AU2013204292B2 - Rock bolt assembly - Google Patents

Abstract

A rock bolt assembly (100) for installation in a bore hole (10) extending through a rock face (11). The rock bolt assembly (100) comprises an elongate load bearing member (110) longitudinally extending between a leading end (111) and a trailing end (112). A head (120) is mounted on the load bearing member (110) towards the trailing end (112). The head (120) is adapted to transfer tensile load, in use, from the load bearing member (110) to the rock face (11) surrounding the bore hole (10) via a plate washer (20). An indicator element (130) is mounted on the load bearing member (110) between the head and the leading end (111). The indicator element (130) is adapted to transfer the load, in use, from the head (120) to the plate washer (20) and to structurally fail if and when the load reaches a predetermined load, so as to provide a visual indication of the load in the load bearing member (110) reaching the predetermined load. The predetermined load is a load lower than a failure load of the head (120) and a failure load of the load bearing member (110). 4j159 toIt N4N \N'Zo \N -5\

Description

1 2013204292 12 Apr 2013

ROCK BOLT ASSEMBLY

FIELD OF THE INVENTION

[0001] The present invention relates to strata control in civil engineering and mining operations, and in particular relates to a rock bolt assembly for securing the roof or a mine, tunnel or other ground excavation.

BACKGROUND OF THE INVENTION

[0002] Rock bolts are commonly utilised to secure the roof or walls of underground mines, tunnels and other ground excavations. Rock bolts comprise elongate load bearing members that may be in the form of a rigid bar or a flexible cable formed of multiple wires wound around a central wire. The flexible cable form of rock bolts are known as cable bolts. In one form of installation, a rock bolt is fixed into a bore hole drilled into a rock face by a mechanical or chemical anchor, creating a point anchor at the leading (upper) end of the rock bolt. The rock bolt is then typically pre-tensioned, by any of various means. In some applications, the annular cavity formed between the rock bolt and the bore hole wall is then filled with grout, which is typically cementitious in form, encapsulating the rock bolt along its length.

[0003] For mechanical anchor point anchoring, the leading end of the rock bolt is typically inserted into the bore hole and point anchored into the upper end of the bore hole by way of an expansion shell mechanical anchor. For rigid rock bolts, the mechanical anchor is typically threaded onto the leading end of the rigid bar. For cable bolts, the mechanical anchor is attached to the leading end of the cable by any of various means. Chemical anchors are equally applicable to both rigid rock bolts and cable bolts. Chemical anchors generally comprise a two-component resin filled cartridge having a frangible casing. The resin filled cartridge is inserted into the bore hole in front of the rock bolt. The rock bolt is rotated as it is thrust towards the top of the bore hole, thereby puncturing the frangible casing of the resin filled cartridge, and mixing the resin. The resin is allowed to set and thus point anchor the leading region of the rock bolt within the bore hole.

[0004] When using either a chemical or mechanical anchor, once the point anchor is in place, rigid rock bolts are typically tensioned by way of a threaded end fitting threadingly mounted onto a threaded trailing region of the rock bolt. The nut, or an intervening dome washer, bears against a plate washer which in turn engages the face of the rock surrounding the bore hole.

73310881vl:PRW 2 2013204292 11 Oct 2016

Cable bolts are typically tensioned by way of a barrel and wedge assembly that bears against the plate washer, with an hydraulic tensioning unit gripping the trailing end of the cable and pulling the cable through the barrel and wedge assembly.

SUMMARY OF INVENTION

[0005] In a first aspect, the present invention provides a rock bolt assembly for installation in a bore hole extending through a rock face, said rock bolt assembly comprising: an elongate load bearing member longitudinally extending between a leading end and a trailing end; a head mounted on said load bearing member towards said trailing end, said head being adapted to transfer tensile load, in use, from said load bearing member to the rock face surrounding the bore hole via a plate washer; an indicator element mounted on said load bearing member between said head and said leading end, said indicator element being adapted to transfer the load, in use, from said head to the plate washer and to structurally fail if and when the load reaches a predetermined load, so as to provide a visual indication of the load in said load bearing member reaching said predetermined load, said predetermined load being a load lower than a failure load of said head and a failure load of said load bearing member; and a leading washer mounted on said load bearing member between said indicator element and said leading end, said leading washer being adapted to transfer the load, in use, from said indicator element to the plate washer, said leading washer comprising a dome washer having a generally domed shaped leading face adapted to engage the plate washer.

[0006] In some embodiments, said predetermined load is substantially equal to a desired pretensioning load to be applied to said load bearing member during installation of said rock bolt assembly.

[0007] In other embodiments, said predetermined load is at least 90% of the lesser of said failure load of said load bearing member and said failure load of said head.

[0008] Typically, said indicator element comprises a body of revolution in the form of a wall defining a central longitudinally extending aperture through which said support member extends. AH26(11861249_2):dah 3 2013204292 11 Oct 2016 [0009] In preferred forms, said wall is inclined with respect to a longitudinal axis by 0 to 60 degrees. In some embodiments, said indicator element has a longitudinal length of at least 15mm.

[0010] In some embodiments, said wall has a thickness of 2 to 6 mm.

[0011] In a preferred form, said indicator element is formed of steel.

[0012] In some embodiments, said wall is adapted to structurally fail by plastically deforming when the load reaches said predetermined load.

[0013] In alternate embodiments, said wall is adapted to structurally fail by rupturing when the load reaches given predetermined load.

[0014] The wall may be adapted to structurally fail by plastically deforming and rupturing when the load reaches the predetermined load.

[0015] In some embodiments, said wall is generally cylindrical.

[0016] In some embodiments, said wall is configured to rupture when the load reaches said predetermined load.

[0017] In some embodiments, said wall is generally frustoconical and is configured to collapse when the load reaches said predetermined load.

[0018] In some embodiments, said indicator element is adapted to transfer the load, in use, from said head to the plate washer via one or more components of said rock bolt assembly mounted on said load bearing member.

[0019] In some embodiments, said load bearing member is a rigid bar.

[0020] In some embodiments, said rigid bar has a threaded trailing region and said head comprises a drive nut threadingly mounted on said threaded trailing region.

[0021] In alternate embodiments, said load bearing member comprises a cable. AH26(11861249 2):dah 4 2013204292 11 Oct 2016 [0022] In such alternate embodiments, said end fitting typically comprises a barrel and wedge fitting.

[0023] In one or more embodiments, said rock bolt assembly further comprises a trailing washer mounted on said load bearing member between said head and said indicator element, said trailing washer being adapted to transfer the load, in use, from said head to said indicator element.

[0024] In one form, said trailing washer has a generally dome shaped leading face.

[0025] In some embodiments, said rock bolt assembly further comprises a tensioning assembly mounted on said support member adjacent said indicator element between said indicator element and said member leading end, said tensioning assembly being adapted to pre-tension said support member and to transfer the load, in use, from said indicator element to the plate washer.

[0026] [Paragraph is intentionally left blank] [0027] In a second aspect, the present invention provides a method of installing the rock bolt assembly defined above, comprising: drilling a bore hole through a rock face; mounting a plate washer on said load bearing member between said dome washer and said leading end; inserting said elongate load bearing member into said bore hole with said leading end leading; point anchoring a leading region of said load bearing member in said bore hole; and pre-tensioning said load bearing member until structural failure of said indicator element.

BRIEF DESCRIPTION OF DRAWINGS

[0028] Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings wherein: AH26(11861249_2):dah 4a 2013204292 11 Oct 2016 [0029] Figure 1 is an isometric view of a rock bolt assembly according to a first embodiment; [0030] Figure 2 is an exploded view of the trailing end portion of the rock bolt assembly of Figure 1; and [0031] Figure 3 is an isometric view of the indicator element of the rock bolt assembly of Figure 1, from the leading end; [0032] Figure 4 is an isometric view of the indicator element of Figure 3, from the trailing end; AH26(11861249_2):dah 5 2013204292 12 Apr 2013 [0033] Figure 5 is a partially cross-sectioned view of a rock bolt installation utilising the rock bolt assembly of Figure 1; [0034] Figure 6 is a front elevation view of the trailing end portion of the rock bolt assembly of Figure 1 with the indicator element in a failed condition; [0035] Figure 7 is an isometric view of the trailing end portion of a rock bolt assembly according to a second embodiment; [0036] Figure 8 is an isometric view of the trailing end portion of a rock bolt assembly according to a third embodiment; [0037] Figure 9 is an isometric view of the trailing end portion of a rock bolt assembly according to a fourth embodiment; [0038] Figure 10 is an exploded view of the trailing end portion of the rock bolt assembly of Figure 9; [0039] Figure 11 is an isometric view of a rock bolt assembly according to a fifth embodiment; [0040] Figure 12 is an exploded view of the trailing end portion of the rock bolt assembly of Figure 11; [0041] Figure 13 is an isometric view of the indicator element of the rock bolt assembly of Figure 11, from the leading end; [0042] Figure 14 is an isometric view of the indicator element of Figure 13, from the trailing end; [0043] Figure 15 is a partially cross-sectioned view of a rock bolt installation utilising the rock bolt assembly of 11; [0044] Figure 16 is a front elevation view of the rock bolt assembly of Figure 11 with the indicator element in a failed condition;

7331088 lvl:PRW 6 2013204292 12 Apr 2013 [0045] Figure 17 is an isometric view of the trailing end portion of a rock bolt assembly according to a sixth embodiment; [0046] Figure 18 is an isometric view of the trailing end portion of a rock bolt assembly according to a seventh embodiment; [0047] Figure 19 is an isometric of the trailing end portion of a rock bolt assembly according to an eighth embodiment; [0048] Figure 20 is an isometric view of the trailing end portion of a rock bolt assembly according to a ninth embodiment; [0049] Figure 21 is an exploded view of the trialing end portion of the rock bolt assembly of Figure 20.

DESCRIPTION OF EMBODIMENTS

[0050] A rock bolt assembly 100 according to a first embodiment is depicted in Figures 1 to 6. The rock bolt assembly 100 has an elongate load bearing member that is in the form of a rigid bar 110. The rigid bar 110 longitudinally extends between a bar leading end 111 and a bar trailing end 112. The rigid bar 110 is of a standard form used in rock bolting applications, being formed of high tensile steel and provided with a series of ribs 113 along its length and here having a threaded trailing region 114. The rock bolt assembly 100 further has a head, here in the form of a drive nut 120, mounted on the bar 110 towards the bar trailing end 112. Specifically, the drive nut 120 is threadingly mounted on the threaded trailing region 114 in the usual manner.

[0051] The rock bolt assembly 100 further comprises an indicator element 130 mounted on the bar 110 between the drive nut 120 and the bar leading end 111. As will be discussed further below with reference to Figure 5, the indicator element 130 is adapted to transfer load, in use, from the drive nut 120 to a plate washer 20 and to structurally fail if and when the load reaches a predetermined load.

[0052] Referring specifically to Figures 3 and 4, the indicator element 130 comprises a body of revolution defining a central aperture 132 through which the bar 110 extends. In this specific embodiment, the body of revolution is in the form of a sleeve defining a generally cylindrical sleeve wall 131. Being generally cylindrical, the sleeve wall 131 is inclined with respect to a 73310881vl:PRW 7 2013204292 12 Apr 2013 longitudinal axis by 0 degrees. The sleeve wall 131 here has a generally planar sleeve leading face 133 with a leading tapered surface 134 being defined between the sleeve leading face 133 and the inner wall 137 of the sleeve 131. The sleeve wall 131 has a generally planar sleeve trailing wall 135 and a trailing tapered surface 136 defined between the sleeve trailing face 135 and the sleeve inner wall 137. The indicator element 130 is thus symmetric, having identical leading and trailing ends such that it may be mounted on the bar 110 either way up.

[0053] In a specific embodiment, the indicator element 130 is formed of steel, and specifically grade 4140 high tensile steel. The specific indicator element 130 depicted has a sleeve wall thickness of approximately 5 mm and an outer diameter of approximately 69 mm. The indicator element 130 will typically have a longitudinal length of at least 15 mm for enhanced visibility.

In the arrangement depicted, the indicator element 130 has a longitudinal length of about 50 mm.

[0054] In the arrangement depicted, the rock bolt assembly 100 further comprises an additional component in the form of a trailing washer 140 mounted on the bar 110 between the drive nut 120 and the indicator element 130 for transferring load from the drive nut 120 to the indicator element 130. Referring specifically to Figure 2, the trailing washer 140 is here in the general form of a dome washer, having a generally planar trailing face 142 that engages the leading face 121 of the drive nut 120 and a generally dome shaped leading face 141 that engages the indicator element 130. In the specific arrangement depicted, the leading face 141 of the trailing washer 140 engages the trailing tapered surface 136 of the sleeve wall 131. Engagement of these surfaces provides a centralizing function, helping to ensure that the indicator element 130 remains centrally located about the bar 110 and the sleeve wall 131 is thus substantially evenly loaded.

[0055] The rock bolt assembly 100 further comprises another additional component in the form of a leading washer 150 mounted on the bar 110 between the indicator element 130 and the bar leading end 111, for transferring load from the indicator element 130 to the plate washer, as will be discussed further below. The leading washer 150 is here in the general form of a dome washer, identical to the trailing washer 140, having a substantially planar trailing face 152 and a generally dome shaped leading face 151. The trailing face 152 of the leading washer 150 engages the sleeve leading face 133. In use, the domed leading face 151 of the leading washer 150 engages the plate washer 20 and allows for misalignment between the plate washer 20 and the bar 110, when the rigid bar is not perpendicular to the rock face, in the usual manner.

7331088 lvl:PRW 8 2013204292 12 Apr 2013 [0056] A rock bolt installation utilising the rock bolt assembly 100 is depicted in Figure 5. The rock bolt assembly 100 is installed by first drilling a bore hole 10 through a rock face 11 in the usual manner, and mounting a plate washer 20 onto the bar 110 so as to engage the domed leading face 151 of the leading washer 150. The bar 110 is then inserted into the bore hole 10 with the bar leading end 111 leading. The leading end region of the bar 110 is point anchored within the bore hole 10, by any suitable means. In the arrangement depicted, the leading end region of the bar 110 is point anchored by way of a two-component resin 12 in the usual manner. A frangible cartridge housing the two-component resin 12 is inserted into the bore hole 10 before the bar 110 and the bar 110 is thrust upwardly and rotated, puncturing the cartridge and mixing the resin 12. The resin 12 is allowed to set to form the point anchor. Alternatively, a mechanical point anchor could be utilized if desired.

[0057] The bar 110 is then pre-tensioned in the usual manner by driving the drive nut 120 along the threaded trailing region 114 of the bar 110. Tensile loads developed in the bar 110 are transferred from the bar 110 to the rock face 11 surrounding the bore hole 10 via the trailing washer 140, indicator element 130, leading washer 150 and plate washer 20. For a typical installation utilising a 22 to 24 mm diameter bar 110, the bar 110 will typically be pre-tensioned to a tensile load of the order of 6 to 10 tonne (60 to 100 kN). In the particular configuration depicted, the indicator element 130 does not fail at this pre-tensioning load.

[0058] Following installation and in service, fracture of the strata anywhere along the length of the bore hole 10, or other movement of the strata, will develop an increasing tensile load in the bar 110. This increased tensile load in the bar 110 may ordinarily lead to catastrophic failure of a rock bolt assembly if the load exceeds the failure load of the bar 110 or the head 120. The failure load of the bar 110, for a 22 to 24 mm diameter high tensile steel bar will typically be of the order of 25 to 34 tonne (250 to 340 kN), depending on the grade of steel. The failure load of the drive nut 120 will typically be of the order of 23 to 31 tonne (300 to 310kN), again depending on the grade of steel. Failure of the drive nut 120 will typically be by stripping of the threaded engagement with the threaded trailing region 114 of the bar 110. Such a catastrophic failure may be preceded by plastic deformation of the plate washer 20, if of a domed configuration, however such plastic deformation may not be visually recognizable, particularly in installations with a large roof height which will place any person inspecting a significant distance from the rock bolt assembly.

73310881vl:PRW 9 2013204292 12 Apr 2013 [0059] The indicator element 130 is configured so as to fail when the load reaches a predetermined load that is lower than the failure load of the drive nut 120 and the failure load of the bar 110. Typically, the indicator element 130 will be configured to fail at a predetermined load of at least 80%, more typically at least 90%, of the lesser of the failure load of the bar 110 and the failure load of the drive nut 120. In the specific arrangement depicted, for a bar 110 with a failure load of 25 tonne (250kN) the indicator element 130 will fail upon application of a predetermined load of the order of 20 to 25 tonne (200 to 250 kN), more typically 22 to 23 tonne (220 to 230 kN). For a bar 110 with a failure load of 34 tonne (348 kN), the indicator element 130 will typically be configured to fail at a load of about 28 to 29 tonne (280 to 290 kN). Failure of the indicator element 130 will thus provide a clear visual indication of the load in the bar 110 having reached the predetermined load so that rectification action can be taken before the rock bolt assembly 100 fails catastrophically, typically by installing additional rock bolts. Upon failure of the indicator element 130, the rock bolt assembly 100 is still fully functional and able to continue supporting the strata up to the full failure load.

[0060] Referring to Figure 6, the indicator element 130 may be configured to fail by plastic deformation and/or rupture. If the indicator element 130 fails by plastic deformation, in the particular arrangement depicted, the trailing region of the sleeve wall 131 will deform outwardly, with the trailing washer 140 and drive nut 120 being drawn into the aperture 132 defined by the sleeve wall 131. This may or may not be associated with a rupture of the sleeve wall 131, extending from the sleeve leading face 133 to the sleeve trailing face 135, which again allows the trailing washer 140 and drive nut 120 to be drawn into the aperture 132 defined by the sleeve wall 131.

[0061] This failure of the indicator element 130 will be readily visibly identifiable during a visual inspection. Identification may be enhanced by applying a contrasting coloured surface coating, such as a light coloured bright paint, to the surface of the trailing washer 140 and/or the drive nut 120. On failure of the sleeve 130, drawing of the trailing washer 140 and drive nut 120 into the aperture 132 will result in it becoming significantly less visible on a rock bolt installation without a failed indicator element 130.

[0062] Whilst, in the rock bolt assembly 100 of the first embodiment, the indicator element 130 transfers the load from the drive nut 120 to the plate washer 20 via additional components of the 73310881vl:PRW ίο 2013204292 12 Apr 2013 rock bolt assembly 100 in the form of the leading and trailing washers 150, 140, it is also envisaged that the indicator element may transfer the load more directly.

[0063] An example of such a rock bolt assembly, according to a second embodiment, is depicted in Figure 7. Features of the rock bolt assembly 200 of the second embodiment that are identical to that of the rock bolt assembly 100 of the first embodiment are provided with identical reference numerals in the accompanying drawings, whilst equivalent features are provided with equivalent reference numerals, increased by 100.

[0064] The rock bolt assembly 200 is generally identical to the rock bolt assembly 100 apart from omission of the leading washer. The rock bolt assembly 200 thus comprises a rigid bar 110, a drive nut 120 threadingly mounted on the threaded trailing region 114 of the rigid bar 110, a trailing washer 140 and an indicator element 230 that is again in the form of a sleeve and is identical to the indicator element 130 apart from having a smaller diameter of about 30 mm. [Please advise typical diameter]. As shown in Figure 7, the indicator element 230 is arranged to directly engage the plate washer 20, so as to transfer load directly to the plate washer, rather than via a leading washer. The drive nut 120 and trailing washer 140 could be integrally formed as a single component in the form of a nut with a dome shaped leading face.

[0065] A rock bolt assembly 300 according to a third embodiment is depicted in Figure 8, and is identical to the rock bolt assembly 200 of the second embodiment, apart from omission of the trailing washer 140. Features of the rock bolt assembly 300 are accordingly provided with identical reference numerals to features of the rock bolt assembly 200. The rock bolt assembly 300 thus comprises a rigid bar 110, a drive nut 120 threadingly mounted on the threaded trailing region 114 of the rigid bar 110 and an indicator element 230 mounted on the bar 110 and directly engaging the drive nut 120 and, in use, the plate washer 20 so as to directly transfer load from the drive nut 120 to the plate washer 20.

[0066] A rock bolt assembly 400 according to a fourth embodiment is depicted in Figures 9 and 10. Again, features of the rock bolt assembly 400 that are identical or equivalent to features of any of the embodiments described above are provided with identical or equivalent reference numerals.

[0067] The rock bolt assembly 400 is of the general form described in International Patent Publication No. WO 2004/013463 to the present applicant and published on 12 February 2004, 73310881vl:PRW 11 2013204292 12 Apr 2013 the entire contents of which are incorporated herewith by cross-reference. The rock bolt assembly 400 comprises a rigid bar 110 having a head in the form of a modified drive nut 420 having a leading portion with three radially spaced protrusions 421 defining grout passages 422 therebetween. The drive nut 420 is threadingly mounted on the threaded trailing region 114 of the bar 110. A grout delivery fitting 460 is mounted on the bar 110 between the drive nut 420 and the bar leading end. The grout delivery fitting 460 comprises a trailing cup-shaped element 461 having a rearwardly facing recess within which the leading portion of the drive nut 420 is received and a leading grout delivery sleeve 462 that has a plastic grout delivery tube 463 extending forward therefrom. The grout delivery fitting 460 and drive nut 420 allow, following pre-tensioning, for grout to be injected through the grout delivery passages 422 and up the annular cavity defined between the rigid bar 110 and the grout delivery sleeve 462 and grout delivery tube 463 towards the bar leading end, where the grout is then allowed to flow back down the annular passage defined between the grout delivery tube 463 and the bore hole wall. The grout also provides double corrosion protection to the bar 110 and also provide for some transfer of load between the bar 110 and the strata via the bore hole wall. An indicator element 130 is mounted on the bar 110 over the grout delivery sleeve 462, with the indicator element 130 engaging the generally dome shaped leading face of the cup-shaped element 461. A leading washer 450, which is here in the form of a dome washer similar to that of the leading washer 150 of the first embodiment, is mounted on the rigid bar 110, also over the grout delivery sleeve 462.

[0068] In use, load is transferred from the modified drive nut 420 to a plate washer via the cupshaped element 461, indicator element 130, and leading washer 450, which directly engages the plate washer.

[0069] A rock bolt assembly 500 according to a fifth embodiment is depicted in Figures 11 through 16. Again, features of the rock bolt assembly 500 that are identical or equivalent to any of the above described embodiments are provided with identical or equivalent reference numerals.

[0070] The rock bolt assembly 500 is identical to the rock bolt assembly 100 of the first embodiment, apart from the configuration of the indicator element 530 and rear washer 540. Specifically, the rock bolt assembly 500 comprises a rigid bar 110 with a drive nut 120 threadingly mounted on the threaded trailing region 114 of the bar 110, with a trailing washer 7331088 lvl:PRW 12 2013204292 12 Apr 2013 540, indicator element 530 and leading washer 150 mounted on the bar 110, in succession, between the drive nut 120 and the bar leading end 111.

[0071] Referring specifically to Figures 13 and 14, the indicator element 530 is again in the form of a body of revolution in the form of a wall 531 defining a central aperture 532 through which the bar 110 extends. The wall 531 is generally frustoconical. In the arrangement depicted, the wall 531 tapers from the wall leading face 533 to the wall trailing face 535 although it is envisaged that the indicator element 530 may have an opposite taper. In the specific arrangement depicted, the wall 531 is inclined with respect to a longitudinal axis by about 45 degrees and has a wall thickness of approximately 3.5 mm. The outer diameter of the wall leading face 533 is approximately 50 mm, whilst the inner diameter of the wall trailing face 535 is approximately 30 mm. In the specific embodiment described, the indicator element 530 is again formed of high tensile steel, particularly 4140 grade. The wall 531 may be inclined with respect to the longitudinal axis by anywhere between 60° and 0° (which would be in the form of a cylinder, as per the indicator element 130). To ensure clear visibility of the indicator element 530, the longitudinal length of the indicator element 530 is preferably at least 15 mm and is here approximately 16 mm.

[0072] In this particular embodiment, the trailing washer 540 is a flat washer, having generally planar leading and trailing faces 551, 552.

[0073] A rock bolt installation utilizing the rock bolt assembly 500 is depicted in Figure 15. The rock bolt assembly 500 is installed in the same maimer as described above in relation to the rock bolt assembly 100 of the first embodiment, point anchoring the leading region of the bar 110 within a bore hole 10 drilled into a rock face 11, here with the use of a two-component resin 12 as discussed above. Tensile loads in the rigid bar 110 are again transferred to the rock face 11 via the drive nut 120, rear washer 540, indicator element 530, leading washer 150 and plate washer 20 in the same general manner as described above in relation to the rock bolt assembly 100 of the first embodiment.

[0074] In this embodiment, however, the indicator element 530 is configured to provide an indication of when the pre-tensioning load in the bar 110 has reached a predetermined load, being a desired pre-tensioning load, rather than providing an indication of imminent failure during use. Specifically, the indicator element 530 is configured to fail once the load reaches the

7331088 lvl:PRW 13 2013204292 12 Apr 2013 order of 10 tonne (approximately 100 kN), which equates to a typical desired pre-tensioning load for a rock bolt installation utilizing a 22 to 24 mm bar 110.

[0075] Accordingly, when installing the rock bolt assembly 500, following establishment of the point anchor, the drive nut 120 is driven along the threaded trailing region 114, gradually increasing the tensile load in the rigid bar 110, until the indicator element 530 fails, providing a visual indication that the appropriate pre-tensioning load has been applied to the bar 110.

Driving of the drive nut 120 will then be ceased.

[0076] Referring to Figure 16, the failure mode of the indicator element 530 will typically be by plastic deformation of the indicator element 540, collapsing the indicator element 530 between the indicator element leading and trailing faces 533, 535. To assist in visually identifying failure of the indicator element 530, the indicator element 530 may be formed in a contrasting colour, or otherwise provided with a surface coating of a contrasting colour, such as a bright coloured paint. After collapse, the indicator element 530 will only be partly visible.

[0077] A rock bolt assembly 600 according to a sixth embodiment is depicted in Figure 17. The rock bolt assembly 600 is identical to the rock bolt assembly 500, apart from omission of the leading and trailing washers 150, 540. Again, features of the rock bolt assembly 600 that are identical to or equivalent to features of earlier described embodiments are provided with identical or equivalent reference numerals. The rock bolt assembly 600 specifically comprises a rigid bar 110 with a drive nut 120 threadingly mounted on the threaded trailing region 114 of the bar 110 and an indicator element 530, as described above in relation to the fifth embodiment, mounted on the rigid bar 110, leading the drive nut 120. As shown in Figure 17, with the omission of the leading and trailing washers, the indicator element 530 directly transfers load from the drive nut 120 to the plate washer 20.

[0078] Use of the indicator elements described above is equally applicable to rock bolt assemblies in the form of cable bolt assemblies, rather than just rigid bars.

[0079] Such an arrangement, forming a rock bolt 700 according to a seventh embodiment, is depicted in Figure 18. Again, features of the rock bolt assembly 700 that are identical or equivalent to features of the embodiments described above have identical or equivalent reference numbers.

7331088 lvl:PRW 14 2013204292 12 Apr 2013 [0080] The rock bolt assembly 700 comprises an elongate load bearing member in the form of a cable 710. A head, in the form of a standard barrel and wedge assembly 720, is mounted on the cable 710 towards the cable trailing end 712. An indicator element 130, of identical form to that described in relation to the first embodiment, is mounted on the cable 710 between the barrel and wedge assembly 720 and the cable leading end. A leading washer 150 is mounted on the cable 710 between the indicator element 130 and the cable leading end. The cable bolt assembly 700 is installed in the usual manner by point anchoring the leading end region of the cable 710 in a bore hole drilled through a rock face, typically by way of a two-component resin that is mixed by rotating the cable 710 by way of a drive fitting 715 welded to the cable trailing end 712. The cable 710 is then pre-tensioned in the usual manner by way of standard hydraulic tensioning equipment which grips the trailing portion of the cable 710 protruding beyond the barrel and wedge assembly 720 bears against the barrel and wedge assembly to draw the trailing portion of the cable 710 through the barrel and wedge assembly 720, thereby tensioning the cable 710.

[0081] Tensile loads in the cable are transferred to the rock face via the barrel and wedge assembly 720, indicator element 130, leading washer 150 and plate washer 20 in the same general manner described above in relation to the rigid bar embodiments of the rock bolt assembly.

[0082] The indicator element 130 is here again configured to provide an early indication of impending failure of the rock bolt assembly 700, by providing a visual indication when tensile load in the cable 710 has reached a predetermined load that is less than the failure load of the barrel and wedge assembly and the failure load of the cable 710. Depending on the form and material of the cable 710, the failure load of the cable 710 will typically be 25 to 60 tonne (250 to 600 kN). Failure of the cable 710 will typically be at the location of the barrel and wedge assembly 720, where a stress concentration will form. The failure load of the barrel and wedge assembly 720 would typically be the load at which the barrel and wedge assembly slips on the cable 710, however, this will generally exceed the failure load of the cable 710 itself.

[0083] Figure 19 depicts a rock bolt assembly 800 according to an eighth embodiment that is identical to the rock bolt assembly 700 described above, but replacing the cylindrical sleeve form of indicator element 130 with the generally frustoconical indicator element 530 as described above in relation to the fifth embodiment. Again, features of the rock bolt assembly 800 of the 73310881vl:PRW 15 2013204292 12 Apr 2013 eighth embodiment that are identical to or equivalent to those of earlier embodiments are provided with identical or equivalent reference numerals.

[0084] The cable bolt assembly 800 thus comprises a cable 710, a barrel and wedge assembly 720 mounted towards the cable trailing end 712, an indicator element 530 and a leading washer 150, successively mounted on the cable 710 between the barrel and wedge assembly 720 and the cable leading end. The rock bolt assembly 800 is installed and acts in the same maimer as the cable bolt assembly 700 described above, except that the indicator element 530 is configured to fail during installation when an appropriate pre-tensioning load has been achieved in the cable 710, in the equivalent maimer to the installation of the rock bolt assembly 500 described above.

[0085] A rock bolt assembly 900 according to a ninth embodiment is depicted in Figures 20 and 21. Features of the rock bolt assembly 900 that are identical or equivalent to features of embodiments described are provided with identical or equivalent reference numerals. The rock bolt assembly 900 is of the same general form of the rock bolt assembly described in International (PCT) Patent Application No. PCT/AU2012/001044 in the name of the present applicant and filed on 4 September 2012, the entire contents of which are incorporated by cross-reference. The cable bolt assembly 900 comprises a cable 910, a barrel and wedge assembly 720 mounted on the cable 710 adjacent the cable trailing end 712, a frustoconical form of indicator element 530 mounted on the cable 710 leading the barrel and wedge assembly 720, a leading washer 950 leading the indicator element 530 and a pre-tensioning assembly 970 mounted on the cable between the leading washer 950 and the cable leading end. In the specific arrangement depicted, the leading washer 950 is a generally flat washer, having generally planar washer leading and trailing faces, 951, 952.

[0086] The pre-tensioning fitting 970, which is described in greater detail in International (PCT) Patent Application No. PCT/AU2012/001044, is used to pre-tension the cable 710 during installation, rather than utilizing hydraulic tensioning equipment. The pre-tensioning assembly 970 comprises a tensioning member 971 having an externally threaded leading tubular element 972 and a trailing drive head 973 fixed in relation to the tubular element 972. The pre-tensioning assembly 970 further comprises an internally threaded dome washer 974 that is threadingly mounted on the externally threaded tubular element 972 and which, in use, bears against the plate washer 20 to transfer loads thereto. An anti-friction washer 975 is mounted between the

73310881vl:PRW 16 2013204292 12 Apr 2013 dome washer 974 and the drive head 973. A thrust bearing 976 is mounted on the cable 710 and located within a recess formed in the trailing face of the drive head 973.

[0087] During installation of the rock bolt assembly 970, following point anchoring of the leading end of the cable 710, the drive head 973 is rotated so as to draw the tensioning member 971 rearwardly through the internally threaded dome washer 974. The tensioning member 971, being drawn rearwardly, engages the thrust bearing 976, which in turn engages the leading washer 950, in turn engaging the indicator element 530 and in turn engaging the barrel and wedge assembly 720, which in turn applies a pre-tensioning load to the cable 710. The thrust bearing 976 avoids the transfer of any significant torque from the drive head 973 to the cable 710 as it is rotated. Tension in the cable 710 is thus transferred to the rock face via the barrel and wedge assembly 720, indicator element 530, leading washer 950, tensioning assembly 970 and plate washer 20.

[0088] During pre-tensioning using the pre-tensioning assembly 970, the indicator element 530 will fail when the pre-tensioning load reaches the predetermined load corresponding to the desired pre-tension load in the cable. Accordingly, once the indicator element 530 fails, by collapse as discussed above in relation to the rock bolt assembly 500, driving of the drive head 973 may be ceased and the installation is complete. When pre-tensioning the cable 710 using a socket spanner extending over the trailing end of the cable bolt assembly 900 to engage the drive head 973, the indicator element 530 will not be visible to enable its failure to be identified during pre-tensioning. In this situation, the spanner may be used to drive the drive head 973 to a pre-set torque and then removed. At this point the condition of the indicator element 530 may be identified, with failure indicating that the desired pre-tensioning cable has been achieved. Again, rather than configuring the indicator element 530 to fail when a predetermined load corresponding to a desired pre-tension load is achieved, the indicator element 530 would be configured to fail at a load closer to the failure load of the cable 710 and barrel and wedge assembly 720 so as to provide an early indication of imminent failure of the rock bolt assembly 900.

[0089] A person skilled in the art will appreciate that various features of the rock bolt assemblies as described above may be interchanged as desired, in particular that different forms of indicator elements, with varying failure loads, may be applied to any of various forms of rock bolt assembly that transfer loads from the rock bolt to the rock face in which it is installed by way of 7331088 lvl:PRW 17 2013204292 12 Apr 2013 a head. It is also envisaged that two indicator elements may be mounted in any rock bolt assembly between the head and plate washer. One indicator element could be configured to fail at a desired pre-tensioning load whilst the other could be configured to fail at a higher load approaching the failure load of the load bearing member or head.

73310881vl:PRW

Claims (21)

1. A rock bolt assembly for installation in a bore hole extending through a rock face, said rock bolt assembly comprising: an elongate load bearing member longitudinally extending between a leading end and a trailing end; a head mounted on said load bearing member towards said trailing end, the head being adapted to transfer tensile load, in use, from the load bearing member to the rock face surrounding the bore hole via a plate washer; an indicator element mounted on the load bearing member between the head and the leading end, the indicator element being adapted to transfer the load, in use, from the head to the plate washer and to structurally fail if and when the load reaches a predetermined load, so as to provide a visual indication of the load in the load bearing member reaching the predetermined load, the predetermined load being a load lower than a failure load of the head and a failure load of the load bearing member; and a leading washer mounted on said load bearing member between said indicator element and said leading end, said leading washer being adapted to transfer the load, in use, from said indicator element to the plate washer, said leading washer comprising a dome washer having a generally domed shaped leading face adapted to engage the plate washer.

2. The rock bolt assembly of claim 1, wherein the predetermined load is substantially equal to a desired pre-tensioning load to be applied to the load bearing member during installation of the rock bolt assembly.

3. The rock bolt assembly of claim 1, wherein the predetermined load is at least 90% of the lesser of the failure load of the load bearing member and the failure load of the head.

4. The rock bolt assembly of either one of claims 1 to 3, wherein said indicator element is formed of steel.

5. The rock bolt assembly of any one of claims 1 to 4, wherein said indicator element has a longitudinal length of at least 15mm.

6. The rock bolt assembly of any one of claims 1 to 5, wherein said indicator element comprises a body of revolution in the form of a wall defining a central longitudinally extending aperture through which the support member extends.

7. The rock bolt assembly of claim 6, wherein said wall is inclined with respect to a longitudinal axis by 0 to 60 degrees.

8. The rock bolt assembly of either one of claims 6 and 7, wherein said wall has a thickness of 2 to 6 mm.

9. The rock bolt assembly of any one of claims 6 to 8, wherein said wall is adapted to structurally fail by plastically deforming when the load reaches said predetermined load.

10. The rock bolt assembly of any one of claims 6 to 9, wherein said wall is adapted to structurally fail by rupturing when the load reaches given predetermined load.

11. The rock bolt assembly of any one of claims 6 to 10, wherein said wall is generally cylindrical.

12. The rock bolt assembly of any one of claims 6 to 8, wherein said wall is generally frustoconical and is configured to collapse when the load reaches said predetermined load.

13. The rock bolt assembly of any one of claims 1 to 12, wherein said indicator element is adapted to transfer the load, in use, from the head to the plate washer via one or more components of the rock bolt assembly mounted on the load bearing member.

14. The rock bolt assembly of any one of claims 1 to 13, wherein said load bearing member is a rigid bar.

15. The rock bolt assembly of claim 14, wherein said rigid bar has a threaded trailing region and said head comprises a drive nut threadingly mounted on the threaded trailing region.

16. The rock bolt assembly of any one of claims 1 to 13, wherein said load bearing member comprises a cable.

17. The rock bolt assembly of claim 16, wherein said end fitting comprises a barrel and wedge fitting.

18. The rock bolt assembly of either one of claims 16 and 17, further comprising a tensioning assembly mounted on said support member adjacent said indicator element between said indicator element and said member leading end, said tensioning assembly being adapted to pretension said support member and to transfer the load, in use, from said indicator element to the plate washer.

19. The rock bolt assembly of any one of claims 1 to 18, further comprising a trailing washer mounted on said load bearing member between said head and said indicator element, said trailing washer being adapted to transfer the load, in use, from said head to said indicator element.

20. The rock bolt assembly of claim 19, wherein said trailing washer has a generally dome shaped leading face.

21. A method of installing the rock bolt assembly of any one of claims 1 to 20, comprising: drilling a bore hole through a rock face; mounting a plate washer on said load bearing member between said dome washer and said leading end; inserting said elongate load bearing member into said bore hole with said leading end leading; point anchoring a leading region of said load bearing member in said bore hole; and pre-tensioning said load bearing member until structural failure of said indicator element.