AU2013203826B2 - Rock bolt - Google Patents

Abstract

A rock bolt is provided, including an elongate member which has an elongate tubular section and a leading end at one end of the tubular section, wherein the leading end has a first part which, in cross-section, is generally of a U-shape and a second part which, in cross-section, is generally of a U-shape and which is located, at least partly, in the first part. '14 40-,, 26' ' A~~ ofN 24 '

Description

Page 1 ROCK BOLT BACKGROUND OF THE INVENTION [0001] This invention relates to a rock bolt. [0002] Rock bolts are used extensively in underground mining. When a 5 narrow seam of ore is to be mined it is cost-effective to remove only sufficient rock so as to allow a worker to access the seam. This can lead to narrow stopes of only 900mm in height. [0003] Various underground support techniques are used. In hard rock mining in South African gold mines, for example, a large diameter hole is 10 drilled into a rock body whereafter a rock bolt is inserted into the hole. Full column grouting into the rock bolt is preferred to obtain superior performance. To achieve this objective without excessive use of steel, a tubular steel rock bolt is used. Typically the tubular steel rock bolt is inserted into a borehole and is fixed in place using a resin mixture. 15 [0004] Normally a capsule which contains resin components is placed in a borehole. The capsule is punctured when the rock bolt is inserted into the hole and the contents of the capsule are mixed by rotating the rock bolt. If the rock bolt is tubular then a leading end of the rock bolt must be closed to prevent the resin mix from flowing into an interior of the rock bolt. One way of 20 sealing the tubular rock bolt is to deform the leading end, so that it is pointed. However, when this point penetrates the capsule a so-called "finger-gloving" effect may arise, when the bolt is rotated, which results in the resin not being adequately mixed or distributed throughout the borehole.

Page 2 [0005] If the leading end of the rock bolt is pressed closed, a flat chisel shape results. This helps to avoid finger-gloving during resin mixing, but the leading end may have a width which exceeds the width of a borehole - a feature which, without further processing of the leading end, works against the 5 adoption of this technique. [0006] In one aspect of the present invention there is provided a rock bolt which enables the aforementioned situation to be addressed. SUMMARY OF INVENTION [0007] The invention provides a rock bolt which includes an elongate 10 member which has an elongate tubular section and a leading end at one end of the tubular section, wherein the leading end has a first part which, in cross section, is generally of a U-shape and a second part which, in cross-section, is generally of a U-shape and which is located, at least partly, in the first part. [0008] Preferably opposing surfaces of the first part and of the second part 15 are in contact with each other. Alternatively the opposing surfaces are sufficiently close to each other to prevent any meaningful flow of material through a gap between the opposing surfaces. These features provide a sealing function which prevents or at least limits resin from flowing into an interior of the tubular section. 20 [0009] The leading end may have a maximum width which is approximately equal to an outer diameter of the tubular section. Thus, when the leading end is used for mixing, it does not come into contact with a wall of a borehole in Page 3 which the rock bolt is located in such a way that rotation of the rock bolt is inhibited. [0010] The tubular section may have a wall of thickness W and the second part may have a centrally positioned channel with a width T and a length L, 5 wherein T is greater than 2W and L is greater than 4W. [0011] T is preferably approximately equal to 3W. [0012] L is preferably at least 8W. [0013] The channel may have a depth D and D may be greater than 2W and less than 3W. 10 [0014] The tubular section may have keying formations on an outer surface. Part of an outer surface of the second part may be devoid of the keying formations. BRIEF DESCRIPTION OF THE DRAWINGS [0015] The invention is further described by way of example with reference to 15 the accompanying drawings in which: Figure 1 shows a leading end of an elongate tubular member; Figure 2 illustrates how the leading end is deformed to achieve a sealing action; and Figure 3 illustrates an aspect of a possible manufacturing step. 20 DESCRIPTION OF PREFERRED EMBODIMENT [0016] Figure 1 of the accompanying drawings illustrates, in perspective, a leading end 10 of an elongate tubular member 12 which is made from a Page 4 suitable metal and which has external keying formations 14 on an outer surface 16. [0017] The member 12 has a bore 18 with a diameter 20. [0018] In a manufacturing step the leading end 10 is inserted into a suitable 5 press, as shown in Figure 3 (by way of example only). The press has a bed 24 which cradles one half-section 26 of the leading end 10. A tool, not shown, which has a narrow elongate shape, is then pressed onto an outer surface 30 of an opposing half-section 32 of the leading end. In this way an elongate U shaped channel 36 is formed in the member, extending towards the half 10 section 26. Opposing surfaces of the channels are thereby brought into contact with each other or are sufficiently close so that any gap between the opposing surfaces is not large enough to allow a meaningful quantity of resin to flow through the gap during use of a rock bolt made from the tubular member. 15 [0019] The member 12 has a wall 40 of thickness W. In the deformation process referred to care must be taken to avoid the wall splitting or fracturing. In other words the deformation force must be applied to the leading end in such a way that the material at the leading end deforms with a yielding action, as opposed to rupturing. This objective can be achieved by deforming the 20 leading end so that the half section 36 is deformed, as shown in Figure 2, into a part which in cross-section is U-shaped and which extends at least partly into the half section 26 which, in cross-section, also constitutes a U-shaped part. Account must be taken though of the relative sizes of the channels, as explained herein.

Page 5 [0020] The channel 36 has a length L, a width T and a depth D. It has been established through experiment that when the elongate member is made from a material which suits the usage of the member in a rock bolt, that the leading end can be formed with the material yielding, as opposed to rupturing, 5 provided the following parameters are met namely: T is greater than 2W and L is greater than 4W. In a preferred form of the invention T is of the order of 3W. Also L should, preferably, be greater than 8W. D should be greater than 2W and generally less than 3W. [0021] When the leading end is deformed in the manner described and 10 subject to the aforementioned limitations, the integrity of the material at the leading end is maintained and there is no cracking or splitting of the metal. Also, the deformation force is such that, once the leading end is satisfactorily deformed, an outer surface of the deformed half section 36 is essentially devoid of the keying formations - this provides a good indicator of the status 15 of the deformation. [0022] The member 12, shaped in the manner referred to, can be processed further, as is known in the art, to make a rock bolt. The rock bolt then has an elongate shank or body which, over a greater portion of its length, is tubular. This feature, as is indicated in the preamble hereof, allows the elongate body 20 to be positioned inside a borehole which is relatively large in diameter but so that an annular gap between a surface of the borehole and an outer surface of the elongate body is relatively small - this helps to conserve the amount of resin which is used during installation of the rock bolt.

Page 6 [0023] When the elongate member is made into a rock bolt the trailing end of the member can be threaded so that a nut, which is engaged with the thread, is actuable to prestress the member once it has been bonded to the resin inside a borehole. During the installation process the shaped leading end of 5 the member 12 is able to puncture a resin capsule which has been positioned inside the borehole and, as the member is thereafter rotated, the shaped leading end can effectively mix the resin without giving rise to the finger gloving effect referred to. Also, the shaped leading end has a maximum transverse dimension which is not materially different from the diameter 22 of 10 the member i.e. away from the leading end. [0024] Throughout this specification and the claims, unless the context requires otherwise, the word "comprise" and its variations, such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers but not the exclusion of any other 15 integer or step or group of integers or steps. [0025] The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that prior art forms part of the common general knowledge in Australia.

Claims (7)

1. A rock bolt which includes an elongate member which, over a greater portion of its length, has a tubular section and a leading end at one end of the tubular section, wherein the elongate member, adjacent the leading end, has a first part which, in 5 cross-section, is generally of a U-shape, and a second part which, in cross-section, is generally of a U-shape and which is located, at least partly, in the first part whereby opposing surfaces of the first part and of the second part are in contact with each other to provide a sealing function which prevents a fluent settable material from flowing into an interior of the tubular section, and wherein the 10 leading end has a width which is approximately equal to an outer diameter of the tubular section, and the first part and the second part have a maximum width which is approximately equal to said outer diameter.

2. A rock bolt according to claim 1 wherein the tubular section has a wall of thickness W, and the second part has a centrally positioned channel with a width T and a 15 length L and T is greater than 2W and L is greater than 4W.

3. A rock bolt according to claim 2 wherein T is approximately equal to 3W.

4. A rock bolt according to claim 2 or 3 wherein L is greater than 8W.

5. A rock bolt according to claim 2, 3 or 4 wherein the channel has a depth D and D is greater than 2W. 20 Page 8

6. A rock bolt according to any one of claims 1 to 5 wherein the tubular section has keying formations on an outer surface and part of an outer surface of the second half section is devoid of keying formations.

7. A rock bolt substantially as herein described. 5 Ref: P23585/mh