FIELD OF INVENTION

The present invention relates to an anchor bolt.

More particularly, the present invention relates to an anchor bolt having an anchor for 5 securing the anchor bolt within a bore hole in a rock body.

BACKGROUND ART

Anchor bolts, also known as rock bolts or friction bolts, are used in the mining industry to support and stabilise a rock body against creep movement or collapse. The anchor bolts are then used to locate bearing plates or thrust plates to apply a compression force onto 10 the rock strata to stabilise the rock body. It is also known to use the anchor bolts to

support a wire mesh adjacent the rock face and to spray a settable concrete over the mesh to strengthen the rock face.

Many types of anchor bolts are known and they are generally in the form of an elongated element such as a tube, cable, rod or combinations thereof that are able to be fitted into a i s hole drilled into the rock body and subsequently secured within the hole. Typically the anchor bolts are secured in the hole by mechanical or chemical means. Mechanical fixations can be friction based, such as by providing a split sleeve surrounding the anchor bolt and pulling or forcing a plunger attached to the anchor bolt into the sleeve. The plunger causes the sleeve to expand and press against the sides of the rock body in the

20 hole to jam the anchor bolt in place. Chemical fixations utilise an epoxy or cement to bond the anchor bolt within the hole.

One drawback encountered with many mechanical fixations in prior art anchor bolts is that the sleeve is expanded from its end furthest within the hole, i.e. the plunger is pulled out from deeper within the hole than the sleeve. This causes the sleeve to expand forming a 25 conical wedge that has its widest base part furthest within the hole and its frusto-conical "wedge apex" being aimed out from the hole. An example of such an anchor bolt configuration is seen in published patent US 7,959,379. Accordingly, when movement in the rock body applies a force onto the anchor bolt in a direction to pull the anchor bolt out of the hole, the wedge is urged to collapse and can slip within the hole. This is particularly a problem in a softer less compact rock body. In an attempt to address this issue, some anchor bolts have a roughened outer surface on their sleeves, e.g. by providing knurling or protrusions on the sleeve outer surface. As can be seen in the referenced

US 7,959,379, its sleeve is provided with a plurality of annular cleats that are able to 5 engage the rock body. However even such cleats can still slide over the rock body

because the overall shape is still an outwardly aimed conical wedge. From a practical viewpoint, the wedge formed by the sleeve faces the wrong direction.

A further drawback with such prior art anchor bolts is that, should the plunger become loose in the sleeve, there would be a reduction of the friction force holding the anchor bolt 10 in the hole, which may lead to further slipping of the sleeve in the hole and subsequent performance loss of the anchor bolt.

Some of the above-mentioned problems have been alleviated by the anchor bolts shown in US 201 1/0038683 and CN 203214519. The bolt of US 201 1/0038683 discloses an anchor having a fixation member mounted on a bolt. The fixation member has an i s expansion end being divided into four expansion pieces that are arranged to face a head of the bolt. A sleeve is mounted on the bolt between the bolt head and the fixation member so that, as the bolt body is rotated, the fixation member moves towards the sleeve so that the expansion end rides over the sleeve and is deflected radially outwardly. The bolt of CN 203214519 operates in a similar manner.

20 A problem encountered with both US 2011/0038683 and CN 203214519 are that they are generally not intended for use in the mining industry and do not permit yielding for gradual dissipation of tensile stresses. Thus, these two bolts provide rigid fixed anchor points and, once the tensile load exceeds the tensile strength of the bolts, the bolts will likely fail.

It is to be understood that, if any prior art publication is referred to herein, such reference 25 does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided an anchor bolt for use as a rock bolt or friction bolt in a bore hole formed in a rock body, the anchor bolt 30 comprising: an elongated load bearing element having a first end and an opposed second end; an anchor supported by the load bearing element near to its first end, wherein the anchor comprises a deflectable part being configured to be deflected outwardly from the load bearing element;

5 a sleeve surrounding the load bearing element between the anchor and the second end, wherein the sleeve is spaced apart from the first end so as to define an axial gap between the sleeve and the anchor;

a deflection arrangement formed on the sleeve and being configured to deflect the deflectable part; and

10 the load bearing element being axially movably within the sleeve so that, in use, the anchor is able to traverse the gap to bring the deflectable part into contact with the deflection arrangement to deflect the deflectable part.

The sleeve may be in frictional engagement with the load bearing element. In one embodiment an inner surface of the sleeve may be roughened or knurled to increase its i s frictional engagement with the load bearing element. Alternatively, an outer surface of the load bearing element may be roughened or knurled to increase its frictional engagement with the sleeve.

The sleeve may be substantially tubular and have an axial slot extending along its length so that the sleeve is substantially C-shaped in end view, whereby the sleeve is 20 circumferential ly compressible around the load bearing element.

The deflection arrangement may comprise a ramp defined by the sleeve. The ramp may be a tapered or bevelled end wall of the sleeve tapering towards the first end of the load bearing element.

The gap between the sleeve and the anchor may extend for about half a length of the load 25 bearing element.

The anchor may comprise a collar and a depending skirt, wherein the collar is joined to the load bearing element and the skirt comprises the deflectable part. The skirt may be provided with a bevelled knife edge along its free edge remote form the collar. One or more slits may extend axially through the skirt to define multiple deflectable tines. The collar may be movably supported on the load bearing element. The anchor bolt may comprise a spigot joined to the first end of the load bearing element, the spigot having a threaded end, wherein the collar comprises a threaded bore for being screwed onto the threaded end of the spigot. Alternatively, the collar may be fixedly attached to the load 5 bearing element.

The anchor may comprise a base member being joined to the load bearing element, wherein the load bearing element extends through a bore passing through the base member. The base member may be welded to the load bearing element fully around the bore. A recess may be formed in the base member, the recess defining a seat for 10 receiving a distal end of the sleeve.

The load bearing element may comprise a tube or hollow bar. The tube may have an axial slot extending along its length so that the tube is substantially C-shaped in end view. The anchor bolt may further comprise a bridging insert secured within the axial slot.

According to another aspect of the present invention, there is provided a method of i s securing an anchor bolt in a bore hole formed in a rock body, the method comprising the steps of:

inserting into the hole an elongated load bearing element having a first end and an opposed second end, the load bearing element supporting an anchor near to its first end, whereby the anchor comprises a deflectable part being configured to be deflected 20 outwardly from the load bearing element;

providing a sleeve to surround the load bearing element between the anchor and the second end, wherein the sleeve is spaced apart from the first end so as to define an axial gap between the sleeve and the anchor, and a deflection arrangement formed on the sleeve being configured to deflect the deflectable part;

25 enabling the load bearing element to be axially movably within the sleeve;

whereby, in use, when a load is imposed onto the load bearing element by the rock body, the load bearing element is pulled out from the hole causing the anchor to traverse the gap, and

whereby, when the deflectable part contacts the deflection arrangement, the 30 deflectable part is outwardly deflected so as to engage the rock body and prevent the load bearing element from being further pulled out from the hole. The method may further comprise the step of, prior to the deflectable part being outwardly deflected to engage the rock body, having the deflectable part at least partially compress or crush either or both of the load bearing element and the sleeve.

BRIEF DESCRIPTION OF DRAWINGS The present invention will now be described, by way of example, with reference to the accompanying schematic drawings, in which:

Figure 1 is a perspective side view of a first embodiment of an anchor bolt;

Figure 2 is a sectional side view of the anchor bolt of Figure 1 , which is shown located in situ within a bore hole formed in a rock body and with its anchor in an initial pre- anchoring position;

Figure 3 is a sectional side view of the anchor bolt of Figure 2, which is shown with its anchor deployed in an operative anchoring position;

Figure 4 is a perspective side view of a second embodiment of an anchor bolt; Figure 5 is a sectional side view of the anchor bolt of Figure 4, which is shown located in situ within a bore hole formed in a rock body and with its anchor in an initial pre- anchoring position;

Figure 6 is a sectional side view of the anchor bolt of Figure 5, which is shown with its anchor deflected into an operative anchoring position;

Figure 7 is a perspective side view of a third embodiment of an anchor bolt;

Figure 8 is a side view of the anchor bolt seen along arrow VI II in Figure 7;

Figure 9 is an exploded perspective view of the anchor bolt of Figure 7; and

Figures 10A through 10D are diagrammatic representations (on a reduced scale) of the anchor bolt shown in progressive operative stages.

DETAILED DESCRIPTION OF THE DRAWINGS Referring to Figures 1 to 3 of the drawings, there is shown a first embodiment of an anchor bolt 10 that is configured for use as a rock bolt for insertion into a bore hole 12 formed in a rock body 14.

The anchor bolt 10 includes elongated load bearing element in the form of a bar 16 having a bar first end 18 and a bar second end 20. During use, the bar first end 18 is arranged to be inserted into the hole 12 while the bar second end 20 is arranged to protrude from the hole 12. The bar 16 is rigid and is typically made of steel so that it has a suitably high tensile strength, e.g. being in the order of 600 MPA to 900 MPA so that it is capable of supporting a load of about 30 metric tonnes.

The bar 16 is formed with a first screw thread 22 extending from the bar first end 18 and 5 with a second screw thread 24 extending from the bar second end 20. In the exemplary embodiment both the first and second screw threads 22, 24 are co-directional, i.e. both being either a right-hand thread or a left-hand thread. In alternate embodiments, opposing turnbuckle threads can be provided. As shown in the drawings, the first and second screw threads 22, 24 are discrete from each other to provide a uniform non- 10 threaded elongated section 26 between them. However, it is envisaged that in other embodiments the first and second threads 22, 24 can run together so that the bar 16 will have a common thread along its entire length.

The bar 16 extends through a sleeve 28. In the exemplary embodiment the sleeve 28 is generally tubular along its length so that it concentrically surrounds the bar 16. In other i s embodiments, the sleeve 28 can be cut along its axial length to form a slot (not shown) resulting in the sleeve 28 being C-shaped in end view. The sleeve 28 has a tapered end portion that narrows towards the bar first end 18 so that the tapered end portion defines a deflection arrangement in the form of a ramp 30. The cone angle of this ramp 30 is relatively shallow, e.g. about 5°-15° with respect to a longitudinal axis of the sleeve 28.

20 An anchor 32 is threadably provided onto the first screw thread 22 and arranged to abut the ramp 30. In this regard the anchor 32 includes a collar 34 integrally formed with a depending skirt 36, whereby collar 34 has a threaded bore for engaging with the first screw thread 22. The collar 34 is in the form of a nut. The skirt 36 is conical in shape so that it can lie relatively flush against the ramp 30, but the skirt 36 is malleable so that, in

25 use, the skirt 36 is able to be deflected to ride over and along the ramp 30 when these two parts are moved towards each other. In the drawings the ramp 30 and the skirt 36 are shown to be substantially geometrically similar, but it should be appreciated that the working of the invention will still be possible even if there is no close similarity, provided the skirt 36 remains able to ride over and along the ramp 30. Two or more slits 38 are cut

30 axially through the skirt 36 from its free edge 42 so that the skirt 36 is sectioned into

separate deflectable tines 40. When the anchor 32 is in its initial pre-anchoring position, the circumference of the edge 42 is substantially similar to or less than the circumference of the sleeve 28, i.e. being smaller than the circumference of the hole 12. However, the skirt 36 (particularly the tines 40) is adapted to flare outwardly during use as will be described in due course so that its edge 42 has a circumference larger than the circumference of the hole 12. The edge 42 can be formed flat or be bevelled to exhibit a 5 knife edge in the form of a smooth or serrated cutting edge so as to be able to cut into the rock body 14.

It will be appreciated that the configuration of the anchor bolt 10 provides an expansion type anchor that works in an opposite or inverted manner from most known prior art anchors, namely in the present invention the flared edge 42 is directed towards the bar 10 second end 20 and is not directed towards the bar first end 18.

The distal end of the sleeve 28 abuts against a washer 44 that is held in place by a nut 46 being threadably provided on the second screw thread 24 of the bar 16. The washer 44 is arranged to press a bearing plate 48 against a face of the rock body 14. As can be more clearly seen in Figure 1 , the nut 46 comprises a shear pin 50 that initially secures the i s nut 46 to the bar 16 so that the bar 16 can be rotated together with the nut 46. The shear pin 50 is configured to fail under a predefined torque, whereafter the nut 46 will be able to move along the second screw thread 24. It will be appreciated that persons skilled in the art will be aware of many other means whereby the nut 46 can be secured to the bar 16 and configured to be released under a predetermined torque.

20 In use, the hole 12 is first drilled into the rock body 14 to a desired depth. The anchor bolt 10 is then suitably inserted into the hole 12 until the bearing plate 48 abuts against a rock face 52 of the rock body 14. This insertion is normally performed by percussion hammering the anchor bolt 10 into the hole 12. A driving tool such as a pneumatic wrench (not shown) is then applied to the nut 46 and operated to rotate the nut 46. The

25 presence of the shear pin 50 causes the rotation of the nut 46 to be transferred to the bar 16 so that the bar 16 rotates within the collar 34, thereby causing the anchor 32 to travel along the first screw thread 22 in a direction towards the bar second end 20 and thus the sleeve 28. During this initial stage of rotation, the shear pin 50 prevents the nut 46 from rotating relative to the bar 16 and thus the nut 46 will not advance along the

30 second screw thread 24. ln the event that the anchor 32 is initially spaced slightly apart from the sleeve 28, the anchor 32 will not experience any deformation as it moves towards and first contacts the sleeve 28. Further rotation of the bar 16 will subsequently cause the skirt 36 to be pulled over the ramp 30 so that the edge 42 rides up along and over the ramp 30 thereby 5 deflecting the tines 37. This results in the skirt 36 flaring outwardly to enlarge the

circumference of the edge 42 until the edge 42 comes into contact with the surrounding rock body 14. Any further rotation will start to cause the edge 42 to press against the rock body 14 (as is shown more clearly in the enlarged image of Figure 3). When the rock body 14 comprises hard rock the edge 42 will not greatly penetrate the rock body 14 but 10 the anchor bolt 10 will be securely held in place by the anchor 32 due to the outward

pressure exerted by the skirt 36 pressing against the rock body 14. When the rock body 14 comprises soft rock the edge can cut into and penetrate the rock body 14.

As the edge 42 presses against or cuts into the rock body 14, the movement of the anchor 32 along the first screw thread 22 will slow and require greater torque to be further i s tightened. Once the torque applied by the pneumatic wrench exceeds the torque at which the shear pin 50 is designed to fail, the nut 46 will be released from the bar 16 and this avoids further expansion of the edge 42 and tightening of the anchor 32. Subsequent rotation of the nut 46 will cause the nut to travel along the second screw thread 24 to tighten the bearing plate 48 against the rock face 52, thereby to tension the bar 16

20 between the anchor 32 and the bearing plate 48. This results in a stabilising compression force being applied onto the rock body 14.

It will be appreciated that the structural characteristics of the shear pin 50 can be selected to alter the shear strength thereof and, accordingly, the torque that will be required to cause the shear pin 50 to fail. This can be achieved, for example, by manufacturing the

25 shear pin 50 from specific materials or by changing the cross-sectional thickness of the shear pin 50. Having a stronger shear pin 50 will cause the anchor 32 to be more firmly embedded in place in the rock body 14 and will create a higher pre-tension within the bar 16 before the shear pin 50 fails. Conversely, a weaker shear pin 50 will cause the anchor 32 to be less firmly embedded in place in the rock body 14 and will create a lower

30 pre-tension within the bar 16 before the shear pin 50 fails. In this manner it is possible to select a desired strength of the shear pin 50 to limit the length of the bar second end 20 that will protrude beyond the nut 46 out from the hole 12 - i.e. after the shear pin 50 fails, the nut 46 will only need to undergo a few rotations before the desired stabilising compression force is applied onto the rock body 14, thereby limiting the extent of travel of the nut 46 along the second screw thread 24.

During operation, when an additional load is applied to the bearing plate 48, for example being caused by movement or fracturing of the rock body 14, this load will bear on the 5 bearing plate 48 and exert a force in a direction that tries to pull the anchor 32 out from the hole 12. Contrary to the prior art discussed above, it will be appreciated that in the exemplary embodiment the additional load imparted onto the bearing plate 48 will be transferred via the bar 16 to the anchor 32. Further, the sleeve 28 will either not move or will only experience slight movement as it is not joined to the bearing plate 48. As such 10 the additional load will cause the skirt 36 to be pulled further over the ramp 30 and result in the edge 42 cutting more deeply into the rock body 14 and thereby to increase the fixation of the anchor point and the strength of the anchor bolt 10.

Referring now to Figures 3 to 6 of the drawings, there is shown a second embodiment of an anchor bolt 60 that is configured for use as a friction bolt for insertion into a bore i s hole 62 formed in a rock body 64.

The anchor bolt 60 includes elongated load bearing element in the form of a hollow bar or tube 66 having a tube first end 68 and a tube second end 70. The tube first end 68 is arranged to be inserted into the hole 62 while the tube second end 70 is arranged to protrude from the hole 62. The tube 66 typically made of steel so that it has a suitably 20 high tensile strength, e.g. being in the order of 500 MPA to 600 MPA so that it is capable of supporting a load of 20 metric tonnes to 25 metric tonnes.

The tube 66 extends through a sleeve 72 and is in abutting contact therewith, but with the tube 66 being axially movable within the sleeve 72. In the exemplary embodiment the sleeve 72 is generally tubular along its length so that it concentrically surrounds the

25 tube 66. The sleeve 72 is shorter than the tube 66 so that the sleeve 72 is located

between the tube first end 68 and the tube second end 70. Both the tube 66 and the sleeve 72 have a slot 74, 75 extending along their axial length resulting in them being C- shaped in end view, i.e. both the tube 66 and the sleeve 72 are in the form of spilt tubes. It will be appreciated that the diameters of the tube 66 and the sleeve 72 at rest, as well

30 as their inherent resilience to compression, can be predetermined so that the anchor bolt 60 exhibits a selective friction coefficient whereby the tube 66 can slide relative to and be extracted from the sleeve 72. Although the embodiment of the anchor bolt 60 shown in the drawings has the slots 74, 75 being of equal or equivalent width, the slots 74, 75 in other embodiments of the anchor bolt 60 can have different widths. In one embodiment the slot 75 of the tube 66 will be narrower than the slot 74 of the sleeve 72, wherein 5 preferably the slot 75 will be sufficiently narrow that it will be substantially closed once the anchor bolt 60 is inserted into the hole 62 during use.

The sleeve 72 has a bevelled end wall 76 that tapers and narrows towards its proximal end nearest the tube first end 68 so that the bevelled end wall 76 defines a deflection arrangement in the form of a ramp 78. The angle of this ramp 78 is relatively shallow, e.g. 10 about 5°-15° with respect to a longitudinal axis of the sleeve 72.

The tube 66 is provided with an anchor 80 at the tube first end 68. The anchor 80 comprises a fixed section or collar 82 that is attached to an outer circumferential wall of the tube first end 68, e.g. by welding, so that the collar 82 is immovable relative to the tube 66. In an alternate embodiment the collar 82 can be integrally formed with the i s tube 66. The collar 82 leads into a depending skirt 84, which is preferably integrally

formed with the collar 82 but can also be welded thereto. The skirt 84 is malleable and separate from the tube 66 to enable the skirt 84 be deflected from the tube 66 and flare outwardly during use. In the exemplary embodiment the collar 82 is tapered to provide a conical formation in the tube first end 68 enabling easier insertion of the anchor bolt 60

20 into the hole 62.

The skirt 84 has a free edge 86 remote form the collar 82, with the edge 86 being inwardly bevelled in the form of a knife edge that can be smooth or serrated, to define a cutting edge able to cut into the rock body 64 as it flares outwardly in use. In the drawings the ramp 78 and the edge 86 are shown to be inclined at a similar angle but it should be 25 appreciated that the working of the invention will still be possible even if there is no close similarity, provided the edge 86 and skirt 84 are able to ride over and along the ramp 78.

As shown in the drawings, the sleeve 72 is spaced apart from the tube first end 68 so as to define an axial gap 87 between the sleeve 72 and the anchor 80, i.e. the edge 86 is axially spaced away from the ramp 78. This gap 87 functions as a yield distance that will 30 be further described in due course. Two or more slits 88 are cut axially through the skirt 84 from its edge 86 so that the skirt 84 is sectioned into separate deflectable tines 90. When the anchor 80 is in its initial pre-anchoring position, the edge 86 lies substantially adjacent to the tube 66, i.e. so that it has a circumference smaller than the circumference of the hole 62. However, the skirt 84 5 (particularly the tines 90) is adapted to flare outwardly during use as will be described in due course so that its edge 86 has a circumference larger than the circumference of the hole 62.

It will be appreciated that, as with the first embodiment, also the configuration of this second embodiment provides an expansion type anchor 80 that works in an opposite or 10 inverted manner from known prior art anchors, namely in the present embodiment the skirt 84 is directed towards the tube second end 70 and is not directed towards the tube first end 68.

The distal end of the tube 66 is joined to a washer 92 being arranged to press a bearing plate 94 against a rock face 96 of the rock body 64. In other embodiments, the bearing i s plate 94 can be directly welded to the tube 66.

In use, the hole 62 is first drilled into the rock body 14 to a desired depth. The anchor bolt 60 is then suitably inserted into the hole 12 until the bearing plate 94 abuts against the rock face 96. This insertion is normally performed by percussion hammering the anchor bolt 60 into the hole 62 so that the tube 66 and its sleeve 72 are circumferentially 20 compressed and held in place by a friction fit with the rock body 64. As such the anchor bolt 60 has the advantage of being of a simpler construction and easier to install than the anchor bolt 10, however it has the drawback of being able to withstand lower loads before failing.

Prior art friction bolts having a single split tube do not permit a full weld joint to be formed 25 to join the single split tube to their washer or bearing plate - i.e. no weld joint can be

formed across the axial slot in the single split tube - and this often provides a point of weakness at which the prior art split tubes can tear and fail. As described above, in the anchor bolt 60 of the present invention the slot 75 will be sufficiently narrow that it will be substantially closed once the anchor bolt 60 is inserted into the hole 62 during use.

30 Furthermore, when installed in use, a terminal section of the tube second end 70 remains protruding from the hole 62 - namely, that section extending through the bearing plate 94. Therefore, this terminal end of the tube second end 70 can be made completely closed because, as it does not enter the hole 62, it does not need to be compressible. The anchor bolt 60 thus permits the formation of a full weld joint to join the tube 66 to the washer 92.

5 The anchor bolt 60, comprising the concentrically arranged tube 66 and sleeve 72, is able to exert a greater outward radial pressure than the prior art friction bolts having a single split tube. In this regard, the sleeve 72 will exert a first outward radial pressure onto the rock body 64 (equivalent to that of the prior art friction bolts) but then the tube 66 will impart its own outward radial force onto the inside of the sleeve 72 and thereby increase 10 the outward radial force exerted by the sleeve 72 onto the rock body 64.

It will be appreciated that when an additional load is applied to the bearing plate 94, for example being caused by movement or fracturing of the rock body 64, this load will bear on the bearing plate 94 and exert a force in a direction that tries to pull the anchor 80 out from the hole 62. This additional load can be applied slowly, for example by squeezing i s ground, or the additional load can be applied quickly, for example by dynamic ground conditions caused by seismic activity. In the exemplary embodiment the additional load imparted onto the bearing plate 96 will be transferred via the tube 66 to the anchor 80. Further, the sleeve 72 will either not move or will only experience slight movement as it is not joined to the bearing plate 94. When the additional load overcomes the frictional

20 engagement existing between the tube 66 and the sleeve 72, it will cause the tube 66 to slide through the sleeve 72 (in a downward direction in the figures). This allows the anchor bolt 60 to yield thereby prolonging its ability to support the rock face. The sliding will thus occur relatively easily over the yield distance provided by the gap 87 between the edge 86 and the ramp 78.

25 The skilled addressee will appreciate that the extend of yielding to be permitted by the anchor bolt 60 can be varied by varying the size of the gap 87 between the edge 86 and the ramp 78, i.e. if more yielding is required, then the gap 87 size can be increased, and conversely if less yielding is required, then the gap 87 size can be reduced. In some embodiments, the gap 87 can be entirely eliminated by having the edge 86 initially abut

30 directly against the ramp 78 - in such case the operation of the second embodiment (anchor bolt 60) will be more closely aligned with the operation of the first embodiment (anchor bolt 10). Once the tube 66 has moved sufficiently through the sleeve 72 for the anchor 80 to traverse the gap 87 and the edge 86 contacts the ramp 78, further movement thereof will cause the edge 86 to be pulled over and ride up the ramp 78 and result in the skirt 84 (or the tines 90) being deflected away from the tube 66 and to flare outwardly. If the rock 5 body 64 comprises hard rock material, the edge 86 will not greatly penetrate the rock body 64 - but if the rock body 64 comprises soft rock material, the edge 86 can cut into and penetrate the rock body 64. Accordingly, at this stage there will no longer be any sliding yielding by the anchor bolt 60, rather the load bearing capacity of the anchor bolt 60 will be determined by the tensile strength of the tube 66. Thereafter the working of 10 this second embodiment will operate in a manner equivalent to the first embodiment.

Referring now to Figures 7 to 10 of the drawings, there is shown a third embodiment of an anchor bolt 100 that is configured for use as a friction bolt for insertion into a bore hole 102 formed in a rock body 104.

The anchor bolt 100 includes elongated load bearing element in the form of a hollow bar i s or tube 106 having a tube first end 108 and a tube second end 1 10. The tube first

end 108 is arranged to be inserted into the hole 102 while the tube second end 1 10 is arranged to protrude from the hole 102. The tube 106 is typically made of steel so that it has a suitably high tensile strength, e.g. being in the order of 500 MPA to 600 MPA capable of supporting a load in excess of 15 metric tonnes.

20 The tube 106 extends through a sleeve 112 and is in abutting contact therewith, but with the tube 106 being axially movable within the sleeve 112. In the exemplary embodiment the sleeve 1 12 is generally tubular along its length so that it concentrically surrounds the tube 106. The sleeve 112 is shorter than the tube 106 so that the sleeve 1 12 is located between the tube first end 108 and the tube second end 1 10.

25 Both the tube 106 and the sleeve 112 have a slot 1 14, 1 16 extending along their axial length resulting in them being C-shaped in end view, i.e. both the tube 106 and the sleeve 112 are in the form of spilt tubes. The tube 106 and sleeve 112 will normally be formed from metal strips which are folded to form the respective split tubes. However, they can also be formed in other known manners, such as a by extruding a pipe and

30 cutting through the side wall thereof or by simply extruding the desired C-shaped tube. It will be appreciated that the diameters of the tube 106 and the sleeve 1 12 at rest, as well as their inherent resilience to compression, can be predetermined so that the anchor bolt 100 exhibits a selective friction coefficient whereby the tube 106 can slide relative to and be extracted from the sleeve 112. Although the embodiment of the anchor bolt 100 5 shown in the drawings has the slots 1 14, 1 16 being of equal or equivalent width, the slots in other embodiments of the anchor bolt 100 can have different widths. In one

embodiment the slot 114 of the tube 106 will be narrower than the slot 116 of the sleeve 112, wherein preferably the slot 116 will be sufficiently narrow that it will be substantially closed once the anchor bolt 100 is inserted into the hole 102 during use. 10 The sleeve 112 is shown having its slot 1 16 rotationally offset from the slot 114 through about 90°. However, it is possible to rotate the sleeve 1 12 relative to the tube 106 so that the slots 114, 116 are aligned with each other (similar to as shown in Figure 4).

The sleeve 1 12 has a bevelled end wall 118 that tapers and narrows towards its proximal end nearest the tube first end 108 so that the bevelled end wall 118 defines a deflection i s arrangement in the form of a ramp 120. The angle of this ramp 120 is about 35°-55° with respect to a longitudinal axis of the sleeve 112. It can be seen in the drawings that the sleeve 112 is shown having a length approximately half that of the length of the tube 106, whereby the ramp 120 is located about midway along the length of the tube 106 and the opposed end of the sleeve 112 end near the tube second end 110. In other embodiments

20 the length of the sleeve 1 12 can be altered so that it is longer or shorter, i.e. respectively positioning the ramp 120 closer to or farther from the tube first end 108. The tube 106 has an outer surface 122 that is configured to contact against an inner surface 124 of the sleeve 112. In the exemplary embodiment of the anchor bolt 100, both the outer surface 122 and the inner surface 124 are smooth. However, in other embodiments,

25 either one of or both the outer surface 122 and the inner surface 124 can be roughed to increase the frictional contact between them. For example, both the outer surface 122 and the inner surface 124 can include knurling.

The tube 106 is provided with an anchor 126 at the tube first end 108. The anchor 126 comprises a substantially cylindrical spigot 128 that is attached internally within the tube 30 first end 108, e.g. by welding, so that the spigot 128 is immovable relative to the tube 106.

The spigot 128 has a screw thread 130 protruding beyond the tube first end 108. The anchor 126 further comprises a fixation member 132 threadably mounted onto the spigot 128. The fixation member 132 has an internally threaded collar 134 for attachment to the spigot 128. The fixation member 132 further has an expansion skirt 136 extending from the collar 134 and flaring outwardly as it extends farther away from the collar 134. Two or more slits 138 are cut axially through the skirt 136 from its free edge so that the skirt 136 is sectioned into separate tines 140. The drawings show two slits 138 arranged 5 perpendicularly to each other so that four congruent tines 140 are formed.

The skirt 136 forms a deflectable part and accordingly is malleable to enable the tines 140 be deflected and flare further outwardly during use. When the anchor 126 is in its initial pre-anchoring position, the skirt's edge 142 lies substantially adjacent to the tube 106, i.e. so that the outer circumference of the skirt 136 is the same as or slightly smaller than the 10 outer circumference of the sleeve 112. In one embodiment the outer dimeter of the

skirt 136 is smaller than the circumference of the hole 102, while the outer diameter of the sleeve 112 is larger than the circumference of the hole 102. However, the skirt 136 (particularly the tines 140) is adapted to flare outwardly during use so that its edge 142 has a circumference larger than the circumference of the hole 102. i s The skirt's edge 142 is inwardly bevelled in the form of a knife edge, that can be smooth or serrated, to define a cutting edge able to cut into the rock body 104 as it flares outwardly in use. In the drawings the ramp 120 and the edge 142 are shown to be inclined at a similar angle but it should be appreciated that the working of the invention will still be possible even if there is no close similarity, provided the edge 142 and skirt 136

20 are able to ride over and along the ramp 120.

In its initial pre-use position, the edge 142 is axially spaced quite a long way away from the ramp 120 to provide an axial gap 144 between the edge 142 and the ramp 120. In one embodiment the gap 144 extends for about half the length of the tube 106. The gap 144 functions as a yield distance that will be further described in due course.

25 A base member in the form of a ring 146 is attached to the distal tube second end 110.

The ring 146 has a substantially quarter round cross-section with a flat base 148. The ring 146 has a cylindrical bore 150 axially aligned with the tube 106 and being

dimensioned to snugly receive the tube 106. The ring 146 is positioned on the tube 106 near to but slightly spaced from the tube second end 110 in a manner that the tube

30 second end 110 projects through and beyond the base 148, e.g. by about 5-15mm and preferably by about 10 mm. The ring 146 is attached to the tube 106, e.g. by welding, with the weld joint 152 (seen in Figure 8) extending fully around the tube 106. As can be seen in the drawings, a bridging insert 154 is inserted into the slot 1 14 where it passes through the ring 146, with the insert 154 also being welded into position. The insert 154 serves as a bridge across the 5 slot 114 to enable a more secure weld joint 152 to be made and also improves the

structural integrity to reduce the possibility of the weld joint 152 breaking prior to and during use of the anchor bolt 100.

An annular seat 156 is formed in the ring 146 for receiving a distal end of the sleeve 1 12.

The ring 146 is arranged to press a bearing plate 158 against a rock face 160 of the rock 10 body 104.

In one embodiment it is envisaged that assembly of the anchor bolt 100 will occur by welding the ring 146 to the tube 106 near to the tube second end 1 10 and also welding the spigot 128 to the tube first end 108. At that stage, if needed, this partial assembly can be galvanised or otherwise surface treated in a known manner to reduce corrosion i s thereof. Also, the sleeve 122 and the fixation member 132 can be galvanised or surface treated in a similar manner. Thereafter the anchor bolt 100 can be further assembled by sliding the sleeve 122 onto the assembly by passing it over the tube 106 from the tube first end 108 and finally threadably attaching the fixation member 132 to the spigot 128.

In use, as shown in Figure 10A through 10D, the hole 102 is first drilled into the rock 20 body 104 to a desired depth (Figure 10A). The anchor bolt 100 is then suitably inserted into the hole 102 until the bearing plate 158 abuts against the rock face 160 (Figure 10B).

This insertion requires insertion force and this is normally performed by percussion hammering the anchor bolt 100 into the hole 102 so that the tube 106 and the sleeve 112 are circumferential ly compressed and held in place within the hole 102 by a friction fit with 25 the rock body 104. The diameter of the hole 102 will always be smaller than the outer diameter of the sleeve 1 16 so that the sleeve 1 16 can be frictionally held in place within the hole 1 12.

However, it should be appreciated that the outer diameter of the edge 142 of the skirt 136 can be selected so that it is either smaller than or larger than the diameter of the hole 102. When the diameter of the edge 142 of the skirt 136 is smaller than the diameter of the hole 102, the fixation member 132 and tube first end 108 can be inserted into the hole 102 in a relatively easy and unobstructed manner. Therefore the insertion force will only be required when the sleeve 1 12 enters into the hole102. Once the anchor bolt 100 is 5 inserted in this manner, the fixation member 132 does not provide any initial anchoring force because the skirt 132 does not engage with the rock body 104.

In other instances, when the diameter of the edge 142 of the skirt 136 is larger than the diameter of the hole 102, the use of insertion force will be required to insert the fixation member 132 and tube first end 108 into the hole 102. During such insertion, the skirt 136

10 will circumferentially compress around the tube first end 108 and such circumferential compression may also crush or compress the tube 106 tightly around the spigot 128 to close any open spaces between the tube 106 and the spigot 128, thereby providing a secure tight grip of the tube first end 108 on the spigot 128. Once the anchor bolt 100 is inserted in this manner, the fixation member 132 will provide some initial anchoring force i s because the skirt 132 frictionally engages with the rock body 104.

In the latter case, the amount of the initial anchoring force can be selected or adjusted by screwing the fixation member 132 further onto the spigot 128, i.e. towards the tube second end 108, prior to inserting the anchor bolt 100 into the hole 102. This will cause the tube first end 108 to enter inside the skirt 136 and subsequently deflect the tines 140 20 outwardly - this can be done until the edge 142 has reached a desired outer diameter.

The tube first end 108 can be chamfered or bevelled to assist entering thereof inside the skirt 136. The installation force will be sufficient to overcome the resistance caused by the anchor 126 and will deform the tines 140 and compress the tube first end 108 sufficiently until they are able to enter the hole 102.

25 The anchor bolt 100, comprising the concentrically arranged tube 106 and sleeve 112, is able to exert a greater outward radial pressure than prior art friction bolts only having a single split tube. In this regard, the sleeve 112 will exert a first outward radial pressure onto the rock body 104 (equivalent to that of the prior art friction bolts). Additionally, the tube 106 will impart its own outward radial force onto the inside of the sleeve 112 and

30 thereby increase the outward radial force exerted by the sleeve 1 12 onto the rock

body 104. It will be appreciated that during subsequent load bearing use, when an additional load is applied to the bearing plate 158, for example being caused by movement or fracturing in the rock body 104, this load will bear on the bearing plate 158 and exert a force in a direction that tries to pull the tube 106 and the anchor 126 out from the hole 62. In the 5 exemplary embodiment the additional load imparted onto the bearing plate 158 will be transferred via the tube 106 to the anchor 126. It will be appreciated that the sleeve 112 will not move (or will only experience very slight movement) as it is not joined to the bearing plate 158.

When the additional load overcomes the frictional engagement existing between the 10 tube 106 and the sleeve 112, the tube 106 will start to slip or slide through the sleeve 1 12 (Figure 10C). This slippage allows the anchor bolt 100 to yield thereby prolonging its ability to support the rock face 160, e.g. by retaining a surface support mesh. The amount of load force that is required to overcome the frictional engagement will be dependent on (a) the amount of compression that is imparted onto the tube 106 and sleeve 1 12 when i s inserted into the hole 102, or (b) the presence of and roughness of any knurling applied to the outer surface 122 and inner surface 124. It will be appreciated that the yielding will continue until the tube 106 is pulled through the sleeve 1 12 for a distance until the gap 144 is closed and the edge 142 of the skirt 136 comes into contact with the ramp 120. Accordingly, the desired extent of yielding to be permitted can be varied by changing the 20 size of the gap 144 between the edge 142 and the ramp 120, i.e. if more yielding is

required then the gap 144 can be enlarged whereas if less yielding is required then the gap 144 can be reduced.

Once the tube 106 has moved sufficiently through the sleeve 1 12 for the anchor 126 to traverse the gap 144 and the edge 142 contacts the ramp 120, further outward movement 25 of the tube 106 will cause the skirt 136 to be pulled over and ride up the ramp 120 and thereby cause the tines 140 to deflect and flare outwardly from the tube 106 so that the edge 142 bites into the rock body 104 (Figure 10D).

In situations when the material of the rock body 104 is harder than the material of the anchor bolt 100, as the skirt 136 initially rides up the ramp 120, the skirt 136 will 30 circumferential ly will crush or compress both the tube 106 and the sleeve 112 tightly

around the spigot 128 to close any open spaces between the skirt 136 and the spigot 128. Thereafter, when further compression ceases (i.e. the force required for compression exceeds the force needed for the tines 140 to bite into the rock body 104), the tines 140 will have nowhere else to go and will deflect outwardly to bite into the rock body 104.

Accordingly, at this stage there will no longer be any sliding yielding by the anchor bolt 100, rather the load bearing capacity of the anchor bolt 100 will be determined by the tensile strength of the tube 106. Thereafter the working of this third embodiment will operate in a manner equivalent to the first embodiment. The above described crushing / compressing of the tube 106 and sleeve 1 12 provides a cushioning effect while the load bearing support provided by the anchor bolt 100 transitions from the sliding yielding stage to the tensile strength anchoring stage. In one example of use, it is expected that during dynamic ground conditions when seismic activity occurs, the yield gap 144 will be close quickly within a few milliseconds (i.e. rather violently) wherafter the cushioning may assist in dissipating the seismic energy to protect against the bearing plate 158 from popping over/off the ring 146 and avoid destruction of any surface support mesh held in place by the anchor bolt 100.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Although all the embodiments described above are illustrated for use in a borehole drilled into a rock body, it should be appreciated that other uses and applications for the anchor bolt 10, 60, 100 are envisaged and thus the hole can be formed by any other means and in other structural bodies, for example, the hole can be a passage pre-formed within or drilled into a concrete slab.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.