AU2003204831A1 - Yielding strata bolt - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name of Applicant: Actual Inventor: Address for Service: Invention Title: Details of Basic Application(s): INDUSTRIAL ROLL FORMERS PTY

LIMITED

JEFFREY ROBERT FERGUSSON FRASER OLD SOHN Patent Attorneys Level 6, 118 Alfred Street MILSONS POINT NSW 2061 YIELDING STRATA ABOLT Australian Provisional Patent Application No. PS3108 Filed 21 June 2002 The following statement is a full description of this invention, including the best method of performing it known to *me/us: 2945BZ-AU YIELDING STRATA BOLT Field of the Invention The present invention relates to strata bolts and, in particular, to a yielding strata bolt intended to control the movement of unstable rock strata into which the bolt is installed. The tendon of the bolt can either be a solid rod or a flexible cable.

Background Art Rock strata are liable to move as a result of various developments including mineinduced seismicity, the excavation of perimeter rock, minor earthquakes, and the like.

Some such movements are termed "rock bursts".

In general, regular rock bolts are insufficient to withstand such movement and snap.

In the past various proposals have been made. One such proposal is the so called DURABAR or DURABOLT (South African Patent No. 94/2177) invented by D Ortlepp which provides a heavy solid steel bar with a wriggle-like deformation. The bar is grouted in place at installation. As a consequence, in the event of ground movement, the deformed portion of the bar pulls through the grout and this absorbs a considerable amount of energy.

A similar arrangement is to shape the far end of the bar into a conical form which is embedded in grout. The shank of the bar is coated with wax which means that this part of the bar does not bond with the grout. In the event of excessive forces being applied to the bar, the conical end is forced or pulled through the grout. Again this absorbs a considerable amount of energy.

An alternative arrangement is to insert a mild steel slug within a multi-strand steel cable. A tapered sleeve is then placed over the cable. In the event of rock movement, the intention is to extrude the slug through the cable wires which are held in place by the tapered sleeve thereby giving a high pull through force and absorbing a considerable amount of energy. This arrangement is difficult to use in such a way as to give reproducible results and is time consuming to assemble.

2945BZ-AU Object of the Invention The object of the present invention is to provide a yielding cable bolt which is easy to fabricate and assemble and which provides a substantially predictable and repeatable result.

Summary of the Invention According to a first aspect of the present invention there is disclosed a yielding grouted rock bolt to control the movement of unstable rock strata into which the bolt is installed, said bolt comprising an elongate tendon, a portion of said tendon having a grout slippage means, and a grout engaging anchor fitted to said tendon portion and thereby at least partially deforming same, whereby in yielding said tendon portion passes through said anchor and is worked thereby.

In accordance with a second aspect of the present invention there is disclosed a two part rock bolt anchor adapted to be fitted to a tendon of a rock bolt, said anchor comprising a body engageable with grout into which said bolt is embedded, and having two parts shaped to be clamped together over said tendon.

In accordance with a third aspect of the present invention there is disclosed a method of permitting a grouted rock bolt having a tendon to yield to control the movement of unstable rock strata into which the bolt is installed, said method comprising the steps of: providing a portion of said tendon with grout slippage means; (ii) fitting at least one grout engaging anchor to said tendon and thereby at least partially deforming same; (iii) installing said rock bolt in a blind hole drilled in said rock strata; (iv) introducing flowing hardenable grout into said hole to surround said bolt tendon and said anchor(s); and permitting said tendon portion to move through said grout but be worked by movement of said portion through said anchor(s) which is/are substantially immobilized in said grout.

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 perspective view of a bulge portion of a prior art multi-strand steel cable; FIG 2 is a longitudinal view, partly in section, of the cable portion of FIG 1 installed as a rockbolt; FIG 3 is an exploded perspective view of a grout engaging anchor; FIG 4 is an exploded perspective view of the anchor of FIG 3 during assembly onto a portion of a cable; FIG 5 is a perspective view showing the assembled grout engaging anchor; FIG 6 is a longitudinal sectional view through the assembled anchor of FIG FIG 7 is a view similar to FIG 6 but of a second embodiment but of the anchor of FIGS 3 to 6; FIG 8 is an exploded perspective view illustrating a yielding cable bolt in accordance with a first embodiment of the present invention; FIG 9 is a longitudinal view, partly in section, illustrating the bolt of the type shown in FIG 8 after installation; FIG 10 is a view similar to FIG 8 but illustrating a yielding cable bolt of a second embodiment; FIG 11 is a view similar to FIG 9 but of a cable bolt of the type shown in FIG FIGS 12 and 13 are views similar to Figs 4 and 5 respectively but of a still further embodiment of the anchor and bolt; and FIG 14 is a perspective view of yet another embodiment of a grout engaging anchor.

Detailed Description Cable bolts are traditionally made from multi-strand steel cable 1 such as that illustrated in FIG 1. The cable is conveniently bulged at 2 in known fashion by gripping the cable 1 at two spaced apart locations and forcing the gripped regions together to permanently spring out, or bulge, the strands 3.

As illustrated in FIG 2, the intention of such a bulge 3 is to enable grout 5 which is normally used to surround the cable 1, to better the grip the cable 1 and so provide 2945BZ-AU good keying between the cable 1 and the grout 5. Such grout 5 is often installed by means of a tube 6 of inexpensive plastics material through which the cable 1 passes.

An alternative to bolts with a cable tendon are bolts with a solid tendon. One such bolt is described in Australian Patent No. 669,393 (WO 94/05900) and known as the "CT" bolt.

Turning now to FIGS 3 to 6, a grout engaging anchor 10 is proposed which is fabricated from two complimentary shells 11 and 12, each of which forms half of the anchor 10. The shell 11 is provided with a single recess 14 whilst the shell 12 is provided with a single protrusion 15. The recess 14 and protrusion 15 are of complimentary shape. In addition, each shell 11 and 12 is provided with a half boss 17 at each end.

As indicated in FIG 4, the anchor 10 is assembled by passing the cable 1 through a close fitting tube 8 of inexpensive plastics material. The tube 8 is preferably manufactured from high density polyethylene (HDPE) and is widely used for irrigation purposes. The two shells 11 and 12 are then positioned as indicated in FIG 4 and clamped together so as to securely grip the cable 1 within the anchor 10. With the two shells 11 and 12 clamped together, a keeper ring 19 is passed over the two adjacent half bosses 17 at each end of the anchor 10. With both keeper rings 19 in place, the clamping force on the anchor 10 can be released since the half bosses 17 are then maintained within the keeper rings 19. This is the situation illustrated in FIGS and 6.

FIG 7 illustrates a second embodiment of the anchor 100 in which the keeper rings 19 are as before but each of the shells 111 and 112 is provided with a recess 114 and a protrusion 115. As will become apparent hereafter, the purpose of the anchor 100 of FIG 7 is to provide a greater degree of work before the cable 1 can be passed therethrough.

Turning now to FIG 8, a first embodiment of a yielding cable bolt 21 is illustrated.

The tendon 22 of the bolt 21 is fabricated from the multi-strand steel cable 1 and the near end is provided with the threaded end fitting 23 which cooperates with a load 2945BZ-AU plate 24, grout injector 25 and nut 26. The grout injector 25 works in the general manner described in the abovementioned Australian Patent No. 669,393 (WO 94/05900). In the embodiment illustrated in FIG 8, a single anchor 10 is secured adjacent the free end of the bolt 21, however, in the embodiment illustrated in FIG 9 a pair of spaced apart anchors 10 are so secured.

In the particular embodiment illustrated in FIG 9, three strata 30, 31 and 32 are illustrated and strata 30 and 32 are relatively strong whereas stratum 31 is relatively weak and liable to movement. The tube 8 covers the cable 1 essentially throughout the stratum 32 but does not cover the cable 1 essentially throughout the strata 30 and 31. As a consequence, there is good keying between the cable 1 and the grout 5 in the area of strata 30 and 31 but the far end of the cable bolt 21 is itself able to move relative to the grout 5 if necessary, notwithstanding that the two anchors 10 are securely fixed within the grout 5 within the stratum 32.

As a consequence, in the event that strata 30 and 31, for example, move to the left as seen in FIG 9, the cable 1 at the near end of the cable bolt 21 will move with the strata and 31 due to the keying between the cable 1 and the grout 5. However at the far end of the cable bolt 21 the cable 1 covered by tube 8 is free to move relative to the grout 5 but the grout anchors 10 remain firmly secured relative to the grout 5. As a consequence, the cable 1 is plastically deformed by the anchors 10 as the cable 1 moves past the interengaged recesses 14 and protrusions As a result, considerable mechanical work is performed in moving the far end of the cable bolt 21 through the anchors 10. In this way, a considerable amount of energy is able to be rapidly dissipated thereby ensuring that the cable bolt 21 yields and absorbs the energy, but does not break.

FIGS 10 and 11 illustrate a second embodiment which is essentially as before save that prior art tube 6 is provided adjacent the near end of the cable bolt 21 and the far end of the cable bolt 21 is provided with an expansion anchor 35 which is formed as part of the anchor 10, or anchor 10 closer(est) to the far tip of the cable. The expansion anchor 35 enables installation of the bolt so as to permit post tensioning 2945BZ-AU grouting. In addition, the uncovered region of the tendon 22 (that is without the tube 8) is provided with a number of bulges 2 which function, as before, to increase the keying or interengagement between, the cable 1 and the grout As explained above in relation to FIG 9, the far end of the cable 21 of FIG 11 is able to plastically deform and move through the anchors 10 thereby absorbing the energy of the moving strata.

Turning now to Figs 12 and 13, a still further embodiment of an anchor 200 is illustrated in which each of the shells 211, 212 is substantially identical having a central protrusion 215. Thus the two opposed protrusions 215 form a pinch point which deforms the solid rod tendon 201 of the rock bolt. Thus the two protrusions 215 securely clamp the solid tendon 201 which is forced to deform and pass between the protrusions 215 as the bar or tendon 201 yields. Thus a considerable amount of energy is dissipated in this plastic deformation of the solid bar 201 and the rock bolt is able to yield without breaking.

Furthermore, where the bar 201 is provided with a pattern of exterior lugs or ribs (either forming a thread, a partial thread or other grout engaging mechanism) then such lugs or ribs are also able to be deformed by the pinch point created between the two opposed protrusions 215. Depending upon the severity of the pinch point only the exterior lugs can be deformed without deforming the central body of the bar 201, or the entirety of the bar 201 can be deformed.

As seen in Fig. 14, in yet another embodiment, a one piece anchor 300 is provided.

The anchor 300 is formed from a tubular piece of steel, and is preferably annular in transverse cross-section having an internal diameter just larger than the exterior diameter of the tube 8, or if tube 8 is omitted and the tendon 301 waxed or greased, for example, then larger than the tendon 301. The tendon 301 can be either a solid rod or bar as illustrated in Fig. 14 or a cable as illustrated previously.

After the anchor 300 has been slid over the far end of the bolt, the anchor is pressed onto the tendon by centrally deforming the anchor 300 as illustrated which partially 2945BZ-AU deforms the tendon 301 thereby creating a pinch point. When a sufficient tensile load is applied to the tendon 301, the tendon 301 pulls through the anchor 300 and is plastically deformed thereby dissipating energy. The deformed anchor 300 remains fixed in the surrounding grout (not illustrated in Fig. 14). If desired, the anchor 300 can be deformed at a number of longitudinally spaced apart locations thereby forming a series of pinch points which are preferably of increasing severity moving away from the far end of the bolt.

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.

The term "comprising" and its grammatical variations as used herein are used in the sense of "including" or "having" and not in the exclusive sense of "consisting only of'.

2945BZ-AU

Claims (22)

1. A yielding grouted rock bolt to control the movement of unstable rock strata into which the bolt is installed, said bolt comprising an elongate tendon, a portion of said tendon having a grout slippage means, and a grout engaging anchor fitted to said tendon portion and thereby at least partially deforming same, whereby in yielding said tendon portion passes through said anchor and is worked thereby.

2. The bolt as claimed in claim 1 wherein said grout slippage means comprises a tube surrounding said tendon portion.

3. The bolt as claimed in claim 2 wherein said tendon is formed from metal and said tube from plastics.

4. The bolt as claimed in any one of claims 1-3 wherein said anchor comprises a body engageable with said grout and formed in two parts which are clamped together over said tendon portion to fit said anchor thereto. The bolt as claimed in claim 4 wherein said parts include at least one complementary protrusion and recess.

6. The bolt as claimed in claim 5 wherein said parts include a plurality of complementary protrusions and recesses.

7. The bolt as claimed in claim 4 wherein said parts include at least one pair of opposed protrusions forming a corresponding pinch point.

8. The bolt as claimed in any one of claims 4-7 wherein said two parts are substantially identical.

9. The bolt as claimed in any one of claims 4-8 wherein said parts are maintained clamped together by keeper rings shaped to mate with said parts. The bolt as claimed in any one of claims 1-9 wherein said tendon comprises a multi-strand cable.

11. The bolt as claimed in any one of claims 1-9 wherein said tendon comprises a bar.

12. The bolt as claimed in any one of claims 1-11 and having a plurality of said grout engaging anchors.

13. A yielding grouted rock bolt to control the movement of unstable rock strata into which said bolt is installed, said bolt being substantially as herein described with reference to Figs. 3-6 and 8 or 9, or Figs. 3-6 and 10 and 11, or Figs. 7 and 8 or Figs 7 and 9, or Figs. 7, 8, 10 and 11, or Figs. 12 and 13 of the drawings. 2945BZ-AU

14. A two part rock bolt anchor adapted to be fitted to a tendon of a rock bolt, said anchor comprising a body engageable with grout into which said bolt is embedded, and having two parts shaped to be clamped together over said tendon. The anchor as claimed in claim 14 wherein said parts include at least one complementary protrusion and recess.

16. The anchor as claimed in claim 15 wherein said parts include a plurality of complementary protrusions and recesses.

17. The anchor as claimed in claim 14 wherein said parts include at least one pair of opposed protrusions forming a corresponding pinch point.

18. The anchor as claimed in any one of claims 14-17 wherein said two parts are substantially identical.

19. The anchor as claimed in any one of claims 14-18 wherein said parts are able to be maintained clamped together by keeper rings shaped to mate with said parts. A two part rock bolt anchor substantially as herein described with reference to Figs. 3-6 or Fig. 7 or Figs. 12 and 13 of the drawings.

21. A method of permitting a grouted rock bolt having a tendon to yield to control the movement of unstable rock strata into which the bolt is installed, said method comprising the steps of: providing a portion of said tendon with grout slippage means; (ii) fitting at least one grout engaging anchor to said tendon and thereby at least partially deforming same; (iii) installing said rock bolt in a blind hole drilled in said rock strata; (iv) introducing flowing hardenable grout into said hole to surround said bolt tendon and said anchor(s); and permitting said tendon portion to move through said grout but be worked by movement of said portion through said anchor(s) which is/are substantially immobilized in said grout.

22. The method as claimed in claim 21 including the further step of forming said tendon as a multi-strand cable.

23. The method as claimed in claim 21 including the further step of forming said tendon as a bar.

24. A method of permitting a grouted rock bolt having a tendon to yield to control the movement of unstable rock strata into which the bolt is installed, said method being substantially as herein described with reference to Figs. 3-13 of the drawings. 2945BZ-AU A rock bolt anchor adapted to be pressed onto a tendon of a rock bolt, said anchor comprising a body engageable with grout into which said bolt is embedded, and said anchor being shaped to be press fitted to said tendon.

26. The rock anchor as claimed in claim 25 and comprising a tube having an interior sized to receive said tendon.

27. The rock anchor as claimed in claim 26 wherein a portion of said tube is crushed to press fit said tube onto said tendon. Dated this 19 th day of June 2003 INDUSTRIAL ROLL FORMERS PTY LIMITED By FRASER OLD SOHN Patent Attorneys for the Applicant 2945BZ-AU