Abstract:
A rock bolt includes an elongate metallic body having a first end and an opposed second end, a threaded portion at the second end, for attaching thereto and locating thereon, a nut and a bearing plate, a mechanical anchor at, or at least partially located on, a first end portion of the body and a first resistive anchor, located between the threaded portion and the mechanical anchor.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to bolting for reinforcement of rock subject to deformation and dilation and, more specifically, to a rock bolt anchor with two anchor types that provide active and passive loading. 
       BACKGROUND OF THE INVENTION 
       [0002]    The prior art teaches a deformable rock anchor that is deformation tolerant, which is used in highly stressed rock masses to achieve reinforcement of these stressed rock masses and prevent large, sudden or catastrophic deformation, movement, dilation or failure of this rock mass. 
         [0003]    This rock bolt includes an elongate cylindrical stem, with a threaded portion at a borehole surface portion of the stem, to which a nut and washer or bearing plate may be attached, and three or more stem portions serially extending along the length of the stem with each stem portion followed by an integral anchor, being of shorter extend than the stem portions. 
         [0004]    Each integral anchor is capable of locally anchoring the rock bolt in a grouted borehole and each stem portion is adapted to elongate, move and slip relatively to the grouted borehole surround and, by the work done by this movement, absorb energy from the surrounding rock and constrain local rock deformation movement, whilst the rock bolt remains locally anchored by each integral anchor. 
         [0005]    The rock bolt of the earlier invention is therefore principally defined by having at least three integral anchors and therefore, in situ, is capable of being locally anchored at three discrete localities along the length of the borehole. These anchor points exclude anchoring, by the bolt and bearing plate, at an entrance of the borehole. 
         [0006]    The problem experienced with such a rock bolt is that it is reliant, for local anchoring, on the interaction of the anchors on the grout within the borehole. 
       SUMMARY OF THE INVENTION 
       [0007]    The invention provides a rock bolt for being grouted in a borehole in a rock which includes:
       a) an elongate body of a suitable steel material having;   b) a first distal end and an opposed second proximal end;   c) a threaded portion at the second end;   d) a mechanical anchor or a composite anchor at, or at least partially located on, a first end portion of the body;   e) a second anchor integrally formed on the body, between the threaded portion and the mechanical or composite anchor;   f) a first stem portion between the mechanical or composite anchor and the second anchor; and   g) a second stem portion between the second anchor and the threaded portion;   h) wherein the first and second stem portions have a smooth cylindrical surface; and wherein the second anchor is adapted to exceed the diameter of the body in at least one radial direction to be locally anchored in a grouted borehole and is adapted to be harder than the stem portions.       
 
         [0016]    The mechanical anchor may be an expansion shell-type mechanical anchor which is actuated to radially expand into frictional engagement with the walls of the borehole. 
         [0017]    The composite anchor may comprise an expansion shell type mechanical anchor component and an integrally formed anchor component which is adapted to exceed the diameter of the body in at least one radial direction, wherein the mechanical anchor component and the integral anchor component are consecutively serially positioned on the rock bolt body. 
         [0018]    The second anchor may be positioned on the body between 400 and 700 mm from the second end. Preferably, the second anchor is positioned 600 mm from the second end. 
         [0019]    A “mechanical anchor” means an anchor engaged with a rock bolt and which is actively actuated to anchor the rock bolt in a rock hole or, in other words, an anchor that is actively loaded. 
         [0020]    A “resistive anchor” means an anchor engaged, or integrally formed, with a rock bolt which is passively actuated to anchor the rock bolt within a rock hole by resistive contact with grout or resin within the hole. 
         [0021]    From another perspective, the invention provides a rock bolt which includes an elongate metallic body having a first end and an opposed second end, a threaded portion at the second end, for attaching thereto and locating thereon, a nut and a bearing plate, a mechanical anchor at, or at least partially located on, a first end portion of the body and a first resistive anchor, located between the threaded portion and the mechanical anchor. 
         [0022]    The mechanical anchor may be an expansion shell-type anchor. 
         [0023]    The first resistive anchor may be integral with the body, formed by adapting a section of the body, between 400 mm and 700 mm from the second end, to exceed the diametric dimension of the cylindrical body at least in one radial direction. 
         [0024]    The rock bolt may include a second resistive anchor, located between the mechanical anchor and the first resistive anchor, preferably consecutively serially positioned relatively to the mechanical anchor. 
         [0025]    Between the mechanical anchor or the second resistive anchor, the first resistive anchor and the threaded portion respectively, first and second stem portions are defined, each of which are adapted to elongate under a tensile load. 
         [0026]    The invention extends to a method of supporting a wall of an excavation which uses a rock bolt having an elongate metallic body with opposed first and second ends, a threaded end portion towards the second end, a mechanical anchor located on the body towards the first end and at least two spaced resistive anchors between the mechanical anchor and the threaded portion, the method including the steps of:
       a) drilling a hole in the wall and inserting the rock bolt into the hole;   b) mechanically anchoring the bolt within the hole with the mechanical anchor to define a first anchor location;   c) pre-tensioning the bolt by applying an axial load to the bolt;   d) holding the bolt in pretension between the first anchor location and a second anchor location defined at the mouth of the rock hole;   e) introducing a settable material into the rock hole to set between the rock bolt and the walls of the rock hole to define a third and fourth anchor location respectively about each of the resistive anchors.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The invention is further described by way of example with relevance to the accompanying drawings in which: 
           [0033]      FIG. 1  is a view in elevation of a rock bolt, in a first embodiment of the invention, inserted into a borehole; 
           [0034]      FIG. 2  is a view in perspective of an integral anchor part of the rock bolt; 
           [0035]      FIGS. 3A and 3B  are isometric illustrations of one end of the rock bolt with a mechanical anchor located thereon; 
           [0036]      FIG. 4  is a view in elevation of a rock bolt, in a second embodiment of the invention, inserted into a borehole; and 
           [0037]      FIGS. 5A to 5C  illustrate, in chronological sequence, the rock bolt of the second embodiment in use. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
       [0038]      FIG. 1  of the accompanying drawings illustrates a rock bolt  10 , in accordance with a first embodiment of the invention, which is adapted to be inserted into a rock hole  12 , anchored within the rock hole  12  by a mechanical anchor  14 , and then, after grout is introduced into the rock hole  12 , to be additionally anchored, at a second locality, by an integral anchor  16  which is designed to resist passage through the grouted rock hole. 
         [0039]    The rock bolt  10  has a solid cylindrical steel body  18 , which extends between a first distal end  20  and a second proximal end  22 , which projects out of the rock hole  12 . 
         [0040]    A section of the rock bolt body  18 , extending from the second end  22  is threaded, to define a threaded portion  24 . 
         [0041]    The mechanical anchor  14 , of an expansion shell-type, is located at the distal end  20 . This expansion shell-type mechanical anchor can be of any suitable configuration known to the art. However a specific preferred expansion shell anchor is described below as a non-limiting example. 
         [0042]    The integral anchor  16  is located between the threaded end section  24  and the mechanical anchor  14 . This anchor  16  is integral with the body in that it is formed from the same blank as the body  18 . 
         [0043]    With reference to  FIG. 2 , the integral anchor, in a preferred embodiment, comprises a pair of end-to-end paddle formations, respectively designated  26 A and  26 B. Each paddle formation  26 A and  26 B lies in a plane which is perpendicular to its counterpart. Each paddle formation  26 A and  26 B is formed by flattening the rod such that the rock bolt body  18  expands in opposed directions which are orthogonal to the direction of the flattening force (these directions of expansion are designated X and Y respectively). This flattening process is a cold forming process that strain hardens the steel material along the length of the anchor  16 . This process also adapts the cylindrical rock bolt body  18  to locally exceed its diameter in radial directions X and Y respectively providing extensions which are resistive to pull through a grouted borehole. 
         [0044]    In recognition that the rock, in a typically South African mine excavation, is most densely fractured within the first 300 mm or so, from a rock face, the integral anchor  16  is optimally and preferably positioned on the rock bolt body  18  about 500 mm from the second end  22 . 
         [0045]    Between the first mechanical anchor  14 , the second integral anchor  16  and the threaded section  24 , first and second, smooth surfaced, stem portions  30  and  32  are respectively defined. 
         [0046]    With reference to  FIGS. 3A and 3B , the expansion shell-type mechanical anchor  14  includes a tapered nut  28  attached to the first end  20 , an expansion shell  34  that abuts the tapered nut  28 , in a dis-engaged position illustrated in  FIG. 3A , at its leading end  36  and a spring  38 , located between a trailing end  40  of the shell  34  and a collar formation  42 . The spring  38  biases the shell  34  towards the tapered nut  28  to ride over the tapered nut  28 , and radially expand, in an engaged position illustrated in  FIG. 3B . 
         [0047]    The advantage of the mechanical anchor  14  as described above is that mere insertion of the rock bolt  10  into the rock hole  12 , and axial retraction, will actuate the anchor  14  into the engaged position. There is no need to spin the rock bolt  10  to actuate the mechanical anchor  14  to radially expand as is typically with many mechanical anchors known in the art. 
         [0048]    A nut  46  and bearing plate  48  are provided, located on the threaded section  24  of the rock bolt body  18 . 
         [0049]    In a variation (not shown), a tapered formation, provided by the nut  28  in the embodiment described above, can be integrally forged with rock bolt body  18  at the first end  20 . 
         [0050]      FIG. 4  illustrates a second embodiment of the invention, a rock bolt  10 A. 
         [0051]    In describing this embodiment, like features bear like designations. This embodiment differs, in essence, from the rock bolt  10  of the first embodiment in that it includes a composite anchor  50  which replaces the mechanical anchor  14  and the collar formation  42  of the first embodiment. 
         [0052]    The composite anchor includes a mechanical anchor component  52 , of the expansion shell-type as described above particularly with reference to  FIGS. 3A and 3B , located at the distal end  20  and an integral anchor component  54  consecutively serially positioned with respect to the anchor component  52 , back from the component  52 . 
         [0053]    The integral anchor component  54 , in the preferred embodiment, is structurally equivalent to the integral anchor  16  of the rock bolt  10 . 
         [0054]    Positioned, as it is, in consecutive serial arrangement relatively to the mechanical anchor component  52 , the integral anchor component  54  not only provides an additional passively loaded anchor to the rock bolt  10 A, it also performs the function provided by the collar formation  42  of the earlier embodiment in that it provides an abutment surface to one end of the spring  38 , located between the trailing end  40  of the shell  34  and one end of the anchor component  54 . 
         [0055]    In use, and with reference to  FIGS. 5A to 5C , the rock bolt  10 A is inserted into a rock hole  12 , first end  20  leading, to a point where the threaded portion  24 , at least, is projecting from the rock hole  12 . The rock bolt  10 A, in this preferred embodiment, includes a bung  56 , located on the body  18 , through which a grout pipe and breather tube (not shown) pass. The bung  56  is located between the threaded portion  24  and the integral anchor  16  and is totally inserted in the rock hole  12 . A holed bearing plate  48  is passed over the second end  22  followed by the threaded engagement of a nut  46  to the threaded portion  24 . 
         [0056]    The bearing plate  48  can be provided with a pair of holes (not shown) on either side of central aperture, to provide respective passage to a grout or resin filler tube and a breather tube. 
         [0057]    To actuate the mechanical anchor component  52  of the composite anchor  50  into the engaged position, the rock bolt body  18  is pulled axially outwardly. This action causes the expansion shell  34 , which is held in place relatively to the rock bolt body by frictional engagement with the walls of the rockhole  12 , to ride over the tapered nut  28 , radially dilating in the process into loaded contact with the walls of the rock hole  12 . The rock bolt  10 A is now locked in the rock hole  12  at this location, a first anchor location (illustrated as a dotted line  60 ). 
         [0058]    With reference to  FIG. 5A , tightening of the nut  46 , along the threaded portion  24 , to bear against the bearing plate  48 , forcing the plate against the rock face  52 , defines a second anchor location (illustrated as a dotted line designated  62 ). 
         [0059]    With further tightening of the nut  46 , the rock bolt body  18  is pre-tensioned (the opposed forces directionally illustrated by arrows in  FIG. 5A ), prior to the grouting of the rock hole  12 , between the first and the second anchor locations ( 60 ,  62 ) thus actively providing reactive load support to the rock mass between the two locations  60  and  62 . 
         [0060]    With reference to  FIG. 5B , grout, from a source  64 , is now introduced into an annular space  66 , via the grout or filler tube, between the walls of the rock hole  12  and the rock bolt  10 A until the annular space  66  is fully grouted as illustrated. As grout fills the annular space  66 , displaced air  68  passes out of the hole through the breather tube. The rock bolt  10 A is now locked in pre-tension. 
         [0061]    The bung  56  seals the rock hole  12  from egress of the grout out of the rock hole  12  once introduced. 
         [0062]      FIG. 5C  illustrates the highly fractured layer of the rock mass described above, dilating about surface parallel stress fractures  70 , forces are imparted on the bearing plate  48  which is translates into a pulling force on the rock bolt  10 A out of the rock hole  12 . This pulling force is resisted by the integral anchor  16 , which is adapted, due to it exceeding the diametric dimension of the cylindrical rock bolt body  18  in at this point, to resist passage through the now hardened grout, thus providing a third anchor location (illustrated by a dotted line designated  72 ). 
         [0063]    Once the rock bolt  10 A is set in the grouted rock hole, with the integral anchor component  54  anchored in the grout, any movement of the surrounding rock mass relatively to the rock bolt  10 A will cause the anchors ( 16 ,  54 ) to become passively loaded and anchored by resistive movement through the grouted annular space  66 . Thus, about integral anchor component  54 , a fourth anchor location (illustrated by a dotted line designated  74 ) is defined. Ahead of this anchor location  74 , the initial anchor location  60 , about the mechanical anchor component  52 , is rendered inutile as reactive load support is now provided between anchor locations  74  and  72  and between  72  and  62 . 
         [0064]    The advantage of the rock bolt  10 A of the invention is that, between the anchor locations  62 ,  72  and  74 , the rock bolt body  18  can stretch along respective first and the second stem portions ( 30  and  32 ) to accommodate any dynamic loading movement. 
         [0065]    The stem portions  30  and  32 &#39;s ability to stretch is uninterrupted along their lengths due to their smooth surface which allows relative movement within the grouted confines of the rock hole  12 . 
         [0066]    However, prior to dynamic rock movement, with quasi-static movement, caused by dilation in the highly fractious rock layer, the second stem portion  32  is further passively pre-loaded, between the second  62  and third  72  anchor locations to provide support to this layer effectively by clamping this layer of rock  70  between the bearing plate  44  and the integral anchor  16 .