AU2016202889B2 - Friction bolt assembly - Google Patents

Abstract

FRICTION BOLT ASSEMBLY ABSTRACT A friction bolt assembly (100) has a generally tubular friction bolt body (110) defining a cavity and having a split (114) extending along the friction bolt body (110). A rod (120) extends through the cavity (113). An expansion element (130) is mounted on the rod (120) and protrudes through the friction bolt body leading end (111). A tubular element (140) is mounted on the rod (120) and extends along the cavity (113). A load transfer head (170) is mounted on, or integrally formed with, the rod (120) within the tubular element (140). The load transfer head (170) has an interference fit with the tubular element (140) inhibiting displacement of the load transfer head (170) and the rod (120) and transfers loads, in use, between the tubular element (140) and the rod (120)up to apredetermined load. A drive head (180) located at or adjacent a trailing end of the friction bolt assembly (100) transfers torque applied to the drive head (180), during installation, to the rod (120) via the tubular element (140) and the load transfer head (170). The rod (120) is rotated to draw the expansion element (130) toward the friction bolt body trailing end (112) such that the engagement surface engages the friction bolt body (110), radially outwardly deforming the friction bolt body (110). A collar (150) defines an external annular shoulder adjacent the friction bolt body trailing end transfers axial loads, in use, between a rock face (12) in which the friction bolt assembly (100) is to be installed and the tubular element (140). 1/13 137 LI 3 13 JZZo k4a Fig.I1

Description

1/13

137 LI 3

13

JZZo

k4a

Fig.I1

FRICTION BOLT ASSEMBLY

Field

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

Background

[0002] A current method of stabilising the roof or wall of an underground mine involves the use of friction bolts, otherwise known as friction rock stabilisers. Friction bolts have a generally cylindrical body and typically have a collar welded to the trailing end of the body. The leading end portion of the body is generally tapered to assist in inserting the friction bolt into a bore hole drilled into the rock strata. The body is split down one side such that, when it is driven into a slightly undersized hole in the rock strata the friction bolt body elastically deforms to reduce the size of the split in the body. This elastic deformation exerts radial forces against the wall of the bore hole, providing a corresponding frictional force, retaining the friction bolt within the bore hole. A plate washer is typically fitted to the body directly above the collar such that the collar bears the plate washer against the rock face of the mine to distribute axial loads carried by the friction bolt across the face of the roof

Summary of Invention

[0003] The present invention provides a friction bolt assembly comprising: a generally tubular friction bolt body longitudinally extending between a friction bolt body leading end and a friction bolt body trailing end, said friction bolt body defining a cavity longitudinally extending through said friction bolt body and having a split longitudinally extending along said friction bolt body to said friction bolt body leading end; a rod longitudinally extending through said cavity between a rod leading end and a rod trailing end; an expansion element mounted on said rod and protruding through said friction bolt body leading end; a tubular element mounted on said rod and longitudinally extending along said cavity between a tubular element leading end and a tubular element trailing end; a load transfer head mounted on, or integrally formed with, said rod and within said tubular element, said load transfer head having an interference fit with said tubular element inhibiting displacement of said load transfer head, and said rod, relative to said tubular element and transferring loads, in use, between said tubular element and said rod up to a predetermined load; a drive head located at or adjacent a trailing end of said friction bolt assembly and adapted to transfer torque applied to said drive head, during installation, to said rod via said tubular element and said load transfer head, thereby rotating said rod to draw said expansion element toward said friction bolt body trailing end such that said engagement surface engages said friction bolt body, radially outwardly deforming said friction bolt body; and a collar defining an external annular shoulder adjacent said friction bold body trailing end and configured to transfer axial loads, in use, between a rock face in which said friction bolt assembly is to be installed and said tubular element.

[0004] Typically, said expansion element is threadingly mounted on a threaded leading portion of said rod such that, upon actuation of said rod by rotation of said drive head, said rod rotates with said drive head, drawing said expansion element along said threaded leading portion of said rod.

[0005] Typically, said expansion element is located at or adjacent said rod leading end.

[0006] Typically, said friction bolt assembly further comprises means for at least substantially preventing rotation of said expansion element relative to said friction bolt body. Said means may comprise a surface feature of said expansion element configured to engage said friction bolt body.

[0007] Typically, said tubular element is provided with a constriction towards said tubular element leading end, said constriction defining an internal annular shoulder for engaging said load transfer head upon displacement of said tubular element relative to said load transfer head, thereby limiting displacement of said tubular element relative to said load transfer head, and said rod.

[0008] In one or more embodiments, said collar is fixedly mounted on, or integrally formed with, said tubular element at said tubular element trailing end.

[0009] In one form, said friction bolt assembly further comprises an anti-friction washer mounted on said tubular element between said friction bolt body trailing end and said collar.

[0010] In one form, said drive head is welded to said collar.

[0011] In another form, said drive head is secured to said collar by way of an external thread formed on said drive head threadingly engaging an internal thread formed in a trailing face of said collar.

[0012] In a preferred form, said drive head has an aperture extending therethrough for receipt of a trailing end portion of said rod, said aperture having a larger transverse cross section than the transverse cross section of said trailing end portion of said rod such that said rod trailing end portion of said rod passes freely through said aperture.

[0013] In one embodiment, a detent is defined in said cavity at or adjacent said friction bolt body trailing end to restrain said tubular element and said rod from ejecting completely from said friction bolt body through said friction bolt trailing end upon failure of said rod.

[0014] In one form, said collar is fixed to said friction bolt body and said detent is defined by said collar.

[0015] In one form, said friction bolt assembly further comprises an adapter extending from an adapter leading end secured to said tubular element at or adjacent said tubular element trailing end, through said collar, to an adapter trailing end secured to said drive head.

Brief Description of Drawings

[0016] Preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings wherein:

Figure 1 is a front elevation view of a friction bolt assembly according to a first embodiment; Figure 2 is a rear elevation view of the friction bolt assembly of Figure 1; Figure 3 is a cross sectional elevation view of the friction bolt assembly of Figure 1; Figure 4 is a fragmentary cross sectional view of a partially completed rock bolt installation incorporating the friction bolt assembly of Figure 1;

Figure 5 is a fragmentary cross sectional view of the friction bolt assembly installation of Figure 4 in a completed state; Figure 6 is a fragmentary cross sectional view of the friction bolt assembly installation of Figure 4 in a partially yield state following a rock burst or other seismic event; Figure 7 is a fragmentary cross sectional view of the rock bolt installation of Figure 4 in a fully yielded state following a rock burst or other seismic event; Figure 8 is a load displacement curve of the friction bolt assembly of Figure 1; Figure 9 is cross sectional view of a friction bolt assembly according to a second embodiment; Figure 10 is a rear elevation view of a friction bolt assembly according to a third embodiment; Figure 11 is a cross sectional view of the friction bolt assembly of Figure 10; Figure 12 is a fragmentary cross sectional view of a friction bolt assembly installation incorporating the friction bolt assembly of Figure 10, in a fully yielded state following a rock burst or other seismic event; and Figure 13 is a fragmentary cross sectional view of the friction bolt assembly installation of Figure 12 following failure.

Description of Embodiments

[0017] A friction bolt assembly 100 according to a first embodiment is depicted in Figures I to 3 of the accompanying drawings. The friction bolt assembly 100 has a generally tubular friction bolt body 110 that longitudinally extends between a friction bolt body leading end 111 and a friction bolt body trailing end 112. The friction bolt body 110 defines a cavity 113 longitudinally extending through the friction bolt body 110. The friction bolt body 110 has a split 114 extending along the friction bolt body 110 to the friction bolt body leading end 111 to allow for radial compression of the friction bolt body 110 in the usual manner. Here the split 114 extends along the full length of the friction bolt body 110 from the friction bolt body trailing end 112. The friction bolt body 110 has a tapered leading portion 115 that tapers toward the friction bolt body leading end 111 in the usual manner to enable the friction bolt body 110 to be driven into a bore hole having a smaller diameter than the constant diameter of the primary portion 116 of the friction bolt body 110. In one embodiment, the external diameter of the primary portion 116 of the friction bolt body 110, being the maximum diameter of the friction bolt body 110, is approximately 47 mm, whilst the cross-section of the leading portion 115 of the friction bolt body 110 at the friction bolt body leading end 111 is of a reduced cross sectional area, being the minimum cross-sectional area of the friction bolt body 110. In one embodiment, the cross-section of the leading portion 115 at the friction bolt body leading end 111 is of an oval configuration having a major axis (maximum) diameter of 40 mm and minor axis diameter of 26 mm, although it is also envisaged that the leading portion 115 at the friction bolt leading 111 may be generally circular. The wall thickness of the friction bolt body 110 is here approximately 3 mm. The friction bolt body 110 is typically formed of structural grade steel. Unlike a typical friction bolt, no load bearing ring is welded to the friction bolt body trailing end 112 for transferring tensile loads to the friction bolt body 110.

[0018] The friction bolt assembly 100 further includes an elongate rod 120 longitudinally extending through the cavity 113 in the friction bolt body 110 between a rod leading end 121 and a rod trailing end 122. The rod 120 is typically formed of rigid steel bar. An expansion element 130 is mounted on the rod 120. The expansion element 130 is typically located toward the rod leading end 121 and in the embodiment depicted the expansion element 130 is located at or adjacent the rod leading end 121. As best shown in Figure 3, in the embodiment depicted, the expansion element 130 is threadingly mounted onto a threaded leading portion 123 of the rod 120. The threaded leading portion 123 of the rod 120 is received within a threaded aperture 133 that extends through the length of the expansion element 130, through the expansion element leading and trailing ends 131, 132. The expansion element 130 is in the general form of a body of revolution having a frusto-conical tapered leading surface 134 extending and tapering to the expansion element leading end 131, a generally cylindrical mid-surface 135 trailing the leading surface 134 and defining the maximum diameter of the expansion element 130 and a trailing generally frusto-conical engagement surface 136 that tapers, here in a non-linear manner, from the mid-surface 135 to the expansion element trailing end 132. Here the engagement surface 136 has a slightly concave form. In the embodiment depicted, the maximum diameter of the expansion element 130, defined by the mid-surface 135, is approximately 43mm. This is greater than the internal diameter of the friction bolt body 110 at the friction bolt body leading end 111 and less than the maximum diameter of the friction bolt body 110.

[0019] As best depicted in Figure 1, the expansion element 130 may further comprise means for at least substantially preventing rotation of the expansion element 130 relative to the friction bolt body 110. In the first embodiment, the means is in the form of a surface feature of the expansion element 130, particularly in the form of a key 137. The key 137 projects from, and is integrally formed with, the engagement surface 136 and extends from the expansion element trailing end 132 to the mid-surface 135. As shown in Figure 1, the key 137 projects into the split 114 formed in the friction bolt body 110. As a result, rotation of the rod 120, which would tend to rotate the expansion element 130, results in the key 137 engaging an edge of the friction bolt body 110 bounding the split 114, preventing relative rotation, at least beyond minor movement associated with the free play of the key 137 within the slightly broader width of the split 114 at the friction bolt body leading end 111. In an alternate form, the means may be in the form of one or more tack welds fixing the engagement surface 136 directly to the friction bolt body leading end 111. The welds ensure that the expansion element 130 is retained in engagement with the friction bolt body leading end 111 during transport and handling, but would fail upon application of load during installation.

[0020] A tubular element 140 is mounted on the rod 120 and longitudinally extends along the cavity 113 in the friction bolt body 110 between a tubular element leading end 141 and tubular element trailing end 142. In the embodiment depicted, the tubular element 140 has a diameter slightly less than that of the friction bolt body 110. The tubular element is typically formed of steel. The tubular element 140 is provided with a constriction 143 toward the tubular element leading end 141. The constriction 143 defines an internal annular shoulder 144.

[0021] In the arrangement of the first embodiment, the tubular element 140 extends beyond the friction bolt body trailing end 112 and a collar 150 is integrally formed with the tubular element 140 at the tubular element trailing end 142. It is also envisaged that the collar 150 could be formed separate to the tubular element 140 and fixedly mounted thereto. The collar defines an external annular shoulder 151 adjacent the friction bolt body trailing end 112. A plastic anti friction washer 160 is mounted on the tubular element 140 between the friction bolt body trailing end 112 and the collar 150. An extra steel washer 161 is mounted on the tubular element 140 between the friction bolt body trailing end 112 and the anti-friction washer 160, for the anti-friction washer 160 to bear against, so that it won't be shredded in use. In use, as will be discussed below, the shoulder 151 of the collar 150 transfers axial loads between a rock face in which the friction bolt assembly 100 is installed and the tubular element 140, via the anti friction washer 160 and washer 161.

[0022] A load transfer head 170 is mounted on the rod 120 toward the rod trailing end 122 and within the tubular element 140. In the first embodiment depicted, the load transfer head 170 is mounted on a threaded trailing portion 124 of the rod 120, particularly with the load transfer head 170 being wound along the threaded trailing portion 124 until it engages the leading end of the thread, thereby effectively locking the load transfer head 170 onto the rod 120, particularly in the event of rotation of the load transfer head 170 in a direction tending to further advance it along the rod 120. It is also envisaged that the load transfer head 170 may be mounted on the road 120 by alternate means, or that the load transfer head 170 is integrally formed with the rod 120. The load transfer head 170 is particularly in the form of a sleeve.

[0023] The load transfer head 170 is sized so as to provide an interference fit with the tubular element 140. In the arrangement depicted, the diameter of the load transfer head 170 tapers toward the load transfer head leading end 171 to assist in assembly of the rod 120 with the load transfer head 170 mounted thereon into the cavity 113 of a friction bolt body 110 through the friction bolt body trailing end 112. During assembly, the rod 120 and mounted load transfer head 170 is forced into the tubular element 140, in one embodiment under a load of approximately 150 kN, so as to set the initial interference fit of the load transfer head 170 with the tubular element 140. After assembly of the rod 120 and load transfer head 170 into the tubular element 140, the expansion element 130 is threaded onto the threaded leading portion 123 of the rod 120.

[0024] The interference fit of the load transfer head 170 within the tubular element 140 inhibits displacement of the load transfer head 170, and thereby the rod 120, relative to the tubular element 140, transferring loads between the tubular element 140 and the rod 120, at least up until a predetermined load, as will be discussed further below, beyond which the frictional force of the interference fit is overcome, allowing the load transfer head 170 (and rod 120) to be displaced relative to the tubular element 140 toward the tubular element leading end 141 into engagement with the shoulder 144 of the constriction 143, as will be further discussed below.

[0025] A drive head 180 is located at or adjacent the trailing end of the friction bolt assembly 100 and, in the embodiment depicted, is fixed to the collar 150 by welding. The drive head 180 has the external form of a hexagonal nut, defining six drive faces 181 and an aperture 182 extending longitudinally through its length. The aperture 182 has a larger transverse cross section than the transverse cross section of the threaded trailing end portion 124 of the rod 120, and in particular the aperture 182 is not internally threaded. Accordingly, the aperture 182 allows receipt of the threaded trailing end portion 124 of the rod 120 so that it may pass freely through the aperture 182. Accordingly, there is no direct load transfer between the drive head 180 and the rod 120. Instead, torque applied to the drive head 180 during installation (which will be discussed below) is transferred to the rod 120 only via the tubular element 140 and the load transfer head 170.

[0026] Installation of the friction bolt assembly 100 will now be described with reference to Figures 4 and 5. Firstly, a bore hole 10 is drilled into the rock face 12 of a rock strata 11 to be stabilized. In the embodiment depicted, the bore hole 10 is drilled with a standard installation rig with a drill bit having a diameter typically of 43 to 44mm, which will typically result in a bore hole diameter of 43 to 45mm, depending on strata type and hardness. Accordingly, the maximum diameter of the friction bolt body 110 (being approximately 47mm in a preferred embodiment) is slightly greater than the diameter of the bore hole 10, so as to provide for an interference fit in the usual manner, whilst the maximum diameter of the expansion element 130, here being approximately 43mm, is less than the maximum diameter of the friction bolt body 110 and slightly less than the diameter of the bore hole 10 such that the expansion element 130 may be readily inserted into the bore hole 10.

[0027] Before inserting the friction bolt assembly 100 into the bore hole 10, a plate washer 50 (and optionally a ball washer) is mounted on the friction bolt body 110 adjacent the steel washer 161 and plastic anti-friction washer 160. The friction bolt assembly 100 is mounted on the installation rig, particularly with the drive head 180 being received within a mating socket of the installation rig. The installation rig then drives the friction bolt assembly 100 into the bore hole , applying percussive force via the collar 150 until the plate washer 50 is firmly engaged with the rock face 12. The frictional forces due to the interference fit between the friction bolt body 110 and bore hole wall 13 retain the friction bolt assembly 100 in the bore hole 10, and allow for the transfer of loads between the rock strata 11 and the friction bolt body 110.

[0028] Additional anchoring of the friction bolt body 110 in the bore hole 10 is then achieved by way of the expansion element 130 by actuating the rod 120 by rotating the drive head 180. Rotation of the drive head 180 transmits torque to the rod 120 via the tubular element 140 and load transfer head 170, resulting in rotation of the rod 120. The anti-friction washer 160 (supported by the steel washer 161) reduces friction between the plate washer 50 and collar 150 as the drive head 180 is rotatably driven. Rotation of the expansion element 130 is at least substantially prevented by way of the key 137. Accordingly, rotation of the rod 120 draws the expansion element 130 along the threaded leading portion 123 of the rod 120 toward the friction bolt body trailing end 112 into the cavity 113, as depicted in Figure 5. In particular, the expansion element 130 is drawn fully into the tapered leading portion 115 of the friction bolt body 110, which is radially outwardly deformed by both the engagement surface 136 and mid surface 135 of the expansion element 130, bearing the leading portion 115 of the friction bolt body 110 against the bore hole 10, thereby point anchoring the friction bolt body 110 within the bore hole 10.

[0029] As well as point anchoring the friction bolt body 110 within the bore hole 10, the rotation of the drive head 180 also acts to pre-tension the rod 120 (and tubular element 140) which is then able to carry the bulk of loads applied to the friction bolt assembly 100 during ground movement in use.

[0030] The friction bolt assembly 100 provides for dynamic yielding of the friction bolt assembly 100 in the event of dynamic load application as may occur as a result of significant rock bursts or other seismic events, as may particularly occur in hard rock mining applications. Referring to Figure 6 and 7 of the accompanying drawings, upon the occurrence of a dynamic load, the friction bolt assembly 100 is able to yield by allowing relative longitudinal displacement between the rod 120 and the tubular element 140. Specifically, once the axial load applied to the friction bolt assembly exceeds a predetermined load at which frictional resulting from the interference fit between the load transfer head 170 and the tubular element 140 is overcome, relative longitudinal displacement between the rod 120 and tubular element 140 occurs, as depicted in Figure 6, with the tubular element 140 being drawn away from the rod leading end 121, drawing the constriction 143 at the tubular element leading end 141 toward the load transfer head 170 as the tubular element 140 moves downwardly with the rock face 12. The friction bolt body 110, being anchored to the rod 120 by way of the expansion element 130 and not being restrained to the rock face 12 by a ring at the friction bolt body trailing end 112 remains fixed in place whilst the tubular element 140 and section of rock strata 11 adjacent the rock face 12 move downwardly. This first stage of yield occurs under a relatively constant load, absorbing energy as the first stage of yielding occurs. Once the load transfer head 170 engages the shoulder 144 of the constriction 143 as shown in Figure 7, the relative displacement between the rod 120 and tubular element 140 ceases, signifying the end of the first stage of yielding. As the load further increases, the rod 120 begins to plastically yield, signifying a second stage of yielding. Throughout the first and second stage yielding processes, the rod 120 remains firmly point anchored to the rock strata 11 by way of the expansion element 130, thereby maintaining containment of the rock strata 11 along the full extent of the bore hole 10. Once the ultimate tensile strength of the rod 120 has been reached, the rod 120, and thereby the friction bolt assembly 100, will fail.

[0031] A load displacement curve for a short test sample friction bolt assembly 100 according to the first embodiment is depicted in Figure 8. The test sample was subject to laboratory loading, initially being subject to load to insert the rod 120 and mounted load transfer head 170 into the tubular element 140 to set the initial interference fit of the load transfer head 170 with the tubular element 140, followed by further loading replicating that of an installed friction bolt assembly 100. In the graph of Figure 8, region A of the graph signifies the load applied during insertion of the rod 120 into the tubular element 140 to set the initial interference fit, up to the load at point B (about 120 kN). Following setting of the interference fit and installation of the friction bolt assembly 100, some elastic yielding of the rod 120 will take place under initial loading up to the load at which the interference fit was set (although this initial loading is not represented in the graph of Figure 8). Once the frictional forces associated with the interference fit between the load transfer head 170 and the tubular element 140 are overcome (which is also represented at point B), the first yielding stage (represented at region C) commences. During this first yielding stage, the tubular element 140 of the test sample was displaced relative to the rod 120 under a relatively constant load of the order of 120 to 130 kN. In the short test sample, displacement of the order of 50 mm during the first stage of yielding was exhibited. This length of displacement during the first stage of yielding is dependent upon the length of the tubular element 140 between the load transfer head 170 and the constriction 143. In commercial embodiments, this length might typically be of the order of 200-300 mm. Once the load transfer head 170 engages the constriction 143 at point D, a further stage of elastic yielding of the rod 120 at region E commences, until the elastic limit of the rod 120 is reached at point F. A stage of plastic yielding occurs at region G up until failure of the rod at point H in the graph depicted, here at a load of about 370 kN. For commercial lengths of friction bolt assembly, with a significantly longer rod 120 than that of the short test sample, the second stage yielding, being the plastic yielding of the rod 120, might be up to another 200 mm or so.

[0032] A friction bolt assembly 200 according to second embodiment is depicted in Figure 9. The friction bolt assembly 200 of the second embodiment is identical to the friction bolt assembly 100 of the first embodiment except that, rather than welding a drive head 180 to the collar 150, as is the case in the first embodiment, a drive head 280 is provided with an external thread 283 which threadingly engages an internal thread 253 formed in a trailing face of the collar 250. Installation and operation of the friction bolt assembly 200 of the second embodiment is otherwise identical to that of the friction bolt assembly 100 of the first embodiment.

[0033] A friction bolt assembly 300 according to a third embodiment is depicted in Figures 10 and 11. Features of the friction bolt assembly 300 that are identical to those of the friction bolt assembly 100 are provided with identical reference numerals in the drawings, with equivalent features being provided with identical reference numerals incremented by 200. The friction bolt assembly 300 of the third embodiment is substantially identical to the friction bolt assembly 100 of the first embodiment, apart from a safety arrangement configured toward the friction bolt assembly trailing end which prevents the rod 320 and tubular element 340 from being ejected completely from the friction bolt body 310 upon failure of the rod 320.

[0034] The friction bolt body 310 of the friction bolt assembly 300 of the third embodiment is identical to the friction bolt body 110 of the friction bolt assembly 100 of the first embodiment, apart from the addition of a plurality of apertures 317 extending through the wall of the friction bolt body 310, radially spaced about the friction bolt body 310 at the same longitudinal position, serving to reduce the cross sectional area of the wall of the friction bolt body 310 at that longitudinal position and thereby weaken the friction bolt body 310 under axial load. Rather than providing a plurality of apertures 317, a single aperture might alternatively be provided, or any other means for weakening the friction bolt body 110. As will be discussed below, this ensures that the friction bolt body 310 fails at the appropriate time under dynamic loading, as will be discussed further below. The expansion element 130 is identical to the expansion element 130 used in the first embodiment.

[0035] The rod 320 of the friction bolt assembly 300 of the third embodiment is identical to the rod 120 of the friction bolt assembly 100 of the first embodiment, except that the threaded trailing portion 324 of the rod 320 is shorter, with the rod trailing end 322 being located within the tubular element 340, adjacent the load transfer head 170. The load transfer had 170 is mounted on the threaded trailing portion 324 of the rod 120 in the same manner as discussed above in relation to the first embodiment.

[0036] The tubular element 340 has a constriction 343 toward the tubular element leading end 341 defining an internal annular shoulder 344 in the same manner as the tubular element 140 of the friction bolt assembly 100 of the first embodiment. The load transfer head 170 is mounted within the tubular element 340 with an interference fit in the same manner as discussed above in relation to the first embodiment. The tubular element trailing end 342 is located within the cavity 313 of the friction bolt body 310, adjacent the friction bolt body trailing end 312. Rather than having a collar integrally formed with the tubular element trailing end 342, a separate collar 350 is fixed to the friction bolt body 310, at the friction bolt body trailing end 312. Inthe embodiment depicted, the collar 350 is fixed to the friction bolt body trailing end 312 by welding. The collar 350 both defines an external annular shoulder 351 for transfer of axial loads from the rock face and an annular detent 353 within the cavity 313 adjacent the friction bolt body trailing end 312.

[0037] The drive head 380 is attached to the tubular element 340 by way of an adapter 390 that extends from an adapter leading end 391, secured to the tubular element 340 adjacent the tubular element trailing end 342, through the collar 350 to an adapter trailing end 392 that is secured to the drive head 380. The adapter leading end 391 is secured to the tubular element 340 by way of a threaded engagement, whilst the drive head 380 is also secured to the adapter trailing end 392 by way of a threaded engagement. The threaded engagements are arranged to enable the transfer of torque applied to the drive head 380, during installation, to the tubular element 340 (and therefore to the rod 320 via the load transfer head 170). An anti-friction washer 360 is located between the collar 350 and the head 380 to reduce friction during rotational driving of the drive head 380 relative to the fixed collar 350 during installation.

[0038] The friction bolt assembly 300 is assembled by first assembling the load transfer head 170 onto the rod 320, driving the rod 320 into the tubular element 340 under load to set the interference fit between the load transfer head 170 and the tubular element 340, mounting the adapter 390 onto the tubular element 340, inserting the assembled rod 320, tubular element 340 and adapter 390 into the cavity 313 of the friction bolt body 310, mounting the expansion element on the threaded leading portion 323 of the rod 320, welding the collar 350 to the friction bolt body trailing end 312 and mounting the drive head 380 onto the adapter 390.

[0039] The friction bolt assembly 300 is installed in the same manner as the friction bolt assembly 100 of the first embodiment as described above, with the collar 350 engaging a plate washer 50 mounted on the friction bolt body 310 and engaging the rock face 12. Rotation of the drive head 380 during installation to set the expansion element 130 and pre-tension the rod 320 transfers torque to the rod 320 via the adapter 390 to the tubular element 340, rather than directly to the tubular element 340 as is the case with the friction bolt assembly 100 of the first embodiment.

[0040] In use, the friction bolt assembly 300 exhibits the same two stage yielding process as is described above in relation to the first embodiment, with Figure 12 depicting a friction bolt assembly installation utilising the friction bolt assembly 300 at the end of the first stage of yielding, where the tubular element 340 has displaced relative to the rod 320 such that the constriction 343 engages the load transfer head 170. During initial yielding of the friction bolt assembly 100, axial load is transferred from the rock face 12 to the friction bolt body 310 by way of the collar 350 welded to the friction bolt body trailing end 312. Inclusionofthe weakening apertures 317, which are here in the form of slots, ensures that the friction bolt body 310 fails prior to failure of the welds securing the collar 350 to the friction bolt body 310, as is shown in Figure 12. The friction bolt body 310 is not required for load carrying purposes, given the bulk of the load is intended to be carried by the rod 320. Following the initial stage of yielding, the friction bolt assembly 300 continues to yield in the same manner as the friction bolt assembly 100 of the first embodiment by plastic yielding of the rod 320.

[0041] Figure 13 depicts the installation with the friction bolt assembly 300 following failure of the rod 320 at point X. Following failure of the rod 320, the lower portion of the rod 320, and the tubular element 340, adapter 390 and drive head 380 that are secured to the lower portion of the rod 320 by way of the constriction 343 and load transfer head 170, fall downwardly through the cavity 313 of the friction bolt body 310. Movement of this failed assembly is, however, arrested by engagement of the adapter 390 by the detent 353 defined by the collar 350, thereby preventing the lower portion of the rod 320 and tubular element 340 from being ejected completely from the bore hole 10 and potentially injuring personnel or damaging equipment located beneath the bore hole 10. In the embodiment depicted, the detent 353 particularly engages an annular flange 393 formed on the adapter 390 and defining an external shoulder. The friction bolt assembly 300 could otherwise be configured such that, for example, the detent 353 engaged the tubular element trailing end 342 directly.

[0042] A person skilled in the art will appreciate various modifications that may be made to the friction bolt assemblies described.

Claims (14)

1. A friction bolt assembly comprising: a generally tubular friction bolt body longitudinally extending between a friction bolt body leading end and a friction bolt body trailing end, said friction bolt body defining a cavity longitudinally extending through said friction bolt body and having a split longitudinally extending along said friction bolt body to said friction bolt body leading end; a rod longitudinally extending through said cavity between a rod leading end and a rod trailing end; an expansion element mounted on said rod and protruding through said friction bolt body leading end; a tubular element mounted on said rod and longitudinally extending along said cavity between a tubular element leading end and a tubular element trailing end; a load transfer head mounted on, or integrally formed with, said rod and within said tubular element, said load transfer head having an interference fit with said tubular element inhibiting displacement of said load transfer head, and said rod, relative to said tubular element and transferring loads, in use, between said tubular element and said rod up to a predetermined load; a drive head located at or adjacent a trailing end of said friction bolt assembly and adapted to transfer torque applied to said drive head, during installation, to said rod via said tubular element and said load transfer head, thereby rotating said rod to draw said expansion element toward said friction bolt body trailing end such that said engagement surface engages said friction bolt body, radially outwardly deforming said friction bolt body; and a collar defining an external annular shoulder adjacent said friction bold body trailing end and configured to transfer axial loads, in use, between a rock face in which said friction bolt assembly is to be installed and said tubular element.

2. The friction bolt assembly of claim 1 wherein said expansion element is threadingly mounted on a threaded leading portion of said rod such that, upon actuation of said rod by rotation of said drive head, said rod rotates with said drive head, drawing said expansion element along said threaded leading portion of said rod.

3. The friction bolt assembly of either one of claims 1 and 2 wherein said expansion element is located at or adjacent said rod leading end.

4. The friction bolt assembly of any one of claims 1 to 3 further comprising means for at least substantially preventing rotation of said expansion element relative to said friction bolt body.

5. The friction bolt assembly of claim 4 wherein said means comprises a surface feature of said expansion element configured to engage said friction bolt body.

6. The friction bolt assembly of any one of claims 1 to 5 wherein said tubular element is provided with a constriction towards said tubular element leading end, said constriction defining an internal annular shoulder for engaging said load transfer head upon displacement of said tubular element relative to said load transfer head, thereby limiting displacement of said tubular element relative to said load transfer head, and said rod.

7. The friction bolt assembly of any one of claims 1 to 6 wherein said collar is fixedly mounted on, or integrally formed with, said tubular element at said tubular element trailing end.

8. The friction bolt assembly of any one of claims 1 to 7 wherein said friction bolt assembly further comprises an anti-friction washer mounted on said tubular element between said friction bolt body trailing end and said collar.

9. The friction bolt assembly of any one of claims 1 to 8 wherein said drive head is welded to said collar.

10. The friction bolt assembly of any one of claims I to 8 wherein said drive head is secured to said collar by way of an external thread formed on said drive head threadingly engaging an internal thread formed in a trailing face of said collar.

11. The friction bolt assembly of any one of claims 1 to 10 wherein said drive head has an aperture extending therethrough for receipt of a trailing end portion of said rod, said aperture having a larger transverse cross section than the transverse cross section of said trailing end portion of said rod such that said rod trailing end portion of said rod passes freely through said aperture.

12. The friction bolt assembly of any one of claims 1 to 11 wherein a detent is defined in said cavity at or adjacent said friction bolt body trailing end to restrain said tubular element and said rod from ejecting completely from said friction bolt body through said friction bolt trailing end upon failure of said rod.

13. The friction bolt assembly of claim 12 wherein said collar is fixed to said friction bolt body and said detent is defined by said collar.

14. The friction bolt assembly of either one of claims 12 and 13 further comprising an adapter extending from an adapter leading end secured to said tubular element at or adjacent said tubular element trailing end, through said collar, to an adapter trailing end secured to said drive head.

DYWIDAG-Systems International Pty Limited Patent Attorneys for the Applicant/Nominated Person SPRUSON & FERGUSON