WO2004063531A1

WO2004063531A1 - Rock bolt re-tensioning

Info

Publication number: WO2004063531A1
Application number: PCT/AU2004/000021
Authority: WO
Inventors: Jeffrey Robert Fergusson
Original assignee: Jeffrey Robert Fergusson
Priority date: 2003-01-10
Filing date: 2004-01-08
Publication date: 2004-07-29

Abstract

The present invention discloses a rock bolt (100,117) which is able to be re-tensioned after initial installation. The rock bolt has a hollow shaft (22, 102) which receives via its trailing end a threaded shank (109, 129). The rock bolt passes through an apertured support member (17, 37). Rotation of the shank (109, 129) causes an engagement member (116, 131) to bear against the support member which in turn bears against the coal, rock or other mineral face into which the rock bolt is inserted. The threaded shank (109, 129) can be received by an internally threaded nut (26) or by an internally threaded tube (106). The shank (109, 129) can be temporarily fixed relative to the rock bolt by means of a frangible pin (121, 210).

Description

ROCK BOLT RE-TENSIONING

Field of the Invention

The present invention relates to rock bolts which are used in coal mining, hard rock mining, and civil engineering projects including tunneling.

Background Art

Rock bolts have a leading end and a trailing end with the leading end being inserted into a blind hole drilled in the coal or rock face. One type of rock bolt has only the leading end secured to the interior of the hole (either by resin or grout located at the leading end only, or by an expansion member carried by the leading end and mechanically manipulated to grip the interior of the hole and thereby anchor the leading end of the bolt). Such rock bolts are tensioned after the leading end is anchored, for example, by tightening a nut which is threadably engaged with the trailing end of the rock bolt and which typically bears on an apertured support plate through which the trailing end passes. The support plate in turn bears on the coal or rock face. Such rock bolts are inherently re-tensionable at any time merely by re- tightening the nut.

The main reason why such rock bolts may require re-tensioning is that the rock or coal face under the support plate is friable or otherwise breaks away. This results in the support plate no longer bearing against the coal or rock and the bolt no longer being in tension. Accordingly, the mechanical support initially provided by the tensioned rock bolt no longer exists in its intended form.

Because the length of thread on the trailing end of the rock bolt tendon is limited, there are limits as to how much the nut can be re-tightened. However, in principle the nut can, if necessary, be removed and various shims or washers placed over the trailing end and the nut then replaced and re-tightened.

However, in order to increase the tensile load able to be carried by the rock bolt and/or to increase the corrosion resistance of the tendon of the rock bolt, most rock bolts have the tendon fully encapsulated in a sleeve of grout or resin. (For yielding rock bolts a portion of the tendon may not be encapsulated in grout or resin but at least the trailing end of the tendon is so encapsulated). Normally this grouting also covers or at least fouls the nut at the trailing end of the bolt thereby making re- tensioning difficult.

The installation of such encapsulated rock bolts is assisted if the nut (or equivalent drive arrangement) is fixed to the trailing end of the rock bolt. This means that the rock bolt can be rotated during installation, for example to mix the resin and/or to expand an anchor at the leading end of the bolt. Unless a post-installation grouting technique is to be used, the machine installing the rock bolt temporarily holds the rock bolt fully driven home for a short period whilst the grout sets. After the installing machine releases the bolt the coal or rock previously compressed by the machine returns to its original dimensions, thereby placing the rock bolt tendon in tension as desired.

If now the coal or rock face crumbles away, the drive nut no longer forces the support plate against the coal or rock face. As it is not possible to rotate the rock bolt tendon, it is not possible to cause the support plate to come into contact with the coal or rock face. That is, the face is unsupported and the rock bolt is not tensioned as originally installed. This can give rise to a dangerous situation.

One particular form of encapsulated rock bolt has a solid tendon in the form of a hollow shaft. Such bolts are sold under the Registered Trade Mark JUMBOLT owned by the present applicant.

This bolt is provided with a castellated ring at its trailing end which is used to rotate the bolt and drive it home during installation. Such bolts have enjoyed widespread commercial acceptance since their commercial release in Australia a few years prior to the priority date of the present invention.

Object of the Invention

The aim of the present invention is to substantially overcome, or at least to some extent ameliorate, the abovementioned problems caused for such hollow shaft rock bolts as a result of the rock or coal face retreating after the bolt has been installed.

Summary of the Invention In accordance with a first aspect of the present invention there is disclosed a method of re-tensioning a rock bolt having a hollow shaft and being adapted to be fixedly secured in a blind hole drilled in a coal or rock face, said method comprising the steps of:

(i) passing a leading end of a shank having a head through an apertured support member dimensioned to permit said head to bear against said support member,

(ii) engaging said shank leading end with said hollow shaft and advancing said shank into said shaft until said support member bears directly, or indirectly, against said coal or rock face to place said hollow shaft in tension, and (iii) if necessary if said rock face crumbles away, further advancing said shank relative to said shaft to re-tension said hollow shaft.

In accordance with a second aspect of the present invention there is disclosed a re-tensioning arrangement for a rock bolt having a hollow shaft and being adapted to be fixedly secured in a blind hole drilled in a coal or rock face, said arrangement comprising a shank having a head and extending through an apertured support member with said head being dimensioned to bear against said support member; said shank and hollow shaft having engagement means whereby said shank can be advanced into said shaft until said support member directly, or indirectly, bears against said coal or rock face to place said hollow shaft in tension.

In accordance with a third aspect of the present invention there is disclosed a rock bolt in combination with the abovementioned re-tensioning arrangement.

Brief Description of the Drawings

Embodiments of the present invention will now be described with reference to the drawings in which: Fig. 1 is a truncated schematic perspective view of a hollow shaft rock bolt having a nut secured to its trailing end,

Fig. 2 is a perspective view of the trailing end of the bolt of Fig. 1,

Fig. 3 is a longitudinal exploded cross-sectional view through a rock bolt with a re-tensioning arrangement in accordance with a first embodiment of the present invention,

Fig. 4 is a view similar to Fig. 3 but of a modified arrangement to that of Fig. 3,

Fig. 5 is a longitudinal exploded cross-sectional view through a rock bolt with a re-tensioning arrangement of a second embodiment, and

Fig. 6 is a view similar to that of Fig. 5 but showing the rock bolt after installation.

Detailed Description As seen in Figs 1 and 2, a rock bolt 117 of the general type of the abovementioned JUMBOLT rock bolts has a shaft 22 which is a hollow cylinder of one of many possible lengths. The shaft 22 is preferably fabricated from conventional steel pipe and preferably has a nominal bore of from 10-30 mm approximately. The shaft 22 has a tip 23 or leading end which is swaged closed so as to create four impellers 24 which are used to shred a resin cartridge (not illustrated) and mix the resin or grout used to hold the shaft 22 of the rib bolt 117 in position. Such a bolt is preferably used in coal mining applications. Alternatively, the bolt is of larger diameter (eg 45- 50mm) for use in hard rock mining applications.

At the opposite, trailing end of the shaft 22 a domed washer 25 and an hexagonal nut 26 are secured by a means of a frictional press fit, by welding, by threaded engagement, or by some other conventional such means. The interior of the nut 26 is provided with an internal thread 27 which, was originally intended to be threadably engaged with an external thread 29 on the shank 30 of a support member 31 (Fig. 2). A typical support member 31 is fabricated from a conventional bolt to which is welded a U-shaped link 32. Such support members can be used to support temporary lights, warning signs, etc. As best seen in Fig 2, prior to insertion of the rock bolt 1 into a hole previously drilled in the coal or a hard rock mineral face, the shaft 22 is passed through a support plate 37 which has a recessed collar 38 which mates with the convex face of the domed washer 25. Following installation of the rock bolt 117, a two-part epoxy resin or grout surrounds the shaft 22 and fills the space between the shaft 22 and the hole (not illustrated). This quick setting resin or grout secures the bolt in position and a hollow tubular spanner (conventional but not illustrated) is used to drive the rock bolt 117 so as to rotate it about its longitudinal axis during the installation procedure. This drives the washer 25 and nut 26 into the recessed collar 38 and ensures that the support plate 37 compresses the coal or rock in the vicinity of the hole. The result is that the coal or rock is stabilized.

In order to ensure adequate keying or bonding between the grout or resin and the shaft 22, the shaft 22 is preferably provided with a plurality of keying deformations 28 (only some of which are illustrated) along its length in known fashion (eg. by helically rolling). This ensures good keying between the grout and the shaft 22.

In accordance with a first embodiment of the present invention, the rock bolt 117 instead of being provided with a services support member 31 can instead be utilized with a re-tensioning member 131 as illustrated in Fig. 3. The re-tensioning member 131 takes the form of a threaded shank 129 mounted within a cup 130 having a cylindrical side wall 140. A nut 126 is welded to the cap of the cup 130. Alternatively, the shank 129 and nut 126 can also be formed as a conventional bolt which passes through the cap of the cup 130.

As also illustrated in Fig. 3, it often happens that the coal or rock face 150 in the vicinity of the bolt 117 crumbles away after the bolt 117 has been installed and initially tensioned. This is schematically illustrated in Fig. 3. A consequence of this change in the coal or rock face 150 is that the support plate 37 no longer supports, or compresses, the coal face 150. Consequently, the bolt 117 is no longer in tension. Since the bolt 117 is securely embedded in resin, it is unable to be moved. So, normally in this circumstance the plate 37 lies free of the coal or rock face 150 adjacent, but not abutting, the nut 26.

This can be remedied in accordance with the first embodiment of the present invention by engaging the shank 129 with the interior of the nut 26, (the shank 129 having an external diameter less than the internal diameter of the bolt shaft 22) thereby causing the cylindrical side wall 140 to pass over the nut 26 and abut the support plate 37. Continued rotation of the nut 126 causes the side wall 140 to drive the support plate 37 into contact with the coal or rock face 150. In this way the coal or rock face 150 can again be supported and the bolt 117 again placed in tension.

The arrangement of Fig. 4 also illustrates a domed washer 125 which is slidably mounted on the bolt 117 rather than being fixed thereto, by welding for example, as illustrated in Figs. 1 and 2.

In a modified embodiment, illustrated in Fig. 5, the nut 26 is provided with a radial hole 200 into which a breakable pin 201 is inserted by means of a friction fit. The shank 129 can then have its tip only partially threadably inserted into the nut 26 until the tip of the shank 129 abuts the pin 201. In this pre-assembled condition, the entire bolt assembly illustrated in Fig. 5 can be mounted in the hole in the rock or coal face 150 but with the conventional driving machinery engaging the nut 126 rather than the nut 26. In this way the support plate 37 is driven against the coal face 150 in the usual way (and without the side wall 140 abutting the support plate 37).

If subsequently the coal or rock face 150 should crumble away, the support plate 37 can be brought to again bear against the new face 150 in the following fashion. The nut 126 is rotated thereby causing the shank 129 to rotate relative to the nut 26 which is now fixed by hardened grout. The shank 129 therefore fractures the pin 201 and progresses through the nut 26 until the front edge of the re-tensioning member 131 (in this instance the front edge of the side wall 140) bears against the support plate 37. This re-engagement of the support plate 31 against the face 150 can be repeated several times, if necessary to re-tension the bolt 117 each time. Also the cup 130 can be replaced with an equivalent cup but with a longer side wall 140, if necessary.

Turning now to Figs. 5 and 6, a second embodiment of a re-tensionable rock bolt in the form of rock bolt 100 is illustrated. The bolt 100 has a shaft 102 and is essentially as before, however, at the trailing end of the shaft a nut, or a length of internally threaded tube, 106 is welded in place of the nut 26 of the first embodiment. The tube 106 preferably has an outer diameter substantially equal to, or less than, that of the shaft 102. In addition, the tube 106 has a hole in one side (similar to hole 200 of Fig. 4) into which a breakable pin 121 is secured by means of a friction fit.

An externally threaded drive member 111 having a shank 110 with thread 109 and a chamfered nut 116 is passed through a support plate 17. The shank 110 can then have its tip only partially threadably inserted into the tube 106 until the tip of the shank 110 abuts the pin 121. In this pre-assembled condition, the entire bolt 100 can be mounted in the hole 140 in the rock or coal face 150 as seen in Fig. 6 but with the conventional driving machinery engaging the nut 116 rather than the tube 106. The tube 106 is driven some distance into the hole 140 during this initial procedure. Also the support plate 17 is driven against the rock or coal face 150 by the shank 110 breaking the pin 121 and passing through the tube 106.

If subsequently the rock or coal face 150 should crumble away, the support plate 17 can be brought to bear against the new face 150 and thereby re-tension the rock bolt 100 in the following fashion. The nut 116 is rotated thereby causing the shank 110 to rotate relative to the tube 106 which is now fixed by hardened grout. The shank 110 therefore progresses through the tube 106 until the front of the support plate 17 again bears against the new face 150. This re-engagement of the support plate 17 against the face 150 can be repeated several times, if necessary. Also the nut 116 and shank 110 can be replaced with an equivalent nut and shank of different length, if necessary.

Furthermore, the keying deformations 28 on the shaft 22 of the bolt 117 of Fig. 1 can form an internal thread on the interior surface of the shaft 22. Where such a shaft is provided with a castellated ring as a drive mechanism (rather than the nut 26) the re- tensioning arrangement can have a shaft 129 dimensioned to mate with the thread on the interior of the shaft 22. Under these circumstances the cup 130 is dimensioned to pass over the castellated ring and abut the support plate 37.

The foregoing describes only some embodiments of the present invention and modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the present invention. For example, rather than providing a threaded engagement between the shank 129 and nut 26, or the shank 110 and tube 106, the shank can be provided with a series of frusto-conical protrusions. Similarly, the nut 26 or tube 106 is provided with a single circular orifice that is slightly smaller in diameter than the maximum diameter of each conical protrusion. Thus the shank can be advanced through the orifice by applying axial blows to the nut 126 or 116 or equivalent, for example by a sledgehammer. The last protrusion to pass through the orifice leaves the plate 37 or 17 compressed against the rock or coal face 150 and the bolt 117 or 100 in tension. If then the face 150 crumbles away, the plate 37 or 17 can be brought back into contact with the new face 150 and the bolt 117 or 100 thereby re- tensioned, merely by applying further axial blows to the nut 126 or 116 or equivalent with the sledgehammer.

In a further modification, the cup 130 can take the form of a U-shaped member having two arms 140 which each bear against the support plate 37. Such an arrangement has the same appearance in cross-section as the cup 130 of Figs. 3 and 4.

In a still further modification, the tip 23 can be crimped onto a threaded rod which carries a substantially conventional expansion anchor. This allows the rock bolt to be initially point anchored. A hole (not illustrated) in the side wall of the tubular shaft 22 near the tip 23 allows for grouting after the initial point anchoring. Thereafter the bolt can be re-tensioned at various time intervals, as needed, in the same manner as the illustrated embodiments.

The term "comprising" as used herein is used in the inclusive sense of "including" or "having" and not in the exclusive sense of "consisting only of.

Claims

1. A method of re-tensioning a rock bolt having a hollow shaft and being adapted to be fixedly secured in a blind hole drilled in a coal or rock face, said method comprising the steps of:

(ii) engaging said shank leading end with said hollow shaft and advancing said shank into said shaft until said support member bears directly, or indirectly, against said coal or rock face to place said hollow shaft in tension, and

(iii) if necessary if said rock face crumbles away, further advancing said shank relative to said shaft to re-tension said hollow shaft.

2. The method as claimed in claim 1 wherein said shank is threaded and said step (ii) comprises the step of threadably engaging said shank and said hollow shaft.

3. The method as claimed in claim 1 or 2 wherein said support member comprises a support plate and step (ii) comprises the step of forcing said support plate into direct contact against said coal or rock face.

4. The method as claimed in claim 1 or 2 wherein said support member is concave and has a leading portion, said hollow shaft is passed through an apertured support plate prior to insertion of said hollow shaft into said hole, and step (ii) comprises the step of bringing said leading portion into engagement with said support plate whereby said support member indirectly bears against said coal or rock face.

5. The method as claimed in any one of claims 1-4 including the step of: (iv) temporarily fixing said shank relative to said hollow shaft prior to advancing said shank into said shaft.

6. The method as claimed in any one of claims 1-5 including the step of: (v) replacing said support member with a similar support member of different length.

7. A re-tensioning arrangement for a rock bolt having a hollow shaft and being adapted to be fixedly secured in a blind hole drilled in a coal or rock face, said arrangement comprising a shank having a head and extending through an apertured support member with said head being dimensioned to bear against said support member; said shank and hollow shaft having engagement means whereby said shank can be advanced into said shaft until said support member directly, or indirectly, bears against said coal or rock face to place said hollow shaft in tension.

8. The arrangement as claimed in claim 7 wherein said engagement means comprises an external thread on said shank and an internally threaded member adjacent a trailing end of said hollow shaft.

9. The arrangement as claimed in claim 8 wherein said internally threaded member comprises a drive nut secured to said trailing end.

10. The arrangement as claimed in claim 9 wherein the transverse dimension of said drive nut exceeds the transverse exterior dimension of said hollow shaft.

11. The arrangement as claimed in claim 10 wherein said hollow shaft passes through an apertured support plate which bears against said coal or rock face.

12. The arrangement as claimed in claim 11 wherein said support member is concave and has a leading portion which bears against said support plate.

13. The arrangement as claimed in claim 12 wherein said support member is cup shaped.

14. The arrangement as claimed in claim 12 wherein said support member is U- shaped and comprises a pair of arms.

15. The arrangement as claimed in any one of claims 9-14 wherein said drive nut includes a frangible pin which limits the depth of engagement of said shank with said drive nut whereby the assembly comprising said rock bolt with said shank limitedly engaged with said drive nut is driveable by said head of said shank.

16. The arrangement as claimed in claim 8 wherein said internally threaded member comprises an internally threaded tube adjacent said trailing end.

17. The arrangement as claimed in claim 16 wherein said internally threaded tube has a transverse dimension that is less than, or substantially equal to, the transverse exterior dimension of said hollow shaft whereby during installation of said rock bolt said internally threaded tube passes into said hole.

18. The arrangement as claimed in claim 17 wherein said apertured support member comprises an apertured support plate which directly bears against said coal or rock face.

19. The arrangement as claimed in any one of claims 16-18 wherein said internally threaded tube includes a frangible pin which limits the depth of engagement of said shank with said tube whereby the assembly comprising said rock bolt with said shank limitedly engaged with said internally threaded tube is driveable by said head of said shank.

20. The arrangement as claimed in any one of claims 7-19 wherein said shank is replaceable by a similar shank of different length.

21. A rock bolt in combination with the re-tensioning arrangement as claimed in any one of claims 7-20.