Patent Publication Number: US-6213212-B1

Title: Spherical stemming plug and method of use

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     “Not Applicable”. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     “Not Applicable”. 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to a method of shattering earth formations by drilling boreholes in the formation and placing explosives therein which are detonated. More particularly, the invention relates to a method of plugging a borehole at any desired position in order to improve the performance of the explosives, and reduce the danger of fly rock and stemming ejection. 
     A conventional method of shattering earth formations in mining or excavation operations includes the steps of drilling a plurality of boreholes in the formation in a predetermined array, placing a predetermined amount of explosives in the boreholes, and detonating the explosives in a conventional sequence. Typically, the boreholes are not completely filled with explosives, but are plugged at the top by a stemming material such as crushed rock. In addition, the stemming at the tops of the boreholes may be spaced from the explosives by an air deck and/or other additional layers of air, explosives and stemming or plugging material. 
     Regardless of the number and arrangement of materials and decks in the boreholes, the upper layer of stemming is intended to plug the top of the borehole and confine the blast for a few extra milliseconds than would otherwise be the case in order to improve the cratering produced by the explosives and the fragmentation of material being shattered and to reduce dangerous fly rock and stemming ejection. As such, it is considered an important step in known processes to provide the stemming layer at the upper ends of the boreholes. 
     Although crushed rock is preferably used as the stemming material in mining and excavation operations, it is not always readily available, and must sometimes be shipped to the mining site for use. Because mining and site preparation is often conducted in urban locations, fly rock and stemming ejection can be dangerous. As such, there is a need for a stemming plug construction capable of use with readily available materials to prevent such dangerous conditions. Unfortunately, drill cuttings are much smaller than crushed rock, usually on the order of 6 mesh down to 200 mesh in size, and are easily blown from the boreholes upon detonation of the explosives. As such, very little confinement of the blast results, reducing the amount of cratering and fragmentation relative to that achieved when crushed rock is employed as the stemming material. The stemming plug in conjunction with crushed rock provides extra safety in confined areas where stemming ejection and fly rock can be dangerous. 
     BRIEF SUMMARY OF INVENTION 
     It is an object of the present invention to solve the technical problems left unaddressed by the prior art, and to provide a method of plugging boreholes in mining and excavation operations that employs readily available material without adversely affecting the amount of cratering and fragmentation and prevents dangerous conditions. 
     In accordance with these and other objects evident from the following description of a preferred embodiment of the invention, a method of plugging a borehole comprises the steps of placing a first layer of stemming in the borehole, placing a spherical body in the borehole on top of the stemming, and placing a second layer of stemming in the borehole over the body. The body includes a hard sphere and a diameter slightly smaller than the diameter of the borehole, and is preferably formed of a hard synthetic resin material such as ultra-high molecular weight polyethylene, or any material of similar minim hardness or a hard shell filled with a fluid, sand or other relatively hard material. The stemming preferably includes crushed rock but may include drill cuttings such as those produced during formation of the boreholes, ranging in size from about 6 mesh down to 200 mesh in size. However, gravel or other conventional stemming materials may also be employed in the method. 
     The borehole plug of the present invention preferably includes a first layer of stemming material adapted to be placed in the borehole, a second layer of stemming material adapted to be placed in the borehole above the first layer of stemming material, and a spherical body positioned between the first and second layers of stemming material, wherein the body is relatively hard and incompressible, and includes a diameter slightly smaller than that of the borehole in which the plug is to be used. 
     By providing a method and/or apparatus in accordance with the present invention, numerous advantages are realized. For example, by providing a method in which a pair of layers of stemming material are placed in a borehole with a relatively hard spherical body, a plug is defined in which the spherical body cooperates with the stemming material to prevent the stemming material from being ejected along the borehole during blasting. As such, the performance of the explosive material is comparable or better than with arrangements in which the stemming material is employed without the plug body. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The preferred embodiment of the present invention is described in detail below with reference to the attached drawing, wherein: 
     FIG. 1 is a schematic view of a borehole including explosives, a primer, and a plug constructed in accordance with the preferred embodiment; 
     FIG. 2 is a schematic view of a borehole including explosives, a primer, an air deck, a gas bag, and a plug constructed in accordance with the preferred embodiment; and 
     FIG. 3 is a schematic view of a borehole including explosives, a primer, a backfill plug, and a stemming plug constructed in accordance with the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A blasting arrangement constructed in accordance with the preferred embodiment of the invention is illustrated in FIG. 1, and broadly includes a borehole  6  formed in a material to be shattered, an explosive material  8  disposed in the bottom of the borehole, a primer  10  embedded in the explosive, a lead  12  extending from the primer to a conventional detonation controller, and a plug  14 . 
     The borehole  6  is drilled into the material, and includes a diameter of about 7.62-40.64 cm (3-16 in). Typically, the drilling process generates drill cuttings ranging in size from about 6 mesh to 200 mesh, which are removed from the borehole during formation. These cuttings may be used as stemming, decking, or backfill in a manner described below. 
     The explosive material  8  is of known composition, e.g. ammonium nitrate fuel oil or the like, and is placed in the borehole  6  in a conventional fashion to a desired height. In the illustrated embodiment, the borehole is 3 m (10 ft) deep, and the explosive material  8  is filled to a height of 1.8 m (6 ft) from the bottom of the borehole. The primer  10  is lowered into the borehole during placement of the explosive material, with the lead  12  protruding from the borehole for connection to the detonation controller. The primer  10  and lead  12  are also of conventional construction. For example, a pentalite primer may be used such as that offered under the trademark “TROJAN™”. 
     The plug  14  broadly includes first and second layers  16 ,  18  of stemming material, and a spherical body  20  interposed between the layers. The stemming material is preferably comprised of crushed rock but may include the drill cuttings produced during drilling of the boreholes at the site being mined or excavated, but may also include other materials such as gravel and the like. The primary advantage of using the drill cuttings is their availability and low cost relative to that of crushed rock or any other material that must be transported to the site for use. 
     The spherical body  20  of the plug is formed of any material or combination of materials that presents a relatively hard, incompressible mass that can be placed in the borehole and substantially retain its spherical shape upon detonation of the explosive material. A preferred material for use in the body is ultra-high molecular weight polyethylene, which possesses a hardness greater than about R50 using the Rockwell hardness test. Other synthetic resin materials may be used that are not as hard as ultra-high molecular weight polyethylene so long as they substantially hold their shape through detonation, or are formed in a shell that is filled with a relatively incompressible material such that the body holds its shape, as shown in FIG.  2 . 
     Although ultra-high molecular weight polyethylene is the preferred material for use in forming the spherical body, other materials exhibiting suitable hardness may also be employed. For example, metals and ceramic materials or plastic shells filled with water, sand, foamed synthetic resins or other incompressible material may be suitable so long as they do not substantially deform upon detonation of the explosive material. 
     The method used to form the spherical body depends on the material used, but may be carried out with most materials by shaping the material on a lathe, injection molded or the like. However, it is understood that the method used to construct the body is not critical to the performance of the plug, and that any method of construction may be employed. 
     In order to plug the borehole, the first layer of stemming is placed in the borehole over the explosive material to a depth of about ¼ of the total stemming, and the spherical body is dropped into the borehole over the first layer  16 . In an exemplary embodiment, the borehole includes a diameter of 7.62 cm (3 in), and the spherical body of the plug includes an outer diameter of 6.985 cm (2.75 in). As such the spherical body can be dropped into the bore without any special alignment or effort. Once the spherical body is placed on top of the first level of stemming material, the remaining space above the body is filled with additional stemming material  18 . The layer of stemming material underlying the body protects the body from heat generated during blasting, and acts as the first blockage achieved by the plug. 
     With reference to FIG. 2, the plug  14  should not be placed in the borehole immediately over the explosive material. It is also possible to employ the plug  14  in an arrangement including one or more air decks  22 . In order to construct this arrangement, the explosives  8  and primer  10  are placed in the same manner as described above with reference to FIG. 1, with the exception that less explosive material need be placed in the borehole than would be required in the absence of the air deck. Once placement of the explosive material and primer is complete, a gas bag  24  or other conventional device is lowered into the borehole and inflated, filled or otherwise engaged with the wall of the borehole  6  to define a space between the bag and the upper surface of the explosive material. Although the deck  22  is illustrated as being an air deck, other materials may be used to form the deck, e.g. dirt or stemming material. 
     Once the gas bag  24  is secured in the borehole, the plug  14  is constructed by placing first and second layers  16 ,  18  of stemming material in the borehole with the spherical body  20  interposed between the layers, just as described above with reference to the arrangement of FIG.  1 . 
     An alternate placement of the plug  14  is illustrated in FIG. 3, and includes using the plug at the bottom of the borehole as backfill in order to prevent coal or other material disposed beneath the material to be shattered from being crushed or chilled by the blasting. In order to use the plug  14  in this environment, a first layer  16  of stemming material is placed in the borehole  6  prior to placement of the explosive material  8  in order to backfill the borehole to the top of the material to be protected. Thereafter, the plug body  20  is dropped into the borehole and the second layer  18  of stemming material is placed over the body to a height of several inches. 
     When used as a backfill plug, the upper layer  18  of stemming material protects the plug body  20  from the heat of the blast, and the body engages the stemming material of the lower layer  16  to plug the bottom end of the borehole and prevent the underlying material from being crushed or chilled. 
     Although the present invention has been described with reference to the preferred embodiment illustrated in the attached drawing figures, it is noted that substitutions may be made and equivalents employed herein without departing from the scope of the invention. For example, although the plug  14  is illustrated as being used to plug either the top or bottom end of a borehole, it may also be used to plug the borehole at other locations to separate decks or layers of materials placed in the borehole, or to block off portions of the borehole from other portions. In each instance, the spherical body  20  cooperates with the stemming material  16 ,  18  to prevent the stemming material from being ejected along the borehole during blasting, improving the performance of the explosive relative to arrangements in which the stemming material is employed without the plug body.