Abstract:
A cable bolt ( 10 ) for use in mining excavations comprising a plurality of generally closely spaced wires or strands ( 11 ) which have a first end for fitting into a bore hole ( 21 ) formed in an excavation and a second end which is terminated within a cylindrical sleeve ( 17 ). The sleeve ( 17 ) has a threaded external surface for receiving a lock nut ( 18 ) which is tightened against a bearing plate ( 19 ) so as to tension the cable bolt ( 10 ). The wires or strands ( 11 ) are separated outwardly from a central longitudinal axis of the cable bolt ( 10 ) at spaced locations along the length of the cable bolt to form a “bird cage” ( 13 ) at each location. Spacers ( 14 ) are provided at each location to cause the outward separation of the wires or strands ( 11 ). A central tubular member ( 12 ) extends along the axis through the sleeve ( 17 ) and each spacer ( 14 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to cable bolts for use in the mining and construction industries to support the walls and roofs of underground excavations and openings and particularly, although not exclusively, to cable bolts for use in coal mining where the softer excavation material generally requires differing considerations to those in metalliferous mining. 
     2. Description of the Related Art 
     In coal mines the cable bolt is normally fixed at the inner end of a hole drilled in the mine roof by means of a two part quick curing resin. The epoxy resin is inserted into the hole in separate packaged parts and pushed to the end of the hole by insertion of the cable bolt which also causes destruction of the packaging and mixing of the parts. Rotation of the bolt during full insertion ensures proper mixing of the resin which quickly cures to secure the top of the cable bolt in the hole. In some circumstances cement grouting is then pumped into the hole to secure the bolt along its entire length, to the sides of the hole. 
     One known form of cable bolt for coal mines is marketed under the name FLEXIBOLT®, a registered trade mark of JJP Geotechnical Engineering Pty. Ltd., and comprises a number of outer wires or strands wound around a central core of smaller diameter wires or strands in a helical fashion to form a bar which constitutes the main part of the cable bolt. The lower end of the bolt, that is, the end external of the hole in use, has a thread rolled into the surface of the bolt which, because of the irregular outer surface of the bolt, is a discontinuous thread only occurring in the outermost surface of each of the outer wires or strands. A nut having a thin metal plate pressed into one end is adapted to be wound on the thread until initially the plate bears on the end of the cable bolt and prevents further progress of the nut. Further rotation of the nut by a standard drive mechanism causes rotation of the entire cable bolt to thereby mix the two part resin until the resin cures, at which time the top end of the bolt is firmly anchored and further rotation of the nut causes the plate to be forced out of the nut and the nut to progress along the threaded end of the cable bolt. A bearing plate pre-arranged on the cable bolt and a conventional barrel and wedge between the bearing plate and nut are therefore forced upwardly by the further rotation of the nut against the roof of the mine until sufficient tension is applied to the cable bolt by the rotation of the nut. While the aforementioned FLEXIBOLT® cable bolt is used quite extensively there are some features thereof which render it less than ideal. Most significantly the combination of the nut, barrel and wedge, does not normally give an anchorage force equivalent to the strength of the cable bolt, as it would require a disproportionately long barrel and wedge which, apart from cost considerations would extend downwardly into the mine an unacceptable distance. Furthermore, it is not possible with the FLEXIBOLT® device to provide additional bonding such as grouting over the remaining length of the bolt as there is no means for getting the grouting into the hole after the bearing plate and barrel and wedge are in position. Therefore, the FLEXIBOLT® device must rely only on the bonding achieved by the resin which extends about two meters down from the top of the bolt and consequently full bonding is difficult to achieve. The nut and rolled thread combination is not capable of developing more than about 30% of the ultimate tensile strength (UTS) of the cable due to the non-continuous thread and therefore the cable bolt cannot be pre-tensioned beyond about 30% of the UTS by rotation of the nut alone. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a cable bolt for use in coal mines which overcomes one or more of the shortcomings of the “Flexibolt”™ or at least provides a useful alternative. 
     Accordingly, one broad form of the invention which may be preferred provides a cable bolt comprising a plurality of generally closely spaced elongate wires or strands, said wires or strands having a first end adapted for fitting into a bore hole and a second end being terminated within a cylindrical sleeve, said cylindrical sleeve having a threaded external surface to receive a lock nut for tightening against a bearing plate so as to tension said cable bolt. 
     Preferably said termination of said second end of said wires or strands comprises a transverse plate in said cylindrical sleeve, said transverse plate having individual holes for each wire or strand and each wire or strand passes through a respective termination hole and has an enlarged head which is larger than the diameter of the termination hole whereby the wire or strand cannot be withdrawn from the said respective termination holes. 
     Preferably said wires or strands are separated outwardly from a central longitudinal axis of the cable bolt at discrete spaced locations along the length of the cable bolt to provide a “bird cage” at each location. 
     In one more specific form of the invention the wires or strands are straight and parallel to the longitudinal axis between each bird cage and in an alternative form said wires or strands have a slight helical configuration over the length of the cable bolt. 
     Preferably, coupling means such as a drive nut suitable for engagement with an industry standard square or hexagonal drive is provided in the lower end of said sleeve for the purpose of rotating said cable bolt. 
     In one more specific form said lock nut has a connection for receiving a grout tube and said connection provides access to an annular chamber formed within said lock nut, said annular chamber being against said bearing plate and said bearing plate having a bore therethrough which is greater in diameter than the outer diameter of said sleeve whereby said annular chamber is open to a space between said cable bolt and a wall of a bore hole in the mine roof into which the cable bolt is inserted, whereby grout is pumped through said grout tube and enters said bore hole in the mine roof. 
     In an alternative more specific form of the invention said grout enters said annular chamber via said coupling means. 
     Preferably, located along the longitudinal axis of said cable bolt is a central elongate tubular member which may be hollow and which extends over a substantial length of said cable bolt to act as an air bleed tube when grout is pumped into said bore hole, said tube extending through said sleeve and coupling means. 
     Preferably, a resin seal is provided on said cable bolt at a spaced location from said first end of said wires or strands to prevent resin, used to bond the first end of said wires or strands to the wall of the bore hole, from passing downwardly below said seal, said resin seal comprising a sleeve that fits over said wires or strands and said elongate tubular member terminating within said resin seal. 
     Preferably, spacers are provided at spaced locations around said elongate tubular member to cause said wires or strands to spread outwardly and form said bird cages. 
     Another preferred form of the invention provides a cable bolt for use in an excavation comprising a plurality of generally closely spaced elongate wires or strands having a first end adapted for fitting into a bore hole formed in said excavation and a second end adapted for bearing against the excavation surface wherein the terminal region of said second end of said wires or strands terminates in a cylindrical sleeve having a threaded external surface adapted to co-operate with a threaded lock nut for tightening said cable bolt against said excavation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention may be more readily understood reference should be made to the accompanying drawings wherein: 
     FIG. 1 is a front elevation of a cable bolt according to a first embodiment of the invention; 
     FIG. 2 is an enlarged view of a portion of the cable bolt shown in FIG. 1; 
     FIG. 3 is a front elevation of a cable bolt according to a second embodiment of the invention; 
     FIG. 4 is an enlarged view of a portion of the cable bolt shown in FIG. 3; 
     FIG. 5 is an enlarged view of a portion at the top end of the cable bolt of FIGS. 1 and 2; 
     FIG. 6 is an enlarged view of a portion of the top end of an alternative cable bolt; and 
     FIG. 7 is an enlarged sectional view of a portion of the lower end of a cable bolt according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The cable bolt  10  consists essentially of a number of wires or strands  11  extending over the length of the cable bolt and spaced around a central member  12  (FIGS. 2,  4 ) which is preferably elongate and tubular. The central member  12  is a hollow metal or plastics tube in this embodiment but in situations where the cable bolt is not to be grouted after installation, the central member  12  may merely comprise a further wire similar to the wires or strands  11  but arranged centrally along the longitudinal axis of the cable bolt  10 . 
     In the embodiments of FIGS. 1 to  5 , the strands  11  are essentially straight and parallel with the central strand  12  except for points along the length of the cable bolt where the strands are flared outwardly to form a “bird cage”. For the purpose of forming each “bird cage”  13 , a spacer  14  in the form of a nut or annular ring (see FIG. 5) is provided around the central member  12  and the strands  11  are caused to bow out over the spacer  14  thus providing the “bird cage”  13 . The bird cage  13  exposes a greater surface area of each of the strands  11  to provide increased bonding for the resin and additionally for grout, where it is used. Spaced collars  15  at either side of each “bird cage” clamp the wires back together against the central strand  12 . 
     The strands  11  are terminated at a first end  44 , in use, by a terminal collar  16  which is welded to the strands  11  or alternatively the strands  11  may be merely welded together at the first end  44 . 
     A second end of the wires or strands  11  enter a cylindrical sleeve  17  in which the second end is terminated. The second end termination of the wires or strands  11  is shown more clearly in FIG.  7  and consists of a transverse plate  47  in the sleeve  17  having individual holes  48  for each strand  11 . Each strand  11  passes through a respective hole and is provided with an enlarged head  49  in the form of a button end which is larger than the diameter of each respective hole  48  thereby preventing withdrawal of the strands  11  from the sleeve  17 . The plate may be an integral part of the sleeve  17  as is the case in FIG.  7 . As shown the sleeve  17  is threaded on its outer surface to receive lock nut  18  which is shown in more detail in FIG.  2 . The lock nut  18  is tightened in use against bearing plate  19  to tension the cable bolt  10  to provide the required tension for supporting the roof of a mine. 
     The bearing plate  19  is provided with a “trumpet”  20  which is welded to the bearing plate  19  and consists of a stepped cylindrical tube adapted to extend a short distance into the bore hole  21  which is drilled in the mine roof for accommodating the cable bolt. Collar pipes  22  and  23  extend concentrically from the sleeve  17  by means of the collar pipe  22  being inserted into the end of the sleeve  17  and the collar pipe  23  being inserted in the end of the collar pipe  22 . The purpose of these collar pipes  22  and  23  is to provide stiffening to the lower end of the cable bolt when required. In an alternative embodiment only a single collar pipe with expanded section is used. 
     As is shown in FIG. 2, the lock nut  18  has a bore  24  in the side of the nut which extends radially inwardly to an annular chamber  25  provided in the upper end of the nut  18 . The bearing plate  19  has a hole through which the sleeve  17  of the cable bolt passes and the hole in the bearing plate has clearance around the sleeve. Thus there is a passage from the annular chamber  25  through the bearing plate  19  and into a space which exists between the trumpet  20  and the sleeve  17 . A grout tube  26  is connected to the bore  24  by means of a suitable connection  24   a.  Thus, grouting, which is typically in the form of “liquid” grout, pumped into the grout tube  26  enters the annular chamber  25  and passes up through the trumpet  20  and into the bore bolt  21  accommodating the cable bolt  10 . In another from the grout may enter via coupling means such as a drive nut  27  which is fixed into the end of the sleeve  17  and is of a size that will be readily engageable with an industry standard hexagonal or square drive for the purpose of rotating the cable bolt during insertion into the bore hole. Alternatively a screw coupler or any other coupling means that is engageable to sleeve  17  may be used. 
     A trumpet seal  30  in the form of either foam or an “O” ring is adapted to fit around the trumpet  20  immediately above either of the stepped sections  28  or  31  for the purpose of sealing the cable bolt against the side of the bore hole. The trumpet seal  30  assists in centering the trumpet  20  and sleeve  17  within the bore  21  to prevent damage to the external thread of sleeve  17  against the surface walls formed by the bore  21 . A further seal in the form of a tapered tubular resin seal  29  is provided over the strands of the cable bolt at a location spaced about 2 meters from the first end  44  of the wires or strands  11 . In the case wherein the central member  12  is a hollow metal or plastics tube, it terminates at an upper end  45  within the resin seal  29  as is shown in FIG.  2 . 
     In the second embodiment shown in FIGS. 3 and 4, the locking nut  18  is substantially thinner in width and does not have an annular chamber compared with the embodiment of FIGS. 1 and 2, to enable the substantially thinner nut  18  to have the required strength. For situations where grouting is required, the grout is pumped into grout tube  26  (FIG. 4) which is connected to coupling means such as a drive nut  27  by suitable connection means and is in communication with a bore  33 . As an alternative to using drive nut  27 , a screw coupler or any coupling device that is welded, press-fitted into sleeve  17  may be used. The screw coupler may be threaded with limited depth so that when it is screwed onto the lower end of sleeve  17 , it will eventually lock so that further rotation of the coupler will result in rotation of the cable bolt  10 . The bore  33  opens up into an annular cavity  34  surrounding the central member  12  in the drive nut  27 . The drive nut  27  is externally threaded at its upper end and engages a corresponding internal thread of sleeve  17 . The annular cavity  34  extends upwardly through sleeve  17 , locking nut  18 , bearing plate  19  into a space  35  between trumpet  20  and collar pipe  23 . The grout fills the annular cavity  34  and space  35  until it is expelled from the top portion  46  of trumpet  20  and fills the bore hole  21 . The trumpet seal  30  prevents any grout flowing downwardly past the seal itself so that with continued pumping the bore  21  will fill until it reaches the upper end termination  45  of the central member  12  in resin seal  29 . During the process, air is bled out from the bore hole  21  and is forced out down the interior of hollow central member  12  until it is expelled from the lower end  36  thereof. The grout follows downwardly through the central member  12  so that when it exits the lower end  36  it provides an indication that the grouting process is complete. A cap  37  may be screwed onto end  36  to prevent any further egress of the grout. 
     There may exist a number of cracks or channels in the mine walls or roof that meet at various openings to the bore hole  21 . It is possible that grout being pumped into the bore hole  21  will fill up these channels with the result that the grout does not reach the resin seal  29  and enter the central member  12 . There will therefore be no indication of the grout covering a substantial length of the cable bolt  10  up to the resin seal  29 . To overcome this, an option to the user is to have the grout pumped in a reverse manner, that is, pumping the grout up the central member  12  first and letting the grout fill up the space between the cable bolt  10  and the walls  32  of the bore hole  21 . To facilitate this, the grout tube  26  is directly connected (not shown) to a lower part of central member  12  so that it is in communication with the central member  12 . Grout, which in this application is typically thixotropic, is then pumped into the central member  12  until it fills whereupon further pumping forces grout to flow out of the upper end  45  of central member  12  in resin seal  29  and then substantially fills up the bore hole  21 . The seal  30  and/or an additional seal  38  prevent the grout from exuding outwardly of the hole  21  past the bearing plate  19 . The air originally existing in the bore hole  21  is expelled through the channels or cracks if they exist. 
     In use, the cable bolt is installed in the roof of a mine by firstly drilling a stepped bore hole  21  in the mine roof to the required length. Next, a two part resin adhesive in separate plastic packs  39 ,  40  and  41  is placed in the hole and is pushed upwardly to the top of the hole by insertion of the cable bolt  10 . When the resin has reached the inner end  42  of the hole further insertion of the cable bolt fractures the packaging and the two parts of the resin are allowed to mix. Rotation of the cable bolt  10  by means of applying driving dolly  43  to the drive nut  27  causes further mixing of the resin which extends from the first end  44  of the wires or strands  11  of the cable bolt to the resin seal  29 . The resin is quick curing and once cured further rotation of the cable bolt is prevented. Thus, the cable bolt is then secured in the bore hole  21  at the upper end  42  and the lock nut  18  is tightened to force the bearing plate  19  against the mine roof. Once the required tension in the cable bolt has been reached, the mine roof is secured. If the cable bolt is to be grouted over its entire length, grouting is pumped via the grout tube  26  until it fills the bore hole and all the spaces surrounding the strands  11  up to the resin seal  29 . During the first application of this process where the grout is first pumped through annular cavity  34 , air is bled from the bore hole via the hollow central tube  12  and since this central tube  12  extends outwardly through the drive nut  27  at the bottom of the cable bolt, evidence that grouting has been completed occurs when the grouting appears at the bottom of the central member  12 . Installation is then complete. 
     As indicated previously, in an alternative embodiment where the cable bolt is not to be grouted into the hole, the central hollow tube  12  may be replaced by a solid wire or strand. 
     FIG. 6 shows an alternative form of the cable bolt where the strands  11  assume a helical path around a central strand which is not evident in the FIGURE. The degree of the helical twisting of the strands is relatively slight. 
     It should be evident from the description hereinabove that the cable bolt of the present invention provides advantages over existing cable bolts. For example, the cylindrical sleeve  17  provides a continuous thread for the lock nut  18  and because of its larger diameter, it is able to withstand much higher forces than previous lock nuts and therefore it is possible to have a bottom end termination which can withstand forces equal to or greater than the minimum tensile capacity of the cable bolt. Whilst it necessitates the reaming of a larger diameter portion at the bottom of the bore hole  21 , this is not a detrimental requirement. Furthermore, by providing in one application an air bleed tube along the central axis of the cable bolt, it achieves the two purposes of providing means for supporting the spacers  14  as well as means for allowing air to be discharged from the bore hole  21 .