Friction rock bolt

Abstract

A friction rock bolt assembly is arranged to frictionally engage an internal surface of a bore formed in rock strata. The rock bolt includes an expander mechanism having at least two radially outer wedge elements engageable by an inner wedge element. The expander mechanism is configured for symmetrical displacement of the expander elements to provide controlled enlargement by the rock bolt within the borehole for secure anchorage.

Claims

1. A friction bolt assembly arranged to frictionally engage an internal surface of a bore formed in rock strata, the assembly comprising: an elongate tube having a leading end, a trailing end and a longitudinally extending primary slot; an expander mechanism located within the tube towards the leading end and configured to apply a radial expansion force to the tube to secure the assembly to the rock strata; and an elongate tendon extending longitudinally within the tube and connected at or towards a first end to the expander mechanism and at or towards a second end to a loading mechanism positioned at or towards the trailing end of the tube that by adjustment is configured to create tension in the tendon to act on the expander mechanism and provide the radial expansion force, the expander mechanism including at least two radially outer wedge elements positionally secured to the tube and a radially inner wedge element secured to the tendon arranged for axial movement relative to the outer wedge elements to apply the radial expansion force to the outer wedge elements, wherein the elongate tube includes at least one secondary slot positioned axially at the expander mechanism and extending beyond the expander mechanism in both the axial forward and rearward directions, the tube being arranged to deform radially at an axial position of the expander mechanism via the primary slot and the at least one secondary slot in response to axial movement of the inner wedge element and the expansion force transmitted by the outer wedge elements.

2. The assembly as claimed in claim 1, wherein the outer wedge elements each have a radially inward facing surface that is oblique relative to a longitudinal axis extending through the assembly and wherein a radially outward facing surface of the inner wedge element extends oblique relative to the longitudinal axis.

3. The assembly as claimed in claim 2, wherein the radially inward facing surface of the outer wedge elements and/or the radially outward facing surface of the inner wedge element are generally planar or are at least part conical.

4. The assembly as claimed in claim 1, wherein the secondary slot is positioned diametrically opposed to the primary slot.

5. The assembly as claimed in claim 1, wherein an axial length of the secondary slot is less than an axial length of the primary slot.

6. The assembly as claimed in claim 5, wherein the axial length of the secondary slot is 0.5 to 40% of a total axial length of the elongate tube.

7. The assembly as claimed in claim 1, wherein the secondary slot has a width being less than a width of the primary slot.

8. The assembly as claimed in claim 1, wherein the outer wedge elements are spaced apart in a circumferential direction by an equal separation distance.

9. The assembly as claimed in claim 1, wherein in a circumferential direction, the outer wedge elements are positioned between and do not overlap with the primary and secondary slots.

10. The assembly as claimed in claim 1, wherein the outer wedge elements are secured to the tube by a weld.

11. The assembly as claimed in claim 1, wherein at least a portion of each of the outer wedge elements extends axially beyond the leading end of the tube.

12. The assembly as claimed in claim 11, wherein a maximum outside diameter of the inner wedge element is greater than an inside diameter of the tube.

13. The assembly as claimed in claim 11, wherein a maximum outside diameter of the inner wedge element is approximately equal to an outside diameter of the tube.

14. The assembly as claimed in claim 1, wherein at least a portion of the radially inner wedge element extends axially beyond the leading end of the tube.

15. The assembly as claimed in claim 1, wherein the tendon is an elongate bar that is radially enlarged at or towards the first end.

16. The assembly as claimed in claim 15, wherein the first end of the bar comprise threads, the threads provided at the radially enlarged first end.

17. The assembly as claimed in claim 16, wherein the inner wedge element is mounted on the bar via the threads.

18. The assembly as claimed in claim 1, comprising a single primary slot, a single secondary slot and two outer wedge elements positioned diametrically opposite one another and spaced apart in a circumferential direction between the primary and secondary slots.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) A specific implementation of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

(2) FIG. 1 is a cross-sectional view of a friction rock bolt according to an aspect of the present invention.

(3) FIG. 2 is a cross-sectional view through AA of FIG. 1.

(4) FIG. 2A is a modified version of FIG. 2 showing an alternative expander mechanism.

(5) FIG. 3 is a cross-sectional view of the leading end of a friction rock bolt according to another aspect of the present invention.

(6) FIG. 4 is a cross-sectional view through BB of FIG. 1.

(7) FIG. 5 is a cross-sectional view of the trailing end of a friction rock bolt according to another aspect of the present invention;

(8) FIG. 6 is a cross sectional view of an axially forward region of friction rock bolt according to a further aspect of the present invention;

(9) FIG. 7 is a cross sectional view of a friction rock bolt according to a further aspect of the present invention;

(10) FIG. 8 is a cross sectional view of the trailing end of a friction rock bolt according to a further aspect of the present invention;

(11) FIG. 9 is a cross sectional view of the trailing end of a friction rock bolt according to a further aspect of the present invention;

(12) FIG. 10 is a cross sectional view of the trailing end of a friction rock bolt according to a further aspect of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION

(13) FIG. 1 is a cross-sectional view of a friction rock bolt 10 according to one embodiment of the invention. The rock bolt 10 includes an elongate generally cylindrical tube 11 (having a circular cross section) with a leading end 12 and a trailing end 13. The length of a typical rock bolt be can in the range of about 1 m to about 5 m.

(14) The tube 11 is split longitudinally along its full length via a primary slot 26 so that it can be expanded radially for improved frictional engagement with the inside surface 14 of a bore which is drilled into a body of rock or a rock strata.

(15) For the purpose of expanding the tube 11 radially, or to increase the frictional contact between the outer surface of the tube 11 and the surface 14 of the bore with or without radial expansion, the rock bolt 10 includes an expander mechanism 15 within the tube 11 and disposed at or towards the leading end 12 of the tube 11. The expander mechanism 15 includes a pair of first wedge like expander elements 16 and 17 that are secured to the inside surface 18 of the tube 11. FIG. 2 also shows this arrangement and in that figure, it is clear that the expander elements 16 and 17 are secured to the inside surface 18 of the tube in positions that are diametrically opposite each other.

(16) The expander mechanism 15 further includes an engagement structure 20 in the form of a radially inner wedge element that is secured to a tendon on the form of an elongate bar 21 (which could alternatively be a cable), and is positioned at the leading end of the bar 21 and for cooperation or engagement with the respective radially outer expander (wedge) elements 16 and 17.

(17) It can be seen from FIG. 1, each of the generally wedge-shaped expander elements 16, 17 comprise a radially inward facing surface 22 that is aligned oblique to a longitudinal axis 67 of the rock bolt 10 so as to be generally tapered. Similarly, the radially inner wedge element 20 comprises a radially outward facing surface 23 that is also aligned oblique to longitudinal axis 67 and parallel to outward facing surface 22 of the outer wedge elements 16, 17. Such an arrangement enables the inner wedge element 20 to slide in frictional contact with outer wedge elements 16, 17 as the elongate bar 21 is actuated and the inner wedge element 20 moved axially relative to the stationary outer wedge elements 16, 17. The complementary aligned surfaces 22, 23 are advantageous to facilitate maximum symmetrical expansion of the expander mechanism 15 and avoid galling of regions of the surfaces 22, 23. In particular, it will be evident from FIG. 1, that as the inner wedge element 20 moves in a direction away from the blind end 25 of the bore, the relative movement and engagement that occurs between the outer elements 16 and 17 and the inner element 20 will tend to cause the tube 11 to expand radially and force the tube 11 into greater frictional contact with the surface 14 of the bore. That radial expansion is facilitated by slot 26 (formed longitudinally of the tube 11 as shown in FIG. 2).

(18) Expander elements 16 and 17 may be secured against the inside surface 18 of the tube 11 in any suitable manner and preferably are secured by weld 68. Likewise, the inner element 20 can be secured to the bar 21 in any suitable manner. In FIG. 1, the leading end 27 of the bar 21 is threaded to threadably engage a threaded bore 28 formed in element 20.

(19) The leading end 12 of the tube 11 is tapered to facilitate insertion of the rock bolt 10 into a bore drilled into a rock strata. FIG. 1 shows a slot or slit 29 formed in the leading end 12 to allow the leading end 12 to compress radially if necessary for insertion into the bore. In practice, there could be two slots 29 formed diametrically opposite each other for this purpose, or three slots at 120° to each other, or four slots at 90° etc.

(20) The expander mechanism 15 is shown in FIG. 1 in an actuated or activated state, in which the inner wedge element 20 has been shifted relative to the outer wedges 16 and 17 to cause an expansion load to be applied to the tube 11. However, when the rock bolt 10 is to be inserted into the bore, the inner wedge element 20 would be in a position in which it would be further towards the leading end 12 of the tube 11. The intention would be that wedge element 20 would be positioned so that the expander mechanism 15 is not imposing an expansion load on the tube 11. Indeed, it is preferred that inner wedge element 20 be positioned such that the tube 11 can radially compress or contract as the bolt 10 is inserted into a bore by the bore being drilled to a diameter which is slightly smaller than the outside diameter of the main portion of the tube 11. This naturally allows the tube 11 to compress or contract radially as the bolt 10 is forced into the bore and thus allows the outside surface of the tube 11 to frictionally engage the inside surface 14 of the bore so that once the rock bolt 10 is fully inserted into the bore, there will already be a frictional engagement between the tube and the inside surface of the bore.

(21) Once the bolt 10 has been fully inserted into the bore, the expander mechanism 15 can be activated, to impose a radial expansion load on the tube 11 and so to increase the frictional engagement between the tube 11 and the inside surface 14 of the bore. As indicated, activation of the expansion mechanism 15 causes wedge element 20 to shift (relative to the stationary elements 16 and 17) in a direction away from the blind end 25 of the bore. This movement may be achieved either by pulling the bar 21 in a direction away from the blind end 25, or by rotating the bar 21 so that by the threaded engagement between wedge element 20 and the bar 21, wedge element 20 is drawn in a direction away from the blind end 25. Rock bolt 10 comprises a nut 30 located at a trailing end 69 of bar 21 to represent a head of the bar 21 and to be configured to brace against the trailing end of tube 11 either directly or indirectly via an axially intermediate washer 48. Nut 30 may be formed integrally (i.e., fixed) at the end 69 of the bar 21. Alternatively, nut 30 may be threadably connected to the end 69 of the bar 21. In that latter arrangement, inner wedge element 20 would shift relative to the elements 16 and 17 with movement of the bar 21 as opposed to the arrangement where the bar 21 rotates and the inner wedge element 20 shifts relative to the bar due to the threaded engagement between the bar 21 and wedge element 20.

(22) In another alternative, the nut can be a blind nut with an internally threaded bore, so that the nut 30 can be threaded onto the threaded free end of the bar 21 to the point at which the blind end of the threaded opening engages the end of the bar, at which point no further threaded movement can take place. Further rotation of the nut then will cause rotation of the bar 21.

(23) The expander mechanism 15, comprising a pair of expander elements 16 and 17 contrasts with earlier arrangements in which only a single wedge element is provided at the tube internal surface. In those arrangements, a wedge element that has been fixed to the bar or cable interacts with the single wedge element that is fixed to the tube, but the expansion available in the arrangements employing a single wedge element is less than that available in the arrangement of the present invention. Thus, by the provision of a pair of expander elements 16 and 17, which are in diametrically opposed positions against the inside surface of the tube 11, there can be an increased level of expansion of the tube 11. In prior art arrangements, the maximum expansion of a tube is in the region of 52 mm, whereas in the new arrangement illustrated in FIG. 1, the expansion can be up to 56 mm. While this increase is only relatively small, the benefits it provides can be significant. For example, in very weak rock where the bore diameter is over drilled, the maximum expansion of prior art bolts might not be sufficient to frictionally engage the bore surface with sufficient force to properly fix the bolt within the bore. However, the extra expansion facilitated in a rock bolt according to the present invention enables greater expansion and thus means it is more likely that a rock bolt expanded in weak rock will be able to sufficiently engage the bore surface to properly anchor the bolt within the bore.

(24) The arrangement of the expander elements 16 and 17 as being diametrically opposed within the tube 11 is further advantageous to ensure that there is no misalignment between the elements 16 and 17 as the expander mechanism is initially activated and under subsequent loading through failure or movement in the rock strata. Where misalignment occurs this can develop torsional loading that could negatively affect the weld connection of the elements 16 and 17 to the inside surface 18 of the tube 11. Moreover, misalignment between the elements 16 and 17 and the structure 20 can result in reduced surface engagement between the respective components which could affect the proper expansion of the expander mechanism 15.

(25) To improve the likelihood of complete alignment between the inner and outer elements 20, 16, 17, a secondary (further) slot or slit 51 is provided opposite the primary tube slot 26 to facilitate symmetric tube expansion as the expander mechanism 15 expands as shown in FIGS. 1 and 2. As illustrated in FIGS. 1 and 2, secondary slot 51 comprises different dimensions to primary slot 26 and for example, includes a width and a length that are less than those of primary slot 26. In particular, slot 51 may comprises a width of about 5 mm and a length of about 200 mm. Such a further slot or slit 51 can also be provided in the FIG. 3 arrangement.

(26) With reference to FIG. 3, an alternative expander mechanism 35 is illustrated which includes a pair of outer wedge elements 36 and 37 that are welded to the free end 38 of the rock bolt tube 39. The elements 36 and 37 are welded via the annular weld 40 to the free end 38 of the tube 39 and therefore the elements 36 and 37 are not only present within the tube 39, but extend out of the tube 39. An engagement structure (inner wedge element) 41 is threadably attached to the threaded end 42 of the bar 43 and relative movement of the inner wedge element 41 relative to the outer (stationary) elements 36 and 37 can be as described in relation to the embodiment of FIGS. 1 and 2 (referring to elements 20, 16 and 17. The arrangement of FIG. 3 facilitates even greater expansion of the tube 39 compared to the tube 11 of FIGS. 1 and 2 because the diameter of the inner wedge element 35 can be greater than the diameter of the wedge 20 of the FIG. 1 embodiment. In particular, inner wedge element 35 is generally frusto-conical along some, most or all of its axial length (consistent with the FIG. 1 embodiment). The inner wedge element 35 may comprise a maximum diameter (at its thickest axial leading end) that is greater than an insider diameter of tube 11 (as defined by tube internal facing surface 18) with the tube compressed and squeezed into the as-formed bore hole 14, in contact with bore surface 14. Moreover, the maximum diameter of inner wedge element 35 is approximately equal to an outside diameter of tube 11 (as defined by tube external surface 71). Such an arrangement is beneficial to strengthen the inner wedge element 35 against compressive stress encounter during use and imparted by bar 21. Additionally, the arrangement of FIG. 3 is expected to gain a further 5 to 6 mm of tube expansion. Slots (not shown) are provided in the tube 39 to extend through the free end 38 facilitate that expansion and are to be considered consistent with the secondary slot 51 of the embodiment of FIGS. 1 and 2.

(27) In other respects, the arrangement of FIG. 3 is the same as FIG. 1, except that it will be apparent that the leading end of the tube 39 is not tapered in the manner shown in FIG. 1 as the tube 39 is required to remain of constant diameter to facilitate attachment of the elements 36 and 37 to the free end 38 of the tube 39.

(28) While the figures show a pair of expander elements 16, 17 and 36, 37, the invention covers arrangements in which an arrangement of three expander elements is provided, or there could more expander elements. These expander elements can be wedge elements of the kind shown in the figures and they can all be fixed to the tube by welding. One or two of the expander elements can be welded in such a position that it or they would extend into or over, or even to substantially cover the longitudinal slot (longitudinal slot 26 as shown in the figures) of the tube. FIG. 2A illustrates a tube 11a having a primary longitudinal slot 26a and a pair of secondary slots 51a. An engagement structure (inner wedge element) 20a cooperates with three outer wedge elements 44, two of which extend into or at least partially over the longitudinal slot 26a. The slots 51a have the same purpose as the slot 51 described earlier, however because there are three expander elements 44, two slots 51a are required.

(29) The arrangement as illustrated in FIG. 2A can advantageously act to prevent the engagement structure attached to the tendon from being dislodged out of the tube by significant impact loading, such as might happen during insertion of the rock bolt into a bore. For example, the rock bolt can be subject to significant impact loading during manoeuvring of the installation machine where the leading end of the bolt might strike the rock surface with a relatively large lateral force. By placing the expander elements in such a position that they extend into or over the longitudinal slot, the engagement structure is less likely to, or will actually be prevented from egress out of the tube during a significant impact event.

(30) Returning to FIG. 1, at the trailing end 13 of the tube 11, a rock plate 45 is shown bearing against the rock face 46. The plate 45 as illustrated is not reflective of the shape of plate that would actually be used in the field, but it is sufficient for the purposes of this description. The plate 45 bears against the rock face 46 and against a ring 47 which is welded to the outside surface of the tube 11. A plate or washer 48 is positioned axially between nut 30 and an axially rearwardmost free end 49 of tube 11. Importantly, a gap G is provided between ring 47 and washer 48. FIG. 4 is a cross-section through B-B of FIG. 1 and shows spot welds 50 for securing ring 47 to an external surface 11a of tube 11. In particular, four spot welds 50 are provided.

(31) The arrangement described above at the trailing end 13 of the tube 11 is a loading mechanism 70 (alternatively termed a support arrangement) for supporting loading that is imposed on the rock bolt 10 by movement or failure in the rock strata and in particular, provides a multi-stage load support. In a first stage, load support is provided by ring 47, whilst in a second stage, rock support is provided by the washer 48 and the nut 30. The operation of the multi-stage loading mechanism 70 is as follows. With the rock bolt 10 inserted within a bore and the expansion mechanism 15 expanded, if a load is applied to the rock bolt (normally a dynamic load), then the first stage of support is provided by loading mechanism 70 between the rock plate 45 and the ring 47. In the event that the load which is applied to the rock bolt exceeds the shear strength of the spot welds 50, then those welds will fail and the ring 47 will shift to take up the gap G and to bear against the washer 48. The first stage of load support thus is provided up to the point at which the spot welds 50 fail. Upon failure of the spot welds 50, the load which is applied to the rock bolt 10 will shift to the washer 48 and the nut 30, so that the load will be reacted by the bar 21 to which the washer 48 and the nut 30 are connected. That load will tend to shift the bar away from the blind end 25 of the bore and thus will cause a shift of inner wedge element 20 relative to the outer elements 16 and 17 of expander mechanism 15. This will have the effect that there will be a greater expansion load applied by the expander mechanism 15 to even more firmly force the tube 11 into frictional engagement with the inside surface 14 of the bore and by that increased frictional engagement, the load applied to the rock bolt 10 will be supported up to the point at which the bar 21 itself fails. In addition, the tube 11 will be prevented from movement relative to the surface 14 of the bore (other than very minor movement) by the increased frictional engagement between the tube 11 and the bore wall as the expander mechanism 15 operates to increase the frictional engagement load. The rock bolt 10 is thus restrained against movement within the rock strata, or is restrained with acceptable levels of movement.

(32) As explained above, the increased expansion available with the expander mechanisms 15 and 35 facilitates improved load support where loads of the above described kinds occur in weak rock. Thus in weak rock, if a dynamic load occurred of a magnitude that caused the spot welds 50 to shear, there is an improved likelihood of the rock bolt absorbing the dynamic load where the ability of the rock bolt to expand radially is greater.

(33) The multi-stage (two stage) load support arrangement discussed above is important and advantageous for the following reasons. When a rock bolt is subject to a significant initial load, such as in seismic rock conditions, the sudden dynamic loading can be greater than the tensile strength of the bar or cable which would typically be expected to absorb the load. For example, when the rock kinetic energy is at a level of about 25 kJ, the impact load may exceed 45 t. However, the tensile strength of bars typically used in rock bolts is not more than 33 t so in such conditions, the bar would break. This obviously could compromise the support role that the rock bolt is intended to have. However, by providing a multi-stage load support arrangement, the initial load can be partly absorbed by the ring 47 up to the point of shear which would occur in the region of 2-10 t. Some of the initial load energy is thus absorbed by the ring up to the point of shearing and thereafter, the load energy is transferred via the washer 48 and nut 30 to the bar 21. By absorbing 2-10 t of the overall load energy initially, the energy which is transferred to the washer and nut is significantly reduced and is then likely to be of a magnitude which will develop a tensile load that is less than the tensile strength of the bar. In the illustrated embodiment, the gap G is important, because it allows the spot welds 50 to shear. If the gap G was not provided, and the ring 47 rested against the washer 48, there would be no first stage of load absorption. The gap G between the ring 47 and the washer 48 is optimally between 5-8 mm. According to some installations procedures this allows for some ‘mushrooming’ of the trailing end of the tube during impact (hammering) installation, which typically is about 2 mm, but does not leave the gap G too large to allow excessive rock displacement as the ring 47 shears. A rock bolt according to the figures is thus expected to provide greater reliability of rock support, particularly in seismic rock conditions or in weak rock.

(34) The multi-stage load support arrangement of FIG. 1 represents just one form of arrangement which provides the support required. In alternative arrangements, multiple load absorbers (optionally in the form of rings 47) could be provided at the rearward tube end 13 to provide further stages of load support or energy absorption. Each of the multiple load absorbers (e.g., rings 47) could be spaced apart sufficient to allow successive energy absorption (e.g., by a shear of the welds 50). The minimum number of load absorbers is one and may comprises one or two rings, while any number of rings beyond two could be provided as required.

(35) A further alternative load absorber is a compressible element and such an arrangement is shown in FIG. 5. In FIG. 5, the same components that have been included in FIG. 1 are given the same reference numerals. Thus, FIG. 5 illustrates a rock bolt tube 11, a bar 21, a nut 30, a rock plate 45 and a washer 48. However, FIG. 5 also illustrates a compressible cylindrical collar 55 which extends axially between the rock plate 45 and the washer 48. The rock plate 45 bears against bearing surface 56 of the collar 55, while the washer 48 bears against bearing surface 57. Between the bearing surfaces 56 and 57 is a neck 58 and it can be seen in FIG. 5, that the outside diameter of the neck 58 is reduced compared to the outside diameters of the collar 55 at the bearing surfaces 56 and 57.

(36) The compressible collar 55 is intended to compress, crush or crumple at a particular load applied to it by the rock plate 45. That load could be the same load that causes the spot welds 50 of the rock bolt 10 to fail or it could be a greater or lower load to cause failure. Regardless, upon the load being sufficient to cause the element 55 to fail, collar 55 will fail by the neck 58 crushing or crumpling. Once the collar 55 has failed to the maximum it can, the load energy that has not already been absorbed by failure of the collar 55 is transferred to the washer 48. Thus, the load energy that is transferred to the washer 48 is reduced compared to the load energy that the collar 55 was exposed to initially. Upon that transfer, the second stage of load support is the same as explained in relation to the rock bolt 10 when the ring 47 shears and engages the washer 48.

(37) FIG. 6 illustrates a further embodiment of the present rock bolt in which elongate bar 21 is radially enlarged at its leading end 27. In particular, bar 21 may be divided axially so as to comprise a main length section 21e having external ribs. Bar 21 then transitions to a generally smooth or unribbed region 21a A radially enlarged section 21b extends axially from section 21a and comprises threads, as described with reference to FIGS. 1 and 3 to mount the radially inner element 20 (in a form of a conical wedge). As described, wedge 20 comprises an internal bore having corresponding threads to mate with the threads on radially expanded section 21b. Such an arrangement is advantageous to strengthen rod 21 at the leading end 27 against tensile forces imposed on bar 21 during use. Preferably, the threads on end section 21b are not typical metric threads and are preferably rounded or rope style threads to minimise the creation of stress concentrations that would otherwise weaken the bar 21 at leading end 27.

(38) FIGS. 7 to 9 illustrate further embodiments of the axially rearward loading mechanism of the present rock bolt. Referring to FIG. 7 and in a further implementation, the loading mechanism, alternatively referred to herein as a load support arrangement, comprises washer 48 positioned axially intermediate rock plate 45 and nut 30. Washer 45 comprises an axially forward facing abutment surface 48a that also extends radially outward beyond a radially outward facing external surface 71 of tube 11 at the tube rearward end 13. Abutment surface 48a is annular and is configured to engage, in a butting contact, a radially inner region of rock plate 45 such that loading forces imposed on rock plate 45 by the rock face 46 are transmitted into washer 48 that is axially spaced from nut 30 by a gap region G. A conical compressible collar 62 is mounted within the gap region G. Collar 62 comprises an axially forward end 62a (in contact with an axially rearward facing face 48b of washer 48) and an axially rearward end 62b (in contact with an axially forward facing face 30a of nut 30).

(39) Collar 62 may be formed from the same material as compressible collar 55 as described referring to FIG. 5 such that collar 62 is capable of compressing via deformation as washer 48 is forced axially rearward by loading forces imposed on rock plate 45 (and hence washer 48) due to movement of the rock surface 46. Collar 62 is dimensioned such that a maximum diameter does not exceed an external diameter of nut 30 such that collar 62 does not extend radially beyond the nut 30. Such an arrangement is advantageous to provide a radially accessible region around nut 30 and collar 62 to receive an axially forward end 60 of a hammer tool used to deliver and force the rock bolt 10 into the bore during initial installation. In particular, the axially forward end of hammer tool 60 is configured for placement in direct contact against the rearward facing surface 48b of washer 48 such that the compressive forces delivered to the rock bolt 10 via the tool 60 are transmitted directly through washer 48 and into tube 11 importantly without being transmitted through nut 30 and compressible collar 62. Such an arrangement is advantageous to avoid unintended and undesirable initial compression of collar 62 due to the hammer driven compressive forces by which rock bolt 10 is driven into the borehole. The further embodiments of FIGS. 8 and 9 are also configured for avoiding a compressive force transmission pathway through the load absorber component (in the form of a compressible washer, gasket, seal, flange etc. as described herein). Accordingly, in some embodiments, preferably washer 48 extends radially outward beyond tube 11, nut 30 and the load absorber, so as to present an accessible rearward facing surface 48b for contact by the leading end of the hammer tool 60.

(40) A further embodiment of the loading mechanism is described referring to FIG. 8 in which flange 48 comprises corresponding surfaces 48a, 48b. However, differing from the embodiment of FIG. 7, a radially inner section 63 of washer 48 is dome-shaped so as to curve in the axial direction towards nut 30 (secured at the rearward end of bar 21). Dome section 63 occupies the gap region G between the main body of washer 48 and nut 30. Accordingly, as load from the rock strata surface 46 is transmitted into rock plate 45 and accordingly into washer 48 via surface 48a, dome section 63 is configured to compress such that the washer 48 flattens to reduce gap G.

(41) FIG. 9 illustrates a further embodiment of the rock bolt of FIG. 7 in which the conical collar 62 is formed as a generally cylindrical deformable collar 64. As with the embodiment of FIG. 7, collar 64 is dimensioned so as to not extend radially outward beyond nut 30 to provide access to the washer surface 48b by the hammer tool 60 and accordingly avoid compressive force transmission through collar 64 during initial hammering of the rock bolt 10 into the borehole as described.

(42) FIG. 10 illustrates a further embodiment of the rock bolt 10 corresponding to the arrangement of FIG. 6 having a radially enlarged section of bar 21. As illustrated in FIG. 10, bar 21 at an axially rearward region of main length section 21e comprises a non-ribbed generally smooth section 21d. A radially enlarged section 21c extends from the rearward end of smooth section 21d and comprises threads to mate with corresponding threads formed on a radially inward facing surface (not shown) of nut 30 so as to secure nut 32 to bar 21. As described referring to FIG. 6, the enlarged section 21c provides reinforcement of the bar 21 against tensile forces encountered during use with the thread configuration at section 21c being preferably the same as described at section 21b.

(43) The expander mechanism as described herein comprising at least two radially outer expander elements 16, 17, 44 is advantageous to maximise the radial expansion force imposed by the axially rearward movement of the inner wedge element 20. As indicated, in contrast to existing rock bolt configurations having a single outer wedging element, the present configuration provides a greater maximum radial expansion (combined radial movement of wedging elements 16, 17, 44) relative to the corresponding maximum radial displacement achievable by a single outer wedging element.

(44) Additionally, the present arrangement, via the plurality of outer wedging elements 16, 17, 44 provides a desired symmetrical tube expansion. This is achieved, in part, via the circumferential spacing between the wedging elements 16, 17, 44, the provision of a secondary elongate slot 51 and the oblique alignment of the inward and outward facing surfaces of the respective outer and inner wedging elements 16, 17, 44 and 20, 20a. The controlled interaction between and parallel alignment of the mating surfaces 22, 23 (of the wedging elements 16, 17, 44, 20, 20a) is beneficial to avoid development of sideways (torsional) forces at the region of the expander mechanism 15, 35 that i) would reduce the desired frictional contact, ii) lead to possible development of galling of the wedging elements 16, 17, 44, 20, 20a and iii) reduce the performance in the clamping action of the expander mechanism 15, 35. Additionally, and as will be appreciated, the provision of a secondary slot 51 in addition to the primary slot 26 reduces the magnitude of force absorbed by the tube 11 as the expander mechanism 15, 35 is expanded which, in turn, maximises the efficiency and effectiveness of the expansion mechanism 15, 35 to deform tube 11 into tight frictional contact with the surrounding rock strata.

(45) As will be appreciated, the present rock bolt may comprise a plurality of secondary elongate slots 51 with each slot 51 spaced apart in a circumferential direction around the central longitudinal axis 67 of rock bolt 10. Similarly, the present rock bolt 10 may comprise a plurality of outer wedging elements 16, 17, 44 (optionally including 2, 3, 4, 5, 6, 7 or 8 separate elements) each spaced apart in a circumferential direction around axis 67. Preferably, to facilitate radial expansion of tube 11 via the slots 51, wedging elements 16, 17, 44 are secured to tube 11 at locations between the slots 26 and 51 and do not bridge or otherwise obstruct slots 51.

(46) The embodiments illustrated in the figures discussed above are expected advantageously to allow for more reliable and secure rock strata support under loading, such as seismic loading or loading due to ground swelling. Failure of a bar or cable (for example due to the bar or cable being effectively ‘pulled-through’ the outer wedges) of a rock bolt according to the invention is expected to be less likely while the greater radial expansion provided in a rock bolt according to the invention is expected to provide more secure anchoring of a rock bolt within a bore.