GB2441018A

GB2441018A - Rock bolt and method of use

Info

Publication number: GB2441018A
Application number: GB0707743A
Authority: GB
Inventors: Stephen Render
Original assignee: Romtech Ltd
Current assignee: Romtech Ltd
Priority date: 2006-04-25
Filing date: 2007-04-23
Publication date: 2008-02-20
Also published as: GB0608092D0; GB0707743D0

Abstract

The rock bolt (10) comprises an elongated tensile member (12), a first mounting member (14) typically in the form of a sleeve surrounding a part of the tensile member (12), a second mounting member (18) typically secured to the distal end of the tensile member (12), and a retaining member (20) located between the first mounting member (14) and the second mounting member (18). The retaining member (20) typically comprises a pair of rigid bars (40, 42), one of the bars (42) being hingedly connected at the first mounting member (14), the other of the bars (40) being hingedly connected at the second mounting member (18), the bars (40, 42) also being hinged together. The rock bolt (10) has means to reduce the distance between the first mounting member (14) and the second mounting member (18) so as to force a part of the retaining member (20) away from the tensile member (12) and into contact with the rock or earth surrounding the rock bolt (10).

Description

ROCK BOLT AND METHOD OF USE

FIELD OF THE INVENTION

This invention relates to a rock bolt and to its method of use.

BACKGROUND TO THE INVENTION

Rock bolts are commonly used in tunnelling or mining to support a rock face and/or to provide an anchorage point adjacent the rock face within the tunnel or mine. Rock bolts comprise elongated bolts which are fitted into holes drilled into a rock face. The rock bolt may be several metres long, the length depending upon the type and condition of the rock. Rock bolts are typically used to support the rock above a ceiling of a tunnel or chamber, but may alternatively be used to support the rock behind a wall thereof.

The present invention may alternatively be used as a ground anchor or soil nail.

In these applications, the ground is not sufficiently hard to support a pre-drilled hole, so that the bolt is driven into the ground or earth to form its own hole.

Notwithstanding these other applications, however, the present invention is likely to find its greatest utility as a rock bolt and so the following description will refer primarily to such applications, but the possible use as a ground anchor or soil nail is not thereby excluded.

DESCRIPTION OF THE PRIOR ART

Many different types of rock bolt are known. Possibly the most common type is a bar of solid metal which is threaded adjacent its proximal end. The bar often has a roughened or deformed outer surface and is inserted into a hole which has been drilled into the rock face. The hole is somewhat larger than the diameter of the bar, and before insertion of the bolt a cement-based or resin grout is inserted into the hole. When the rock bolt is subsequently pressed into the hole the grout substantially fills the gap between the bolt and the hole, and when the grout has set this secures the bolt within the hole.

When the grout has hardened, a face plate is fitted over the proximal end of the bolt, and a nut applied to the threads thereof in order to clamp the face plate against the rock face. The nut and face plate will typically have corresponding part-circular formations to permit the face plate to lie other than perpendicular to the bolt, since the surface of the rock adjacent the proximal end of the bolt may be other than perpendicular to the bolt.

It is a recognised disadvantage of this type of rock bolt that tension cannot be apphed between its ends, i.e. along the whole length of the bolt. in other words the rock bolt cannot clamp, or apply compression to, the rock between its distal end and the face plate. Such clamping is desired if the rock is soft, or is highly fractured or jointed, and the rock face is liable to movement or subsidence.

In order to enable tension to be applied to substantially the whole length of the bolt it is known to provide a two-stage setting resin grout system in place of the cement-based grout or the single-stage setting resin grout described above. The resin grout comprises a combination of two chemicals which harden after being mixed together. The chemicals are provided in cartridges in an unmixed state, and the cartridges are inserted into the drilled hole prior to the insertion of the bolt.

The bolt is subsequently inserted into the hole and ruptures the cartridges of resin, the bolt being rotated as it is inserted to mix the chemicals together. When using the two-stage setting resin grout system it is arranged that several cartridges are used in the hole and the compositions of the chemicals vary between the cartridges. In this way, it can be arranged that the resin adjacent the distal end of the hole hardens quickly, whilst the resin adjacent the proximal end of the hole hardens more slowly. Accordingly, tension can be applied to a greater length of the bolt by tightening (or partially-tightening) the nut against the face plate as soon as the resin adjacent the distal end has hardened, and before the resin adjacent the proximal end has hardened.

It is another known disadvantage of both of these designs that the separate insertion of the grout and bolt is a time consuming procedure and is also messy.

In addition, some means needs to be provided to retain the bolt within the hole whilst the grout hardens. Furthermore, these methods are not always able to accommodate voids created adjacent the hole, i.e. where the cross-section of the hole is locally larger than the drill diameter because of the displacement of rock during the drilling procedure, or natural cavities within the rock.

In addition, the rock adjacent the hole can move (and perhaps block the hole) during the time between extraction of the drill and insertion of the bolt, the time taken to insert the grout delaying the insertion of the bolt, If the rock moves significantly and blocks the hole, a new hole will need to be drilled.

Another design of rock bolt is known in which a locking shell is fitted to the distal end of the bolt, the locking shell being mounted in threaded engagement with the bolt; following insertion of the bolt (with a loosely-fitted shell) the bolt is rotated relative to the shell so as to cause the shell to be expanded into secure engagement with the surrounding rock. Whilst such rock bolts avoid the requirement for grout, they are not in widespread use since the failure of the shell to bind with the rock (and so simply rotate with the bolt) is commonplace.

Two alternative types of rock bolt have been designed, which also avoid the requirement for grout. Avoiding the requirement for grout reduces the time taken to fit the rock bolt, which is beneficial in itself, and also reduces the likelihood of the rock adjacent the hole becoming displaced and blocking the hole prior to insertion of the bolt. The avoidance of grout also makes the fitment of a rock bolt a cleaner and tidier job.

One of the alternative designs of rock bolt is made and sold by Ingersoll Rand, and is referred to as "Split Set" (TM). The bolt comprises a tube of high strength steel which has a slot running along its length to provide a C-shaped cross section. The bolt is resiliently deformable, i.e. the diameter can be reduced by closing up the slot, and the resilience of the bolt will seek to open up the slot and increase the diameter. The hole drilled in the rock face is sized to be smaller than the rest condition of the bolt, so that when the bolt has been inserted its resilience results in a frictional engagement with the hole.

Another alternative is the "Swellex" (TM) rock bolt manufactured by Atlas-Copco.

The bolt comprises a tube of ductile steel which is deformed by being partially folded within itself so as to reduce its effective diameter by around 40%. The hole drilled in the rock face is arranged to be of larger diameter than the deformed tube, but of smaller diameter than the enlarged tube. Following insertion of the deformed tube into the hole, pressurised fluid (typically water) is passed into the tube, causing it to unfold itself and expand into contact with the wall of the hole. .4-

Both of the "Split set" and "Swellex" rock bolts are in widespread use, and are distinguished by the speed with which they may be inserted into a drilled hole; despite the piece cost penalty over many of the other designs of rock bolt, they can offer a reduced cost overall when the time of installation is taken into account.

However, both of these alternative designs have their own disadvantages. Firstly, they are both susceptible to corrosion damage since in each case the metal of which they are made is in intimate contact with the rock surrounding the hole. In addition, the load which the "Split set" design can withstand is limited by the resilience of the tube. The "Swellex" design on the other hand can withstand greater loads, but requires a source of pressurised water which complicates the procedure involved in fitment of the bolt.

Neither of these alternative designs can have tension applied between its ends so as to clamp the rock therebetween.

Also, neither of the alternative designs is able to cope well with voids which have a greater cross-section than the diameter of the "Split set" tube (when relaxed) or the "Swellex" tube when fully expanded. Accordingly, these designs of rock bolt will provide little or no support adjacent such a void, and there will be no way of knowing that such a void exists, or the extent (length) of such a void as a proportion of the overall length of the bolt. Thus, a fitted bolt of the "Split set" or "Swellex" type may provide a significantly reduced load capacity because of a void along a significant proportion of the length of the bolt, without the operator being aware of this.

In WO 02/099251 we have disclosed a rock bolt and method of use which overcomes or reduces the above-stated disadvantages, that rock bolt employing a number of deformable members surrounding a central tensile member. Upon tensioning of the tensile member the ends of the deformable member are forced towards each other, which causes each deformable member to move outwardly at spaced positions along its length into engagement with the rock surrounding the hole which has been drilled thereinto. It is arranged that the tension in the tensile member is sufficient to force the deformable members into engagement with the rock so that the frictional engagement therebetween can hold the rock bolt securely within the hole. In addition, the action of the deformable members causes a clamping force upon the rock along the length of the rock bolt.

The rock bolt disclosed in WO 02/099251 therefore has considerable advantages over prior art rock bolts, and is achieving commercial success as a result.

However, there remain a number of drawbacks to that design, which the present invention seeks to avoid or reduce.

The first drawback is the requirement to deform the deformable member.

Notwithstanding the possibility of deformation zones being created therein by cut-outs in the deformable member, or the pre- formation of a desired deformation pattern, the deformation requires significant force, and there is substantially a compromise between the strength of the deformable member required to hold the rock bolt in place and the force required to deform it.

The second drawback is that there is substantially no limit upon the deformation which wilt occur. This has an advantage when there is a void adjacent to the hole, since the deformable member(s) can deform into the void as described in that document. However, it is preferable that some limit upon the deformation be available so as to avoid the possibility of over-tightening of the rock bolt and irrevocable damage to one or more of the deformable members.

SUMMARY OF THE INVENTION

The present invention seeks to provide a rock bolt which shares the advantages of the rock bolt disclosed in WO 02/09925 1 over the other prior art rock bolts, and yet which reduces or avoids the stated drawbacks thereof.

According to the invention therefore, there is provided a rock bolt comprising an elongated tensile member, a sleeve surrounding a part of the tensile member, a mounting member carried by the tensile member and spaced from the sleeve, and a retaining member hingedly connected at its first end at the sleeve, and hingedly connected at its second end at the mounting member, the rock bolt having means to reduce the distance between the mounting member and the sleeve so as to force a part of the retaining member away from the tensile member.

When the rock bolt is in use and is located in a hole which has been drilled or otherwise formed into rock or earth, the distance between the mounting member and the sleeve can be reduced so as to force the part of the retaining member into engagement (or into greater engagement) with the surface of the hole (or into the earth surrounding the hole), increasing the frictional or interference grip provided by the rock bolt.

In the present invention therefore, hinged connections are provided for the retaining member, the hinged connections avoiding any requirement to deform or distort any part of the retaining member and requiring only a small force to move the part of the retaining member relative to the tensile member. Alternatively stated, the retaining member is not required to deform, but is required only to hinge about its hinged connections, so that the material from which the retaining member is made can be substantially resistant to deformation and distortion.

Desirably, the retaining member is hingedly connected to the sleeve, and hingedly connected to the mounting member. Alternatively, the hinged connections are provided adjacent to the sleeve andlor adjacent to the mounting member respectively.

Preferably, the retaining member comprises a first substantially rigid bar hingedly connected at the sleeve, a second substantially rigid bar hingedly connected at the mounting member, the first and second bars also being hingedly connected together. As the mounting member is moved towards the sleeve the hinged connection between the first and second substantially rigid bars is forced away from the tensile member.

Desirably, there are two or more retaining members, each connected (preferably independently) at the sleeve and at the mounting member, and each acting substantially independently. Alternatively, the two (or more) retaining members can be connected at the sleeve and/or at the mounting member by a common hinge.

Movement of the mounting member towards the sleeve can be effected in several ways. In one example, the mounting member is securely fixed to the distal end of the tensile member, and the proximal end of the tensile member is threaded and carries a nut. Rotation of the nut in a chosen direction causes the tensile member and mounting member to be pulled outwardly of the hole. It will be understood -.7-that in such embodiments an increasing length of tensile member projects from the hole as the nut is tightened.

Alternatively, the mounting member can comprise (or be engaged by) a nut which is threaded onto the distal end of the tensile member, rotation of the tensile member in a chosen direction causing the mounting member to move towards the sleeve.

There is also provided a method of using a rock bolt as herein defined, comprising the steps of {i} inserting a rock bolt as herein defined into a hole which has been drilled or otherwise formed, so that a part of the proximal end of the tensile member projects from the hole, (iii) placing a face plate over the projecting part, {iv) locating a nut member upon the projecting part so as to clamp the face plate against the rock face, and {v} rotating the nut member so as to move the mounting member towards the sleeve and force a part of the retaining member against the surface of the hole. In ground anchor and soil nail applications, the hole may be formed by the rock bolt itself as this is forced into the ground or earth.

The sleeve can be a substantial proportion of the length of the tensile member, in which case the retaining member is relatively short and retention of the rock bolt is effected over a relatively short length of the rock bolt (adjacent the distal end of the rock bolt). This would be suitable for applications in which the force acting upon the rock bolt is small, or in which the rock is sufficiently soft for a secure retention to be provided with a small contact area.

Alternatively, the sleeve can be short in relation to the length of the rock bolt, perhaps effectively comprising a collar at the proximal end, in which case the retaining member is almost as long as the tensile member and retention of the rock bolt is effected substantially over the full length of the rock bolt.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig.1 is a side view of part of a first embodiment of rock bolt according to the invention; Fig.2 is a section along the lines Il-Il of Fig.1; Fig.3 is a section along the lines Ill-Ill of Fig.1; and Fig.4 is a side view of part of a second embodiment of rock bolt according to the invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The rock bolt 10 of Fig. I comprises a tensile member 12, around a part of which is located a sleeve 14. The distal end 16 of the rock bolt, i.e. the end which will be inserted first into a hole in use, carries a mounting member 18 to which is mounted one end of a pair of retaining members 20, as further described below.

Adjacent the proximal end 22 of the bolt 10, i.e. the end of the bolt which will be adjacent the rock face in the fitted condition of the bolt, the tensile member 12 has a threaded length 24. A plate washer 26 is be fitted over the threaded length 24, into engagement with the proximal end 30 of the sleeve 14. A washer 32 is then fitted over the threaded length 24, followed by a nut 34.

If desired, the plate washer can have a hemispherical mounting surface engageable by a hemispherical washer, whereby the plate washer may lie at an angle other than perpendicular to the tensile member 12, as is commonly used upon rock bolts.

In this embodiment there are two retaining members 20, and each comprises a pair of substantially rigid bars 40, 42. As shown in Fig.2, the bars 40 are each hingedly mounted to the mounting member 18 by a pivot pin 44 (or a pair of pivot pins as desired). As shown in Fig.3, the bars 42 are each hingedly mounted to the distal end of the sleeve 14 by way of a boss 46 and a pivot pin 50 (or pair of pivot pins as desired). As also shown in Fig.3, the bars 40 and 42 are hingedly connected together by a pivot pin 52.

It will be understood that as the nut 34 is tightened upon the threaded length 24 of the tensile member 12, the distance between the mounting member 18 and the sleeve 14 (or more specifically the distance between the pivot pin 44 and the pivot pin 50) is reduced, which forces the connection defined by the pivot pin 52 away from the tensile member 12 (i.e. upwardly as drawn). In a typical application, it will be understood that the rock bolt is located in a hole which has been drilled into rock, and upwards movement of the connection defined by the pivot pin 52 will drive a part of the bars 40, 42 into engagement with the surface of the hole.

Since the bars 40 and 42 are substantially rigid, and there is a relative angle between the bars only slightly greater than 900, the force which can be applied upon the surface is considerable, and the fnctional engagement of the rock bolt 10 can be made sufficient to retain the rock bolt within the hole in the presence of large forces.

It will also be understood that as the nut 34 is tightened, the action of the rock bolt is to compress the rock adjacent to the hole between the engagement of the bars 40, 42 and the plate washer 26, and as above indicated the ability of the rock bolt to compress the rock in this way is highly advantageous.

To further increase the retention of the rock bolt 10, the bars 40 and 42 carry respective projecting pins 54, which pins 54 are positioned and angled so that they engage the surface of the hole as the mounting member 18 moves towards the sleeve 14.

It will be understood from Fig.3 in particular that in the free condition shown, the bars 40, 42 lie substantially within the projected area of the sleeve 14. It is arranged that the sleeve 14 is a sliding fit within the hole drilled into the rock, so that the mounting member 18 does not need to move far in order to force the bars 40, 42 into engagement with the surface of the hole.

It is a particular advantage of the present invention utilising hinged connections that during the first stage of movement of the mounting member, i.e. before the bars 40, 42 engage the surface of the hole, the resistance to rotation of the nut 34 is very small, but the force increases noticeably when the bars 40, 42 engage the surface of the hole. The nut 34 can be further tightened by a specified amount, or can be further tightened until a specified torque is reached, the degree of further tightening being associated with the retention force available, and perhaps being predetermined for different rock types.

It will be understood that as the connection between the bars 40 and 42 which is defined by the pivot pin 52 is forced upwardly as drawn in Figs. 1 and 3, the circular form of the drilled hole acts to force the two bars 40, and also the two bars 42 together, i.e. towards the centre-line of the hole. The pivot pin 52 (and similarly the pivot pins 44 and 50) should resist that force, and in one suitable embodiment (not shown) the pin 52 for example carries a sleeve to prevent the bars 42 (and consequently the bars 40) moving towards each other.

As is clear from Fig.3, the end of the bar 42 is bifurcated and the end of the bar 40 is received between the bifurcated ends of the bar 42. Clearly, the arrangement can be reversed, and many other forms for the ends of the respective bars can be used, without departing from the scope of the present invention.

In the embodiment of Figs. 1-3 the angle between the bars 40 and 42 is increased by positioning the pivot pins 44 and 50 to one side (i.e. below) the tensile member 12, and the pivot pin 52 to the other side (i.e. above) the tensile member. Clearly, other arrangements could work also, with for example one or both of the pivot pins 44 and 50 being alongside the tensile member 12, for example, although in such embodiments the angle between the bars 40 and 42 would thereby be reduced and the force which could be imparted against the surface of the hole reduced.

Clearly also, reducing the length of the bars 40 and 42 will increase the angle between them and increase the force which can be imparted to the surface of the hole, but at a cost of limiting the overall outwards movement which is possible.

In the embodiment of Fig.4 on the other hand, the pivot pins 144, 150 and 152 of each retaining member 120 are all to the same side of the tensile member 112.

This arrangement has the advantage of permitting a number of retaining members 120 to be grouped around the tensile member, for example three, four, six, or perhaps more retaining members 120, as desired.

Also in the embodiment of Fig.4, the mounting member 118 is formed as a nose for the rock bolt 110, which enables this rock bolt 110 to be used as a ground anchor or soil nail and be forced into the ground or earth as well as being fittable into a pre-drilled hole.

Though not shown in Fig.4, the bars 140, 142 adjacent the connection defined by the pivot pins 152 can carry projecting pins such as 54 to increase the likelihood that the retaining member can be forced into the rock surrounding a pre-drilled hole.

Only the distal end of the rock bolt 110 is shown in Fig.4, but it will be understood that the proximal end can be similar to that described form Figs. 1-3.

In both of the embodiments shown, the mounting member 18, 118 is securely fixed to the distal end of the tensile member 12, 112 respectively. Accordingly, as the nut 34 is tightened, the length of the tensile member which projects beyond the hole increases. In tunnelling or mining operations, it may be desired to avoid such projecting tensile members, and rather than require the projecting lengths to be cut off, it can instead be arranged that the distal end of the tensile member is threaded and the mounting member is threadedly engaged therewith. The proximal end of the tensile member can carry a fixed nut (or a nut which can be threaded onto a relatively short threaded length at the end of the tensile member, so that further rotation of that nut causes the tensile member to rotate. It can be arranged that the sleeve, the retaining member(s) and/or the mounting member engages the hole to resist rotation, so that as the nut is tightened the mounting member is caused to move along the tensile member towards the sleeve.

Also, in both of the embodiments shown there is only a single set of retaining members between the mounting member and the sleeve. In these embodiments, it would be typical that the sleeve occupy a large proportion of the length of the tensile member, so that the retaining member acts only close to the distal end of the rock bolt. However, it may be that in some applications such an arrangement cannot provide the retention force required, or cannot provide the support to the rock surrounding the hole which is required, and in those applications it can be arranged that the sleeve is made shorter in length and two or more sets of retaining members are arranged along the tensile member. In one such embodiment, for example, two (or more) sets of retaining members similar to those of Fig.4 could be provided, with the pivot pins 150 being carried upon one side of a short collar, and another set of pivot pins such as I 44 being carried on the other side of the collar. Alternatively, the pivot pins 150 could be replaced by a polygonal "ring" providing the pivot pins 150 for one set of retaining members 120, and the pivot pins 144 of an adjacent set of retaining members 120.

It is of course not necessary that the bars 40, 42 (or 140, 142) be hingedly mounted to the mounting member and/or to the sleeve, and instead the bars could be hingedly mounted upon a collar or other member which abutted the mounting member or the sleeve in use. Thus, since the force acting upon the ends of the bars is compressive, all that is required is that a compressive force be communicated from the mounting member and sleeve to the bars, and that can be achieved by abutment as well as by a physical connection. Embodiments in which the bars are connected to the mounting member and sleeve are preferred, however, as these embodiments are more likely to be easier to handle loose, with the bars of the retaining members secured at both of their respective ends.

Another benefit of using hinged connections as in the present invention is that the maximum movement available to the retaining member(s) can be predetermined.

Thus, in the embodiment of Figs. 1-3 for example it can be arranged that the bifurcated end of each of the bars 42 is formed so that the minimum relative angle which can be adopted by the bars 40 and 42 is 900 for example, the bar 40 fouling the bifurcated end of the bar 42 at that orientation. This would ensure that the bars 40 and 42 do not continue to move relative to one another in an uncontrolled and unknown way, as may occur in the presence of voids for example. This is of particular advantage in arrangements utilising more than one set of retaining members along the tensile member as it enables the user more readily to appreciate what is happening (unseen) within the hole.

In ground anchor and soil nail applications, it is usually desirable for the outwardly moving parts of the retaining members to project well beyond the periphery of the sleeve 14, 114 so as to increase the amount of earth available to resist forced removal of the anchor. This could readily be accommodated with the present invention, and yet still allow a limit to be placed upon the degree of movement available to the retaining members. In the embodiment of Fig.4 for example the bars 140 and 142 could cooperate to limit the minimum possible angle between the bars to 60 or thereabouts, for example.

In both of the embodiments shown the bar 40, 140 is approximately the same length as the bar 42, 142 respectively. This is not necessarily the case, and it will be possible to vary the frictional force obtained by varying the relative lengths of the bars. Also, in ground anchor and soilnail applications, in an embodiment -13-similar to that of Fig.4 it could be arranged that the bar 140 was around twice as long as the bar 142 for example, and such an arrangement could allow the bars 142 to be moved to a position substantially perpendicular to the longitudinal axis of the rock bolt, it being understood that such an arrangement would maximise the resistance to forced removal of the anchor.

It will be understood that the hinged arrangements described are substantially totally reversible, and facilitate removal of the rock bolt if desired. Thus, the hinged connections offer as little resistance to inwards movements of the retaining members as they do to outwards movements thereof.

As indicated in the preceding Background to the Invention section, the present invention has utility for other applications besides providing an anchorage in, or supporting, a rock face. It will be understood that the clamping action of the "rock bolt" within a hole could permit its alternative use in securing train or tram rails onto their underlying concrete sleepers, for example. Alternatively again, the "rock bolt" could also be used as a lifting member, i.e. it could be inserted into a hole drilled into a block of concrete or the like, and used to anchor a lifting eye by which the block can be lifted. Notwithstanding that in these alternative uses the name "rock bolt" is not strictly accurate, the term will be used to describe the product even in these alternative applications, for ease of understanding.

The use of a sleeve surrounding a part of the tensile member is preferred but not essential. The sleeve shown in Figs. I and 4 could for example be replaced by a number of bars lying substantially parallel to the tensile member. In addition, the tensile member can be substantially rigid such as a rod or bar, or it can be a substantially flexible cable or the like.

Claims

CLAIMS

1. A rock bolt comprising: an elongated tensile member, a first mounting member, a second mounting member spaced from the first mounting member, and a retaining member located between the first mounting member and the second mounting member, the retaining member being hingedly connected at the first mounting member, and hingedly connected at the second mounting member, the rock bolt having means to reduce the distance between the first mounting member and the second mounting member so as to force a part of the retaining member away from the tensile member.

2. A rock bolt according to Claim I in which the first mounting member is a sleeve surrounding a part of the tensile member.

3. A rock bolt according to Claim I or Claim 2 in which the second mounting member is carried by the distal end of the tensile member.

4. A rock bolt according to any one of Claims 1-3 in which the retaining member is hingedly connected to the first mounting member, and hingedly connected to the second mounting member.

5. A rock bolt according to any one of Claims 1-4 in which the retaining member comprises a first substantially rigid bar hingedly connected at the first mounting member, and a second substantially rigid bar hingedly connected at the second mounting member, the first and second bars also being hingedly connected together.

6. A rock bolt according to Claim 5 in which the first bar and the second bar are approximately the same length.

7. A rock bolt according to any one of Claims I -6 in which there are two or more retaining members arranged around the tensile member, each retaining member being hingedly connected at the first mounting member and at the second mounting member. -15-

8. A rock bolt according to Claim 7 in which the retaining members are independently connected at the first mounting member and at the second mounting member.

9. A rock bolt according to Claim 7 or Claim 8 in which the retaining members are connected at the first mounting member and/or at the second mounting member by a common hinge.

10. A rock bolt according to any one of Claims 1-9 in which there are a plurality of retaining members arranged along the tensile member, one of the retaining members being hingedly connected at the first mounting member, another of the retaining members being hingedly connected at the second mounting member, there being at least one additional mounting member between the first mounting member and the second mounting member with hinged connections to the retaining members.

11. A rock bolt according to any one of Claims 1-10 in which the second mounting member is securely fixed to the distal end of the tensile member, and the proximal end of the tensile member is threaded and carries a nut.

12. A rock bolt according to any one of Claims 1-10 in which the second mounting member comprises a nut which is threaded onto the distal end of the tensile member, rotation of the tensile member in a chosen direction causing the second mounting member to move along the tensile member.

13. A method of using a rock bolt comprising the steps of {i} inserting a rock bolt according to any one of Claims 1-12 into a hole which has been drilled or otherwise formed, so that a part of the proximal end of the tensile member projects from the hole, {iii} placing a face plate over the projecting part, {iv} locating a nut member upon the projecting part so as to clamp the face plate against the rock face, and (v} rotating the nut member so as to move the first mounting member towards the second mounting member and force a part of the retaining member against the surface of the hole.

14. A rock bolt constructed and arranged substantially as described in relation to Figs. 1-3, or Fig.4, of the accompanying drawings.