WO2011020144A1

WO2011020144A1 - Rock bolt anchor and nut

Info

Publication number: WO2011020144A1
Application number: PCT/AU2010/001053
Authority: WO
Inventors: Peter Gray
Original assignee: Peter Gray
Priority date: 2009-08-19
Filing date: 2010-08-18
Publication date: 2011-02-24

Abstract

A point anchoring and pre-tensioning system used on a bar or bolt or self drilling rock bolt comprising a friction anchor device at the leading end of the bolt wherein the friction anchor device comprises two or more expansion shells which are forced into the borehole wall by a cone or cones or a wedge or wedges and wherein the diameter of the friction anchor device in its closed position is substantially the same as the maximum diameter of the main tensile support bar in the bolt. A break-out nut and washer is also claimed.

Description

ROCK BOLT ANCHOR AND NUT

Technical Field

The invention broadly relates to an anchoring and pre-tensioning system for rock bolts, and more specifically, relates to a system which enables pre-tensioning of rock bolts using a combination of a friction anchoring device and pumpable resin. More specifically still, the invention relates to a system which enables a self drilling rock bolt to be pre-tensioned using a friction anchor at the leading end of the bolt, and then enables a pumpable resin or chemical or cement grout to be used to fully encapsulate the bolt in resin, and once cured, the resin or grout then locks the pre-tensioned- force within the bolt.

The invention also specifically enables a friction anchoring device to be used on the leading end of a rock bolt without a significant increase in the diameter of the bolt, such that the borehole annulus space that needs to be filled with resin or grout is minimised. Furthermore, the invention also enables rotation of a self drilling bolt in one direction to enable a borehole to be drilled, then enables rotation of the self drilling bolt in the opposite direction to enable a friction anchor device to be secured to the leading end of the borehole wall, then finally enables a nut to be tensioned up against a rock face, by using a new nut drive and locking mechanism. The invention enables all of the above operations to be completed automatically by simply rotating a nut on the end of a self drilling bolt, and does not necessitate removal of the nut or bolt from the drilling chuck or drilling machine.

Background to the Invention

Rock bolts and cable bolts are used around the world to reinforce and stabilise tunnels, mines, and rock faces. These rock bolts and cable bolts are installed into ^• boreholes drilled into the rock, and are then anchored into those boreholes. Typically rock bolts can be anchored into boreholes either by using some form of mechanical friction anchor where some part of the bolt presses against the internal surface of the borehole thus creating friction and forming an anchor for the rock bolt; or the rock bolt can be anchored by using a cement based grout or a chemical resin in the borehole which cures and hardens thus anchoring the bolt in the borehole. There are two main types of friction anchor used to secure a rock bolt in a borehole. The first type is where almost the whole length of the bolt is pressed against the borehole wall, and the friction between the bolt and the borehole wall secures the bolt in the borehole. Common rock bolts that use this type of friction anchor are the split set bolt designed by lngersoll Rand, and the swellex bolt designed by Atlas Copco.

The second type of friction anchor bolt uses a substantially point anchor at the. leading end or top end of the rock bolt. This type of point anchor device is known as an expansion shell type anchor. An expansion shell type anchor typically has two or more steel shells which are forced against the internal sides of the borehole wall to create a friction anchor against the rock surface of the borehole wall. The steel shells are forced outwards from the bolt and against the borehole wall by forcing a wedge between the shells. This wedge. is typically pulled between the shells by a screw thread, whereby the leading end of the rock bolt is threaded and the wedge is also threaded such that rotation of the bolt causes the wedge to move along the thread on the bolt. As the bolt is rotated the wedge moves along the bolt between the shells and forces the shells outwards against the borehole wall thus anchoring .the leading end of the bolt into the borehole.

Typically the wedge has two planar inclined faces which contact two planar inclined faces on the inside of the shells. Typically the wedge moves backwards from the leading end of the bolt towards the trailing end or nut end of the bolt with the narrow end of the wedge facing backwards towards the nut end of the bolt. As the wedge is forced between the shells, the top of the shells are the first part of the shells to be forced outwards by the wedge. Alternatively, some expansion shell systems operate in the opposite direction where the wedge moves towards the leading end of the bolt and the bottom part of the shells are the first part of the shells to be forced outwards by the wedges. However, both systems described above use the same principle of operation qf a wedge or wedges forcing two or more shells outwards against a borehole wall.

In addition, many expansion shell systems use three or four shells which can be expanded against the borehole wall. The advantage of these multi shell designs is that each shell can be forced against the borehole wall and therefore it maximises the borehole circumferential area covered by the shells. Conversely, in a two shell design, only the top and bottom of the shells can be forced into the borehole wall, and the sides of the shells do not expand against the borehole wall.

However a disadvantage of existing two, three and four shell designs is that the shells do not contact or support each other when they are expanded against the borehole wall. The shells in their expanded position only contact the borehole wall on the outside of the shells, and only contact the wedges on the inside of the shells. The shells do not contact each other along their adjacent longitudinal edges. Consequently the shells do not support each other if high rotational torque is applied to the bolt. High torque applied to the bolt can cause the wedges to rotate inside the shells, and or, cause the shells to twist and buckle and ultimately cause bolt failure.

In addition, another disadvantage of many existing expansion shell systems is that they use a wedge to expand the shell. Typically the wedge has a planar surface inclined at approximately 5 to 10 degrees such that, as it is forced against the underside of a shell, the shell is forced to expand outwards. Consequently the maximum diameter that the shell can expand to is limited by the maximum diameter of the wedge. Since the wedge is inclined at a relatively flat angle as indicated previously, the maximum diameter of the wedge is limited. This can be a disadvantage in soft rock or fractured rock where the shells need to be expanded to a large diameter in order to achieve good anchorage in the borehole.

A further disadvantage of existing expansion shell systems is that they typically have a rough external surface on the outside of the shell which is designed to dig into the surface of the borehole wall. This rough surface typically comprises a series of parallel ridges and grooves which are almost always orientated at right angles to the longitudinal axis of the borehole. These ridges and grooves at right angles to the borehole axis are designed to resist the axial tensile force that can develop in the bolt.

However the friction generated between the outside surface of the shell and the borehole wall has to resist both the rotational torsional force as the shell is expanded, as well as the axial tensile force as the nut on the end of the. bolt is tensioned up. If the ridges and grooves on the outside surface of the shell are orientated at right angles to the borehole axis and hence parallel to the rotational torsional force, then they will provide little resistance to this torsional force.

In addition, if the ridges and grooves are at right angles to the borehole axis, then the width of these ridges and grooves on the outside of the shell are relatively narrow and are typically only 2 to 10mm wide. Consequently as the shell is forced into the borehole wall, the rock bridges formed by the shell grooves, are also narrow. Once the axial tensile force exceeds the shear strength of these rock bridges, then failure between the outside surface of the shell and the borehole wall will occur. It would be preferable to have rock bridges that were very wide, such that they would be stronger, but the problem of having the ridges and grooves on the outside of the shell spaced widely apart, is that there would then be fewer rock bridges formed and the total shear strength would be reduced.

This invention solves this problem by having the ridges and grooves on the outside of the shell at an angle of approximately 45 to 80 degrees to the borehole axis such that the rock bridges formed are also at an angle. In this way the shell can resist both the rotational torsional force as the shell is expanded, and the axial force as the nut on the trailing end of the bolt is tensioned up.

Once the expansion shells cannot be expanded against the borehole wall any further, a nut on the bottom or trailing end of the bolt can then be tensioned up against the rock face. As the nut is tensioned up against the rock face, a tensile force is generated in the rock bolt between the nut at the bottom of the bolt, and the expansion shell anchor at the top of the bolt. This type of expansion shell friction anchor is well known and is common in the mining industry. .

Typically in an expansion shell bolt, the wedge and shells are fitted over a solid steel bar to form the bolt such that the overall diameter of the bolt is much greater than the diameter of the main steel bar in the bolt. In practice, the diameter of the borehole drilled is just large enough to enable the expansion shells to be forced into the borehole, but small enough such that the expansion shells are just touching the borehole wall such that they do not rotate as the steel bar of the bolt is rotated. The inner planar faces of the shells are then contacting the leading part of the planar faces of the wedges such that the wedges do not rotate as the bar is rotated, and the wedges can then be pulled between the shells by the thread on the bar and through the wedge expansion washer. For example, an expansion shell bolt with a solid steel bar of 16mm in diameter would typically require a 30mm diameter borehole to accommodate the large diameter expansion shells. Solid steel bars of 20mm and 24mm in diameter would require boreholes of 35 and 45mm in diameter respectively to accommodate their respective large diameter expansion shells. Therefore the increase in borehole diameter over the bar diameter just to accommodate the large diameter expansion shells for 16mm, 20mm and 24mm diameter bars respectively, is typically 14mm, 15mm and 21mm.

By having expansion shells and a wedge expansion washer mounted onto a steel bar necessitates the use of a borehole diameter that will accommodate the outer diameter of the shells. However, by keeping the bar diameter substantially the same along its entire length, and in particular where the wedge is screwed onto it, means that the tensile strength of the bolt is same along its entire length and that there is no loss in tensile strength of the bolt where the expansion shells are located. For this reason all expansion shell bolts have a much larger overall diameter across the expansion shells, than across the main bar of the bolt.

The large diameter borehole which is used to accommodate an expansion shell . bolt can then be filled with cement grout or resin. If a grout tube is then fed into the bottom of the borehole, cement grout can then be pumped into the borehole to fill the annulus space between the bolt and the borehole wall such that the bolt can then be fully encapsulated with cement grout.

Expansion shell bolts are sometimes referred to as being pre-tensioned, because a tensile force can be generated in the bolt between the anchor point at the leading end of the borehole, and the nut at the trailing end of the rock bolt.

In some applications, an expansion shell bolt can also be installed with a resin cartridge. In this case, a borehole is drilled in the normal manner with a drill rod and drill bit, then a frangible resin cartridge is installed into the borehole. An expansion shell bolt is then pushed into the borehole behind the resin cartridge until the bolt pushes the resin cartridge to the leading end of the borehole. The expansion shell bolt is then pushed through the frangible resin cartridge which then ruptures releasing resin into the borehole. The bolt is then rotated to mix the resin and to expand the shells to anchor the bolt into the borehole, The resin then subsequently cures and hardens fixing the shells in their expanded position against the borehole wall. However, such bolt installations require a relatively large volume of resin to completely fill the borehole, and in practice expansion shell bolts installed with resin cartridges are not fully encapsulated with resin, and the unencapsulated length of bolt can be subject to corrosion.

If rock bolts are to be fully encapsulated with cement grout, then the cement grout is often pumped into the borehole either through a hollow bolt or hollow cable or through a separate grout tube which is inserted into the borehole. A grout pump is used to create sufficient pressure in the grout to enable it to flow through the hollow bolt or grout tube to surround the rock bolt and fully encapsulate the bolt with cement grout in ^"" the borehole. The cement grout is then allowed to cure and harden, and once the . cement grout has hardened, it anchors the rock bolt in the borehole.

Expansion shell bolts can be installed with or without resin cartridges, or with or without pumpable cement grout or pumpable resin.

When a chemical resin is used to anchor a conventional solid rock bolt in a borehole, the resin is typically contained within a frangible plastic cartridge which is inserted into the borehole in front of the rock bolt. The rock bolt, which in this case is typically solid, is then pushed and rotated into the resin cartridge by the drilling machine which ruptures the resin cartridge and thoroughly mixes the resin causing the resin to start to cure as it is forced to flow back down around the annulus between the rock bolt and the borehole wall. This curing process is typically between 10 and 60 seconds, and once this resin has cured and hardened the rock bolt is then anchored into the borehole. This is also known as a fully encapsulated and anchored bolt or fully anchored dowel, but it is not considered to be a pre-tensioned bolt.

A chemical resin can also be pumped through a hollow rock bolt or hollow tube in a similar manner to cement grout described above. In this case the curing time on the resin must be slow enough to allow the resin to be pumped into the bolt and to fill the borehole. The pumping pressure depends on several factors including the viscosity of the resin, the diameter of the hole it is being pumped through, the length of the flow path for the resin, and the time taken to fill the borehole with resin. Typically the time taken to fill a borehole with resin is less than 5 minutes and more commonly is less than 1 minute. The curing times for the resin are therefore adjusted according to the time to fill the borehole with resin.

Developments in resin cartridges have led to cartridges which have a fast curing resin at one end of the resin cartridge, and a slower curing resin at the other end of the resin cartridge. The fast curing resin typically has a curing time of less than 60 seconds, and the slow curing resin typically has a curing time of more than 60 seconds. The fast curing resin is normally placed at the top or leading end of the borehole such that the leading end of the rock bolt is first anchored at the top or leading end of the borehole by the fast curing resin. Once the fast curing resin has anchored the top of the rock bolt, the nut on the opposite end of the rock bolt is tensioned up against the rock face to be supported such that a tensile force is then generated within the bolt between the nut and the section of the bolt which is anchored by the fast curing resin. The slower curing resin subsequently cures and anchors the remaining length of the rock bolt in the borehole but which already has a tensile force applied to it. By using resin cartridges with a fast curing resin and a slow curing resin within them, known as two speed resin cartridges, a pre-tensioning force can be applied to rock bolts. This is known as having a pre- tensioned bolt.

It is also possible to pre-tension cable bolts into boreholes. Cable bolts are installed into long boreholes, typically 4 to 10 metres long, and they can be anchored at the top of a long borehole by using a frangible resin cartridge which is broken and the resin is mixed by the leading end of the cable bolt. Once the resin cures and hardens it anchors the leading end of the cable bolt into the borehole. Alternatively a mechanical anchoring device can be used on the leading end of the cable bolt to anchor the cable bolt in the borehole. A hydraulic jack is then typically placed over the trailing end of the cable bolt and a tensile force is applied to the cable between the anchor point at the leading end of the cable and the supported rock face at the trailing end of the cable bolt. This tensile force is then maintained in the cable bolt by fixing a barrel and wedge or nut to the trailing end of the cable bolt. The borehole is then subsequently completely filled with cement grout to fully encapsulate the cable bolt and to maintain the pre-tensile force in the cable bolt.

It is also possible to pre-tension self drilling rock bolts into boreholes. Current self drilling rock bolts can be pre-tensioned by using a large diameter friction anchor device ^• on the leading end of the bolt and then subsequently filling the borehole with cement grout or resin. However the disadvantage of such bolts is that they require the use of a large diameter borehole which necessitates the use of a large volume of cement grout or resin to fully encapsulate the bolt. In addition, such bolts often use small diameter bars to try and minimise the borehole diameter required, but this then has the disadvantage that the tensile strength of the bar and hence the bolt, will be limited.

Alternatively, self drilling bolts can be pre-tensioned by using a two speed resin cartridge which is contained inside a hollow self drilling rock bolt. The self drilling bolt can drill its own borehole in a conventional manner, and then the resin cartridge is ruptured and the resin is forced out of the leading end of the bolt, the first resin to flow out of the bolt and back down the annulus is resin with a slow curing speed and this resin ends up encapsulating the lower part of the bolt. However, the last resin to flow out of the bolt is resin with a fast curing speed and this resin ends up encapsulating the upper part of the bolt. As soon as the fast curing resin has hardened, a nut at the end of the bolt can be tensioned up to create a tensile force in the bolt before the slow curing resin has hardened. In this manner, a pre-tensioned force can be applied to self drilling rock bolts which contain a two speed resin cartridge. However, the disadvantage of such bolts is that the bolt with the two speed resin cartridge inside it, necessitates the use of a large diameter tube and hence a large diameter borehole for the bolt to fit into. In addition, the volume of resin available is limited to the volume of resin in the resin cartridge, and if the borehole requires additional resin to fill it, then the borehole will not be completely filled. Finally, the bolt must be kept relatively cool and cannot be left out in the sun in a mine storage area and this is a disadvantage when the bolts are used in hot locations. However, with the current process of pumping resin or cement grout through hollow rock bolts, the resin or the grout do not have two speed curing times, and therefore it is not possible to apply a pre-tension force onto bolts installed with pumpable resin or pumpable cement grout using existing technology. With current technology, the only ways to apply a pre-tension force to a rock bolt are by:

• using a friction anchor device such as an expansion shell anchor on the leading end of a rock bolt where the diameter of the expansion shell is much larger than the diameter of the main bar of the bolt which necessitates a large diameter borehole to be used, and then anchoring . the bolt at the leading end of the borehole, and then tensioning the bolt at the trailing end of the bolt with a nut or forged drive nut, and then subsequently fully encapsulating the bolt with cement grout or resin; or by,

• using a two speed resin cartridge such that a fast curing resin surrounds- the bolt at the leading end of the borehole, and having a slow curing resin surrounding the bolt at the trailing end of the borehole, and then tensioning the bolt at the trailing end of the bolt once the fast curing resin has hardened by rotating a nut or forged drive nut on the trailing end of the bolt; or by,

• using a point anchor on a cable bolt and then using a hydraulic jack to apply a tensile force to the cable, and then subsequently fully grouting the borehole with cement grout.

All of the above current methods of applying a pre-tensioned load to a rock bolt or to a cable bolt have disadvantages.

All of the above Background to the Invention is prior art.

It is an objective of the current invention to overcome the above limitations of anchoring and pre-tensioning rock bolts and cable bolts and to provide a system which will enable a pre-tensile force to be applied to self drilling rock bolts or to rock bolts or to cable bolts installed with pumpable resin or be installed with pumpable cement grout and also to provide other advantages of installing self drilling rock bolts or rock bolts or cable bolts.

The present invention relates to a system which enables a pre-tensile force to be applied to self drilling rock bolts or to rock bolts or to cable bolts which use pumpable cement grout or pumpable chemical resin to anchor the bolt in the borehole. More particularly, the present invention relates to system which enables a pre- tensile force to be applied to a self drilling rock bolts or to a rock bolt or to a cable bolt by using pumpable cement grout or pumpable chemical resin without the need for two separate curing times for the cement grout or the chemical resin.

More particularly still, the present invention relates to system which enables a pre-tensile force to be applied to hollow rock bolts or to hollow self drilling rock bolts or to hollow injection rods or to hollow cable bolts by using pumpable cement grout or pumpable chemical resin without the need for two separate curing times for the cement grout or the chemical resin and without the requirement to change over fittings or couplings during the pumping cycle.

The present inventor has developed a system that enables a pre-tensile force to be applied to self drilling rock bolts or to rock bolts or to cable bolts.

More particularly, the present inventor has developed a system that enables the leading or front end of a self drilling rock bolt or a rock bolt or a cable bolt to be anchored into a borehole by using a friction anchor device such that a pre-tensile force can then be applied to the bolt between the leading or front end of the bolt and the trailing or rear end of the bolt, and subsequently enables the remaining length of the bolt to be anchored into a borehole by using a pumpable cement grout or by using a pumpable chemical resin without the need for a large diameter borehole to be drilled.

Furthermore, the present inventor has developed a system that incorporates a new friction anchor expansion shell mechanism that can be used with any bolt, or rock bolt, or self drilling rock bolt, that does not increase the overall diameter of the bolt assembly such that the annulus space between the borehole and the bolt assembly along the length of the bolt assembly is substantially the same and is relatively small. Moreover the new friction anchor expansion shell mechanism has the significant advantage of having a rotational hinge point which enables the expansion shells to expand to a larger diameter than is possible with existing expansion shell wedge mechanisms.

In addition, the new friction anchor expansion shells have the additional advantage that they have external surfaces which contact the borehole wall which comprise a series of grooves and ridges which are designed to resist both the rotational torque which is applied as the shells are expanded, and the axial force as tension is applied to the bolt. The external surfaces of the expansion shells. provide a more effective anchor with the borehole wall than the external surfaces of existing expansion shells. Even furthermore, the inventor has invented a system that enables:

• a self drilling rock bolt to drill its own borehole with a small annulus space between the almost all of the bolt length and the borehole wall; and, • enables the leading or front end of the self drilling rock bolt to be rapidly anchored into the leading end of the borehole wall; and,

• enables a tensile force to be rapidly applied to the bolt; and,

• enables the bolt to be fully encapsulated with cement grout or pumpable resin such that the bolt is then fully encapsulated with grout or resin; and, • enables all of the above to be accomplished without any manual handling or manual attachment of fittings.

This has considerable advantages for the installation of rock bolts in that the invention provides for:

• the rapid installation of self drilling rock bolts such that the installation of self drilling bolts is suited to automation; and,

• the rapid application of a pre-tensile force to a rock bolt to support a rock face; and,

• the ability to fully encapsulate the bolt either with a cement grout or with a chemical resin; and,

• the ability to have a very small annulus space between the bolt assembly and the borehole wall to minimise the volume of resin required to fully encapsulate the bolt, and to maximise the pull out capacity of the bolt.

The above advantages can enable the installation of rock bolts to be faster and be more productive, and can maximise the support capacity of the rock bolts.

Summary of the Invention

According to the present invention there is provided:

• a friction anchor device fixed to the leading end of a rock bolt that it does not significantly increase the overall outside diameter of the rock bolt such that the friction anchor device has a similar diameter to the main bar of the rock bolt; and, • a friction anchor device partly comprising two or more expansion shells that preferably pivot or hinge to expand to maximise the amount of expansion possible and where the expansion shells are designed to provide an efficient anchor against the borehole wall to resist both rotational torque and axial tensile force; and,

• a cone that can force two or more expansion shells to expand and

anchor against a borehole wall; and,

• a one way rotation system that enables the tensioning cone to rotate in one direction in a borehole, but prevents the tensioning cone from rotating in the opposite direction in a borehole such that this tensioning cone can rotate with the self drilling rock bolt during the drilling cycle, but when the bolt is rotated in the opposite direction, it can prevent the tensioning cone from rotating such that the tensioning cone can move along a thread and force the expansion shells to expand against the borehole wall ; and,

• a threaded tube to carry the tensioning cone to which it is threadably attached, and to carry the expansion shells, and to carry the drill tip, whereby the threaded tube is fixed to the end of the main tensile support bar of the rock bolt such that the threaded tube typically has a smaller diameter than the main bar of the rock bolt such that the overall diameter of the friction anchor device is substantially the same as the diameter of the main bar of the rock bolt; and,

• a nut drive and nut locking mechanism which enables a nut to rotate a self drilling bolt in one direction to drill a borehole and then enables the nut to rotate a self drilling bolt in the opposite direction, and then enables the nut to advance along a thread on the trailing end of a self drilling rock bolt to tension up the self drilling bolt against a rock face once a certain torque value has been exceeded. '

The invention is typically used with self drilling rock bolts but is not so limited. In practice, the nut on the trailing or bottom end of a self drilling rock bolt is placed in the drill chuck of a drilling machine and the drilling machine rotates the nut. The nut drive and locking mechanism enables the nut to also rotate the whole self drilling rock bolt assembly such that the self drilling rock bolt can drill its own borehole. Typically this borehole is drilled using left hand rotation. As the self drilling rock bolt is rotated, water or air is pumped through the bolt to remove rock cuttings generated during the drilling process.

Once the borehole has been drilled to its full depth, drilling rotation is stopped and the flushing fluid is also turned off. At this point, the self drilling rock bolt is typically substantially contained within the borehole with only the nut and domed ball at the trailing end of the bolt are protruding from the borehole. Typically the drill bit on the leading end of the self drilling rock bolt has drilled a borehole which is only slightly bigger than the outside diameter of the self drilling rock bolt. Typically the borehole is less than 10mm larger in diameter than the main bar diameter of the self drilling rock bolt, and may be only 5mm larger in diameter than the diameter of the borehole.

The drilling rotation is then reversed such that the drilling machine then rotates the nut in right hand rotation. The nut drive and locking mechanism still prevents the nut from rotating with respect to the self drilling rock bolt such that the self drilling rock bolt assembly is now rotated with right hand rotation. However, as the self drilling rock bolt is rotated in right hand rotation, the one way rotation tensioning washer near the leading end of the bolt, jams or wedges or otherwise locks against the inside of the borehole such that it cannot rotate in right hand rotation.

This one way rotation tensioning washer is screwed onto a threaded tube which is fixed to the leading end of the bolt such that the threaded tube rotates as part of the whole bolt assembly. Therefore as the threaded tube is rotated as part of the bolt, the one way rotation tensioning washer does not rotate, and this washer then moves axially along the thread on the tube. Typically the one way rotation tensioning washer screws along the tube away from the leading end of the bolt and towards the trailing end of the bolt.

Typically two or more shells or expansion shells are located on or around the threaded tube such that the one way rotation tensioning washer is screwed towards them. As the one way washer contacts the shells it forces the shells to move over one or more cone shaped members. Typically this cone shaped member is formed by a cone or cones on one side of the tensioning washer or could be formed by a tapered end on the main bar of the bolt. As the shells are forced over the cone shaped member, the shells are forced to expand and as the shells expand, part or all of the shells contact the borehole wall. Rotation of the bolt is continued until the shells are forced into hard contact with the borehole wall. Typically the self drilling rock bolt is rotated until a pre-determined torque value is reached, at which point the expansion shells will have anchored the self drilling rock bolt into the borehole at the leading or front end of the borehole.

Once the self drilling rock bolt has been rotated up to a pre-determined torque value, further rotation of the nut causes the nut drive and locking mechanism to release and enable the nut to now advance along the thread at the trailing end of the bolt. The nut can now be screwed up against the rock face until the full torque capacity of the drilling machine is reached, at which point further tensioning rotation of the nut by the machine is not possible.

At this point a tensile force now exists in the bolt between the nut at the trailing end of the bolt and the expansion shells at the leading end of the bolt. This tensile force in the bolt may be up to 10 tonnes or more but is typically between 2 to 5 tonnes. This tensile force in the bolt is then transferred through to the rock between the nut and the expansion shells, and this has some geotechnical benefits.

Once the nut is fully tensioned up against the rock face, an anchoring fluid such as a cement grout or a chemical resin can now be pumped through the hollow self drilling rock bolt to completely fill the borehole and fully encapsulate the bolt. Since the annulus space between the outer surface of the bolt and the borehole wall is relatively small, typically 2 to 5mm, then the volume of anchoring fluid is kept to a minimum. Once the anchoring fluid cures and hardens, the tensile force in the bolt is locked into the bolt by the anchoring fluid. However, the cured and hardened anchoring fluid can now fully support the bolt along its entire length in the borehole.

By using the invention as described above, the invention enables self drilling rock bolts to be installed with a pre-tensioned force and then be fully encapsulated with grout or resin, and all this can be achieved without removal of the bolt from the drilling chuck or any manual handling. In practice, the invention enables pre-tensioned, fully encapsulated self drilling rock bolts to be installed very rapidly, and typically be installed in less than 60 seconds.

The present invention overcomes the problems of using conventional large diameter friction anchor devices which necessitate the use of large diameter boreholes which in turn create a large annulus space between the main bar in a bolt and the borehole wall. A large annulus space requires a large volume of cement grout or resin to fill and also provides a lower load transfer capacity than a small annulus space. In addition, the present invention overcomes the problems of using conventional large diameter friction anchor devices which often necessitate the use of small diameter bars for the rock bolt such that the tensile strength of the main bar of the rock bolt is restricted.

Moreover, the present invention overcomes the problems of using conventional large diameter friction anchor devices which are designed to maximise their anchoring load transfer capacity with the rock inside a borehole, but almost always have a lower load transfer capacity than the tensile capacity of the main bar of the rock bolt. The present invention overcomes this problem by having a friction anchor device to only support an initial pre-tensile force in the bolt, and the main anchor support for the bolt is provided by full encapsulation of the bolt with cement grout or resin.

The present invention overcomes the problems of using two speed resin cartridges where there is always the risk that the pre-tensile force may be applied to a bolt before the fast set resin has cured and anchored the leading end of the bolt in the borehole.

The present invention overcomes the problems of using two speed resin cartridges inside a hollow self drilling bolt where there is the risk that the resin may have over heated and become unusable if the bolts are left out in the sun, and also

overcomes the problem of not completely filling a borehole with resin particularly if the bolt is installed in broken or fractured rocks.

The present invention overcomes the problems of using conventional hydraulic jacks to tension up cable bolts where the hydraulic jacks are heavy and require considerable manual handling.

The present invention overcomes the problems of the considerable manual handling associated with installing pre-tensioned fully encapsulated rock bolts or cable ■ bolts and enables such bolts to be installed automatically by a drilling machine.

The present invention overcomes the problems of installing conventional, point anchored hollow rock bolts in large diameter boreholes with a large annulus, and enables such bolts to be installed in boreholes that are only slightly larger than the diameter of the main bar of the rock bolt.

Preferably the expansion shells expand such that the trailing end of the shells expand and the leading end of the shells rotate about a pivot point or hinge point. This has the advantage that a tensile force on the bolt would tend to further open the shells and increase the diameter of the shells. Preferably as the expansion shells expand they rotate about a pivot point or hinge point such that one end of a shell remains in contact with one end of another shell, but at the opposite end of the shells the shells expand away from each other as a wedge or wedges are forced between one end of the shells. This has the advantage that the shells can expand to a larger diameter than is possible if the shells expand parallel to each other.

The expansion shells may open either towards to top or leading end of the bolt, or open towards the bottom or trailing end of the bolt.

Preferably as the expansion shells are curved or form part of a circle in cross section such that the shells fit closely around a circular threaded tube or circular threaded bar.

Preferably the threaded, tube is fixed to the end of the main tensile support bar of the rock bolt and has a smaller outside diameter than the outside diameter of the main bar. In practice this enables the expansion shells to fit over the threaded tube without increasing the overall diameter of the rock bolt. In practice the one way rotation washer is screwed onto the threaded tube.

In the case of self drilling rock bolts, the drill bit is preferably fixed to the leading end of the threaded tube.

Preferably the one way rotation washer may have one or more arms or lugs that allow the washer to rotate in one direction inside a borehole but prevent the washer from rotating in the opposite direction inside a borehole. Preferably the arms are spring loaded against the inside of the borehole. Preferably the one way rotation tensioning washer has one or more pins or bars or lugs or other protruding shape on the cone face of the washer that prevents the washer from rotating with respect to the shells.

Preferably the leading end of the one way rotation tensioning washer is tapered to have a cone shaped end which acts as the cone to open the expansion shells.

Preferably a cone shape is used to open the expansion shells rather than a planar wedge shape.

Preferably the pre-tensioning system is used with self drilling rock bolts but is not so limited.

Persons skilled in the art would appreciate that different embodiments of the invention could be used with self drilling rock bolts, hollow rock bolts, hollow injection tubes, hollow cable bolts or any other bar or bolt that needs to be anchored or be anchored and pre-tensioned. Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of each claim of this specification.

In order that the present invention may be more clearly understood, preferred embodiments will be described with reference to the following drawings and examples.

Brief Description of the Drawings

Figure 1 is a schematic side view of a self drilling rock bolt with the pre- tensioning system.

Figure 2 is a schematic sectional view of the leading end of a self drilling rock bolt with the pre-tensioning system.

Figure 3 is a schematic side view of the leading end of a self drilling rock bolt with the pre-tensioning system with the expansion shells fully closed.

Figure 4 is a schematic side view of the leading end of a self drilling rock bolt with the pre-tensioning system with the expansion shells fully expanded.

Figure 5 is a schematic sectional view of the leading end of a self drilling rock bolt with the expansion shells opening in the opposite direction to that shown in Figures 1 to 4.

Figure 6 is a three dimensional view of the expansion shells in their closed position.

Figure 7 is a three dimensional view of the expansion shells in their open position.

Figure 8 is an end view of the expansion shells in their closed position closely fitting around a threaded tube.

Figure 9 is a three dimensional view of the one way rotation washer. Figure 10 is an end sectional view of the one way rotation washer on a threaded tube inside a borehole with the arms of the one way washer in the closed position.

Figure 11 is an end sectional view of the one way rotation washer on a threaded tube inside a borehole with the arms of the one way washer in the open position. .

Figure 12 is an expanded three dimensional view of the break out washer assembly on the nut end of a self drilling rock bolt.

Figure 13 is a closed three dimensional view of the break out washer assembly on the nut end of a self drilling rock bolt.

Figure 14 is a three dimensional view of the break out washer.

Detailed Description of the Preferred Embodiments

Where the specification refers to a "bar" or to a "bolt" or to a "rock bolt" or to a "cable bolt" or to a "self drilling rock bolt" or to a "hollow rock bolt" or to a "hollow self drilling rock bolt" or to a "hollow cable bolt" or to a "hollow elongate member" or to a "hollow injection tube" or to a "hollow injection rod" or to a "bar" or to a "hollow bar" or to a "hollow elongate bar" or to a "hollow drill rod" or to a "tube" or to a "pipe" it is to be understood that the invention includes all such variations and modifications of the above including any long hollow elongate member including self drilling rock bolts.

Where the specification refers to a "steel bar" or to a "main bar" or to a "support bar" or to a "main tensile support bar" it is to be understood that the invention includes all such variations and modifications of a bar including both solid and hollow bars and bars which are used as the main tensile support bar in a rock bolt.

Where the specification refers to a "drill bit" or to a "drilling bit" or to a "drill tip" or to a "drilling tip" or to a "cutting tip" it is to be understood that the invention includes all such variations and modifications of the above, including any device that can be use to drill a borehole in rock.

Where the specification refers to a "threaded tube" or to a "threaded bar" it is to be understood that the invention includes all such variations and modifications of a threaded elongate member including hollow and solid bars.

Where the specification refers to a "one-way rotational washer" or to a "one-way rotation washer" or to a "one-way washer" or to a "one way rotation tensioning washer" it is to be understood that the invention includes any device that can screw onto a threaded bar or threaded tube and can rotate in one direction inside a borehole but cannot rotate in the opposite direction inside the same borehole. Where the specification refers to an "arm" or to a "bar" or to a "lug" or to a "strap" or to a "spring" it is to be understood that the invention includes all such variations and modifications of an arm that could be used to enable a one way rotation washer to rotate in one direction inside a borehole, but prevent the one way rotation washer from rotating in the opposite direction inside the same borehole.

Where the specification refers to an "expansion shell" or to "expansion shells" or to a "shell" it is to be understood that the invention includes all such variations and modifications of the above including any member or members that can be forced outwards from a rock bolt or bar to contact against the inside surface of a borehole wall.

Where the specification refers to a "closed position" or to "shells in their closed position" or to "expansion shells in their closed position" it is to be understood that the invention refers to the smallest diameter formed around the outside of two or more expansion shells when they are assembled on a rock bolt and when a wedge or a cone has not been forced between them.

Where the specification refers to a "cone" or to a "wedge" or to a "tapered end" it is to be understood that the invention includes all such variations and modifications of a member with one or more inclined surfaces that could be used to force one or more expansion shells outwards to contact against the inside surface of a borehole wall.

Where the specification refers to a "main tensile support bar" or to a "main tensile bar" or to a "main bar" it is to be understood that the invention includes all such variations and modifications of the bar used for the majority of the length of a rock bolt, and where more than one bar is used in a rock bolt assembly, it is the bar with the largest external diameter and which is used for most of the length of the rock bolt.

Where the specification refers to a "domed ball" or to a "domed washer" or to a "hemispherical washer" it is to be understood that the invention includes all such variations and modifications of a washer that can be used to accommodate rock bolts which are installed at an inclined angle to a rock surface.

Where the specification refers to a "nut" or to a "drive nut" or to a "main drive nut" it is to be understood that the invention includes all such variations and modifications of a nut including hexagonal nuts, square nuts, castellated nuts, or any other device that could be used to apply tension to a bar or bolt along a thread.

Where the specification refers to a "break out washer" or to a "plastic break out washer" or to a "plastic washer" it is to be understood that the invention includes all such variations and modifications of a washer that can prevent rotation of a nut on a thread up to a certain torque value and once this torque value is exceeded the washer can enable the nut to advance along the thread.

Where the specification refers to a "lug" or to a "ridge" it is to be understood that the invention includes all such variations and modifications of a raised portion of a washer that can fit into a slot or into a gap or into a space or into a keyway in a nut or in a bar or between two expansion shells.

Where the specification refers to a "keyway" or to a "slot" it is to be understood that the invention includes all such variations and modifications of a groove that could be used to contain a lug or a ridge which may be part of a break out washer.

Where the specification refers to a "sleeve" or to an "injection sleeve" it is to be understood that the invention includes all such variations and modifications of a member that could be used to contain a break out washer between it and a nut.

For consistency, in the Figures, item numbers refer to the same feature or design component.

One embodiment of the pre-tensioning system (the "system") used with a self drilling rock bolt 36 is shown in Figure 1 and the self drilling rock bolt 36 comprises a hollow elongate bar 1 in a borehole 2. The bar 1 has an injection sleeve 3 which retains a break out washer 4 between the sleeve 3 and the nut 5 and the nut 5 is screwed onto a thread 7 on the trailing end of the bar 1. The nut 5 also supports a hemispherical washer 6 against a bearing plate (not shown) which then contacts the rock face.

The bar 1 may have a coarse thread profile or ribbed profile 8 along most of its length. The difference between the borehole 2 diameter and the maximum outer diameter of the bar 1 is small and is typically less than 10mm and often less than 5mm such that the gap or annulus between the borehole 2 and the bar 1 is only a few millimetres.

The bar 1 typically has a tapered end forming a cone 9. This cone 9 just fits under the trailing end of two or more expansion shells 11. The expansion shells 11 fit closely around a threaded tube 10 which is screwed into or otherwise fixed to the leading end of the bar 1. The leading end of the expansion shells 11 are just contacting the trailing face of the one way washer 12 which has one or more pins or lugs 15 which engage with the leading end of the expansion shells 11 such that the expansion shells 11 cannot rotate with respect to the one way washer 12.

The one way washer 12 also has one or more arms 13 and one or more spacer pins 14 on the leading face of the one way washer 12. The arms 13 are shown in Figure 1 in their closed position. The spacer pins 14 enable the arms 13 to operate freely and prevent the arms 13 from contacting the drill bit holder 16 at the end of the threaded tube 10. The drill bit holder 16 at the end of the threaded tube 10 retains a drill bit 17 which drills the borehole 2.

Figure 2 shows the same embodiment as shown in Figure 1 and shows the leading end of a self drilling rock bolt 36 with the pre-tensioning system. The threaded tube 10 is fixed to the bar 1 by a thread 29 or by any other suitable means. The leading end of the bar 1 has a tapered cone 9 and the trailing ends of the expansion shells 11 just fit over the cone 9 when the shells are in their closed position 18. The expanded position of the shells is shown as position 19. However, when the shells are in their closed position 18 the external diameter of the shells 11 is substantially the same as the external diameter of the bar 1 and the external diameter of the one way rotation washer 12. The one way rotation washer 12 is shown at the leading end of the threaded tube 10 at position 20 which enables the expansion shells 11 to be in their closed position 18.

Figure 3 is the same embodiment as shown in Figure 2. The one way washer 12 has pins or lugs or bars 15 that engage with the shells 11 such that the one way washer 12 cannot rotate with respect to the shells 11.

Figure 4 is the same embodiment as shown in Figure 3 except that the shells 11 are shown in their fully expanded or open position 19. The bar 1 and the threaded tube 10 fixed to the leading end of it have been rotated in a clockwise direction which has caused the arms 13 on the one way washer 12 to open and jam against the borehole wall (not shown) preventing the one way washer 12 from rotating. Therefore as the threaded tube 10 is rotated the one way washer 12 is screwed along the threaded tube 10 and pushes the shells 11 over the cone 9 on the end of the bar 1. The washer 12 is screwed along the threaded tube 10 to position 21 where the shells 11 are fully expanded 19. As the shells 11 are forced over the cone 9 they expand to position 19 by substantially rotating about a hinge or pivot point 22 where the shells 11 contact each other. As the expanded position 19 of the shells 11 is forced into the borehole wall (not shown) the shells 11 will not rotate in the borehole 2 and the pins 15 which engage the shells 11 with the one way washer 12 also ensure that the shells 11 do not rotate in the borehole 2. By having the shells 11 expanding by rotating about a hinge point 22 enables the shells 11 to expand to a larger diameter or position 19 than would be possible if the shells 11 expanded parallel to each other (not shown).

Figure 5 shows another embodiment where the cone 9 is positioned on the trailing face of the one way washer 12 and the leading end of the shells 11 expand over this cone 9. The leading end of the bar 1 does not have a tapered end and the shells 11 rotate at this end of the bar 1. In this embodiment the shells 11 expand by the same ■ process as shown in Figure 4 except that the shells 11 expand at the leading end of the shells 11 not the trailing end of the shells 11 as shown in Figure 4, and this may have some advantages for certain rock types.

Figure 6 shows one embodiment of the expansion shells 11 in their closed position 18. The shells 11 contact each other at pivot or hinge points 22 and the shells 11 are substantially curved at these contact hinge points 22 to enable the shells 11 to easily rotate about these hinge points 22. The external surface 24 of the shells 11 may have a rough profile 30 to assist with anchoring against the borehole wall.

Figure 7 shows the same embodiment of the expansion shells 11 as shown in Figure 6 except that the shells 11 are in their fully open or fully expanded position 19. The shells 11 are still contacting each other at hinge points 22 on the curved contact edge of the shells 11.

Figure 8 shows an end view of the same embodiment of the expansion shells 11 as shown in Figure 6 with the shells 11 in their closed position. The shells 11 fit closely around the hollow threaded tube 10 which is circular in cross section. The end view of . the shells 11 when they are in the closed position is also substantially circular in cross section as shown in Figure 8. The inside surface 23 of the shell 11 fits closely around the external surface of the threaded tube 10. The external surface 24 of the shell 11 fits closely inside the borehole 2 (not shown).

Figure 9 shows the one way rotation washer 12 which has a central threaded hole 31 which enables it to be screwed onto the threaded tube 10 (not shown). On one side of the washer 12 are one or more arms 13 that can swing out and open and jam against a borehole wall (not shown) and one or more spacer pins 14 that prevent the arms 13 from contacting the underside of the drill bit holder 16 on the leading end of the threaded tube 10. On the other side of the washer 12 are one or more pins or lugs 15 that engage with the shells 11 (not shown) and which prevent the washer 12 from rotating with respect to the shells 11.

Figure 10 shows an end view of the same embodiment of the washer 12 as shown in Figure 9 whereby the washer 12 is being rotated in a borehole 2 in the direction as shown by the arrows 32. The washer 12 is screwed onto a threaded tube 10 and as the threaded tube 10 is rotated in the direction of the arrows 32 the arms 13 are in their closed position and are just clearing the borehole 2 such that the washer 12 can rotate with the threaded tube 10 in the direction of the arrows 32. Figure 11 shows an end view of the same embodiment of the washer 12 as shown in Figure 10 except that the threaded tube 10 is being rotated in the opposite direction to that shown in Figure 10 as shown by the arrows 37 in Figure 11. As the threaded tube 10 rotates in the direction of the arrows 37, the washer 12 initially tries to rotate in this same direction 37. However, rotation in this direction 37 causes the arms 13 on the washer 12 to open and jam into the sides of the borehole wall at position 25 and prevent the washer 12 from rotating in this direction 37. If the sides of the borehole wall at position 25 consist of soft rock and are initially crushed by the arms 13, the arms 13 can continue to open to a large diameter to ensure that the washer 12 will not rotate in this direction 37.

Figure 12 shows one embodiment of the nut break out washer 4 used with the pre-tensioning system that enables a nut 5 to rotate a bar 1 to pre-tension a rock bolt. Figure 12 shows a nut 5 which is screwed onto a thread 7 on a bar 1 and the nut 5 has one or more slots or keyways 27 on the trailing face of the nut 5. The bar 1 also has one or more slots or keyways 26 on the trailing end of the bar 1 such that the keyways 27 in the nut 5 can be aligned with the keyways 26 in the bar 1. A frangible break out washer 4 with a central hole 33 has one or more lugs 28 on its leading face which can fit into the keyways 26 and 27. Typically the number of lugs 28 on the break out washer 4 matches the combined number of keyways 26 and 27 in the bar 1 and in the nut 5 but is not so restricted. Once the keyways 26 and 27 in the bar 1 and in the nut 5 are aligned, the lugs 28 on the break out washer 4 are pushed into these keyways 26 and 27. An injection sleeve 3 or other member has a threaded end 34 which fits through the central hole 33 in the washer 4 is then screwed into a female thread 35 in the end of the bar 1 to contain the break out washer 4 between the sleeve 3 and the nut 5.

Figure 13 shows the same embodiment as shown in Figure 12 with the break out washer 4 assembled on the trailing end of the bar 1. Figure 13 shows that at least one lug 28 on the washer 4 is located in at least one keyway 27 in the nut 5. Figure 13 shows that for the nut 5 to rotate on the thread 7 on the bar 1 the lugs 28 on the frangible break out washer 4 must shear through and the combined shear strength of all the lugs 28 on the washer 4 determines the break out torque of the nut 5. Since the size of the lugs 28 can be varied, and the lugs 28 and washer 4 can be typically made from different grades of plastic, the break out torque can be varied over a wide range of torque values to suit the application. In addition, it is important to note that the lugs 28 and the break out washer 4 do not damage the thread in the nut 5 or the thread 7 on the bar 1 such that the nut 5 will easily screw along the thread 7 once the break out washer 4 has failed.

Figure 14 shows one embodiment of the break out washer 4 with a central hole 33. The washer 4 is shown with four lugs 28. However the washer 4 could have any number of lugs 28 and the lugs 28 could be any size of shape. The washer 4 could have a different number and size of lugs 28 positioned in the nut 5 keyways 27 compared those located in the bar 1 keyways 26.

In preferred embodiments, the break out washer 4 is injection moulded from plastic such that there is a high degree of precision over the shape and size of the lugs 28 so that the break out torque is repeatable for each bolt. In practice, the break out torque can simply be varied by changing to a washer 4 made from a different strength grade of plastic.

In preferred embodiments, the present invention is used with self drilling rock bolts but is not so limited and could be used to install any solid or hollow bolts or bars.

In preferred embodiments, the present invention has been described using left hand rotation for drilling a borehole and using right hand rotation for applying a pre- tensile force and for nut tensioning but is not so limited and could be used with the opposite rotation to that described.

It should be noted that the present invention enables self drilling rock bolts to be installed quickly with a pre-tensioned force and to be fully grouted with resin or grout without the requirement for manual handling.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

Claims:

1. A point anchoring and pre-tensioning system (the "system") used on a bar or bolt or self drilling rock bolt comprising a friction anchor device at the leading end of the bolt wherein the friction anchor device comprises two or more expansion shells which are forced into the borehole wall by a cone or cones or a wedge or wedges and wherein the diameter of the friction anchor device in its closed position is substantially the same as the maximum diameter of the main tensile support bar in the bolt.

2. The pre-tensioning system referred to in Claim 1 wherein the diameter of the friction anchor device in its closed position is no more than 5mm larger than the maximum diameter of the main tensile support bar in the bolt.

3. The pre-tensioning system referred to in Claim 1 wherein the diameter of the friction anchor device in its closed position is no more than 10mm larger than the maximum diameter of the main tensile support bar in the bolt.

4. The pre-tensioning system referred to in Claim 1 wherein the friction anchor device comprises two or more expansion shells wherein the outer diameter of the expansion shells in their closed position is substantially the same as the maximum diameter of the main tensile support bar in the bolt.

5. The pre-tensioning system referred to in Claim 4 wherein the two or more

expansion shells in the friction anchor device fit closely around a threaded tube or threaded bar in their closed position.

6. The pre-tensioning system referred to in Claim 5 wherein the friction anchor device comprises two or more expansion shells which are expanded by being forced over a cone or cones or over a wedge or wedges.

7. The pre-tensioning system referred to in Claim 6 wherein the friction anchor device comprises two or more expansion shells which have a series of grooves or ridges or other deformations on their outside surfaces which contact the inner surface of a borehole and which are substantially orientated at an angle of between 20 degrees and 80 degrees to the borehole axis.

8. The pre-tensioning system referred to in Claim 7 wherein the friction anchor device comprises two or more expansion shells which expand and open by pivoting or hinging about one end of the expansion shells such that only one end of the expansion shells open and the other end of the expansion shells remain in contact with each other at the pivot or hinge point or points.

9. The pre-tensioning system referred to in Claim 8 wherein the threaded tube or threaded bar has a threaded washer screwed onto it which can threadably move along the threaded tube or threaded bar in an axial direction and which can force a cone or cones or a wedge or wedges between the expansion shells.

10. The pre-tensioning system referred to in Claim 9 wherein the threaded washer has a cone or cones or a wedge or wedges on one side of the threaded washer which can be forced between the expansion shells.

11. The pre-tensioning system referred to in Claim 10 wherein the threaded washer is a one way rotation washer which can rotate in one direction inside a borehole but cannot rotate in the opposite direction inside a borehole.

12. The pre-tensioning system referred to in Claim 11 wherein the one way rotation washer has one or more arms or lugs or bars which enable the washer to rotate in one direction inside a borehole but prevent the washer from rotating in the opposite direction inside a borehole.

13. The pre-tensioning system referred to in Claim 12 wherein the one way rotation washer has one or more contact faces or sides which contact the expansion shells and these contact faces may be any suitable shape including flat or concave or convex or be cone shaped and be at any suitable orientation.

14. The pre-tensioning system referred to in Claim 13 wherein the one way rotation

-o

washer engages with the expansion shells such that the one way rotation washer cannot rotate with respect to the expansion shells.

15. The pre-tensioning system referred to in Claim 1 wherein the system also

comprises a nut break-out out device that will prevent a nut from rotating on a thread on a bar up until a certain torque value is applied to the nut but will enable the nut to rotate along the thread once that torque value has been exceeded and consists of a frangible break out washer which has one or more lugs which are keyed into one or more slots or keyways in the nut and in the bar.

16. The pre-tensioning system referred to in Claim 15 wherein the break out washer is made from plastic or fibreglass or any suitable material.

17. The pre-tensioning system referred to in Claim 16 wherein the lugs on the break out washer are held in position in the keyways in the nut and in the bar by a threaded sleeve or by any threaded member or by glue or by any suitable means.