruptly, causing the propellant burning rate to also rise abruptly. The incompletely burned propellant then burns extremely rapidly, driving up the pressure in the bottom of the hole. This causes a large pressure wave to be reflected back up the injector barrel. As illustrated in Figure 4, the pressure in this reflected wave equals 1,100 MPa and could substantially exceed the strength capacity of the injector barrel causing severe deformation or rupture of the muzzle end of the barrel.

Another advantage of the HBP method relative to the Injector method is the requirement to burn additional propellant in the injector to pressurize the internal volume of the injector. The internal volume of the gas- injector barrel and combustion chamber are comparable to the volume of the hole bottom that is desired to be pressurized. This additional propellant, when burned, ultimately contributes to the air-blast, ground vibration and flyrock energies, all of which are unwanted by-products of the rock-breaking process. The HBP approach eliminates the additional gas-injector internal volume since the hole bottom is acting as the combustion chamber.

As any additional volume required for proper propellant burning is contained internally within the cartridge, the HBP cartridge may be used in either dry or water filled holes. In the case of wet holes, the cartridge, which can be fully water-proofed, displaces the water in the hole bottom as it is inserted into the hole bottom by the stemming bar.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cutaway side view of the present HBP controlled fracture process (shown here with a penetrating cone type fracture) with a propellant containing cartridge held in the bottom of a drill hole by a massive stemming bar.

Figure 2 is a cutaway showing the prior art gas- injector method for controlled fracture of rock (also shown here with a penetrating cone type fracture) .

Figures 3 and 4 illustrate calculated pressure histories of the prior art gas-injector system.

Figure 5 is a cutaway close up side view of an HBP cartridge and stemming means showing the recoiling base plug design for sealing of the cartridge.

Figure 6 is a cutaway view of the present HBP process showing the stemming bar and cartridge in the drill hole prior to initiating the propellant. Figure 7 is a cutaway view of the present HBP process after the propellant has been initiated showing the sealing action by the recoiling base plug of the HBP cartridge when the cartridge wall does not rupture near the end of the stemming bar. Figure 8 is a cutaway view of the present HBP process after the propellant has been initiated showing the sealing action by the back-up sealing ring when the cartridge wall does rupture near the end of the stemming bar.

Figure 9 illustrates a calculated pressure history for a hole-bottom pressure cartridge.

Figure 10 shows the calculated gas distribution in the system during the HBP process where leakage occurs while fracture volume is opened up. Figure 11 shows the present invention in use with a typical carrier having plural booms, each boom comprising a means for drilling and then indexing the present hole- bottom pressure cartridge into the hole. Figure 12 shows an alternative cartridge configuration in which the internal wall is tapered to thicken the wall towards the base of the cartridge. The internal wall transitions to the interior of the base by a large radius so as to reduce the tendency to rupture prematurely from the combined action of the wall of the cartridge being pinned against the wall of the drill hole and the base of the cartridge following the recoil of the stemming bar.

DETAILED DESCRIPTION The present invention represents a significantly different means to induce hole-bottom controlled fracturing, such as the Penetrating Cone Fracture (PCF) type of rock fracture. However, it retains the major advantages of the Injector method in that rock is broken efficiently and the resulting flyrock is so benign that equipment can remain at the working face while the rock is being broken. It differs from the Injector method in that a propellant cartridge, containing a solid or liquid propellant, is placed directly into the bottom of a percussively drilled hole, as illustrated in Figure 1. In the Hole-Bottom Pressurization (HBP) method, the cartridge is stemmed by a massive bar which contains a device for igniting the propellant charge. The stemming bar, which can be made of high-density, high-strength material such as steel, provides inertial confinement for the high-pressure gases developed when the propellant is burned.

The confinement of the high-pressure gases to the hole bottom is realized by the proper interaction of the inertia of the stemming bar which minimizes the recoil displacement of the cartridge, the expansion of the cartridge to the drill hole walls without rupturing and a small clearance between the end of the stemming bar and the hole wall which nearly eliminates the escape of high-pressure gases past the bar during the brief time it takes to initiate, propagate and complete a controlled fracture.

Since the downhole end of the stemming bar fills most of the cross section of the drill hole, it provides adequate sealing of the gas pressures generated by the propellant charge. When the propellant is initiated properly and burns quickly to its peak design pressure, only a small fraction of the propellant gases escape up the gap between the stemming bar and the drill hole walls. This residual gas leak, although it does not seriously degrade the pressure in the hole bottom, can cause damage to the stemming bar over a large number of shots. Design of high-pressure gas sealing features into the cartridge base or downhole end of the stemming bar can reduce or eliminate the residual leakage of propellant gases.

Figure 5 shows an HBP cartridge geometry including: the downhole end of the stemming bar; a tapered base plug that can slide within the cartridge wall; an internal relief volume to control the peak propellant pressure; and a back-up metal sealing ring in the event the cartridge wall ruptures near the base.

The cartridge may be destroyed in one shot. The end of the stemming bar is exposed to a controlled pressure pulse similar to that generated inside a propellant-driven gun and, if protected such as by the tapered base plug of the cartridge, is unlikely to sustain damage over a large number of firings. Even if the end of the stemming bar adjacent to the cartridge is damaged from time to time, it is a relatively simple, low-cost operation to replace or repair the damaged end.

The wall of the cartridge is designed to expand to the drill hole wall without rupturing, thus preventing the high-pressure propellant gases from acting directly on the hole wall or in any fractures (natural or induced) along the hole wall. This containment of propellant gases maintains the gas pressure so that the propellant gases act predominantly to form and pressurize the desired controlled fractures, such as a penetrating-cone-fracture originating at the stress concentration developed at the bottom of the hole. It is important to prevent hot gases from escaping up the hole around the steel bar. Such gas escape can reduce, by a small amount, both the pressure and volume of gas available for the desired HBP controlled fracturing. Also the escaping gases could damage the stemming bar by convective heat transfer erosion processes. As noted above, the escape of gases past the bar may be reduced by having a small clearance between the bar and the hole wall. Both experimental and calculational results indicate that an annular clearance of less than 0.38 mm in a 76-mm borehole will adequately minimize the escape of high- pressure gases. An HBP cartridge and stemming bar may be readily inserted into a hole with such small clearances by drilling a stepped drill hole with a larger-diameter upper- portion section, as illustrated in Figure 5.

Additional cartridge integrity is obtained by including a sliding conical base plug in the cartridge. In this embodiment the cartridge comprises a tapered wall section with a cylindrical exterior and a conical interior and a basal sealing plug of mating conical shape which can move inside the conical interior wall of the cartridge. As the stemming bar is displaced out of the hole by the pressure of the gases, the basal plug can follow and thus maintain a seal against the propellant gases for a time long enough to complete the controlled hole-bottom fracture process. In addition to or as an alternative to the sealing and gas containment provided by the charge cartridge as described above, sealing may be provided at the cartridge end of the stemming bar. Any of several sealing techniques, such as V-seals, O-rings, unsupported area seals, wedge seals, et cetera may be employed. The seals may be replaced each time a cartridge is fired or, preferably, the seals may be reusable. When the primary sealing function is provided only by the stemming bar, the design of the cartridge may be simplified considerably.

The HBP method may be used in either a constant diameter drill hole or a stepped drill hole. In the case of a stepped drill hole, the hole bottom is drilled at a slightly smaller diameter than the top of the hole. This can be accomplished by a pilot bit with a following reamer bit. The length of the smaller diameter pilot hole is slightly longer than the HBP cartridge. The main purpose of the stepped hole is to make it easier to insert the cartridge with the stemming bar. The stepped hole also allows the cartridge to be inserted with a closer tolerance fit than would be the case with a constant diameter drill hole, since alignment of the stemming bar with the drill hole is less critical.

Figures 6, 7 and 8 illustrate the HBP process. Figure 6 shows the system before initiating the propellant. Two possibilities are envisioned for the behavior of the rear of the cartridge. In the fir t case, shown in Figure 7, the tapered base plug recoils with the stemming bar and the walls of the cartridge are held against the drill hole walls by the gas pressure. In this case, there is no leakage of propellant gases out of the rear of the cartridge. The disk which initially separates the propellant from the internal relief volume and the front end of the cartridge have been fragmented, and the hole bottom is exposed to the full gas pressure. In the second case, shown in Figure 8, the wall of the cartridge near the base plug has been ruptured. The high pressure gas has forced some of the wall material and the steel back-up ring into the gap between the stemming bar and the walls of the drill hole to seal any further leakage of gas past the stemming bar. Otherwise the operation of the system is the same as in Figure 7.

Figure 9 illustrates the pressure history in the hole bottom as calculated using a finite difference computer code. This code models the burning of the propellant in the cartridge, the leakage of gas past the stemming bar and the evolution of a typical fracture volume. Two pressure histories are shown. The first is the hole bottom pressure with only recoil of the stemming bar. The second includes the recoil of the stemming bar, some gas leakage past the stemming bar and fracture volume opening up at the hole bottom. Figure 10 shows the gas distribution history for the gas remaining within the cartridge volume, the gas leaked out of the base of the cartridge (assuming no sealing action) , and the gas injected into the hole bottom and the rock fractures. In this calculation, the base of the cartridge is assumed to have ruptured and the gas leaks out the gap between the stemming bar and the drill hole walls. Initially the calculation begins with 150 grams of propellant in the cartridge inserted at the bottom of a 76- mm diameter drill hole. After 2.65 milliseconds, 37 grams of gas remain within the original cartridge volume, 15 grams have leaked past the stemming bar and 98 grams have been injected into the hole bottom and developing fractures. After 2.65 milliseconds, the fracture has propagated over a meter and the rock has been effectively excavated. From the perspective of gas leakage, this is a worst case situation since the gap between the stemming bar and drill hole walls is assumed to be wide open and not blocked by any cartridge material or a back-up metal sealing ring.

Figure 9 illustrates a typical pressure history in the hole bottom calculated for a HBP method. This pressure history can be compared to the hole bottom pressure history of Figure 3 for the injector method of inducing PCF type fractures. The pressure history in the HBP cartridge is much less dynamic than that in the Injector system. This is because the propellant gases in the HBP cartridge need only expand into the small relief volume at the bottom of the HBP cartridge and the pressure increases by small reflection pulses to a maximum of 400 MPa. In the Injector method, the propellant gases developed in the combustion chamber must expand down the injector barrel to reach the bottom of the drill hole. Through this expansion, internal energy is converted to kinetic energy over the length of the barrel. As a result, the gas pressure decreases and the gas velocity increases. When the high-velocity, low- pressure gases encounter the bottom of the hole, kinetic energy is abruptly converted back to internal energy and the gas pressure rises abruptly. In the Injector method, pressure waves reflect back and forth in the injector and hole bottom in a much more dynamic fashion than in the HBP method, causing much higher pressure transients.

Hole sealing can be assisted and apparatus weight can be reduced by accelerating the stemming bar toward the hole bottom just prior to igniting the propellant in the cartridge. The stemming bar can be accelerated by the hydraulic or pneumatic power source that is used to move the boom or carrier for the HBP apparatus, or by any other means that are available. The stemming bar is accelerated to a velocity directed towards the hole bottom, which is comparable to the oppositely directed recoil velocity induced by burning the propellant. These velocities are on the order of 5 to 50 feet per second. The pre-firing acceleration must be sufficient to achieve the desired velocity in a short distance, on the order of a third of a hole diameter (an inch or less in a 3-inch diameter hole) . This technique is referred to as "firing out-of-battery" and is sometimes employed in the operation of large guns to reduce recoil forces. Since the recoil velocity of the HBP apparatus plays an important role in the hole sealing process, it is desirable to minimize recoil velocity. The firing out-of- battery technique can accomplish this. Alternatively, if recoil velocity is acceptable, this technique can be employed to reduce the recoil mass. In the HBP method, the HBP apparatus serves as a large part of the recoil mass and thus the weight of the apparatus may be reduced. Weight reduction is an important goal since the carrier and boom can operate more efficiently with less weight associated with the drill and HBP apparatus.

The firing out-of-battery technique can also be used to assist the sealing operation when sealing is provided by the propellant cartridge. The seal provided by the cartridge is usually broken when the base of the cartridge ruptures and separates from the body of the cartridge as the stemming bar recoils out of the hole (the body of the cartridge is held against the drill hole walls by the high- pressure propellant gases and cannot move relative to the hole) . By firing out-of-battery, the recoil velocity of the stemming bar can be reduced and the out-of-hole displacement of the stemming bar can be delayed, giving the high-pressure propellant gases significantly more time to act on the hole bottom and drive the desired controlled fracturing to completion.

The disk that partitions off the internal relief volume of the cartridge adjacent to the bottom of the drill hole is designed to rupture or disintegrate when the propellant is burned so that the hole bottom is exposed to the high-pressure gases. Alternately, the cartridge can be manufactured as a molded part with the internal relief volume as an integral part of the cartridge as shown in Figure 5. The downhole end of the relief volume would be designed with a thinner wall section or with burst grooves to ensure that it ruptures in a way to expose only the hole bottom to the initial gas pressure pulse. These gases can then cause a PCF type or other controlled fracture to develop and the gases can then drive this fracture deep into the rock. A space between the closure disk and the hole bottom provides a volume into which the burning propellant can expand. This volume is important to the control of the peak propellant burn pressures and provides, through control of the volume, the means to control the gas pressures applied to the material to be fractured and the cartridge. Gas pressures sufficient for controlled fracture development but below those which would rupture the cartridge may thus be attained in a controlled manner.

The pressures thus developed are maintained below those which would deform or damage the end of the stemming bar and below those which would crush the rock around the hole.

An important feature of the HBP process is the elimination of crushed rock which is a primary source of dust. Excess dust requires additional equipment and time to control and can, in some types of excavation operations, lead to secondary explosions which are a safety hazard. The basic components of the HBP system are: ■ boom assembly and carrier

■ drill mounted on the boom assembly

■ the cartridge magazine and loading mechanism

■ the stemming bar and propellant ignition mechanism ■ the cartridge and primer

■ the propellant

The basic components of the system are shown schematically in Figure 11. The following paragraphs describe the envisioned characteristics of the various components.

The boom assembly and carrier The carrier may be any standard mining or construction carrier or any specially designed carrier for mounting the boom assembly or boom assemblies. Special carriers for shaft sinking, stope mining, narrow vein mining and military operations, such as trenching, fighting position construction and demolition charge placement, may be built.

The boom assembly may be comprised of any standard mining or construction articulated boom or any modified or customized boom. The function of the boom assembly is to orient and locate the drill and HBP device to the desired location. The boom assembly may be used to mount an indexer assembly. The indexer holds both the rock drill and the HBP stemming bar assembly and rotates about an axis aligned with both the rock drill and the HBP stemming assembly. After the rock drill drills a short hole in the rock face, the indexer is rotated to align the stemming bar assembly for ready insertion into the drill hole. The indexer assembly removes the need for separate booms for the rock drill and the stemming bar assembly. The mass of the boom and indexer also serves to provide recoil mass and stability for the drill and HBP device.

Drill The drill consists of the drill motor, drill steel and drill bit, and the drill motor may be pneumatically or hydraulically powered.

The preferred drill type is a percussive drill because a percussive drill creates micro-fractures at the bottom of the drill hole which act as initiation points for penetrating-cone fracture. Rotary, diamond or other mechanical drills may be used also. In these cases the bottom of the hole may have to be specially conditioned to promote the PCF type of fracture.

Standard drill steels can be used and these can be shortened to meet the short hole requirements of the HBP method.

Standard mining or construction drill bits can be used to drill the holes. Percussive drill bits that enhance micro-fracturing may be developed. Drill hole sizes may range from 1-inch to 20-inches in diameter and depths are typically 3 to 15 hole diameters deep.

Drill bits to form a stepped hole for easier insertion of the stemming bar assembly may consist of a pilot bit with a slightly larger diameter reamer bit, which is a standard bit configuration offered by manufacturers of rock drill bits.

HBP cartridge magazine and loading mechanism

The HBP cartridges are stored in a magazine in the manner of an ammunition magazine for an autoloaded gun. The loading mechanism is a standard mechanical device that retrieves a cartridge from the magazine and inserts it into the drill hole. The stemming bar described below may be used to provide some or all of this function.

The loading mechanism will have to cycle a cartridge from the magazine to the drill hole in no less than 10 seconds and more typically in 30 seconds or more. This is slow compared to modern high firing-rate gun autoloaders and therefore does not involve high-acceleration loads on the HBP propellant cartridge. Variants of military autoloading techniques or of industrial bottle and container handling systems may be used.

The stemming bar and firing mechanism

This is a major component of the present invention. It provides inertial confinement for the high-pressure propellant gases and provides primary sealing of the gases in the hole bottom by blocking off most of the cross- sectional area of the hole. The stemming bar can be made from a high-strength steel with good fracture toughness characteristics. It can also be made from other materials that combine high density and mass for inertia, strength to withstand the pressure loads without deformation and toughness for durability. Alternately, a high-strength steel stemming bar with a non-metallic end section can be employed. This end section can be made from a high-impact material such as urethane to help isolate the main stemming bar from occasional high-pressure overloads. The stemming bar is attached to the main indexing boom mechanism as illustrated in Figure 11. The stemming bar typically extends well into the drill hole. The stemming bar makes firm contact with the propellant cartridge to provide good contact for initiating the primer and to confine the cartridge at the bottom of the drill hole as the propellant is burned. The diameter of the stemming bar is just less than the drill hole diameter, enough to provide clearance for the bar in the hole. The stemming bar contains the firing mechanism for the propellant cartridge. This firing mechanism may be mechanical (percussive) , electrical or optical in function.

Additional sealing against the escape of the propellant generated gases may be provided at the cartridge end of the stemming bar. Any of several conventional sealing techniques, such as V-seals, O-rings, unsupported area seals et cetera, may be employed. The additional sealing would serve to further limit the undesirable escape of propellant generated gases from the cartridge and the bottom of the hole. Additional sealing of the propellant generated gases may be achieved also by accelerating the stemming bar into the hole just prior to ignition of the propellant charge such that the inertia of the stemming bar into the hole provides additional forces against the displacement of the cartridge out of the hole and the consequent cartridge rupture and loss of high-pressure propellant gases. The HBP cartridge and primer

The HBP cartridge is a major component of the present invention. Its function is to act as a storage container for the solid or liquid propellant, to serve as a means of transporting the propellant from the storage magazine to the bottom of the drill hole, to serve as a combustion chamber for the propellant and to provide a sealing mechanism for the propellant gases as the propellant is burned in the drill hole. In addition to containing the propellant charge, the HBP cartridge contains an internal relief volume as illustrated in Figure 5. This relief volume is necessary to control the peak propellant pressures developed as the propellant is burned. Without this relief volume, the propellant burning could accelerate uncontrollably and the propellant could even detonate in the confined space. Such detonation could cause high-pressure shock waves that might damage the end of the inertial confinement bar. Such rapid burning or detonation of the propellant is also not suitable for inducing PCF or other controlled fracturing, as the process is too abrupt to properly pressurize the desired fractures without creating undesirable fractures and/or crushing the material. The fines generated by such crushing could plug the fractures, thus preventing their proper pressurization by the propellant gases. Also the generation of fines represents an undesirable energy loss. This rapid burning is also likely to rupture the HBP cartridge sealing action along the cartridge wall or at the end of the cartridge adjacent to the stemming bar, causing gas pressure to drop prematurely and or thermal ablation damage to the bar.

One of the main design criteria for the cartridge is to provide proper sealing in the drill hole for the burning or burnt propellant gases under controlled burning conditions. The cartridge may be designed to seal adjacent to the stemming bar, around the drill hole walls. This will prevent high-pressure gases from leaking between the stemming bar and the walls of the drill hole, and better contain the high-pressure propellant gases in the bottom of the drill hole. The cartridge may also be designed to seal around the primer hole to prevent damage to the firing pin from the hot, high-pressure propellant products. A simple cartridge design with features to ensure proper drill hole sealing and containment of the propellant gases is shown in Figure 5. Through several field tests, it has been established that the HBP cartridge must have a combination of the proper geometry and the proper material properties to prevent premature cartridge rupture, which results in the premature loss of propellant gas pressure, which, in turn, reduces the effectiveness of the desired hole-bottom controlled-fracture process. The cartridge design illustrated in Figure 5 satisfies the general requirements by combining a tapered wall and similarly tapered base plug, both of which tend to prevent the premature failure of the cartridge near the cartridge base. Wall tapers in the range of 1 to 10 degrees are satisfactory, with tapers between 3 and 5 degrees being preferred.

The cartridge may be made from any tough and pliable material, including most plastics, ductile metals, and properly constructed composites. The cartridge must be made of a material which can deform either elastically and/or plastically, with sufficient deformation prior to rupture to allow the cartridge containment to follow both the expansion of the drill hole walls and the recoil of the stemming bar during the rapid borehole pressurization and controlled-fracture process. The cartridge may also be made from a combustible or consumable material such as used in combustible cartridges occasionally used in gun ammunition. The preferred materials are those that will provide the required sealing and that can be made for the lowest cost per part.

Reusable cartridges can also be employed. In these, the end adjacent to the bottom of the drill hole would be consumed with each shot. The remainder of the cartridge must be recovered, reprimed, refilled with propellant and refitted with a new bottom disk to hold the propellant in the cartridge.

In the design shown in Figure 5, a mechanical action is used to reduce some of the geometry and material property requirements of the first cartridge design. This HBP cartridge is constructed of a pliable sleeve and basal sealing plug. The pliable sleeve is tapered to provide a greater resistance to premature rupturing of the cartridge near its base and to provide an interference seal with the basal sealing plug, which is also tapered. The basal sealing plug can be constructed from any solid material, such as a plastic, a metal or a composite. The preferred materials are those that can be made for the lowest cost per part. The basal sealing plug contains the primer required to ignite the propellant charge.

The primer fits into the cartridge at the end adjacent to the stemming bar. Its function is to initiate propellant burning when actuated by a command from the operator. Standard or novel propellant initiation techniques may be employed. These include percussive primers, where a mechanical hammer or firing pin detonates the primer charge; electrical primers, where a capacitor discharge circuit provides a spark to detonate the primer charge; thermal primers, where a battery or capacitor discharge heats a glow wire; or an optical primer, where a laser pulse initiates a light sensitive primer charge.

An alternate cartridge design is shown in Figure 12. This cartridge design is of simpler construction than the cartridge design shown in Figure 5. This alternate design satisfies the general sealing requirements by combining a tapered wall and large internal radius at the base, both of which tend to prevent the premature failure of the cartridge near the cartridge base. Wall tapers in the range of 1 to 10 degrees are satisfactory, with tapers between 3 and 5 degrees being preferred. -28-

The propellant

Propellants rather than explosives are employed in the present invention. Propellants burn sub-sonically and pressure build-up is controlled by the propellant geometry; propellant chemistry; propellant loading density; ullage or empty space in the cartridge; and confinement of the cartridge/propellant system between the walls of the drill hole and the stemming bar. With this control, the bottom of the drill hole can be pressurized until a penetrating cone fracture or other controlled fractures are initiated along the line of maximum stress concentration on the perimeter of the hole bottom. The propellant gases then expand into the fracture(s) and drive the fracture(s) deep into the rock and/or to nearby free surfaces. An explosive charge, on the other hand, would detonate which is a supersonic type of burning that generates strong shock waves. This would also pressurize the bottom of the drill hole but pressure build-up would be so abrupt that the rock around the borehole would excessively fractured and crushed. As a result, the fractured and crushed rock around the drill hole would allow the explosive product gases to escape prematurely and would consume energy in an undesirable mode. The amount of rock broken would be less than that from a hole-bottom controlled fracture or a PCF type of fracture pattern; there would be considerably more dust from the pulverized rock; and the broken rock flyrock, would be propelled away from the face at considerably higher throw velocities. The propellants that would be used in the present invention may be in granular form or may be in single-grain solid form. These solid propellants may contain one or more of the following components: ■ nitrocellulose

■ nitroglycerine

■ nitroguanadine

■ black powder

The propellants may also have an oxidizing agent added to reduce the amount of carbon monoxide generated during combustion.

Liquid propellants can also be employed. These include LGP 1846 and its derivatives, the JP4/nitric acid system and any other liquid propellant that can be controllably initiated and burned. One of the main requirements for the propellant is low cost and high- production capacity.

APPLICATIONS This method of breaking soft, medium and hard rock as well as concrete has many applications in the mining, construction and rock quarrying industries and military opera ions. These include: tunneling cavern excavation shaft-sinking adit and drift development in mining long wall mining room and pillar mining stoping methods (shrinkage, cut & fill and narrow-vein) selective mining undercut development for vertical crater retreat

(VCR) mining draw-point development for block caving and shrinkage stoping secondary breakage and reduction of oversize ■ trenching raise-boring rock cuts precision blasting demolition ■ open pit bench cleanup open pit bench blasting boulder breaking and benching in rock quarries construction of fighting positions and personnel shelters in rock ■ development of large holes or chambers for placing demolition charges reduction of natural and man-made obstacles to military movement The hole-bottom controlled-fracture system utilizing the hole-bottom pressure (HBP) method and apparatus 1 of the present invention, as shown in Figure 1, has a high- inertia stemming bar 3 with a propellant ignition system 2 for transporting, igniting and stemming a propellant cartridge 5 with combustible propellant 4. Ignition of the propellant generates high-pressure gases which rapidly expands into and pressurizes the external relief volume 20 as shown by arrows 7, causing a controlled fracture, such as a PCF type fracture 11, to be initiated at the corners 8 of the hole bottom 10, fracturing the rock along fracture line 11 and throwing the rock debris as shown by arrows 13. External relief volume 20 provides void space for proper propellant burning. A borehole 6 is percussively drilled in the surface 12 of a rock or concrete material allowing placement of the cartridge 5 on the end of the stemming bar 3 in the hole. The cartridge 5 has a tapered body 15 with a generally cylindrical outer wall 16 and a sloping inner wall 17. A propellant charge 4 is held by a propellant containment disk 18 separating a space 19 above the bottom 10 of the hole 6.

The prior patent, as shown in Figures 2, 3 and 4, provided a penetrating cone fracture 11 through expansion of a gas from an injector 41. The injector has an inertial stemming bar 9 backing a propellant charge holder, causing a fracture 11 to be initiated at the hole bottom corner 8 and a low energy rock throw 13, as previously described. The gas-injector is shown with a tapered tip 21. When inserted into a stepped hole 22, the tapered tip provides an initial seal for the pressures generated in the hole bottom 10. ^• The problems inherent with the prior method gas-injector hole pressures are illustrated by the calculated pressure histories shown in Figures 3 and 4, as previously discussed.

Figure 5 shows an alternate version of an HBP cartridge 23, which incorporates a sliding plug 24 fitted within the conically tapered wall 25 of the cartridge. The propellant 4 is contained in the volume between the sliding base plug 24 and the propellant containment disk 30. An internal relief volume 31 is contained between disk 30 and the front of the cartridge 14 and has the function of providing a controlled volume for the burning propellant to expand into, thus preventing pressures that are too high for the apparatus or even a detonation of the propellant. The internal relief volume 31 performs the same function as the external relief volxune 20 of Figure 1. When the stemming bar 3 inserts the cartridge 23 and bar 3 into the drilled hole 6, a small space 26 typically surrounds the cartridge and could provide an avenue for propellant gas loss and fracture pressure reduction. Upon ignition of the propellant 4, the cartridge 23 expands to provide a tight fit between its outer wall 27 and the bore hole 6 and the tapered sliding plug 24 is forced by the high-pressure propellant gases to move rearwards and continue to maintain contact against the tapered wall 25 during its recoil motion by the high gas pressure. Due to the tapered relation between the plug 24 and the inner wall 25, a seal is maintained at the base of the cartridge 23. Additional sealing action may be provided by the action of a backup metal sealing ring 28. Figure 6 shows the HBP system 1 positioned in a borehole 6 prior to firing. The cartridge base 24 is attached to the end of the stemming bar 3 for insertion into the bottom of the drill hole. A propellant ignition system 2 is located coaxially in the stemming bar and is used to strike the primer in the base 24 of the cartridge 23. A disk 30 is located in the cartridge to separate the propellant charge 4 from the relief volume 31. This internal relief volume provides the free volume required for the proper controlled burn of the propellant such that the HBP method may be used in either a gas-filled or a water filled hole 6. In a water-filled hole, the cartridge will displace most of the water from the hole bottom 10. Figure 7 shows the HBP system 1 after firing in the situation where the cartridge wall 27 does not rupture near the end of the stemming bar 3. This is the intended mode of sealing. The propellant 4 has been initiated and the pressures developed causes the stemming bar 3 and cartridge base plug 24 to recoil whilst expanding the cartridge walls 27 against the wall of the drill hole 6. The propellant containment disk has been fragmented causing the hole to fill with propellant gases initiating a controlled fracture 11 at or near the bottom of the drill hole 10. The pressure forces the taper of the base plug 24 against the taper of the cartridge wall 25 during recoil to maintain a dynamic seal while the rock breaking process occurs. Figure 8 shows the HBP system 1 after firing in the situation where the cartridge wall 32 ruptures 33 near the end of the stemming bar 3. This is not the intended mode of sealing but is a possible situation which is provided for in the present design. The cartridge wall 32 near the base plug 24 is assumed to have ruptured 33 and the high pressure propellant gases then force the metal back-up ring 28 into the gap 29 between the end of the stemming bar 3 and the wall of the drill hole 6, sealing the system against leakage of gas from the hole bottom 10.

The performance of the HBP method is shown by the calculated pressure history illustrated in Figure 9. The present HBP approach provides a more desirable hole and fracture pressurization which may be seen in comparison with Figure 4. After initiation, the pressure builds up rapidly filling the relief volume 31 and entire bottom of the drill hole 10. If no gas leaks and no fractures develop, the pressure remains and will increase towards an equilibration pressure determined by the energy released, the volume available and the ratio of specific heats of the propellant products, but will not reach this state as a result of recoil motion of the stemming bar which may provide only inertial containment. Once a fracture is initiated or opened up, the gas pressure drops as the fracture is driven by the gas pressure.

The calculated gas distribution within the HBP cartridge 23 and hole bottom 10 is shown in Figure 10. Initially all the propellant 4 is contained within the volume closest to the stemming bar 3. After initiation, the propellant gases expand into the relief volume 31 and into the hole bottom 10. When the pressure reaches a critical threshold (on the order of 30% of the unconfined compressive strength of the rock) , a penetrating cone fracture 11 is initiated. Gas continues to flow from the cartridge into the expanding fracture system. Concurrently, in this calculation, the cartridge wall 32 near the cartridge base plug 24 is assumed to rupture 33 and allow gas to leak through the gap 29 between the stemming bar 3 and the wall of the drill hole 6. The mass flow rate of gas is assumed to leak at the sonic choke condition which is dictated by the cross-sectional area of the gap 29 and the local gas sound speed and density. This calculation illustrates that, with no sealing action from the cartridge 23 or from a special sealing mechanism on the end of the stemming bar 3, the cross-sectional area of the stemming bar 3 is sufficient to prevent all but a small percentage of the high pressure propellant gases from escaping from the bottom of the hole 10 and the evolving fracture system.

The present excavating system 34 shown in Figure 11, has a conventional carrier 35 such as a tracked carrier, which has at least one boom. A preferred embodiment has two articulated booms 36, each with indexing extensions 37. Percussive drills 39 provide drilling 40 of boreholes 6. HBP autoloaders 38 are mounted on the indexing extensions 37. A high-inertia stemming bar 3 inserts, holds and stems a cartridge 5 to create PCF, or other hole-bottom controlled-fracture breakage 11 in the rock face 12. An autoloader 38 acts to place a propellant loaded cartridge

5 on the end of stemming bar 3 prior to insertion of the bar and cartridge into hole 6. To simultaneously drill and blast, both indexing extensions 37 are provided with drills, stemming bars and loaders. Cartridge loading by the autoloader and insertion and igniting the cartridge with one boom may occur while the other boom is drilling a new hole. The operation may be automatic. Once the operator selects a drill spot, drilling, indexing, loading and inserting automatically occur.

Figure 12 shows the basic fracturing system of the present invention incorporating the stemming bar 3 abutting and backing an HBP cartridge 42 positioned in the borehole

6 drilled in the rock surface 12. The stemming bar 3 has a pneumatically, hydraulically or mechanically propellant ignition system 2 in the base of cartridge 42 for igniting the propellant charge 4. The propellant charge 4 is held within the cartridge by a propellant containment disk 30, providing an internal relief volume 31. The resultant propellant burning rapidly pressurizes the space between the cartridge closure disk 44 and the bottom of the borehole 10. The cartridge 42 has a large internal radius 43 in the base, which, in conjunction with the internal tapered wall 15, inhibits cartridge rupture and directs propellant generated high-pressure gases downward against' the hole bottom 10. While the invention has been described with reference to specific embodiments, modifications and variations of the invention may be constructed without departing from the scope of the invention, which is defined in the following claims.

Claims

We claim:

1. An apparatus for breaking rock, concrete and other hard materials with a controlled fracturing technique, comprising: a carrier for underground or surface operation; one or more articulated booms mounted on the carrier; a drill mounted on one or more of the booms for drilling a hole in the material to be broken; a stemming bar carried on one or more of the booms which carries at its end a propellant-containing cartridge; an ignition system for igniting the propellant charge such that high pressure gases are generated and act to fracture the material through the initiation and propagation of controlled fractures from a bottom of the hole and thus effectively break and remove a volume of the material.

2. The apparatus of claim 1, wherein the end of the cartridge towards the bottom of the hole is partitioned by a disk which is positioned within the cartridge and from the bottom of the hole so as to separate the propellant from a controlled volume for the expansion of the burning propellant, thus controlling the propellant burning rates and peak pressures such that the pressure behavior for optimum rock fracture is achieved.

3. The apparatus of claim 1, wherein the cartridge has a cylindrical external wall with a diameter slightly less than the hole drilled in the material and a conical interior wall such that the cartridge wall is thicker nearer the end towards the stemming bar.

4. The apparatus of claim 1, wherein the interior base of the cartridge has a large radius so as to reduce stress concentrations in the cartridge and thus minimize the consequent rupture of the cartridge due to the high pressures occurring with propellant ignition.

5. The apparatus of claim 1, wherein the cartridge comprises a tapered wall se-.ction with a cylindrical exterior and a conical interior and a basal sealing plug of mating conical shape which can move inside the conical interior wall of the cartridge so as to maintain a seal against the propellant gases as the bar, which positions and stems the cartridge in the hole, is displaced out of the hole by the pressure of the gases.

6. The apparatus of claim 2, wherein the partitioning disk contained ir the cartridge is made of a solid propellant or other consumable material which could add to the total energy delivered by the propellant cartridge upon ignition.

7. The apparatus of claim 1, wherein the propellant is composed of a granular solid propellant, a single-grain solid propellant, a single-component liquid propellant, a two-component liquid propellant or any combination of the four propellant types.

8. The apparatus of claim 1, wherein the drill and the stemming bar are both carried on the same boom with an indexing mechanism allowing for the hole to be drilled, the drill retracted, the drill to be indexed out of alignment with the hole and the cartridge carrying stemming bar to be aligned with the hole and the cartridge inserted into the hole.

9. The apparatus of claim 1, wherein each cartridge is positioned on the end of the stemming bar by an automated cartridge handling autoloader.

10. The apparatus of claim 1, wherein the stemming bar has at its cartridge end, an additional or alternative sealing means to prevent the escape of high-pressure gases from the hole.

11. The apparatus of claim 1, wherein the stemming bar is accelerated into the hole just prior to ignition of the propellant charge such that a velocity of the stemming bar into the hole further prevents the displacement of the cartridge out of the hole and the consequent cartridge rupture and loss of high-pressure propellant gases, and reduces the recoil forces on the apparatus.

12. A method for breaking rock, concrete and other hard materials involving: drilling a hole in a material by percussive or other mechanical means; inserting into the hole a cartridge containing a propellant charge and having an external diameter only slightly less than the diameter of the hole; stemming the hole with a relatively heavy bar having a diameter only slightly less than the diameter of the hole and with a mass sufficient to limit bar recoil to less than one third the hole diameter; igniting the propellant by any one of electrical, optical or percussive means; fracturing the material by the initiation and propagation of controlled fractures from the hole bottom.

13. The method of claim 12, whereby the cartridge has a cylindrical external wall with a diameter slightly less than the drill hole and a conical interior wall such that the cartridge wall is thicker nearer the end towards the stemming bar.

14. The method of claim 12, whereby an interior base of the cartridge has a large radius so as to reduce stress concentrations in the cartridge and thus minimize the consequent rupture of the cartridge due to the high pressures occurring with propellant ignition.

15. The method of claim 12, whereby the cartridge has an internal relief volume to provide for the controlled pressurization of the hole bottom in such a way that permits initiation of the controlled fracture process, eliminates damage to the end of the stemming bar, eliminates crushing of the rock around the drill hole and minimizes the tendency of the cartridge to rupture during the fracture initiation and propagation process.

16. The method of claim 12, whereby the cartridge includes a basal sealing plug which can move inside a conical interior wall of the cartridge so as to maintain a seal against the propellant gases as the bar, which positions and stems the cartridge in the hole is displaced out of the hole by the pressure of the gases.

17. The method of claim 12, whereby the drilling is effected by percussive means thus increasing the number and size of microfractures at the hole bottom and thereby improving initiation of the controlled fractures.

18. The method of claim 12, whereby the drill and the stemming bar are both carried on the same boom with an indexing mechanism, allowing for the hole to be drilled, the drill retracted, the drill to be indexed out of alignment with the hole and the stemming bar with a propellant cartridge carried on its end to be aligned with the hole and inserted into the hole.

19. The method of claim 12, whereby an automated cartridge handling autoloader is used to position the cartridge on an end of the stemming bar prior to its insertion into the hole by the stemming bar.

20. The method of claim 12, whereby an end of the stemming bar towards the cartridge provides the primary sealing of the hole bottom.

21. The method of claim 12, whereby the stemming bar provides a secondary seal for the hole bottom and the cartridge provides the primary seal.

22. The method of claim 12, whereby the stemming bar is accelerated towards the hole bottom just prior to firing the propellant charge to provide a reduction of recoil velocity of the stemming bar and an enhancement of sealing by the propellant cartridge as a result of reducing the displacement of a base of the cartridge relative to walls of the cartridge during the rock fracture process.

23. A cartridge apparatus for breaking rock, concrete and other hard materials by a hole-bottom pressurization process comprising: a cartridge containing a propellant charge; a primer at a base or up-hole end of the cartridge for igniting the propellant charge within the cartridge; a tapered wall of the cartridge giving a conical interior shape to the cartridge with a thicker wall towards the base the cartridge thus increasing the cartridge's resistance to rupture by the high-pressure gases; an internal relief volume within the cartridge to control the rate of propellant burning, hole pressurization and the maximum hole pressure; the burning of the propellant such that high-pressure gases are generated and serve to fracture the material through the initiation and propagation of controlled fractures from a bottom of a drilled hole; and thus effectively breaking and removing a volume of the material.

24. The apparatus of claim 23, wherein the cartridge has a cylindrical external wall with a diameter slightly less than the diameter of the drilled hole and a tapered interior wall such that a cartridge body between the external and internal walls is thicker near the up-hole end, and the cartridge having a base with a large interior radius so as to reduce stress concentrations and thus rupture of the cartridge by the high-pressure gases occurring upon ignition of the propellant.

25. The apparatus of claim 23, wherein the cartridge interior wall comprises a conically tapered wall and the base comprises an equally tapered sealing plug which can move inside the conical interior wall of the cartridge for maintaining a seal against the high-pressure gases occurring upon ignition of the propellant.

26. A stemming bar apparatus for breaking rock, concrete and other hard materials by a controlled fracture process comprising: a massive bar, made from steel or other high strength material, which is slightly less in diameter than a drilled hole; a cartridge which contains a propellant charge and which is positioned at the end of the bar; a firing pin or other firing mechanism to initiate a primer or other propellant initiating device contained in the cartridge; a burning of the propellant charge such that high- pressure gases are generated and act to fracture the material through the initiation and propagation of controlled fractures from a corner of a bottom of the drilled hole; and thus effectively breaking and removing a volume of the material.

27. The apparatus of claim 26, wherein a hole-sealing system is employed to seal the drill hole near a bottom or in-hole end of the stemming bar to prevent high-pressure gases generated by the propellant from escaping from the bottom of the hole.

28. The apparatus of claim 26, wherein recoil forces on the stemming bar are reduced and bottom hole sealing is enhanced by accelerating the stemming bar towards the hole bottom, using hydraulic, pneumatic or other power sources available, just prior to initiating the propellant charge.

29. The apparatus of claim 27, wherein the hole- sealing system is selected from a group consisting of V- seals, O-rings, unsupported area seals, wedge seals and the like.