AU2014203250B2 - Rock bolt - Google Patents

Abstract

A rock bolt (100) has an elongate load bearing element (101) longitudinally extending between an element leading end (102) and element trailing end (103). A wire (110) helically extends about a leading end portion (109) of the load bearing elements (101) from a wire leading end (111) located adjacent the element leading end (102) to a wire trailing end (112). The wire (110) comprises a first wire portion (114) located toward the wire leading end (111) and a second wire portion (115) located toward the wire trailing end (112). The first wire portion (114) has a greater helical pitch than the second wire portion (115). AlliI 0o 6 c, Fig. 2 Fig. 1

Description

ROCK BOLT

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

Background [0002] One known method of stabilising the wall or roof of an underground mine is to secure a rock bolt into a bore hole drilled in to the face of the rock to be stabilised. Rock bolts are formed with an elongate load bearing element, typically formed of a rigid steel bar, or, in the case of a cable bolt form of rock bolt, a steel strand.

[0003] A known method of installing such a rock bolt involves first drilling a bore hole into the rock face. A sausage-like two-component resin filled cartridge is then inserted into the bore hole, followed by the rock bolt which pushes the resin filled cartridge towards the blind end of the bore hole. The rock bolt is then rotated by the installation rig which also thrusts the rock bolt upwardly whilst it is rotating to mix the resin components and shred the frangible cartridge casing, pushing it towards the blind end of the bore hole. Various means have previously been proposed to be formed with, or mounted on, the leading end portion of the rigid bar to assist in mixing of the resin components and/or assisting in anchoring the rock bolt within the resin. Rotation of the rock bolt is then stopped for a few seconds to allow the resin to cure.

[0004] In a typical configuration, the trailing end portion of the bar is threaded and the rock bolt is tensioned after curing of the resin by threading the nut along the threaded trailing end portion of the rock bolt, bearing the nut against a plate washer that engages the rock face adjacent the bore hole opening.

Object of the Invention [0005] It is an object of the invention to provide an improved rock bolt, or at least to provide a useful alternative to presently available rock bolts.

Summary of Invention [0006] In a first aspect, the present invention provides a rock bolt comprising: an elongate load bearing element longitudinally extending between an element leading end and an element trailing end; and a wire helically extending about a leading end portion of said load bearing element from a wire leading end located adjacent said element leading end to a wire trailing end; wherein said wire comprises a first wire portion located towards said wire leading end and a second wire portion of said wire located towards said wire trailing end, said first wire portion having a greater helical pitch than said second wire portion, said first and second wire portions both extending about said leading end portion of said load bearing element.

[0007] In a preferred form, said first portion of said wire extends to at least adjacent to said wire leading end.

[0008] In a preferred form, said second portion of said wire extends to said wire trailing end.

[0009] In an alternate form, said wire further comprises a third wire portion located between said first wire portion and said wire leading end, said third wire portion having a shorter helical pitch than said first wire portion.

[0010] Typically, said load bearing element comprises a rigid bar.

[0011] Typically, said wire is welded to said rigid bar.

[0012] In a preferred form, said wire leading end projects outwardly from the helix defined by said wire so as to form a projection for shredding a resin cartridge casing during installation.

[0013] In a particularly preferred form, said projection is bent so as to extend towards said wire trailing end.

[0014] In a preferred form, said wire is formed with a right-handed helix.

[0015] In some embodiments, said wire comprises a single length of wire defining said first and second wire segments. In an alternative embodiment, said wire comprises separate first and second wire segments, said first wire segment comprising said first wire portion and said second wire segment comprising said second wire portion.

[0016] In a further alternative embodiment, the helical pitch of said wire varies along its length.

[0017] Also disclosed is a rock bolt comprising: an elongate load bearing element longitudinally extending between an element leading end and an element trailing end; a first wire segment helically extending about a leading end portion of said load bearing element; and a second wire segment helically extending about said load bearing element, said first wire segment being located between said second wire segment and said element leading end.

[0018] Typically said first wire segment has a greater helical pitch than said second wire segment.

[0019] In one form, said second wire segment is spaced from said first wire segment.

[0020] In a second aspect, the present invention provides a method of installing any one of the rock bolts defined above comprising the steps of: drilling a bore hole with a blind end into a rock face to be stabilized; inserting a two-component resin filled cartridge having a frangible casing into said bore hole; inserting said rock bolt into said bore hole with said element leading end leading; thrusting said rock bolt towards said blind end whilst rotating said rock bolt in a direction opposing the direction of helical winding of said wire, puncturing said frangible casing, mixing said resin and pumping said resin towards said blind end; and stopping thrusting and rotation of said rock bolt, allowing said resin to cure.

Brief Description of Drawings [0021] Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings wherein: [0022] Figure lisa fragmentary front elevation view of a rock bolt according to a first embodiment; [0023] Figure 2 is an isometric view of the rock bolt of Figure 1; [0024] Figure 3 is a partially cross-sectioned front elevation view of a partially completed rock bolt installation utilizing the rock bolt of Figure 1; [0025] Figure 4 is a partially cross-sectioned front elevation view of a completed rock bolt installation utilizing the rock bolt of Figure 1; [0026] Figure 5 is a fragmentary front elevation view of a rock bolt according to a second embodiment; [0027] Figure 6 is a fragmentary front elevation view of a rock bolt according to a third embodiment; [0028] Figure 7 is a fragmentary front elevation view of a rock bolt according to a fourth embodiment; and [0029] Figure 8 is a fragmentary front elevation view of a rock bolt according to a fifth embodiment.

Description of Embodiments [0030] Referring to Figures 1 to 3, a rock bolt 100 has an elongate load bearing element in the form of a rigid bar 101 which longitudinally extends between a bar leading end 102 (forming the load bearing element leading end) and a bar trailing end 103 (forming the load bearing element trailing end). The bar 101 may be of any of various known forms used in rock bolting applications. The bar will typically be formed of steel and here has a coarse left-handed thread 105 that is hot rolled into the bar 101 along its length. It is envisaged that the thread 105 might extend over a threaded trailing end portion 104 of the bar 101 only. It is also envisaged that a finer thread may be rolled onto a trailing end portion of a plain or ribbed bar. In arrangements with a thread only provided on the threaded trailing end portion 104 of the bar 101, other forms of protrusion, such as ribs, will typically be formed along the remaining length of the bar to assist load transfer.

[0031] The bar 101 typically has a nominal diameter of between 12 and 30 mm, with common nominal diameters being 16, 20 and 24 mm. The bar 101 will typically have a length of the order of 1.8 to 2.4 m, but this may vary to suit the particular application.

[0032] In the arrangement depicted, an end fitting in the form of a hexagonal threaded drive nut 106 and dome washer 107 is mounted on the threaded trailing end portion 104, with the drive nut 106 threadingly engaging the thread 105 of the threaded trailing end portion 104 of the bar 101. A standard plate washer 108 is mounted on the rock bolt 100, engaging the dome washer 107 in a known manner.

[0033] The drive nut 106 will typically be fixed in relation to the bar 101 by way of a locking means that is configured to fail upon application of a predetermined torque during the installation process. This allows the drive nut 106 to advance along the threaded trailing end portion 104 of the bar 101 upon application of torque exceeding the predetermined torque during post-tensioning of the rock bolt 100, as will be discussed further below. In the configuration depicted, the locking means is in the form of a shear pin 106a that locks the drive nut 106 to the bar 101. The locking means could alternatively be in the form of a slightly deformed thread of the threaded trailing end portion 104 of the bar 101. In another alternate form, the locking means could be in the form of a disc mounted in the trailing end of the drive nut 106 and configured to prevent the bar 101 from passing through the trailing end of the drive nut 106 until application of a torque exceeding the predetermined torque, at which the disc ruptures. The locking means may also take any other suitable form.

[0034] A wire 110 helically extends about a leading end portion 109 of the bar 101. The wire 110 extends from a wire leading end 111 to a wire trailing end 112. The wire 110 is typically formed of steel and, in the arrangement depicted, has a diameter of about 3.15 mm although wire diameters of up to about 6.3 mm or greater may also be suitable for some applications. The wire helically extends in a direction opposing the intended direction of rotation of the rock bolt 100 during installation. Accordingly, for a standard left-handed installation rig, the wire 110 will helically extend in a right-handed direction as depicted in the accompanying drawings, and in an opposing direction to the left-handed coarse thread 105 formed on the bar 101 (particularly on the threaded trailing portion 104 of the bar 101). The wire 110 will typically extend over the length of the leading end portion 109 of the bar 101 which is intended to be resin encapsulated in a resin point anchored installation. Wire lengths of the order of 500 mm for shorter bars or 1100 mm for longer bars will be typical.

[0035] In the arrangement depicted, the wire 110 is fixed to the leading end portion 109 of the bar 101 by welding, here being welded towards the wire leading and trailing ends 111, 112 and partway along the length of the wire 110. Alternatively, the leading end 111 of the wire 110 may be secured in a slot formed in the leading end 102 of the bar 101. In a still further alternative, particularly suitable for applications with high tensile steel bars which might otherwise be weakened through inadvertent heat treatment of the bar during a welding process, the wire trailing end 112 may be fixed to the bar 101 by way of a ferrule that is crimped onto the bar 101. A person skilled in the art will appreciate other suitable means for securing the wire 110 to the bar 101.

[0036] In the arrangement depicted, the wire leading end 111 projects outwardly from the helix defined by the wire 110, so as to form a projection 113, best depicted in Figure 2, which is useful in shredding the resin cartridge casing during installation as will be described below. In the embodiment depicted, the projection 113 is bent so as to extend towards the wire trailing end 112. This assists in preventing the projection 113 from catching on the bore hole side wall during installation.

[0037] The wire 110 serves three basic purposes, being to mix the two-component resin during the installation process, to pump the resin towards the blind end of the bore hole during installation and to assist in anchoring the rock bolt 100 within the resin upon the application of tensile load to the rock bolt 100 in service.

[0038] The wire 110 has a first wire portion 114 located towards the wire leading end 111 and a second wire portion 115 that is located towards the wire trailing end 112. The helix defined by the wire 110 is configured such that the first wire portion 114 has a greater helical pitch than that of the second wire portion 115.

[0039] In one example, the first wire portion has a helical pitch of about 75 mm. The helical pitch of the first wire portion 115 will typically be between 50mm and 100 mm. In one example, the second wire portion 115 has a helical pitch of about 25 mm. The helical pitch of the second wire portion 115 will typically be between 10 mm and 50 mm. In the configuration depicted, the first wire portion 114 extends to adjacent the wire leading end 111, particularly to the trailing end of the projection 113, whilst the second wire portion 115 extends from the first wire portion 114 to the wire trailing end 112. In one example, the first wire portion 114 extends over a length of approximately 500 mm. The first wire portion 114 will typically extend over a length of between 300 mm and 1000 mm, and between 3 and 20 revolutions. The second wire portion 115 will typically extend over a length of between 50 mm and 500 mm, and over at least 3 revolutions. The second wire portion 115 will typically extend over a shorter length than the first wire portion 114.

[0040] The larger helical pitch of the first wire portion 114 results in a reduced resistance, and hence back pressure, on the first wire portion 114 as it is pushed through the bulk of the resin during installation, thereby enabling a more rapid insertion of the rock bolt 101 through the resin into its final installed position. The increased pitch of the first wire portion 114 also reduces the pumping effect of that portion of the wire as the rock bolt 100 is rotated whilst the rock bolt advances through the resin. The shorter helical pitch of the second wire portion 115 provides a relatively greater resistance to passage of the rock bolt 100 through the resin. However, given that the second wire portion 115 is located towards the wire trailing end 112, it only needs to pass through a relatively short length of the resin, such that the overall effect of this increased resistance is minimal. The relatively shorter helical pitch of the second wire portion 115 also increases the pumping effect of the second wire portion 115, thus acting to consolidate the resin towards the blind end of the bore hole and effectively acting as a resin dam, reducing the possibility of leakage of resin down the rigid bar 101.

[0041] Installation of the rock bolt 100 will now be described with reference to Figures 3 and 4. A bore hole 51 is first drilled into the rock face 50. A two-component resin filled cartridge 130 is then inserted into the bore hole 51. The rock bolt 100, with the plate washer 108 mounted on the bar 101 in front of the dome washer 107, is then inserted into the bore hole 51 with the bar leading end 102 leading. The rock bolt 100 is thrust towards the blind end 52 of the bore hole 51, utilizing a standard rock bolt installation rig, thereby pushing the resin filled cartridge against the bore hole blind end 52 and rupturing the frangible casing of the cartridge 130.

[0042] Whilst the installation rig thrusts the rock bolt 100 towards the bore hole blind end 52, it also rotates the drive nut 106, here in an anticlockwise direction, thereby rotating the entire rock bolt 100 in a direction opposing the direction of helical winding of the wire 110. The projection 113 at the wire leading end 111 pierces and shreds the casing of the cartridge 130 as the rock bolt 100 is rotated and advanced and the leading end portion 109 of the bar 101, and particularly the wire 110, actively mixes the two components of the resin. As noted above, the relatively greater helical pitch of the first wire portion 114 allows the rock bolt 100 to be advanced more rapidly than would otherwise be provided for by the resistance associated with a shorter helical pitch. With the wire 110 helically extending in an opposing direction to the direction of rotation of the rock bolt 100, the resin is actively pumped towards the bore hole blind end 52, with the shorter pitched second wire portion 115 providing an enhanced pumping effect to assist in better consolidating the resin into the leading end portion of the bore hole 51. Thrusting and rotation of the rock bolt 100 is then ceased, allowing the resin to cure for a few seconds.

[0043] With the configuration of the wire 110 allowing for more rapid insertion of the rock bolt 100, two component resins with faster curing times than those that could be utilized with a helical wire with a constant pitch equal to the average pitch of the wire 110 (and thus having an equal number of turns and at least a substantially equal anchoring effect) may be utilized, thereby reducing the total installation time. After the resin has cured, the drive nut 106 is driven by the installation rig at an increased torque that is equal to or in excess of the predetermined torque at which the shear pin 106a fails. The torque applied to the drive nut 106 then threads the drive nut 106 along the threaded trailing portion 104 of the bar 101. Continued driving of the drive head 106 bears the drive nut 106 against the dome washer 107, which in turn bears against the plate washer 108 and in turn the rock face 50, thereby resulting in tensioning of the bar 101.

[0044] It is also envisaged that the rock bolt 100 may be installed to form a fully resin encapsulated rock bolt installation, with a first two-component resin filled cartridge containing a relatively fast setting resin being used to point anchor the leading end portion of the rock bolt 100, and a second two-component resin filled cartridge containing a slower setting resin, which is inserted before the first resin filled cartridge, encapsulating the remaining length of the rock bolt 100. In such an installation, the rock bolt 100 will be pretensioned once the first, faster setting resin has anchored the leading end portion of the rock bolt but prior to the second, slower resin curing, thereby enabling elongation of the rock bolt within the second, slower resin during the pretensioning process.

[0045] A rock bolt 200 according to a second embodiment is depicted in Figure 5. The rock bolt 200 has the same bar 101, a drive nut 106 and dome washer 107 as the rock bolt 100 of the first embodiment and a modified form of wire 210 helically extending about the leading end portion 109 of the bar 101. The wire 210 is identical to the wire 110 used with the rock bolt 100 of the first embodiment, except that, rather than having only a first wire portion with a greater helical pitch and a second wire portion with a lesser helical pitch, the wire 210 has a first wire portion 214 with a relatively longer helical pitch located between second and third wire portions 215, 216 each having a relatively shorter helical pitch. In the arrangement depicted, the second wire portion 215 extends from the wire trailing end 212 to the first wire portion 214, whilst the third wire portion 216 extends from the first wire portion 214 to the projection 213 formed at the wire leading end 211. The second and third wire portions 215, 216 are shorter than the first wire portion 214. Whilst the third wire portion 216 does increase the resistance to passage of the rock bolt 200 through the resin, given that it only extends over a relatively short length, which is typically 50 mm to 100 mm, this effect is relatively minimal. Providing the third wire portion 216 with the relatively shorter helical pitch at the leading end of the wire 210 provides for quick initial mixing of the resin during installation, without excessive drag as would be generated if the shorter helical pitch also extended along the first wire portion 214.

[0046] It is further envisaged that additional shorter and greater pitched wire portions may be provided along the length of the wire. Such a configuration is provided in the rock bolt 300 according to a third embodiment as depicted in Figure 6. The rock bolt 300 according to the third embodiment is effectively equivalent to the rock bolt 200 of the second embodiment, with the addition of a fourth wire portion of greater helical pitch at the leading end of the wire. Specifically, the wire 310 helically extending about the leading end portion 109 of the bar 101 has first and fourth wire portions 314, 317 with relatively longer helical pitches and second and third wire portions 315, 316 with relatively shorter helical pitches. The leading end of the wire 310 may be provided with a projection for shredding the resin cartridge casing during installation, as per the projection 113 formed on the wire 110 of the rock bolt 100 of the first embodiment.

[0047] It is still further envisaged that the helical pitch of the wire may vary continuously, or discontinuously, along the length of the wire from a greater pitch towards the wire leading end to a shorter pitch towards the wire trailing end. Such an arrangement where the helical pitch varies continuously is provided in a rock bolt 400 according to a fourth embodiment as depicted in Figure 7. In the rock bolt 400, a wire 410 helically extends about the leading end portion of the bar 101, with the helical pitch of the wire 410 gradually reducing along the length of the leading first wire portion 414 and trailing second wire portion 415 from the wire leading end 411 to the wire trailing end 412. Again, a projection useful in shredding the resin cartridge may be formed at the wire leading end 411.

[0048] It is still further envisaged that the wire may be formed of two (or more) separate and distinct wire segments. Such an arrangement is provided in a rock bolt 500 according to a fifth embodiment as depicted in Figure 8. In the rock bolt 500, a wire 510 helically extends about the leading end portion 109 of the bar 101. The wire 510 comprises a first wire segment 514 located adjacent the bar leading end 102 and a second wire segment 515 trailing the first wire segment 514, such that the first wire segment 514 is located between the second wire segment 515 and the bar leading end 102. The first wire segment 514 has a greater helical pitch than that of the second wire segment 515. It is also envisaged, however, that, in some embodiments, the first and second wire segments may have similar helical pitches, or the first wire segment 514 may even have a smaller helical pitch than that of the second wire segment 515. The first and second wire segments 514, 515 may abut each other or may be spaced. The first and second wire segments 514, 515 will typically helically extend in the same direction, opposing the intended direction of rotation of the rock bolt 500 during installation. A projection useful in shredding the resin cartridge may again be provided at the leading end of the first wire segment 514.

[0049] It is also envisaged that the rock bolt described may be in the form of a cable bolt, with the elongate load bearing element being in the form of a strand rather than a rigid bar. In such a cable bolt, the wire would typically be secured to the strand by way of a swage fitting or the like.

[0050] A person skilled in the art will appreciate other possible modifications and variations of the rock bolts described.

Claims (13)

1. A rock bolt comprising: an elongate load bearing element longitudinally extending between an element leading end and an element trailing end; and a wire helically extending about a leading end portion of said load bearing element from a wire leading end located adjacent said element leading end to a wire trailing end; wherein said wire comprises a first wire portion located towards said wire leading end and a second wire portion of said wire located towards said wire trailing end, said first wire portion having a greater helical pitch than said second wire portion, said first and second wire portions both extending about said leading end portion of said load bearing element.

2. The rock bolt of claim 1, wherein said first portion of said wire extends to at least adjacent to said wire leading end.

3. The rock bolt of either one of claims 1 and 2, wherein said second portion of said wire extends to said wire trailing end.

4. The rock bolt of either one of claims 1 and 2, wherein said wire further comprises a third wire portion located between said first wire portion and said wire leading end, said third wire portion having a shorter helical pitch than said first wire portion.

5. The rock bolt of any one of claims 1 to 4, wherein said load bearing element comprises a rigid bar.

6. The rock bolt of claim 5, wherein said wire is welded to said rigid bar.

7. The rock bolt of any one of claims 1 to 6 wherein said wire leading end projects outwardly from the helix defined by said wire so as to form a projection for shredding a resin cartridge casing during installation.

8. The rock bolt of claim 7, wherein said projection is bent so as to extend towards said wire trailing end.

9. The rock bolt of any one of claims 1 to 8, wherein said wire is formed with a right-handed helix.

10. The rock bolt of any one of claims 1 to 9, wherein said wire comprises separate first and second wire segments, said first wire segment comprising said first wire portion and said second wire segment comprising said second wire portion.

11. The rock bolt of any one of claims 1 to 9, wherein said length of wire comprises a single wire defining said first and second wire segments.

12. The rock bolt of any one of claims 1 to 11, wherein the helical pitch of said wire varies along its length.

13. A method of installing the rock bolt of any one of claims 1 to 12, comprising the steps of: drilling a bore hole with a blind end into a rock face to be stabilized; inserting a two-component resin filled cartridge having a frangible casing into said bore hole; inserting said rock bolt into said bore hole with said element leading end leading; thrusting said rock bolt towards said blind end whilst rotating said rock bolt in a direction opposing the direction of helical winding of said wire, puncturing said frangible casing, mixing said resin and pumping said resin towards said blind end; and stopping thrusting and rotation of said rock bolt, allowing said resin to cure.