WO2008134798A1

WO2008134798A1 - Energy absorbing rock bolt accessory, rock bolt assembly, and method of installing a rock bolt

Info

Publication number: WO2008134798A1
Application number: PCT/AU2008/000599
Authority: WO
Inventors: Alan Thompson
Original assignee: Cmte Development Limited
Priority date: 2007-05-04
Filing date: 2008-04-30
Publication date: 2008-11-13

Abstract

The present invention provides an energy absorbing accessory (1) for a rock bolt (2) having a longitudinal bolt shaft (3), the accessory including: a substantially tubular body (4) having a tube side wall (5) and a tube axis (6); a pair of apertures (7, 8) in the side wall (5), whereby in use the bolt shaft (3) extends through the apertures (7, 8) substantially transverse of the tube axis (6). The present invention also provides a rock bolt assembly (11) including the energy absorbing accessory (1), and a related method of installing a rock bolt with the energy absorbing accessory (1).

Description

ENERGY ABSORBING ROCK BOLT ACCESSORY, ROCK BOLT ASSEMBLY, AND METHOD OF INSTALLING A ROCK BOLT

FIELD OF THE INVENTION

The present invention relates to rock bolts, and in particular, rock bolts used in mining operations.

The invention has been developed primarily for use in underground mining operations, and is described herein with reference to this exemplary application. However, it will be appreciated that the invention is not limited to this particular use.

BACKGROUND OF THE INVENTION

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

There is a demonstrated increasing need for energy absorbing rock bolts and cable bolts. This need was first identified in the deep South African gold mines where "rockbursts" and the requirement to control the rock mass behaviour after failure led to the development of the "Cone Bolt". Subsequently, mines mainly in Canada and Australia, have experienced similar rockbursts due to high tectonic and mining induced stresses as mines progress to greater depths. Currently, in Canada, the "Modified Cone Bolt" is being used at a limited number of mines. In Australia, mines have used the locally manufactured "Garford Dynamic Cable Bolt". More recently, the "Garford Sliding Anchor Rock Bolt" has been developed in Australia, and the "Durabar", "Durabolt", and "Duracable" energy absorbing rock bolts have been developed in South Africa. A "Cone Cable" has also been developed, and is based on the same principle as the Cone Bolt.

The rock bolts and cable bolts mentioned above will be described in more detail below. However, it will be apparent that all these systems rely on a mechanism at the "toe" or distal end of the borehole for energy absorption.

The Cone Bolt developed in South Africa is believed to be the first bolt designed to use a "sliding" mechanism to absorb energy. The bolt consists of a plain bar with an expanded cross-section at the toe end and a thread, nut, washer and plate at the collar, which is at the proximal end. The initial prototypes of the Cone Bolt were manufactured firom 16 mm diameter bar. Subsequently, the Cone Bolt was also made from 20mm diameter bar.

With the Cone Bolt, the expanded cross-section of the bar is designed to provide resistance to pull out that is controlled by the strength and stiffness of the cement grout that encapsulates the bolt within a borehole. The shaft of the bolt is coated with saponified wax so that there is little or no resistance to movement of the bolt relative to the cement grout.

Observations of in situ performance have demonstrated that the Cone Bolt may not pull through the cement grout as intended. For example, nuts have been pulled through plates or the bolts have failed in tension at the collar. It has been deduced that initial tests of the Cone Bolt were performed with cement grout strengths lower than those used currently in many mines. This clearly demonstrates that the strength and stiffness of the cement grout used influence the response of the Cone Bolt. The performance of the bolt would, therefore, depend on the water/cement ratio used for grout mixing and pumping. In practice, this would be highly variable.

The design of the cement grout should be such that the anchor ploughs (pulls) through the grout column at a force less than the yield strength of the bolt. Both static and dynamic tests have shown that this is not the case with "strong" grouts and much of the elongation is stretch of the bar. It is therefore critical that both: • the cement grout properties are designed to ensure that the "cone" pulls through the grout at a force less than the yield capacity of the bar; and

• the equipment and procedures used for mixing and placing the cement grout paste in a borehole result in consistent strength and stiffness of the hardened cement grout.

Typical force-displacement responses for the Cone Bolt have been documented. Referring to the response for the 22 mm bar, it is considered highly likely that the displacement is a combination of cone movement through the cement grout and elongation of the bar between the cone and nut. Referring to the response for the 16 mm bar, it is more likely that the bar pulled through the grout. Testing has also demonstrated that the bar yields plastically in the length between the cone anchor and collar. The Modified Cone Bolt, developed in Canada, is based on the use of resin to replace the cement grout used with the Cone Bolt described above. Both in situ static pull tests and dynamic loading tests of the Modified Cone Bolt have demonstrated unacceptably high variability in measured performance. Therefore, the use of resin to anchor reinforcement, such as rock bolts, may be inconsistent and unreliable.

Indeed, previously published dynamic test results are somewhat inconsistent in that some indicate that the Modified Cone Bolt produces about 130 mm of anchor displacement per drop of approximately 15kJ of energy, and other results indicate almost zero anchor movement and bar elongation. The fact that the bolt eventually breaks suggests that the ploughing effect eventually ceases.

In summary, it is considered that there are a number of basic problems with the Modified Cone Bolt. Firstly, if the resin were well mixed then it would be difficult for one to imagine the cone ploughing through the high quality resin material. Secondly, if the resin were not so well mixed but had some gloving, voids and variable strength, then one would imagine that the cone would perform inconsistently. That is to say, the cone may or may not move. The latter would appear to be in accord with published results.

It is also considered that mixing resin is difficult, even at the best of times. The problems that can occur are exacerbated if a large borehole and a relatively small bar are used, with only the spade at the toe end of the bar to mix the resin.

The Durabar or Durabolt provides a solid steel bar with a wriggle-like deformation at the end. The original prototype interacted directly with the cement grout in which it was embedded. As expected, this mechanism had some of the inconsistencies that were associated with the Cone Bolt. In particular, the response and energy absorbed were related to grout strength and stiffness and were highly variable.

Subsequent modifications have resulted in the bar having a steel sleeve. The contact between the steel bar and steel sleeve is greased. Energy is absorbed mainly due to the requirement of the bar to deform plastically as it is pulled through the steel sleeve. The important features of the static and dynamic responses of the Durabar are the relatively low force (less than 100 kN) at which sliding occurs, and the relatively high total displacement required to achieve high energy absorption compared with some of the other rock bolts described here. It is considered that energy absorption should occur at reasonably small displacement to minimise gross rock mass loosening. The Garford Sliding Anchor Bolt is a simple concept that requires an oversized bar to be pulled through a smaller diameter hole in a ferrule embedded in cement or resin grout in the borehole. Energy absorption is associated with the plastic deformation - A - caused by the reduction in the diameter of the bar material as the bar is pulled through the ferrule.

Static and dynamic tests performed on the Garford Sliding Anchor Bolt show that the patented mechanism allows the bar to slide at a force that is less than the yield force of the bar. The force at which the anchor slides when subjected to dynamic loading is less than the force required to cause sliding when loaded statically. This is because dynamic friction is lower than that associated with static pull testing. This phenomenon is common to frictional sliding mechanisms.

The Garford Sliding Anchor Bolt mechanism differs from the Cone Bolt in that the anchor is manufactured in a factory to close tolerances with quality assurance testing. This results in an anchor where the force at which sliding occurs is consistent, and therefore, depends less on the strength and stiffness of the cement or resin grout. The strength and stiffness of the hardened grout must exceed certain minimum values that are generally easily achieved by specifying and using water/cement ratios that do not exceed 0.45. The most recent version of this bolt incorporates a mechanism to facilitate mixing of resin grout cartridges.

The Cone Cable, based on the same principle as the Cone Bolt, uses a multiple steel wire strand where the end is expanded to form an anchor that absorbs energy as it is pulled through the grout in which it is encapsulated. Preliminary results indicated that the Cone Cable yielded at nominally 10OkN or 20OkN when encapsulated in resin or cement grout respectively.

The Duracable cable bolt is similar in design to the Durabar, and in particular, the solid bar is replaced by a multiple steel wire strand that is required to deform as it is pulled through a "wiggle" shaped anchor debonding sleeve. The important feature to note in a typical force-displacement response for the Duracable is, again, the low force (less than 100 kN) at which sliding occurs.

The Garford Dynamic Cable Bolt uses a principle similar to the Garford Sliding Anchor Bolt. In the Garford Dynamic Cable Bolt, however, the outside diameter of the standard 15.2 mm nominal diameter cable strand is increased by replacing the central king-wire by a larger diameter straight wire made of a softer steel material. Energy is absorbed in the fiictional sliding and plastic deformation of the replacement king-wire material. The static response for this anchor is stiff up to the force at which sliding occurs. However, the overall response will not be stiff due to the extension of the free length between the anchor and the collar.

In order to address some of the-disadvantages of the forgoing rock and cable bolts, all of which rely on a mechanism at the toe end of the borehole for energy absorption, Australian Patent Numbers 754645 and 2002313905 by Rainsford disclose a rock bolt that relies on an energy absorption mechanism outside the borehole, beyond the collar, at the head end of the rock bolt. In particular, the Rainsford patents describe an open- ended tubular stress-absorbing member located between the rock face and the head end of the rock bolt, with the tubular axis of the member parallel to the shaft of the rock bolt. Thus, the rock bolt shaft passes from one end of the stress-absorbing member to the other end, so that the tubular wall of the member surrounds the shaft.

One disadvantage of the stress-absorbing member of the Rainsford patents is that tubes of suitable dimensions and materials must be sourced. Tubes with diameters that are significantly larger than the shaft of the rock bolt are loose-fitting and larger washer plates are required to hold them in place. Thus, customised tube stock is required for rock bolts having different shaft diameters.

It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

SUMMARY OF THE INVENTION

In a first aspect of the invention, there is provided an energy absorbing accessory for a rock bolt having a longitudinal bolt shaft, the accessory including: a substantially tubular body having a tube side wall and a tube axis; a pair of apertures in the side wall, whereby in use the bolt shaft extends through the apertures substantially transverse of the tube axis.

Preferably, the tubular body is a right cylinder. More preferably, the tubular body is a circular right cylinder. Even more preferably, the apertures are diametrically opposed. Preferably, the energy absorbing accessory is resilient and elastically deformable at a force less than the force capacity of the rock bolt. Preferably, the energy absorbing accessory is plastically deformable at a force less than the force capacity of the rock bolt.

Preferably, the tubular body is metallic. More preferably, the tubular body is steel. Even more preferably, the tubular body is cut from thick-walled steel pipe stock. In a preferred embodiment, the tubular body has an outside diameter of about 119 mm and an internal diameter of about 96 mm. Preferably, the tube side wall has a thickness of about 23 mm. Preferably, the tubular body has a length of about 90 mm.

In other embodiments, the energy absorbing accessory includes a plurality of the tubular bodies, each tubular body being configured to fit inside a previous tubular body. Preferably, the tubular bodies are concentrically located.

Preferably, the energy absorbing accessory has an applied force capacity of at least 150 kN before collapse of the tubular body. Preferably, the energy absorbing accessory absorbs at least 12 kJ of energy before collapse of the tubular body.

In a second aspect of the invention, there is provided a rock bolt assembly including: a rock bolt having a longitudinal bolt shaft and a head end at one end of the bolt shaft, the bolt shaft being insertable into a rock borehole extending through a rock face into a rock mass, and anchorable in the rock borehole, such that the head end is outside of the rock borehole adjacent the rock face; and an energy absorbing accessory as described above positionable between the head end and the rock face; whereby at least some of the energy applied to the rock bolt assembly is absorbed by the energy absorbing accessory. Preferably, the energy absorbing accessory is resilient and configurable to undergo elastic deformation before failure of the rock bolt. Preferably, the energy absorbing accessory is configurable to undergo plastic deformation before failure of the rock bolt. More preferably, the tubular body is configurable to collapse before failure of the rock bolt. Even more preferably, the energy absorbing accessory has physical characteristics selected such that the energy absorbing accessory is configurable to deform preferentially to the rock bolt. These physical characteristics include dimensions such as length and wall thickness, material properties such as yield strength, and mechanical properties such as energy absorption capacities, deformation and energy absorption profiles in response to forces applied over time.

Preferably, the head end includes an enlarged portion such that, in use, the energy absorbing accessory abuts the enlarged portion and is interposed between the enlarged portion and the rock face. More preferably, the head end includes a threaded portion and the enlarged portion includes a nut threadedly engaged with the threaded portion, such that the nut is threadedly adjustable to abut the energy absorbing accessory and captively locate the energy absorbing accessory between the enlarged portion and the rock face. Even more preferably, the enlarged portion includes a washer positionable between the nut and the energy absorbing accessory. Preferably, the rock bolt includes a plate positionable between the energy absorbing accessory and the rock face. In one embodiment, the rock bolt includes a cement grout for placement in the rock borehole to anchor the rock bolt in the rock borehole. In another embodiment, the rock bolt includes a resin for placement in the rock borehole to anchor the rock bolt in the rock borehole. Preferably, the rock bolt has a special geometric arrangement at the distal end of the bolt shaft, which is opposite the head end, to facilitate anchoring of the rock bolt in the rock borehole. In embodiments where the bolt shaft is a solid bar, this geometric arrangement can be an expanded cross section. In embodiments where the bolt shaft is a multiple steel wire strand, the geometric arrangement can be a "bulb". In other embodiments, the geometric arrangement is a swaged fitting or barrel and wedge anchor, or the result of some other method of creating a larger cross section. In further embodiments, the bolt shaft has a thread to which a mechanical "expansion shell" type anchor can be attached to anchor the rock bolt in the rock borehole. In a third aspect of the invention, there is provided a method of installing a rock bolt having a longitudinal bolt shaft and a head end at one end of the bolt shaft, the method including: providing an energy absorbing accessory as described above; inserting the bolt shaft into a rock borehole extending through a rock face into a rock mass; anchoring the bolt shaft in the rock borehole, such that the head end is outside of the rock borehole adjacent the rock face; and mounting the energy absorbing accessory onto the bolt shaft whereby the bolt shaft passes through the apertures, such that the energy absorbing accessory is interposed between the head end and the rock face; whereby at least some of the energy applied to the rock bolt is absorbed by the energy absorbing accessory.

Preferably, the method includes configuring the energy absorbing accessory to undergo elastic deformation before failure of the rock bolt. Preferably, the method includes configuring the energy absorbing accessory to undergo plastic deformation before failure of the rock bolt. More preferably, the method includes configuring the tubular body to collapse before failure of the rock bolt. Even more preferably, the method includes selecting the physical characteristics of the energy absorbing accessory such that the energy absorbing accessory is configurable to deform preferentially to the rock bolt. These physical characteristics include dimensions such as length and wall thickness, material properties such as yield strength, and mechanical properties such as energy absorption capacities, deformation and energy absorption profiles in response to forces applied over time. In some embodiments, the energy absorbing accessory includes a plurality of the tubular bodies, and the method includes fitting each tubular body inside a previous tubular body, and passing the bolt shaft through the apertures of each tubular body once each tubular body has been fitted inside the previous tubular body. Preferably, the tubular bodies are fitted concentrically. Preferably, the head end includes an enlarged portion, and the method includes positioning the enlarged portion such that the energy absorbing accessory abuts the enlarged portion and is interposed between the enlarged portion and the rock face. More preferably, the head end includes a threaded portion, the enlarged portion includes a nut threadedly engaged with the threaded portion, and the method includes threadedly adjusting the nut to abut the energy absorbing accessory and captively locate the energy absorbing accessory between the enlarged portion and the rock face. Even more preferably, the enlarged portion includes a washer, and the method includes positioning the washer between the nut and the energy absorbing accessory. Preferably, the rock bolt includes a plate, and the method includes positioning the plate between the energy absorbing accessory and the rock face.

In one embodiment, the rock bolt includes a cement grout, and the method includes placing the cement grout in the rock borehole to anchor the rock bolt in the rock borehole. In another embodiment, the rock bolt includes a resin, and the method includes placing the resin in the rock borehole to anchor the rock bolt in the rock borehole. Preferably, the method includes providing the rock bolt with a special geometric arrangement at the distal end of the bolt shaft, which is opposite the head end, to facilitate anchoring of the rock bolt in the rock borehole. In embodiments where the bolt shaft is a solid bar, and the method includes expanding a portion of the bolt shaft to form an expanded cross section to define this geometric arrangement. In embodiments where the bolt shaft is a multiple steel wire strand, and the method includes forming a bulbous formation to define the geometric arrangement. In other embodiments, the geometric arrangement is a swaged fitting or barrel and wedge anchor, or the result of some other method of creating a larger cross section. In these embodiments, the method includes fitting and configuring the swaged fitting or barrel and wedge anchor, or carrying out the other method of creating a larger cross section. In further embodiments, the bolt shaft has a thread, and the method includes attaching a mechanical "expansion shell" type anchor onto the thread to anchor the rock bolt in the rock borehole. In all of the aspects of the invention described above, the bolt shaft is a solid bar in some embodiments. In other embodiments, the rock bolt is a cable bolt, with the bolt shaft being a multiple steel wire strand. Preferably, the bolt shaft is metallic.

BRIEF DECRIPTION OF THE FIGURES Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures, in which:

Figure 1 is a front view of an energy absorbing accessory for a rock bolt according to the invention; Figure 2 is a top view of the energy absorbing accessory showing one of the two apertures in the side wall; and

Figure 3 is a longitudinal cross-sectional view of a rock bolt assembly according to the invention, showing the energy absorbing accessory of Figures 1 and 2 positioned between the head end of the rock bolt and a rock face.

Figure 4 is a longitudinal cross-sectional view of the rock bolt in the rock bolt assembly of Figure 3.

Figure 5 is a side view of the rock bolt in another embodiment of a rock bolt assembly according to the invention, showing the head end of the rock bolt in the form of a barrel and a wedge.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring to the figures, the energy absorbing accessory 1 for a rock bolt 2 having a longitudinal bolt shaft 3 includes a substantially tubular body 4 having a tube side wall 5 and a tube axis 6. A pair of apertures 7 and 8 are in the side wall 5, whereby in use the bolt shaft 3 extends through the apertures substantially transverse of the tube axis 6.

In the present embodiment, the tubular body 4 is a substantially circular right cylinder having two tube ends 9 and 10, with the tube axis 6 extending between the tube ends parallel to the tube side wall 5. The apertures 7 and 8 are also diametrically opposed in the present embodiment. In other embodiments, the tubular body has other shapes.

Returning to the present embodiment, the energy absorbing accessory 1 is resilient and is elastically and plastically deformable at a force less than the force capacity of the rock bolt 2, with the tubular body 4 being cut from thick- walled steel pipe stock. Preferably, the tubular body has an outside diameter of about 119 mm, an internal diameter of about 96 mm, and a length of about 90 mm. It is also preferred that the energy absorbing accessory 1 has an applied force capacity of at least 150 IdSf before collapse of the tubular body 4, with the accessory absorbing at least 12 kJ of energy before collapse. In other embodiments, the tubular bodies have varied physical characteristics including varied dimensions such as length and wall thickness, and varied material properties such as yield strengths, and varied mechanical properties such as energy absorption capacities, deformation and energy absorption profiles in response to forces applied over time. In further embodiments, the energy absorbing accessory includes a plurality of the tubular bodies, each tubular body being configured to fit inside a previous tubular body, and preferably, the tubular bodies are concentrically located. Thus, advantageously, the physical characteristics and configuration of the energy absorbing accessory can be varied to customise the energy absorbing accessory to the particular application at hand.

The rock bolt assembly 11 shown in Figure 3, and in accordance with a second aspect of the invention, includes the rock bolt 2 having the longitudinal shaft 3. A head end 12 is at one end of the bolt shaft 3, the bolt shaft 3 being insertable into a rock borehole 13 extending through a rock face 14 into a rock mass 15. The bolt shaft 3 is anchorable in the rock borehole 13, such that the head end 12 is outside of the rock borehole 13 adjacent the rock face 14. The energy absorbing accessory 1 is positionable between the head end 12 and the rock face 14, whereby at least some of the energy applied to the rock bolt assembly 11 is absorbed by the energy absorbing accessory 1. Preferably, the energy absorbing accessory is positionable such that, at least initially, the force in the energy absorbing accessory 1 is equal to the force in the rock bolt 2.

In the present embodiment, the bolt shaft 3 is a solid metallic bar, while in other embodiments, the rock bolt is a cable bolt, with the bolt shaft 3 being a multiple steel wire strand. Typically, the anchored bolt shaft 3 experiences tensile forces resulting from movement of the rock caused by stresses. These stresses can be tectonic or mining induced stresses that can cause rockbursts that are sudden failures of the rock with the release of energy. The rock movement pushes the rock face 14 into the head end 12 of the rock bolt 2, working to pull the bolt shaft 3 out of the rock mass 15, and thereby applying tensile forces to the bolt shaft 3. However, the energy absorbing accessory 1 has physical characteristics selected such that the energy absorbing accessory 1 is configured to undergo elastic and plastic deformation before failure of the rock bolt 2. The tubular body 4 is also configured to collapse before failure of the rock bolt 2.

The head end 12 includes an enlarged portion 16 such that, in use, the energy absorbing accessory 1 abuts the enlarged portion 16 and is interposed between the enlarged portion 16 and the rock face 14. The head end 12 also includes a threaded portion 17 and the enlarged portion 16 includes a nut 18 threadedly engaged with the threaded portion 17. Thus, the nut 18 is threadedly adjustable to abut the energy absorbing accessory 1 and captively locate the energy absorbing accessory 1 between the enlarged portion 16 and the rock face 14, and to preferably induce tension in the bolt shaft 3.

In another embodiment, where the bolt shaft is a multiple steel wire strand, a barrel 19 and a wedge 20 form the enlarged portion 16. The combination of the barrel 19 and the wedge 20 is adjustable to abut the energy absorbing accessory 1 and captively locate the energy absorbing accessory 1 between the enlarged portion 16 and the rock face 14, and to preferably induce tension in the bolt shaft 3.

The enlarged portion 16 also includes a washer 21 positionable between the nut 18 and the energy absorbing accessory 1, or between the barrel 19 and the energy absorbing accessory 1, depending on the specific embodiment. Further, the rock bolt 2 also includes a plate 22 positionable between the energy absorbing accessory 1 and the rock face 14. The washer 21 and the plate 22 can be any of the types generally distributed with rock bolts. Similarly, the nut 18 and the barrel 19 and the wedge 20 can be any of the types generally distributed with rock bolts.

The rock bolt 2 includes, in some embodiments, a cement grout, and in other embodiments, a resin, for placement in the rock borehole 13 to secure a distal or toe end 23 of the bolt shaft 3, which is opposite the head end 12, by forming an anchor 24 between the bolt shaft 3 and the rock mass 15. Preferably, the rock bolt 2 has a special geometric arrangement at the distal end 23 to facilitate anchoring of the rock bolt in the rock borehole 13. In embodiments where the bolt shaft is a solid bar, this geometric arrangement can be an expanded cross section. In embodiments where the bolt shaft is a multiple steel wire strand, the geometric arrangement can be a "bulb". In other embodiments, the geometric arrangement is a swaged fitting or barrel and wedge anchor, or the result of some other method of creating a larger cross section. In further embodiments, the bolt shaft 3 has a thread at the toe end 23 to which a mechanical "expansion shell" can be attached to form the anchor 24 between the bolt shaft 3 and the rock mass 15.

The anchor 24 resists pull out of the bolt shaft 3 from the rock borehole 13. In the relevant embodiments, the toe end 23 also assists in mixing the cement grout or resin in the rock borehole 13 during the anchoring operation. In embodiments where frangible capsules of resin are used, the toe end 23 can be used to rupture these capsules once the capsules have been inserted into the rock borehole 13. The invention also provides, in a third aspect, a method of installing a rock bolt. A preferred embodiment of the method is for installing the rock bolt 2 described above. In this preferred embodiment, the method includes the step of providing the energy absorbing accessory 1 described above. The bolt shaft 3 is inserted into the rock borehole 13 extending through the rock face 14 into the rock mass 15, and anchored in the rock borehole 13, such that the head end 12 is outside of the rock borehole 13 and adjacent the rock face 14.

The energy absorbing accessory 1 is then mounted onto the bolt shaft 3 whereby the bolt shaft 3 passes through the apertures 7 and 8 such that the energy absorbing accessory 1 is between the head end 12 and the rock face 14. The enlarged portion 16 is positioned such that the energy absorbing accessory 1 abuts the enlarged portion 16 and is between the enlarged portion 16 and the rock face 14. Preferably, the enlarged portion includes the washer 21 and the nut 18, whereby the washer 21 is positioned between the nut 18 and the energy absorbing accessory 1, and the nut 18 is threadedly engaged with the threaded portion 17. Preferably, the rock bolt includes the plate 22, and the plate is positioned onto the bolt shaft 3 before the energy absorbing accessory 1 so that the plate 22 is between the energy absorbing accessory 1 and the rock face 14.

The nut 18 is then threadedly adjusted to abut the washer 21 against the energy absorbing accessory 1 and captively locate the washer 21, the energy absorbing accessory 1, and the plate 22 between the nut 18 and the rock face 14. In doing so, the energy absorbing accessory 1 is interposed between the enlarged portion 16 and the rock face 14, whereby at least some of the energy applied to the rock bolt 2 is absorbed by the energy absorbing accessory 1.

In another embodiment, where the bolt shaft is a multiple steel wire strand, the enlarged portion 16 takes the form of the barrel 19 and the wedge 20. In this embodiment, the washer 21 is positioned between the barrel 19 and the energy absorbing accessory 1. The plate 22 is positioned onto the bolt shaft 3 before the energy absorbing accessory 1 so that the plate 22 is between the energy absorbing accessory 1 and the rock face 14. The barrel 19 and the wedge 20 are then adjusted so that the barrel 19 abuts the washer 21 against the energy absorbing accessory 1 and captively locates the washer 21, the energy absorbing accessory 1, and the plate 22 between the barrel 19 and the rock face 14. In doing so, the energy absorbing accessory 1 is interposed between the enlarged portion 16 and the rock face 14, whereby at least some of the energy applied to the rock bolt 2 is absorbed by the energy absorbing accessory 1.

When subjected to force, the energy absorbing accessory 1 undergoes elastic and plastic deformation before failure of the rock bolt 2. In particular, the tubular body 4 collapses before failure of the rock bolt 2. The physical characteristics, including the material and mechanical properties, of the energy absorbing accessory 1 are selected to ensure that the energy absorbing accessory 1 deforms preferentially to the rock bolt 2.

In embodiments where the energy absorbing accessory includes a plurality of the tubular bodies, with each tubular body being configured to fit inside a previous tubular body, each tubular body is fitted, preferably concentrically, inside the previous tubular body. Each tubular body is then mounted onto the bolt shaft whereby the bolt shaft passes through the apertures of each tubular body.

Returning to the present embodiment in further detail, the cement grout or the resin is placed into the rock borehole 13 before insertion of the bolt shaft 3. When the bolt shaft 3 is inserted, the toe end 23 is used to mix the cement grout or resin by moving the rock bolt 2 around inside the rock borehole 13, so that the rock bolt 2 is anchored in the rock borehole 13 once the cement grout or resin sets. In embodiments where frangible capsules of resin are used, the capsules are inserted into the rock borehole in front of the toe end 23. The bolt shaft 3 is then inserted into the rock borehole 13 so that the toe end 23 ruptures the capsule by forcing the capsule against the rock borehole 13, thereby releasing the resin inside the capsule.

In embodiments where the rock bolt has different geometric arrangements at the toe end to facilitate anchoring, the steps for providing these geometric arrangements are carried out. For example, in the embodiments where the bolt shaft 3 has a thread at the toe end 23 for attaching a mechanical "expansion shell", the "expansion shell" is threadedly engaged to the thread, whereby it is expanded, forming the anchor 24 between the bolt shaft 3 and the rock mass 15, in order to anchor the rock bolt 2 in the rock borehole 13.

Advantageously, the energy absorbing accessory of the invention can be used with a variety of existing rock bolts and cable bolts by simply adjusting the size of the apertures. Further advantages include that significant deformation of the energy absorbing accessory only occurs after a certain threshold force is reached, and that deformation of the energy absorbing accessory occurs at approximately constant force until the tubular body collapses. Advantageously, the force capacity of the rock bolt or cable bolt is unaffected. After the tubular body collapses, forces will be transferred to the bolt shaft and internal fixing. These advantages allow for more energy absorption and higher force capacities. Also advantageously, the deformation of the tubular body provides easily discernible visual indication of the force applied to the rock bolt or cable bolt. Another advantage is that the energy absorbing accessory is suitable for use with the domed plates, spherical washers, and standard fixtures, such as nuts, barrels and wedges, that are used with existing rock bolts or cable bolts.

Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:-

1. An energy absorbing accessory for a rock bolt having a longitudinal bolt shaft, the accessory including: a substantially tubular body having a tube side wall and a tube axis; a pair of apertures in the side wall, whereby in use the bolt shaft extends through the apertures substantially transverse of the tube axis.

2. An energy absorbing accessory according to claim 1, wherein the tubular body is a right cylinder.

3. An energy absorbing accessory according to claim 1, wherein the tubular body is a circular right cylinder.

4. An energy absorbing accessory according to claim 3, wherein the tubular body has an outside diameter of about 119 mm.

5. An energy absorbing accessory according to any one of claims 3 to 4, wherein the tubular body has an internal diameter of about 96 mm.

6. An energy absorbing accessory according to any one of the preceding claims, wherein the apertures are diametrically opposed.

7. An energy absorbing accessory according to any one of the preceding claims, wherein the energy absorbing accessory is resilient and elastically deformable at a force less than the force capacity of the rock bolt.

8. An energy absorbing accessory according to any one of the preceding claims, wherein the energy absorbing accessory is plastically deformable at a force less than the force capacity of the rock bolt.

9. An energy absorbing accessory according to any one of the preceding claims, wherein the tubular body is metallic.

10. An energy absorbing accessory according to any one of claims 1 to 8, wherein the tubular body is steel.

11. An energy absorbing accessory according to any one of claims 1 to 8, wherein the tubular body is cut from thick-walled steel pipe stock.

12. An energy absorbing accessory according to any one of the preceding claims, wherein the tube side wall has a thickness of about 23 mm.

13. An energy absorbing accessory according to any one of the preceding claims, wherein the tubular body has a length of about 90 mm.

14. An energy absorbing accessory according to any one of the preceding claims, wherein the energy absorbing accessory includes a plurality of the tubular bodies, each tubular body being configured to fit inside a previous tubular body.

15. An energy absorbing accessory according to claim 14, wherein the tubular bodies are concentrically located.

16. An energy absorbing accessory according to any one of the preceding claims, wherein the energy absorbing accessory has an applied force capacity of at least 150 kN before collapse of the tubular body.

17. An energy absorbing accessory according to any one of the preceding claims, wherein the energy absorbing accessory absorbs at least 12 kJ of energy before collapse of the tubular body.

18. A rock bolt assembly including: a rock bolt having a longitudinal bolt shaft and a head end at one end of the bolt shaft, the bolt shaft being insertable into a rock borehole extending through a rock face into a rock mass, and anchorable in the rock borehole, such that the head end is outside of the rock borehole adjacent the rock face; and an energy absorbing accessory according to any one of claims 1 to 17 positionable between the head end and the rock face; whereby at least some of the energy applied to the rock bolt assembly is absorbed by the energy absorbing accessory.

19. A rock bolt assembly according to claim 18, wherein the energy absorbing accessory is resilient and configurable to undergo elastic deformation before failure of the rock bolt.

20. A rock bolt assembly according to any one of claims 18 to 19, wherein the energy absorbing accessory is configurable to undergo plastic deformation before failure of the rock bolt.

21. A rock bolt assembly according to any one of claims 18 to 20, wherein the tubular body is configurable to collapse before failure of the rock bolt.

22. A rock bolt assembly according to any one of claims 18 to 21, wherein the energy absorbing accessory has physical characteristics selected such that the energy absorbing accessory is configurable to deform preferentially to the rock bolt.

23. A rock bolt assembly according to claim 22, wherein the physical characteristics include one or more of the following: length, wall thickness, yield strength, energy absorption capacities, and deformation and energy absorption profiles in response to forces applied over time.

24. A rock bolt assembly according to any one of claims 18 to 23, wherein the head end includes an enlarged portion such that, in use, the energy absorbing accessory abuts the enlarged portion and is interposed between the enlarged portion and the rock face.

25. A rock bolt assembly according to claim 24, wherein the head end includes a threaded portion and the enlarged portion includes a nut threadedly engaged with the threaded portion, such that the nut is threadedly adjustable to abut the energy absorbing accessory and captively locate the energy absorbing accessory between the enlarged portion and the rock face.

26. A rock bolt assembly according to claim 25, wherein the enlarged portion includes a washer positionable between the nut and the energy absorbing accessory.

27. A rock bolt assembly according to any one of claims 18 to 26, wherein the rock bolt includes a plate positionable between the energy absorbing accessory and the rock face.

28. A rock bolt assembly according to any one of claims 18 to 27, wherein the rock bolt includes a cement grout for placement in the rock borehole to anchor the rock bolt in the rock borehole.

29. A rock bolt assembly according to any one of claims 18 to 27, wherein the rock bolt includes a resin for placement in the rock borehole to anchor the rock bolt in the rock borehole.

30. A rock bolt assembly according to any one of claims 18 to 29, wherein the bolt shaft has a thread for attaching a mechanical "expansion shell" type anchor to anchor the rock bolt in the rock borehole.

31. A rock bolt assembly according to any one of claims 18 to 29, wherein the rock bolt has a special geometric arrangement at the distal end of the bolt shaft, which is opposite the head end, to facilitate anchoring of the rock bolt in the rock borehole.

32. A rock bolt assembly according to claim 31, wherein the special geometric arrangement is a swaged fitting.

33. A rock bolt assembly according to claim 31, wherein the special geometric arrangement is a barrel and wedge anchor.

34. A rock bolt assembly according to claim 31, wherein the bolt shaft is a solid bar, and the special geometric arrangement is an expanded cross section.

35. A rock bolt assembly according to claim 31, wherein the bolt shaft is a multiple wire strand, and the special geometric arrangement is a bulbous formation.

36. A rock bolt assembly according to any one of claims 18 to 33, wherein the bolt shaft is a solid bar.

37. A rock bolt assembly according to any one of claims 18 to 33, wherein the bolt shaft is a multiple wire strand, the rock bolt thereby being a cable bolt.

38. A rock bolt assembly according to any one of claims 18 to 37, wherein the bolt shaft is metallic.

39. A method of installing a rock bolt having a longitudinal bolt shaft and a head end at one end of the bolt shaft, the method including: providing an energy absorbing accessory according to any one of claims 1 to 17; inserting the bolt shaft into a rock borehole extending through a rock face into a rock mass; anchoring the bolt shaft in the rock borehole, such that the head end is outside of the rock borehole adjacent the rock face; and mounting the energy absorbing accessory onto the bolt shaft whereby the bolt shaft passes through the apertures, such that the energy absorbing accessory is interposed between the head end and the rock face; whereby at least some of the energy applied to the rock bolt is absorbed by the energy absorbing accessory.

40. A method according to claim 39, including configuring the energy absorbing accessory to undergo elastic deformation before failure of the rock bolt.

41. A method according to any one of claims 39 to 40, including configuring the energy absorbing accessory to undergo plastic deformation before failure of the rock bolt.

42. A method according to any one of claims 39 to 41, including configuring the tubular body to collapse before failure of the rock bolt.

43. A method according to any one of claims 39 to 42, including selecting the physical characteristics of the energy absorbing accessory such that the energy absorbing accessory is configurable to deform preferentially to the rock bolt.

44. A method according to claim 43, wherein the physical characteristics include one or more of the following: length, wall thickness, yield strength, energy absorption capacities, and deformation and energy absorption profiles in response to forces applied over time.

45. A method according to any one of claims 39 to 44, including configuring the energy absorbing accessory to deform preferentially to the rock bolt.

46. A method according to any one of claims 39 to 45, wherein the energy absorbing accessory includes a plurality of the tubular bodies, and the method includes fitting each tubular body inside a previous tubular body.

47. A method according to claim 46, wherein the tubular bodies are fitted concentrically.

48. A method according to any one of claims 46 to 47, including passing the bolt shaft through the apertures of each tubular body once each tubular body has been fitted inside the previous tubular body.

49. A method according to any one of claims 39 to 48, wherein the head end includes an enlarged portion, and the method includes positioning the enlarged portion such that the energy absorbing accessory abuts the enlarged portion and is interposed between the enlarged portion and the rock face.

50. A method according to claim 49, wherein the head end includes a threaded portion, the enlarged portion includes a nut threadedly engaged with the threaded portion, and the method includes threadedly adjusting the nut to abut the energy absorbing accessory and captively locate the energy absorbing accessory between the enlarged portion and the rock face.

51. A method according to claim 50, wherein the enlarged portion includes a washer, and the method includes positioning the washer between the nut and the energy absorbing accessory.

52. A method according to any one of claims 39 to 51, wherein the rock bolt includes a plate, and the method includes positioning the plate between the energy absorbing accessory and the rock face.

53. A method according to any one of claims 39 to 52, wherein the rock bolt includes a cement grout, and the method includes placing the cement grout in the rock borehole to anchor the rock bolt in the rock borehole.

54. A method according to any one of claims 39 to 52, wherein the rock bolt includes a resin, and the method includes placing the resin in the rock borehole to anchor the rock bolt in the rock borehole.

55. A method according to any one of claims 39 to 54, wherein the bolt shaft has a thread, and the method includes attaching a mechanical "expansion shell" type anchor onto the thread to anchor the rock bolt in the rock borehole.

56. A method according to any one of claims 39 to 54, including providing the rock bolt with a special geometric arrangement at the distal end of the bolt shaft, which is opposite the head end, to facilitate anchoring of the rock bolt in the rock borehole.

57. A method according to claim 56, wherein the special geometric arrangement is a swaged fitting, and the method includes fitting and configuring the swaged fitting.

58. A method according to claim 56, wherein the special geometric arrangement is a barrel and wedge anchor, and the method includes fitting and configuring the barrel and wedge anchor.

59. A method according to claim 56, wherein the bolt shaft is a solid bar, and the method includes expanding a portion of the bolt shaft to form an expanded cross section to define the special geometric arrangement.

60. A method according to claim 56, wherein the bolt shaft is a multiple wire strand, and the method includes forming a bulbous formation to define the special geometric arrangement.

61. A method according to any one of claims 39 to 58, wherein the bolt shaft is a solid bar.

62. A method according to any one of claims 39 to 58, wherein the bolt shaft is a multiple wire strand, the rock bolt thereby being a cable bolt.

63. A method according to any one of claims 39 to 62, wherein the bolt shaft is metallic.

64. An energy absorbing accessory for a rock bolt, the accessory being substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

65. A rock bolt assembly substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

66. A method of installing a rock bolt, the method being substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.