Abstract:
A resin bonded mine roof bolt having an elongated rod with a drive head at one end and an expansion anchor threaded onto the other end. A segmented resin compression layer covers a portion of the rod below the expansion anchor. When installed in a mine roof bore hole with curable resin, the resin compression layer mixes the resin and partially fills the bore hole to minimize the amount of resin needed to anchor the bolt. Individual segments of the layer are tapered to create a wedging force on resin with the bore hole. The expansion anchor is expandable upon initial hardening of the resin to tension the bolt.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/613,150 entitled “Point Anchor Resin Bolt” filed Sep. 24, 2004. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a mine roof bolt anchored in a bore hole by mechanical anchoring and resin bonding, and more particularly to a mine roof bolt bearing an expansion assembly and a segmented resin compression layer that exerts a compressive force on resin within a bore hole. 
     2. Prior Art 
     The roof of a mine conventionally is supported by tensioning the roof with 4 to 6 feet long steel bolts inserted into bore holes drilled in the mine roof that reinforce the unsupported rock formation above the mine roof. The end of the mine roof bolt may be anchored mechanically to the rock formation by engagement of an expansion assembly on the end of the mine roof bolt with the rock formation. Alternatively, the mine roof bolt may be adhesively bonded to the rock formation with a resin bonding material inserted into the bore hole. Alternatively, a combination of mechanical anchoring and resin bonding can be employed by using both an expansion assembly and resin bonding material. 
     A mechanically anchored mine roof bolt typically includes an expansion assembly threaded onto one end of the bolt shaft and a drive head for rotating the bolt. A mine roof plate is positioned between the drive head and the mine roof surface. The expansion assembly generally includes a multi-prong shell supported by a threaded ring and a plug threaded onto the end of the bolt. When the prongs of the shell engage with rock surrounding a bore hole, and the bolt is rotated about its longitudinal axis, the plug threads downwardly on the shaft to expand the shell into tight engagement with the rock thereby placing the bolt in tension between the expansion assembly and the mine roof surface. 
     When resin bonding material is used, it penetrates the surrounding rock formation to adhesively unite the rock strata and to firmly hold the roof bolt within the bore hole. Resin is typically inserted into the mine roof bore hole in the form of a two component plastic cartridge having one component containing a curable resin composition and another component containing a curing agent (catalyst). The two component resin cartridge is inserted into the blind end of the bore hole and the mine roof bolt is inserted into the bore hole such that the end of the mine roof bolt ruptures the two component resin cartridge. Upon rotation of the mine roof bolt about its longitudinal axis, the compartments within the resin cartridge are shredded and the components are mixed. The resin mixture fills the annular area between the bore hole wall and the shaft of the mine roof bolt. The mixed resin cures and binds the mine roof bolt to the surrounding rock. The typical diameter of a mine roof bore hole is one inch. Mine roof bolts anchored with resin bonding are often ¾ inch in diameter, and more recently ⅝ inch in diameter. The mine roof bolt is generally centered within the bore hole creating a circular annulus that becomes filled with bonding resin. The larger diameter bolts (¾ inch) offer performance advantages over ⅝ inch bolts in that the annulus provided between the bore hole wall and a ¾ inch bolt is smaller than that of smaller diameter bolts. A smaller annulus provided between the bolt and the bore hole wall improves mixing of the resin and catalyst in the annulus. In addition, when the resin cartridge is shredded upon insertion of the mine roof bolt and rotation thereof in an annulus larger than ⅛ inch (as for mine roof bolts having less than ¾ inch diameter installed in one inch bore holes), the shredded cartridge can interfere with the resin and catalyst mixing. Poor mixing results in an inferior cured resin and results in poor bond strength between the bolt and bore hole wall. This phenomenon of “glove fingering” occurs when the plastic film that forms the cartridge lodges in the bore hole proximate the surrounding rock thereby interrupting the mechanical interlock desired between the resin and bore hole wall. In addition, the larger annulus created by using a ⅝ inch bolt in a one inch bore hole requires more resin to bond the bolt to the rock than does a larger diameter bolt, thereby adding to the cost of installing a smaller diameter bolt. While one solution would be to proportionally reduce the size of the bore hole to less than one inch, this is not practicable. The mine roof drilling equipment in use is conventionally produced for drilling one inch bore holes. Moreover, there are significant technical difficulties in drilling small diameter bore holes in mine roofs. 
     Despite these drawbacks of using mine roof bolts having a diameter of less than ¾ inch, the popularity of smaller diameter mine roof bolts is increasing. A ⅝ inch bolt is lighter and easier to use than a ¾ inch bolt and can be produced at lower cost. One solution for overcoming the need for extra resin and avoiding the glove fingering problem of smaller diameter bolts installed in one inch bore holes has been provided in a proposed mining bolt which includes an elongated rod that forms the main structure of the mine roof bolt as disclosed in U.S. Patent Application Publication No. 2005/0134104. A portion of the rod in between a drive head and the end of the bolt is coated with a layer of material having a lower specific gravity than the rod, such as a polymer. The polymeric coating layer may have external texturing which can help with mixing of resin in the mine roof bore hole. The coating on the mine roof bolt also helps to fill some of the annulus at a minimal increase in weight to the bolt and minimizes the amount of resin that is required for bonding the bolt to rock strata. This coated mine roof bolt can be produced from a ⅝ inch metal rod with a polymeric coating layer about 1/16 inch thick. The coated mine roof bolt uses only resin bonding to anchor the mine roof bolt to a rock formation. 
     However, the combination of both mechanical anchoring and resin bonding of mine roof bolts has been found to provide superior mine roof control. A mine roof bolt having an expansion assembly with expansion shell and plug is held against the surface of a mine roof by a plate. Rotation of the bolt mixes the resin components and expands the expansion shell. The resin mixture surrounds the expansion assembly and several feet of the mine roof bolt. Upon hardening of the resin mixture, the bolt is anchored to the rock strata by the resin and the expansion assembly. In some mine roof bolts that are anchored by a combination of resin bonding and expansion assembly anchoring, a device is used to delay relative rotation between the expansion assembly and the mine roof bolt until the resin is hardened so that the bolt can be tensioned after the resin begins to harden. An anti-rotation device prevents relative rotation between the plug of an expansion assembly and the bolt so that the plug does not thread down the bolt during mixing of the resin components. One suitable anti-rotation device is a shear pin extending through the plug. The resin components are thoroughly mixed before the shell of the expansion assembly is expanded. The end of the bolt abuts the pin to prevent initial downward movement of the plug on the bolt during rotation of the bolt to effect mixing of the resin components. Once the resin begins to set, the force on the shear pin exceeds its strength and continued rotation of the bolt shears through the pin and allows the plug to advance downwardly on the bolt to expand the shell of the expansion assembly outwardly to grip the bore hole wall. 
     For mine roof bolts that are anchored using a combination of a mechanical anchor and resin bonding and for coated mining bolts that are anchored with resin, the resin is desirably maintained in an upper region of the bore hole. However, retention of the resin adjacent the upper portion of the mine roof bolt is problematic. One solution has been to include a resin retaining washer at a position intermediate the end of the mine roof bolt and the mine roof for restricting the annular area in which the resin may flow. The upward thrust of a mine roof bolt bearing a resin retaining washer can exert a hydraulic force on the resin to confine it within the restricted annular area at the end of the mine roof bolt and forcibly drive the resin into the cracks and crevices on the inside of the bore hole and into the surrounding rock formation to more solidly lock the mine roof bolt within the rock formation. However, such resin retaining washers are limited in their ability to block resin from flowing downwardly along the bolt. While a resin retaining washer can withstand the hydraulic pressure created when the mine roof bolt shreds the resin capsule, nothing on the mine roof bolt urges the resin back upwardly into the bore hole. 
     Accordingly, a need remains for a mine roof bolt which utilizes a combination of mechanical anchoring and resin bonding to anchor the mine roof bolt in a bore hole (particularly for a small diameter mine roof bolt such as ⅝ inch) where the resin mixing and distribution is controlled by the bolt. 
     SUMMARY OF THE INVENTION 
     This need is met by the mine roof bolt of the present invention which includes an elongated rod having a threaded end and a drive end. An expansion assembly composed of an expansion shell and plug are threaded onto the threaded end. A segmented resin compression layer covers a portion of the elongated rod between the threaded end and drive end. The segmented layer includes a plurality of tapered segments with each segment having a first portion that is thicker than a second portion. Each segment also includes an exterior thread that is discontinuous with the thread of an adjacent segment. The surface of each segment may be textured such as by a plurality of ridges extending between the first and second portions. The segmented layer may also include a tapered portion that extends and tapers from a first portion of a terminal segment in closest proximity to the expansion anchor to a position spaced therefrom. The mine roof bolt may further include a resin retaining ring adjacent the end of the segmented layer that is closest to the drive end. The elongated member may be a smooth bar or a textured bar such as rebar. The segmented resin compression layer may be produced from a polymeric material. 
     When the mine roof bolt of the present invention is installed in the mine roof bore hole, a frangible curable resin cartridge is inserted into the bore hole. The mine roof bolt is inserted into the bore hole and ruptures the resin cartridge. The mine roof bolt is rotated along its longitudinal axis such that the resin compression layer contributes to mixing the contents of the resin cartridge and compresses the resin between the mine roof bolt and the bore hole wall. Rotation of the bolt causes the expansion assembly to engage with the bore hole wall. The expansion assembly may include a delay mechanism for delaying the time at which the expansion assembly expands to engage with the bore hole wall. The resin compression layer includes a plurality of tapered segments, whereby a thicker portion of each segment compresses the resin within the bore hole. In addition, the surface of each segment includes a spiral thread that urges the resin toward the threaded end upon rotation of the mine roof bolt. 
     The mine roof bolt of the present invention may be produced by providing an elongated rod and applying a segmented layer to the rod intermediate the ends thereof. An expansion assembly is threaded onto one end and a drive head is attached to the other end of the rod. The segmented layer may be polymeric and may be applied to the rod by injection molding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevational view of a mine roof bolt having a segmented resin compression layer of the present invention, an expansion assembly, a resin retaining ring and a drive head; 
         FIG. 2  is a side elevational view of the mine roof bolt of  FIG. 1 , from an opposing side thereof; 
         FIG. 3  is a cross section of the mine roof bolt of  FIG. 1  taken along lines  3 — 3 ; 
         FIG. 4  is a plan view of the resin retaining ring shown in  FIG. 1 ; 
         FIG. 5  is a side elevational view of another embodiment of the mine roof bolt of the present invention wherein the segmented resin compression layer includes a terminal tapered portion; 
         FIG. 6  is a side elevation partially in section of one step of the method of installing the mine roof bolt of the present invention, illustrating the resin cartridge in position at the end of the bore hole for rupture by the expansion assembly; 
         FIG. 7  is a view similar to  FIG. 6 , illustrating mixing of the components of the ruptured cartridge by rotation of the bolt; 
         FIG. 8  is a graph of the deflection of mine roof bolts versus load for the mine roof bolt of the present invention conducted in a laboratory; and 
         FIG. 9  is a graph similar to  FIG. 8  for a mine test. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A complete understanding of the present invention will be obtained from the following description taken in connection with the accompanying drawing figures, wherein like reference characters identify like parts throughout. 
     For the purposes of the description hereinafter, the terms “upper”, “lower”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom” and derivatives thereof relate to the invention as it is oriented in the drawing figures. However, it is to be understood that the invention may assume alternative variations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings and described in the following specification are exemplary embodiments of the invention. Specific dimensions and other physical characteristics related to the embodiments disclosed herein are not considered to be limiting. 
     Referring to the drawings and particularly to  FIGS. 1–3 , there is illustrated a mine roof bolt  10  for securing in a bore hole  12  drilled in a rock formation  14  to support the rock formation  14  that overlies an underground excavation such as a mine passageway or the like. The bore hole  12  is drilled to a pre-selected depth into the rock formation  14  as determined by the load bearing properties to be provided by the mine roof bolt  10 . 
     The bolt  10  includes an elongated rod  16  having a threaded end  18  for positioning in the upper blind end  20  of the bore hole  12  and a drive end  22  having a drive head  24  that extends into the mine passageway from the open end of the bore hole  12 . A roof or bearing plate  26  is retained by the drive head  24  on the end  22  of the bolt  10 . The drive head  24  generally includes a shoulder  28  and a plurality of drive faces  30 . The rod  16 , roof plate  26  and drive head  24  typically are produced from steel. An expansion assembly  32  is threaded onto the threaded end  18  of the bolt  10 . The expansion assembly  32  shown in  FIGS. 1–3  includes an expansion shell  34  having a base portion  36  in the configuration of a ring or collar to which are integrally attached a plurality of upwardly extending expansion leaves  38  that are spaced from one another and having free ends. A tapered plug  40  is threaded on the rod  16  into the inside of the expansion shell  34 . The tapered plug  40  is configured to move downwardly toward the base  36  of the expansion shell  34  upon rotation of the bolt  10  while the expansion leaves  38  bend outwardly into gripping engagement with the rock formation  14 . Other expansion shell assemblies that may be used in the present invention include bail type shells in which two expansion leaves are supported by a bail that extends over the end of the mine roof bolt and prevents expansion of the leaves from moving axially relative to the bolt until desired. In addition, the expansion assembly  32  may include a stop mechanism (not shown) such as disclosed in U.S. Pat. No. 4,419,805 to Calandara, Jr., incorporated herein by reference. An expansion shell assembly having a stop device prevents expansion of the shell assembly during the stage of mixing resin with the bolt. When the torque applied to the bolt exceeds a pre-determined torque as determined by the time for mixing the bonding material, the stop device fractures and the expansion shell assembly is then free to expand into gripping engagement with the wall of the bore hole as the plug is threaded downwardly on the bolt. In any of these expansion shell assemblies, the bolt  10  is both mechanically anchored and adhesively bonded in the bore hole to prevent slippage of the expansion assembly  32  so that the bolt remains tensioned to support the rock formation  14 . 
     A portion of the elongated rod  16  between the threaded end  18  and the drive end  22  is covered with a resin compression layer  42 . The elongated rod  16  may be a smooth rod or a textured rod such as rebar, with a smooth rod being shown in the drawings herein. In one embodiment of the invention, the resin compression layer  42  extends from a position about one inch from the lower end of the expansion assembly  32  for about sixteen to twenty inches down the length of a four foot mine roof bolt  10 . Other lengths of the resin compression layer  42  may be selected relative to the length of the bolt  10 , depending on the roof anchoring needs. 
     The resin compression layer  42  includes a plurality of tapered segments  44 . Each tapered segment has a first portion  46  that is thicker than a second portion  48  as shown in  FIG. 3 . The tapered segments  44  create a mechanical wedging force when load is applied to the bolt  10 . The surface of segment  44  includes a spiral thread  50 , each spiral thread  50  of a segment  44  being discontinuous with the thread  50  of an adjacent segment  44 . The spiral threads  50  may be ribbed as shown ( FIG. 3 ) or may be smooth. The spiral threads  50  of the tapered segments  44  urge resin upwardly into the bore hole  12  upon rotation of the bolt  10  during mixing of resin. The tapered segments  44  may also include texturing such as a plurality of ridges  52  that extend between the first and second portions  46 ,  48 . The texturing further assists in mixing and distributing the resin around the mine roof bolt  10 . 
     Referring to  FIG. 5 , a resin retaining ring  54  may also be used for maintaining resin within the annulus between the bolt and the bore hole in the location of the resin compression layer  42 . The resin retaining ring  54  may be generally circular shaped with recessed portions  56  that allow for adjustment of the diameter of the ring  54  when compressed within the bore hole  12 . 
     In another embodiment of the invention shown in  FIG. 5 , a mine roof bolt  110  includes a resin compression layer  142  having a plurality of tapered segments  44  and a terminal tapered portion  144  that extends from a terminal segment  44   a  to a position spaced apart from the threaded end  18 . This tapered portion  144  smoothes the transition between the tapered segments  44  and the elongated rod  16  and eases insertion of the bolt  110  into a bore hole. Hereinafter, all references to the mine roof bolt  10  are applicable to mine roof bolt  110 . 
     The mine roof bolt  10  of the present invention may be produced by coating the elongated rod  16  with a flowable polymer so that the coating has a thickness such as of about at least 1 mm. The polymer is allowed to solidify on the elongated rod  16  and texturing is applied to the exterior of the polymer to form the spiral threads  50  and ridges  52 . The coating step may be performed by dip coating, injection molding and/or hot forging of the polymer resulting in an outer layer of a low density hard coating of the resin compression layer  42  on an inner portion of higher density material (e.g., steel) of the elongated rod  16 . Because the resin compression layer  42  is typically formed from a polymer, the low density hard coating that is applied as a resin compression layer  42  increases the overall diameter of a portion of the bolt  10  with a minimal increase in weight. Hence, while realizing the weight advantages of polymers as compared to metals used in an elongated rod  16 , such a composite bolt  10  can be advantageously sized to provide improved mixing of resin by creating a smaller annulus between the bolt in the location of the resin compression layer  42  and the rock  14  surrounding the bore hole  12 . Likewise, with reduced annulus dimensions, less resin is required for bonding the bolt  10  within the bore hole  12  with concomitant reduction in the size and quantity of shredded resin packaging film that remains after mixing. 
     In one embodiment of the invention, the elongated rod  16  is a smooth rod and the polymer coating is produced by molding to create the ridges  52  and spiral threads  50 . Typically, the thickness of the coating is sufficient to minimize the annulus between the resin compression layer and the bore hole wall at less than ⅛ inch or less than 1/16 inch. This reduces the overall weight of the mine roof bolt  10 , particularly if the coating is a polymer of low density, such as about 2.0 g/ml or less. 
     Referring to  FIGS. 6 and 7 , in accordance with the present invention, the mine roof bolt  10  may be installed in a mine roof to provide support to the rock formation  14 . In one embodiment of the method of supporting a mine roof, the mine roof bolt  10  is installed by inserting a frangible resin cartridge  58  into a bore hole  12  and inserting the mine roof bolt  10  into the bore hole  12 . The mine roof bolt  10  includes an elongated rod  16  having a threaded end  18  onto which an expansion assembly  32  is threaded and a drive end  22  extending out of the bore hole  12 . A resin compression layer  42  covers a portion of elongated rod  16  intermediate the drive end  22  and expansion assembly  32 . When the threaded end  18  of the mine roof bolt  10  contacts the resin cartridge  58 , the cartridge  58  ruptures releasing a curable resin  60 . The mine roof bolt  10  is rotated about its longitudinal axis so that the expansion assembly  32 , resin compression layer  42  and any exposed portion of elongated rod  16  mixes the contents of the resin cartridge  58 . The tapered segments  44  of the resin compression layer  42  compress the resin  60  between the exterior of the mine roof bolt  10  and the bore hole wall. The expansion assembly  32  may include a stop mechanism that resists relative rotation between the bolt  10  and the plug  40  until a torque in excess of a predetermined torque is applied to the drive end  22  of the bolt  10 . At this torque, the resistance offered by the curing resin  60  to rotation of the plug  40  fractures the stop mechanism. When the torque for breaking the stop mechanism is reached, resin mixing is complete and the plug  40  travels downwardly into the expansion shell  34 . In this manner, expansion of the shell  34  is delayed until the resin  60  is mixed, but not after the resin  60  completely rigidifies in the bore hole  12 . The stop mechanism includes any suitable device that restrains axial movement of the plug  40  on the bolt  10  beyond a pre-selected point on the threaded end  18  of the bolt  10 , such as a breakable obstruction member (e.g., a shear pin) suitably retained within the plug  40 . 
     The resin compression layer  42  serves several functions during installation of the mine roof bolt  10  and after it is installed in a mine roof. As the bolt  10  is rotated about its longitudinal axis, the spiral threads  50  on the resin compression layer urge resin upwardly toward the blind end  20  of the bore hole  12 . Retention of resin  60  at the blind end  20  of the bore hole  12  is desired to ensure good bonding between the mine roof bolt  10  and the surrounding rock  14  and to concentrate the anchoring function at the threaded end  18  of the bolt  10 . Sufficient resin is required in the annulus between the mine roof bolt  10  and the bore hole wall to completely fill the annulus and allow for some of the resin  60  to fill cracks and crevices in the rock  14  to enhance the interlock between the rock  14  and the mine roof bolt  10 . In addition, such bolts that are anchored by a combination of mechanical components (expansion shells) and resin bonding, the location of the mechanical/resin anchor spaced apart from the mine roof surface creates a “point anchor” that permits tensioning of the bolt between the mechanical/resin point anchor and the mine roof surface. Retention of the resin at the upper end of the bolt is required to achieve a point anchor system that is tensionable. 
     The resin compression layer  42  also serves to mix the resin  58 . The spiral threads  50  and the ridges  52  provide mixing surfaces to enhance mixing of the curable resin  58 . The segmented arrangement of the resin compression layer  42  also provides surface disruptions that enhance mixing. 
     Upon application of load to the mine roof bolt, the tapered surfaces of the segments  44  create mechanical wedging forces that resist pull out of the bolt  10  from the bore holes. The thicker portion (upper end)  46  of each segment  44  compresses the resin  58  towards the bore hole wall. 
     In certain applications, the mine roof bolt  110  shown in  FIG. 5  having a resin compression layer  142  with a terminal tapered portion  144  improves installation in a mine roof bore hole  12 . The terminal tapered portion  144  provides a transition surface from the rod  16  to the resin compression layer  142 , which eases insertion into a bore hole  12 . 
     Experiments were conducted to determine the performance of the mine roof bolts of the present invention. 
     A laboratory pull test was conducted on bolts produced according to the present invention. Four bolts produced according to the present invention were used. For two of the bolts, prior to coating with the resin compression layer, the elongated rod was wiped with a cloth to remove contaminants such as oil, dirt or grease. The other two rods were not cleaned prior to coating. The bolts were installed in threaded steel bore holes and resin bonded using Insta&#39;l 2 resin cartridges available from Jennmar Corporation of Pittsburgh, Pa. (two minute gel time, 1¼ inch diameter×13 inch long) in a 22 inch bore hole. Bolting machine thrust was set at 3000 pounds. After curing of the resin, the ends of the bolts bearing the expansion assembly were cut off and the remaining portions of the mine roof bolt were tested in a hydraulic pull apparatus to measure deflection as function of load. The test was designed to determine the load that is required to debond the resin compression layer from the elongated rod. The results of the pull test are shown in  FIG. 8 . Bolts A and B (cleaned bolts) exhibited respective maximum loads of 13,000 pounds and 13,500 pounds at an average unit strength of 806 pounds per inch. Bolts C and D (uncleaned) exhibited maximum loads of 12,000 pounds and 10,500 pounds, respectively, with an average unit strength of 683 pounds per inch. 
     The mine roof bolts of the present invention were tested for deflection in the roof of a coal mine along with bolts of the prior art. Two bolts of the present invention included a tapered portion at the end of the resin compression layer and two bolts had no tapered portion. Three bolts of the prior art (Insta&#39;l 2 bolts available from Jennmar Corporation) were tested for comparison. 
     The resin used for bonding all bolts was H2 resin with one minute gel time. The mine roof bolts of the present invention were installed with resin 1¼ inch diameter×14 inch long cartridges and the prior art bolts were installed with 1¼ inch×20 inch resin cartridges. Less rotation was required to install the bolts of the present invention than the prior art bolts. The bolts having a tapered end portion were easier to insert into the bore holes than the bolts not having the tapered portion. The results of a pull test are shown in  FIG. 9 . For loads up to about 10–11 tons, the bolts of the present invention (“A” no tapered portion, “B” with tapered portion and “Average” thereof) and prior art bolts exhibited similar deflection. At higher loads, greater deflection was exhibited by the bolts of the present invention, which may have been due to debonding of the resin compression layer from the elongated rod. 
     While the present invention has been described with reference to particular embodiments of a mine roof bolt and methods associated therewith, those skilled in the art may make modifications and alterations to the present invention without departing from the spirit and scope of the invention. Accordingly, the foregoing detailed description is intended to be illustrative rather than restrictive. The invention is defined by the appended claims, and all changes to the invention that fall within the meaning and the range of equivalency of the claims are embraced within their scope.