Ammunition and explosives – Blasting – Patterned blasting
Reexamination Certificate
1999-04-09
2002-01-22
Nelson, Peter A. (Department: 3641)
Ammunition and explosives
Blasting
Patterned blasting
C102S302000, C102S313000, C089S014300, C042S079000
Reexamination Certificate
active
06339992
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed generally to devices for small charge blasting of rock and other hard materials and specifically to devices for sealing pressurized fluids in holes in the rock and other hard materials.
BACKGROUND
In mining and civil excavation work, small charge blasting or controlled fracture techniques are being introduced as alternatives to conventional drill-and-blast, mechanical breakers, chemical expansion agents and in some cases hand methods. “Small-charge blasting” as used herein includes any excavation method where relatively small amounts of an energetic substance (typically a few kilograms or less) are consumed for each hole in a rock breaking sequence as well as any method in which a pressurized fluid such as a gas, liquid, or foam, is sealed in the bottom of a drill hole to initiate and propagate a fracture. “Sealing” refers to the partial or total blockage of the hole to impede the escape of the pressurized fluid from the hole. Examples of small charge blasting devices and methods are described in U.S. Pat. Nos. 5,765,923; 5,308,149, and 5,098,163.
In many small charge blasting methods, a machine drills a hole into the rock to be broken and then inserts a stemming bar or gun-like barrel into the hole. A pressurized working fluid, such as a gas, water, or foam, is released rapidly into a portion of the hole, usually the bottom portion. The pressurized fluid is typically generated by combustion of a propellant or explosive source, by electrical discharge into a conductive fluid, by inducing a rapid phase change or by mechanical compression of a working fluid. The stemming bar or barrel seals and stems the pressurized fluid in the hole bottom and thereby causes fracturing of the rock. Small charge blasting can be highly mechanized and automated to increase productivity, can permit excavation machinery to remain near the face due to reduced fly rock discharge, and can have a seismic signature that is relatively small because of the small amount of blasting agent used in the blasting sequence.
In designing a small charge blasting apparatus, there are a number of objectives. For example, the apparatus should be able to excavate rock at as low a cost as possible to make it commercially viable. This means that it should excavate rock efficiently in the desired quantities; it should have a low per-shot consumable cost (energetic substance and cartridge); and it should be capable of fast cycle times (drill, shoot, scale, and muck). The sealing device employed in the apparatus should inhibit and control leakage of pressurized working fluid from the hole bottom to enable the cartridge to use the least amount of energetic substance (e.g., explosives or propellants) for generating the pressurized working fluid and initiating and propagating controlled fractures. In penetrating cone fracture techniques, for example, the pressure from the working fluid in the hole bottom should be maintained at high levels (about 50,000 to about 75,000 psi typical) for long periods (2 to 6 milliseconds typical) to break hard rock. To achieve such a pressure profile, a practical down hole sealing method should be relatively easy to operate and able to seal against rock walls of unknown condition. The apparatus should be designed to operate effectively in the presence of extraneous downhole fluids, such as water and/or mud. The presence or absence of such fluids cannot generally be controlled. Extraneous fluids not only can remove volume available for expansion of the working fluid and therefore contribute to unnecessarily and often unacceptably high downhole pressures but also can plug the barrel of the apparatus causing the barrel to be damaged during release of the pressurized working fluid into the hole. Finally, the apparatus should be of robust construction and easy to use.
SUMMARY OF THE INVENTION
These and other objectives are realized by the apparatuses and methods of the present invention.
In a first embodiment of the present invention, a small charge blasting system for breaking hard materials is provided that includes:
(a) a chamber for receiving an energetic substance; and
(b) a barrel in communication with the chamber for extending into a hole in the material and releasing a pressurized working fluid (e.g., a gas, foam, or liquid) generated by the energetic substance into the hole to initiate and propagate a fracture in the material. The energetic substance can be a propellant, explosive, a fluid energized by electrical discharge, or a fluid that is caused to undergo a rapid phase change from liquid to gas.
The barrel has a bore having a first cross-sectional area normal to the bore's central axis at an interior (or uphole) portion of the bore and a second cross-sectional area normal to the bore's central axis at or near an exterior (or downhole) end portion of the bore. The first cross-sectional area is less than the second cross-sectional area which provides an expanded volume (a “relief volume”) (that is preferably substantially free of the energetic substance) at or near the downhole end of the bore for controlled expansion of the pressurized working fluid in the bore prior to release of the working fluid in the hole; that is, the diameter of the interior portion of the bore is less than the diameter of the downhole end portion of the bore to provide the expanded volume. The second cross-sectional area is preferably at least about 300% and more preferably at least about 400% and even more preferably ranges from about 300% to about 1700% of the first cross sectional area. The diameter of the interior portion of the bore is preferably no more than about 60%, more preferably no more than about 45%, and even more preferably ranges from about 25 to about 45% of the diameter of the hole bottom. The diameter of the downhole end portion of the bore is preferably no more than about 80%, more preferably no more than about 75%, and even more preferably ranges from about 50% to about 75% of the diameter of the hole bottom. Both the interior and exterior portions are located at a distance from the discharge opening of the barrel. The system can be simple in design and operation, of robust construction, and highly effective in breaking rock, particularly hard rock. The use of the relief volume permits controlled pressurization of the bottom of the hole by the working fluid and thereby prevents over pressuring the hole and causing the rock wall to fail in hoop tension. Once this occurs, longitudinal fractures may form which become additional leakage paths for the pressurizing fluid. In addition, the rock walls expand faster than the steel walls of the barrel which tends to increase the leakage gap during pressurization.
Controlled hole pressurization thus can facilitate more effective formation and propagation of fractures in the material to be broken, which reduces operating costs and permits the use of relatively low amounts of the energetic substance in the cartridge.
The relief volume in the downhole end of the bore is measured relative to a reference volume that is equal to the cross-sectional area of the hole bottom (normal to the longitudinal axis of the hole) times a depth equal to the hole diameter (hereinafter “the reference volume”). The internal relief volume preferably ranges from about 25% to about 125%, more preferably from about 40% to about 100%, and even more preferably from about 50% to about 75% of the reference volume.
The transition from the first cross-sectional area to the enlarged second transitional area is preferably made gradually using an outward curve or taper. The angle of taper (measured relative to a line parallel to the longitudinal axis of the bore) preferably ranges from about 10 to about 60 degrees, more preferably from about 15 to about 50 degrees, and even more preferably from about 20 to about 40 degrees.
The exterior of the distal end of the barrel forms a dynamic seal in the hole and thereby impedes leakage of pressurized working fluid during hole pressurization by allowing only a small annular gap (o
RockTek Limited
Sheridan & Ross P.C.
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