Yielding strata bolt
The present invention discloses a rock bolt having a tendon (1, 201) which is able to yield and thereby control movement of unstable rock strata into which the bolt is installed. The tendon has a portion which is provided with a grout slippage mechanism, preferably in the form of a tube (8) which surrounds the tendon. A two part grout anchor (10, 100, 200) is clamped to the tendon. The grout anchor is embedded in grout and is therefore essentially immovable. The tendon passing through the anchor is worked and thereby dissipates energy. The tendon can be either a multi-strand cable (1) or a solid bar (200). Various forms of anchor (10, 200) are disclosed with either complementary recess(es) (14) and protrusion(s) (15) or opposed protrusions (115) which form pinch point(s). A one piece grout anchor (300) is also disclosed.
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 ARTRock strata are liable to move as a result of various developments including mine-induced 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.
OBJECT OF THE INVENTIONThe 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 INVENTIONAccording 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:
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- (i) 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.
Embodiments of the present invention will now be described with reference to the drawings in which:
Cable bolts are traditionally made from multi-strand steel cable 1 such as that illustrated in
As illustrated in
Turning now to FIGS. 3 to 6, a grout engaging anchor 10 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
Turning now to
In the particular embodiment illustrated in
As a consequence, in the event that strata 30 and 31, for example, move to the left as seen in
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.
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”.
Claims
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 claim 1 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.
5. (canceled)
6. The bolt as claimed in claim 5 wherein said parts include a plurality of complementary protrusions and recesses.
7. (canceled)
8. The bolt as claimed in claim 4 wherein said two parts are substantially identical.
9. The bolt as claimed in claim 4 wherein said parts are maintained clamped together by keeper rings shaped to mate with said parts.
10. The bolt as claimed in claim 1 wherein said tendon comprises a multi-strand cable.
11. The bolt as claimed in claim 1 wherein said tendon comprises a bar.
12. The bolt as claimed in claim 1 and having a plurality of said grout engaging anchors.
13. 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, wherein said tendon moves through said parts in yielding.
14. (canceled)
15. The anchor as claimed in claim 14 wherein said parts include a plurality of complementary protrusions and recesses.
16. (canceled)
17. The anchor as claimed in claim 13 wherein said two parts are substantially identical.
18-21. (canceled)
22. 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, wherein said tendon moves through said parts in yielding.
23. The rock anchor as claimed in claim 22 and comprising a tube having an interior sized to receive said tendon.
24. The rock anchor as claimed in claim 23 wherein a portion of said tube is crushed to press fit said tube onto said tendon.
Type: Application
Filed: Jan 21, 2004
Publication Date: Jul 21, 2005
Inventor: Jeffrey Fergusson (Glenorie)
Application Number: 10/761,872