AU2018204352B2 - Improved rock bolt - Google Patents

Abstract

A friction bolt (10) for frictionally engaging with the internal surface of a bore drilled into rock strata includes a generally circular tube (12) defining a longitudinal split (14). The tube is expandable, having a first leading end for insertion into a bore and a second end defining a head. The bolt includes an expander assembly (80) comprising first (82) and second (84) expander elements arranged so that relative movement of the two elements causes the diameter of the tube to expand at that location. The first expander element (82) is mounted on a rod (30) which extends along the tube towards the head of the bolt. The second expander element (84) locates between the rod and the tube and is not secured to the tube. An elongate rod (30) extends along at least part of the length of the tube which can be tensioned in use. A protruding portion (36) extends radially outwards from the elongate rod. At least the part of the rod (30) that defines the protruding portion (36) is enclosed in a polymer sleeve (70) which is itself enclosed by a further rigid sleeve (38). - I V4 0 94 0 Cs a 3 is c-4 w 0 S C m

Description

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Improved rock bolt

Cross-Reference to Related Applications

[0001] The present application claims priority from Australian Provisional Patent Application No 2017902333 filed on 19 June 2017, the content of which is incorporated herein by reference.

Technical Field

[0002] This invention relates to rock bolts.

Background

[0003] Rock bolts are used in rock reinforcement for the purpose of stabilising the rockmass. One type of rock bolt commonly used in hard rock mines is known as a friction bolt. This type of bolt comprises a tube, typically made of steel, that is split longitudinally and which is forced into a bore drilled into a rock strata, which is marginally smaller than the diameter of the tube. The tube becomes compressed so that the external surface of the tube engages the internal surface of the bore anchoring the rock bolt inside the bore by friction forces.

[0004] Friction bolts are relatively cheap to manufacture and are easy to use compared with some other types of rock bolts which often require resin or cement to lock them into the bore. However, friction bolts do have a number of drawbacks. One significant drawback is the tendency for friction bolts to disengage from the bore when a sufficiently large force is applied to the bolt.

[0005] With certain mining conditions, particularly those found in hard rock mining, the rock formations surrounding the bores are susceptible to movement or rock burst as a result of mining induced stress, the excavation closure of perimeter rock, minor earthquakes, and the like. Under dynamic loading caused by rock bursts, mine rock bolts are vulnerable to failure. In order to address this problem, the applicant developed a yielding rock bolt which is the subject of International Patent application No WO 2011/028790. The bolt is made of a steel bar which includes an upset to specified dimensions at one end and is partially or fully encapsulated in an engineered polymer to achieve yielding performance under quasi-static or dynamic loading. The bolt is installed in resin or cement to fix the bolt in the bore. The engineered polymer is locked into the solidified resin/cement and this inhibited from moving. In high energy loading conditions, the upset transfers the impacts onto the surrounding polymer coating and energy is dissipated by ploughing the upset through the polymer. While the yielding rock bolt of WO 2011/028790 works well, it is quite expensive to manufacture and somewhat time consuming to install in poor ground conditions.

[0006] The present invention aims to provide further improvements to the rock bolt disclosed in WO 2011/028790.

[0007] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.

[0008] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps

Summary

[0009] According to the present invention there is provided a friction bolt for frictionally engaging with the internal surface of a bore drilled into rock strata, the friction bolt comprising a generally circular tube defining a longitudinal split, the tube being radially expandable, the bolt having a first leading end for insertion into a bore and a second end defining a head; and

an expander assembly for expanding the diameter of the tube at least one location along the tube, the expander assembly comprising first and second expander elements arranged so that relative movement of the two elements causes the diameter of the tube to expand at that location; wherein

the first expander element is mounted on a rod which extends along the tube towards the head of the bolt, the second expander element locates between the rod and the tube and is not secured to the tube; and wherein an elongate rod extends along at least part of the length of the tube which can be tensioned in use; and wherein a protruding portion extends radially outwards from the elongate rod, and wherein at least the part of the rod that defines the protruding portion is enclosed in a polymer sleeve which is itself enclosed by a further rigid sleeve.

[0010] The provision of the expander assembly allows the rock bolt to be installed without the need for resin or grout and thus speeds up the installation as well as reducing complexity and allowing possible efficiency cost savings. The rigid sleeve encloses the polymer and secures it in position against movement of the protruding portion through the polymer sleeve.

[0011] Typically, the rigid sleeve is a steel sleeve. End caps may be provided at each end of the rigid sleeve to help retain the polymer sleeve within the rigid sleeve.

[0012] The polymer sleeve is preferably preformed as a annular sleeve having a cylindrical cross-section. This makes assembly of the rock bolt simpler and cheaper since the polymer does not have to be moulded/formed onto the elongate rod. The material forming the polymer sleeve may be varied so that the properties of the sleeve may be changed to suit particular applications and underground conditions. One polymer which may be used is polyethylene, although other suitable polymers may be used including, but not limited to, nylon, including glass filled nylon, polycarbonate or polyethylene terephthalate (PET).

[0013] One end cap distal from the head of the rock bolt may define a closed end which abuts one end of the elongate rod and a blind bore which is internally threaded and receives an externally threaded stud drive which connects to the expander assembly and may rotate with the elongate rod.

[0014] Typically, the first expander element is a wedge which defines an internally threaded bore and is mounted on the stud drive and rotation of the rod draws the wedge towards the head of the bolt. The second expander element may be a shell which locates between the wedge and the tube, and is preferably not secured to the tube.

[0015] Typically, the second expander element defines a projection which projects, preferably radially, outwardly from a longitudinal axis of the bolt and which locates in the longitudinal split to inhibit rotation of the second expander element about the longitudinal axis.

[0016] The shell may define a plurality of leaves which are spaced from one another and the first expander element may define a radially extending projection which locates in a longitudinal gap between two of the leaves to inhibit relative rotation of the first and second expander elements.

[0017] Typically, the radially extending projection on the first expander element is a fin.

[0018] Typically, the protruding portion is positioned adjacent the leading end of the bolt.

[0019] The polymer sleeve may have a first thickness at the protruding portion and a second thickness over the remaining portion of the elongate rod.

Brief Description of the Drawings

[0020] A specific embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 is a front view of a rock bolt embodying the present invention;

Figure 2 is a section on A-A shown in Figure 1;

Figure 3 is a detail on part B shown in Figure 2;

Figure 4 is a detail on part C shown in Figure 2;

Figure 5 is an exploded view of the components of the rock bolt shown in Figure 1;

Figure 6a is an isometric view of an end cap component shown in Figure 5;

Figure 6b shows a section through the end cap of Figure 6a;

Figure 7a is an isometric view of another end cap component shown in Figure 5;

Figure 7b shows a section through the end cap of Figure 7a;

Figure 8a is an isometric view of a nut shown in Figure 5;

Figure 8b is a side view of the nut shown in Figure 8a;

Figure 8c shows a section through the nut of Figure 8b on lines A-A; and

Figure 9 shows an enlarged view of part of an expander assembly shown in Figure 5.

Description of Embodiments

[0021] Referring to the drawings, and to Figures 1 and 5 in particular, a rock bolt in the form of a friction bolt 10 includes an elongate tube 12 made of steel, which is in the order of 2m long but whose length can vary from 1 to 5m, depending on the particular application, and is about 47mm in diameter. The tube 12 is split longitudinally at 14. The split 14 extends along the length of the tube. The tube has a proximal end or head 16 and a distal or leading end 18. The tube tapers towards the leading end 18 of the bolt. The tapered end makes it easier to insert the tube into a pre-drilled bore.

[0022] Approximately 300mm from the leading end of the tube there is an indent 22 which is rolled/crimped into the tube which narrows the internal diameter of the tube at that point.

[0023] The bolt or rod 30 which locates in the tube is best seen in Figure 5. It is typically made from steel. One end 32 which in use is proximal to the head of the rockbolt is threaded. The opposite end 34 defines an enlarged diameter portion and a protruding portion/step or "upset" 36 where the diameter of the rod increases. The enlarged diameter portion includes diametrically opposed flats to inhibit its rotation. The upset is typically machined and threaded onto the bolt , for ease of manufacture.

[0024] Figure 5 also shows a steel tube or sleeve 38 and a proximal or front end cap 40 and a distal or rear end cap 42. The end caps are externally threaded and the steel tube 38 which is internally threaded at both ends, screws onto the end caps. The end caps are shown in more detail in Figures 6a to 7b.

[0025] The front end cap 40 is tubular defining a smooth interior through bore 50 a larger diameter end portion 52 which is generally cylindrical but defines diametrically opposed flats 54, 56. An opposite end portion 58 of the front end cap is externally threaded.

[0026] The rear end cap 42 defines a closed end 60 which, in use abuts the distal end of the rod and a larger diameter end portion 62 which defines a blind bore 64 which is internally threaded and, in use, receives one proximal end of an externally threaded stud drive 64. The closed end 60 is externally threaded. The larger diameter end portion 62 is generally cylindrical but defines diametrically opposed flats 64, 66.

[0027] As is best seen by reference to Figure 4, the bolt 30 abuts the rear end cap 42 and passes through the front end cap 40 and is partly enclosed within the steel tube 38 which screws onto each of the end caps. An engineered polymer sleeve 70 locates between the steel tube 38 and the rod 30. The steel tube 42 confines the polymer sleeve. The front end cap 40 prevents the engineered polymer sleeve 70 from pulling out of the steel tube 38.

[0028] With reference to Figures 3 and 5, and in particular to Figure 9, the expansion assembly/anchor 80 comprises a first expander element in the form of a conical wedge element 82 and a second expander element in the form of an external shell 84. As is best seen in Figure 9, the external shell comprises four leaves 86 which are generally arcuate in a cross-section transverse to the longitudinal axis of the bolt and subtend an angle of about 90. (In alterative embodiments, the shell could comprise only two leaves or four leaves). The inner surface of the leaves 86 is smooth and part cylindrical. The external surface defines a series of ridges 88 which, in use, engage with the internal walls of the tube 12. The thickness of the leaves gradually increases from the end 90 of the leaves closest to the wedge element to the distal end 91.

[0029] Longitudinal gaps 92 are defined between the leaves, three of which are closed at one end, and one of which 92a is not. The wedge element 82 defines an external fin 85 which locates in the gap 92a between two adjacent leaves to prevent the wedge rotating relative to the shell. Gap 92a is a through gap and is not closed at the distal end of the shell furthest from the wedge. The second expander element/shell 84 defines two fins 93 and 94 which are located at the distal end 92 of the shell, spaced apart on either side of the through gap 92a.

[0030] In use, the two fins 93 and 94 locate in the split 14 in the tube 12. The distance between the two fins is about the same/slightly larger than the width of the split 14 so that the fins help to centre and steady the position of the shell 84 in the tube 12, as well as preventing rotation of the shell 84 relative to the tube.

[0031] The wedge element 82 is generally conical and tapers towards the external shell, having a wider end 100 and a narrower end 102. The wedge defines a central through hole 103 which is internally threaded (M24 thread) to engage with the externally threaded part of a stud drive 64.

[0032] Figures 8a to 8c show a nut 120 which threads onto the proximal end of the bolt 30. The nut 120 defines a flange 122 whose front face 124 is generally conically tapered. The front face of the nut defines an annular and circular slot 126 which is configured to receive the proximal end of the tube 12. The engagement of the end of the tube in the slot 126 allows the nut to be used to drive the rock bolt into a bore using percussion while assisting in maintaining the original shape of the tube 12. The nut 120 can be turned by a drive to tension the bolt 14 once the rock bolt 10 has been inserted into the bore.

[0033] In use, the friction bolt is inserted in a pre-drilled bore in the rock strata which is marginally smaller than the external diameter of the tube using percussion. Once the bolt 10 is completely installed, the nut is rotated anti-clockwise which rotates the rod 14 and stud drive 30 and draws the wedge 82 towards the head end of the bolt. The rotating wedge 82 is drawn along the stud drive into the expansion shell 84. The expansion shell 84 cannot move beyond the constriction 22 and the wedge thus causes the leaves to move outwards and the shell 84 to expand, and this expansion of the shell point anchors the friction bolt 10. This friction bolt may be used even in poor ground. Further anti-clockwise rotation of the nut tensions the rod 14.

[0034] The point anchoring of the friction bolt avoids the requirement for anchoring the friction bolt with resin and speeds up the installation process and provides a more reliable result. Once installed the bolt can be tensioned using the nut 120

[0035] During dynamic loading of the installed rock bolt all components of the rock bolt are substantially immovable apart from the rod 14 which is movable relative to the polymer sleeve in the longitudinal/axial direction. The rod may be axially displaced relative to the sleeve as it moves towards the head of the rock bolt. As is does so, the upset 36 ploughs through the material of the polymer sleeve, typically leaving a cavity where the protruding portion was previously. The dynamic loading generates a complex interaction between the protruding portion/upset and the contained polymer including confined compression and deformation of the polymer sleeve. This tends to absorb the dynamic energy in a controlled manner over the controlled displacement of the rod though the polymer sleeve. Compression and fiction engagement typically cause the polymer to heat and soften and become more flowable at and behind the protruding portion/upset 36. This yielding of the rock bolt under dynamic loading absorbs energy and reduces the likelihood of a catastrophic failure of the rock bolt, as well as providing an early indication of unstable mine conditions so that workers may reinforce the mine walls or roof and/or evacuate.

[0036] Providing the polymer as a simple preformed sleeve makes assembly of the rock bolt easier and cheaper. It is also possible to change the capacity of the rock bolt to absorb rock bursts and the like by changing the hardness of the polymer sleeve to change the resistance to movement of the upset 36 through the polymer.

[0037] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims (11)

CLAIMS:

1. A friction bolt for frictionally engaging with the internal surface of a bore drilled into rock strata, the friction bolt comprising a generally circular tube defining a longitudinal split, the tube being radially expandable, the bolt having a first leading end for insertion into a bore and a second end defining a head; and

an expander assembly for expanding the diameter of the tube at least one location along the tube, the expander assembly comprising first and second expander elements arranged so that relative movement of the two elements causes the diameter of the tube to expand at that location; wherein

the first expander element is mounted on a rod which extends along the tube towards the head of the bolt,

the second expander element locates between the rod and the tube and is not secured to the tube; and

wherein an elongate rod extends along at least part of the length of the tube which can be tensioned in use; and wherein

a protruding portion extends radially outwards from the elongate rod, and wherein at least the part of the rod that defines the protruding portion is enclosed in a polymer sleeve which is itself enclosed by a further rigid sleeve.

2. A friction bolt as claimed in claim 1 wherein the rigid sleeve is a metal sleeve and first and second end caps are provided at each end of the rigid sleeve to retain the polymer sleeve within the rigid sleeve.

3. A friction bolt as claimed in claim 2 wherein the rigid sleeve is made from steel.

4. A friction bolt as claimed in claim 2 or 3 wherein the first end cap distal from the head of the rock bolt defines a closed end which abuts one end of the elongate rod and a blind bore which is internally threaded and receives an externally threaded stud drive which connects to the expander means.

5. A friction bolt as claimed in any preceding claim wherein the first expander element is a wedge which defines an internally threaded bore and is mounted on the stud drive and wherein rotation of the rod draws the wedge towards the head of the bolt and wherein the second expander element is a shell which locates between the wedge and the tube, and is not secured to the tube.

6. A friction bolt as claimed in any preceding claim wherein the second expander element defines a projection which projects radially outwardly from a longitudinal axis of the bolt and which locates in the longitudinal split to inhibit rotation of the second expander element about the longitudinal axis.

7. A friction bolt as claimed in claim 5 wherein the shell defines a plurality of leaves which are spaced from one another and wherein the first expander element defines a radially extending projection which locates in a longitudinal gap between two of the leaves to inhibit relative rotation of the first and second expander elements.

8. A friction bolt as claimed in claim 7 wherein the radially extending projection on the first expander element is a fin.

9. A friction bolt as claimed in any preceding claim wherein the protruding portion is positioned adjacent the leading end of the bolt.

10. A friction bolt as claimed in any preceding claim wherein the polymer sleeve has a first thickness at the protruding portion and a second thickness over the remaining portion of the elongate rod.

11. A method of installing a friction bolt as claimed in any one of claims 1 to 10 including the steps of:

forming a bore in rock strata;

inserting the friction bolt in the bore, typically using percussion;

rotating the elongate rod about its longitudinal axis to anchor the friction bolt in the bore and to tension the elongate rod.