WO2019211475A1

WO2019211475A1 - Rock bolts

Info

Publication number: WO2019211475A1
Application number: PCT/EP2019/061460
Authority: WO
Inventors: Are Håvard HØIEN
Original assignee: Norwegian University Of Science And Technology (Ntnu)
Priority date: 2018-05-04
Filing date: 2019-05-03
Publication date: 2019-11-07
Also published as: GB201807373D0

Abstract

A deformable element (250) for a rock bolt comprises a sleeve (255) and a plug (260). The plug (260) is configured to be received at least partially within the sleeve (255) at a first end (256) of the sleeve, and is configured to be held in frictional engagement with the sleeve (255). The plug (260) is configured to be fixed in place relative to an end of the rock bolt by tightening a fastener (2) onto the end of the rock bolt against the plug (260) to secure the plug between the fastener (2) and the sleeve (255). The sleeve (255) has an inner diameter which decreases from a maximum diameter at the first end (256) of the sleeve to a minimum diameter at an opposed second end (257) of the sleeve, the maximum diameter of the sleeve (255) being the same as or larger than a maximum diameter of the plug (260), and the minimum diameter being smaller than the maximum diameter of the plug (260). When a load is applied to the deformable element (250) the plug (260) is drawn through the sleeve (255) towards the second end (257) of the sleeve.

Description

ROCK BOLTS

The present application relates to a deformable element for a rock bolt, and to a rock bolt incorporating a deformable element. It also relates to a method of stabilising a rock mass using rock bolts.

Herein, the term rock bolt encompasses also rock anchors, which are similar to rock bolts but generally have a higher load capacity.

Rock bolts are used for stabilizing excavations (for example in structures such as tunnels or caverns, or at rock cuts). Such bolts may also be used to anchor a slab of rock which has partially cracked away from the rock below, for example on a slope.

One example of usage of rock bolts is in drill and blast construction of a tunnel. Tunnel construction generally follows a cycle as follows. Firstly, the volume that rock is to be removed from is drilled. Then, blasting agent is filled in the holes and detonated to advance the tunnel. The excavated rock (muck) is then removed and scaling is then carried out to remove loose rocks and smooth the rock face.

The tunnel walls and roof may then be covered with sprayed concrete (concrete which is sprayed through a nozzle and projected at a high velocity onto the rock surface, which compacts the concrete on impact) to reinforce them. The concrete sprayed rock surface is then drilled to provide installation holes for the rock bolts. Finally, the rock bolts are installed. The cycle repeats, and each repetition of the cycle advances the tunnel further into the rock mass.

As an alternative to drill and blast construction, a tunnel boring machine may be used. Rock bolts may be installed in the excavated tunnel walls and roof, behind the advancing cutting head of the tunnel boring machine.

The process of excavating may cause large deformations of the rock mass. For example, the excavation process may relieve the surrounding rock of large stresses, which can cause a spontaneous, violent fracture of the rock within the interior of the rock mass. This is known as“spalling" or“rock burst”. Rock bursts are particularly common in competent rock (rock in which an unsupported opening can be made), especially when the rock is under high stress.

Large deformations of the rock mass may also result from overstressing weaker rock, leading to squeezing of the rock. Large deformations other than rock burst or squeezing may also be caused by the excavation process, and the use of rock bolts may be appropriate in any situation where any form of large deformations are possible or likely.

The purpose of rock bolts is to transfer load from the unstable rock surface to the interior of the rock mass, which strengthens the structure. In effect, the rock bolts“knit” the rock mass together by increasing friction in the rock joints. That is, the rock bolts hold the rock mass together so that the rock cannot loosen and fail.

The rock bolts do not necessarily prevent the rock bursts or the deformation in the rock mass. That said, in areas that are particularly prone to rock bursts (where the rock is competent and under high pressure), the counter pressure from rock bolts may create a big enough confinement pressure in the rock mass to reduce the rock burst effect and also to some degree the deformation from squeezing.

A rock bolt comprises a rod for installation within a hole drilled into the rock mass. It comprises a faceplate and nut at the rock face and may comprise a mechanical or chemical anchor at the other end (within the rock mass). The space between the rod and the rock may be filled with a grout comprising cement or resin, for example.

Rock bursts, or deformation within the rock mass, may change the loading on the rock bolts, which can lead to deformation of the rock bolts and potentially to failure of the rock bolts. This can lead to the rock face becoming unstable, potentially resulting in destabilisation of the structure. Inspection of the rock face may not be sufficient to determine if a rock burst has occurred. Nor is it generally possible to measure the resultant change in the loading on the rock bolts, unless a load indicator is provided in the rock bolt.

A known load indicator is the TITAN load indicator, manufactured by Friedr. Ischebeck GmbH. The TITAN load indicator is installed at the bolt head between the faceplate and the nut of the rock bolt. It comprises a thick-walled hollow cylinder with three circumferential grooves machined in the outer surface of the cylinder wall. Each groove has a different depth (such that the wall thickness of the cylinder at each groove is different). As increasing load is applied, the grooves close in a predefined sequence; the groove with the thinnest wall collapses first, followed by the next thicker walled groove, and then the thickest walled groove.

The thinnest groove is used as a pre-stress groove to check that the bolt is fixed and can be loaded. It is designed to close at 60 kN. The other two grooves are designed to close at a load of approximately 180 kN. During the yielding process the length of the indicator is reduced by approximately 30 mm.

The load on the bolt can be assessed, to a limited degree, by checking how many grooves have closed. However, the degree of deformation is not great, and occurs at discrete values of load, so that it is not possible to accurately determine the applied load. Moreover, the TITAN load indicator has a stepwise deformation as a function of the load. That is, the rock in which the rock bolt is installed can expand without applying additional load when a groove collapses. This reduces friction in the rock joints, which reduces the strength of the rock mass.

In view of the shortcomings of existing solutions, an improved rock bolt is sought.

A first aspect of the invention provides a deformable element for a rock bolt, wherein the deformable element comprises a sleeve and a plug, wherein the plug is configured to be received at least partially within the sleeve at a first end of the sleeve, and is configured to be held in frictional engagement with the sleeve, and is configured to be fixed in place relative to an end of the rock bolt by securing a fastener onto the end of the rock bolt against the plug to secure the plug between the fastener and the sleeve, wherein the sleeve has an inner diameter which decreases from a maximum diameter at the first end of the sleeve to a minimum diameter at an opposed second end of the sleeve, the maximum diameter of the sleeve being the same as or larger than a maximum diameter of the plug, and the minimum diameter being smaller than the maximum diameter of the plug, wherein the deformable element is configured such that, in use, when a load is applied to the deformable element (i.e. when load is applied to the rock bolt on which the deformable element is installed), the plug is drawn through the sleeve towards the second end of the sleeve.

Advantages and optional or preferred features of the deformable element of the first aspect of the invention are discussed below.

The sleeve may be open at its first end. The sleeve may be a closed at its second end, except that a through-hole may pass through the otherwise closed end, to enable the sleeve to be slid onto a rod of a rock bolt.

The plug may comprise a through-hole sized so that the rod of the rock bolt can pass through the through-hole, i.e. so that the plug can be installed onto the rod of the rock bolt. When installed, loading on the deformable element causes relative movement of the plug and the sleeve, such that the plug moves along the sleeve from the first end to the second end. Due to the decreasing inner diameter of the sleeve, the plug experiences an increasing resistance as it moves through the sleeve.

Optionally, the inner surface of the sleeve comprises the surfaces of a plurality of notional adjacent truncated conical sections, each having a different base angle, wherein a first truncated conical section that is closer to the first end of the sleeve than an adjoining truncated conical section has a smaller base angle than the adjoining truncated conical section.

The“first end” and“second end” of the sleeve may not be at the extreme ends of the sleeve as a whole - rather they may denote the extreme ends of the part of the sleeve forming a plurality of notional adjacent truncated conical sections.

The ideal deformation-load curve for a deformation element in general may be one in which load initially increases rapidly with low deformation (i.e. an initial steep gradient portion) with a slower development under increasing load (i.e. a shallower gradient portion) as the bolt yield strength (or strength of the deformable element) is approached. This allows pre-tensioning of the rock bolt without a great deal of deformation, and then as the load increases, the deformation gradually increases until the deformation element/bolt starts yielding. A deformation-load curve of this kind may be achieved by the foregoing structure. For example, a first truncated conical section (i.e. the section that the plug passes through first under initial loading) is provided with a shallower slope than the adjoining truncated conical section (i.e. the section that the plug passes through next, under increased loading). This means that it is harder for the plug to pass through the first truncated conical section than the adjoining truncated conical section.

The deformable element may be configured to deform such that the magnitude of the displacement is an increasing function of the load over the working range of the deformable element. This means that deformation-load curve may be a smooth curve, with no steps, spikes or dips, over the working range of the deformable element (i.e. until the plug reaches the end of the sleeve).

The inner surface of the deformable element may comprise the surface of two stacked truncated conical sections, wherein the first truncated conical section has a base angle of between 80 and 85 degrees, for example 83 degrees, and the adjoining truncated conical section has a base angle of between 86 and 89 degrees, for example approximately 88 degrees.

The inner surface of the deformable element may comprise the surface of two stacked truncated conical sections, wherein the first conical section has a height of 10 to 30mm, optionally approximately 15mm, and the adjoining conical section has a height of 50 to 90mm, optionally approximately 70mm.

The inner surface of the deformable element may comprise the surface of more than two stacked truncated conical sections, for example three or four stacked truncated conical sections. Each truncated conical section may be steeper than the last, considering the truncated conical sections in the order in which the plug is pulled through them.

The sleeve may be is shaped and sized to engage with a faceplate of the rock bolt. Described herein are two variations of the sleeve - an external variant, and an in-hole variant.

The external variant may comprise a hemispherical portion (having a through-hole) at the second end for engaging with the faceplate of the rock bolt.

The hemispherical portion may engage with the faceplate in such a way as to allow for the rock bolt to be placed at an angle other than perpendicular to the rock face.

The internal variant may also comprise a hemispherical portion at the second end. Alternatively, the second end need not be hemispherical, but may be closed (except for the through-hole) in order to avoid the plug being pulled out of the sleeve.

Instead of engaging the faceplate at the second end (as in the external variant), the in-hole variant may engage the faceplate towards the first end. Then, the sleeve may comprise a widest portion having a surface shaped as a partial sphere, for engaging with the faceplate of the rock bolt. Advantageously, the widest section forms the surface of a partial sphere to allow for the rock bolt to be placed at an angle other than perpendicular to the rock face.

In the in-hole variant, the outer diameter of the first end may be larger than a diameter of a hole in a faceplate for the rock bolt, and the outer diameter of a majority of the length of the sleeve may be smaller than the diameter of the hole in the faceplate, such that the sleeve can pass through the hole in the faceplate and into the hole in the rock mass, up until the widest portion abuts the faceplate.

For the external variant, in use, a rock bolt is installed into a rock mass, and a faceplate is located over the end of the rock bolt which protrudes from the rock face. The sleeve of the deformable element is then slid onto the rod, until the hemispherical end of the sleeve (i.e. the hemispherical end at the second end of the sleeve which has a smaller inner diameter and which is closed except for the through-hole) abuts the faceplate. Then, the plug is slid onto the rod, to be received (at least partially) within the sleeve at its first end (i.e. the end of the sleeve which has a larger inner diameter). The rod may be threaded at the location where the plug is provided, and a nut may be screwed onto the rod to secure the plug (and hence also the sleeve) in place.

For the external variant, the sleeve may be shorter than the length of the rock bolt which protrudes from the rock mass.

For the in-hole variant, in use, a rock bolt is installed into a rock mass, and a faceplate is located over the end of the rock bolt which protrudes from the rock face. The sleeve of the deformable element is then slid onto the rod, and the majority of the sleeve passes through the hole in the faceplate and into the hole in the rock mass. The sleeve is moved into the hole up until the point where the widest portion at the first end of the sleeve (forming the surface of a partial sphere) abuts the faceplate. Then, the plug is slid onto the rod, to be received (at least partially) within the sleeve at its first end (i.e. the end of the sleeve which has a larger inner diameter). The rod may be threaded at the location where the plug is provided, and a nut may be screwed onto the rod to secure the plug (and hence also the sleeve) in place.

For the in-hole variant, the height of the widest portion may be shorter than the length of the rock bolt which protrudes from the rock mass.

In an alternative embodiment, the inner diameter of the sleeve may decrease linearly from the first end to the second end. The inner diameter of the sleeve at the second end may be 80% to 95% of the inner diameter at the first end. Optionally, the sleeve comprises an inner surface forming the shape of a truncated cone, with the wide end of the truncated cone at the first end and the narrow end of the truncated cone at the second end. The sleeve may comprise an outer surface forming a cylinder, such that the walls of the sleeve increase in thickness from the first end to the second end. An annular disk having a broadly hemispherical shape may be provided between the faceplate and the sleeve (i.e. it may be slid onto the rod before the sleeve is slid on), with the curved surface of the hemisphere providing a good interface with the faceplate, and the flat surface of the hemisphere providing a good interface with the sleeve. The plug is optionally a sphere, a spherical cap, or a spherical segment. Here, a spherical cap is a portion of a sphere cut off by a plane. The spherical cap may be a hemisphere, such that the height of the cap is equal to the radius of the sphere. The spherical cap may be larger than a hemisphere, such that (for example) the height of the cap is equal to 2/3 of the diameter of the sphere, or 3/4 of the diameter of the sphere.

Here, a spherical segment is the solid defined by cutting a sphere with a pair of parallel planes. That is, a spherical segment is a spherical cap with the top truncated, i.e. a spherical frustum.

Alternatively, the plug may have a structure wherein the plug extends between a first plug end having a first outer diameter to a second plug end having a second outer diameter, and tapers outwardly from each of the first and second plug ends to a maximum diameter portion between the first and second plug ends.

Other forms of plug may also be used.

The sleeve may comprise a material with a minimum yield strength of greater than 250 N/mm², optionally greater than 300 N/mm², for example 355 N/mm².

The sleeve may comprise a material with a minimum yield strength of less than 650 N/mm².

The sleeve may comprise a material with a minimum yield strength of between 250 and 650 N/mm².

The sleeve may comprise a material with a Young’s modulus (E) of approximately 210,000 N/mm².

The sleeve may comprise a material with a Poisson’s ratio of 0.26 to 0.31 , for example approximately 0.29.

The sleeve and plug may comprise metal, for example steel, and optionally comprise carbon steel, for example S355 steel or stainless steel.

The sleeve may have a length of between 30mm and 300mm, and optionally has a length between 70mm and 150mm, and for example has a length of approximately 100mm to 120mm. The deformation length of the deformable element broadly corresponds to the length of the sleeve.

The deformation element may be configured to deform up to a maximum load of for example, approximately 100 kN, approximately 150 kN, approximately 200 kN, or approximately 250 kN. Such deformation elements may be particularly suited for use with standard rock bolts. These generally have a diameter of 20mm or 25 mm. For rods with a yield strength of 500 MPa, the maximum loads borne by the rods are 157 kN (for a 20 mm diameter rod) and 245 kN (for a 25 mm diameter rod).

When used with a rock anchor (which generally must withstand higher loads than rock bolts), the deformation element may be configured to deform up to a maximum load of approximately 500 kN, 1000 kN, 1500 kN or 2000 kN, for example.

Optionally, the deformable element comprises a deformation indicator which is configured to indicate whether the deformable element has been deformed.

When a deformation indicator is provided, the degree of deformation can be measured quantitatively and/or monitored. Additionally, the remaining capacity of the deformable element (and hence, of the rock bolt itself) can be assessed.

It is advantageous to be able to survey the degree of deformation in existing rock bolts to determine if the installation of additional bolts is required, to provide increased support. In particular, this can be assessed before the maximum strength limit of the rock bolt is reached. Moreover, it is possible to determine when deformation within the rock mass has stopped, at which point it can be concluded that the tunnel/cavern etc. is safe. At this stage, the bolt head can be covered with sprayed concrete if necessary, for example, if that is needed to obtain a sufficient lifespan for permanent support.

The deformation indicator may give a visual indication of the degree of deformation. For example, the deformation indicator may be a length gauge.

Optionally, load is not applied to the length gauge, such that its length remains unchanged. The length gauge may be read visually.

The length gauge may comprise a scale to read off the degree of displacement. The scale may be marked with units of measurement. Alternatively or additionally, the scale may be marked with a colour-coding. Advantageously, this provides a clear visual cue which can readily be correlated to the magnitude of the displacement.

The length gauge may comprise a scale marked on a rigid rod projecting from the top surface of the plug, or a scale marked on the exterior of the sleeve.

The deformation indicator may comprise a flexible strip having

approximately the same length as the deformation length of the deformable element. The flexible strip may be attached at one end to the plug and the majority of the flexible strip including its second end may be received within a cover portion. The cover portion does not cover the entirety of the strip, leaving an exposed position where the strip can be seen. The strip may be colour-coded such that as the plug is pulled through the sleeve, the strip is pulled out of the cover portion and different colours of the strip are visible at the exposed position.

For an external deformable element, the cover portion may be provided on the exterior of the sleeve. For an in-hole deformable element, the cover portion may be provided on the faceplate.

The deformation indicator may comprise a sensor. The deformation indicator may comprise a passive sensor (i.e. one which can operate as a sensor without requiring a power source such as a battery). Such sensors rely entirely on an external reader device for power. Data stored by such a sensor is read in the following way. Firstly, the sensor receives (via an antenna) electromagnetic energy from an active reader device. Using power harvested from the reader device's electromagnetic field, the sensor sends a signal (i.e. in the form of electromagnetic energy) back to the reader device. The reader device then receives and interprets the signal from the sensor. The signal transmitted from the sensor may include a sensor ID (i.e. a unique identification code or number), and sensor information (i.e. a reading relating to the sensed parameter).

The deformable element may comprise a passive RFID tag. The RFID tag may be integrated with a sensor. In other embodiments, RFID tags may be provided even where the deformable element does not comprise a sensor. The use of such RFID tags allows a large number of bolts to be monitored easily and quickly. By comparing the deformation/loading at each bolt, and optionally with knowledge of the relative positions of each bolt, the stability of the rock mass as a whole can be monitored, rather than merely the deformation at each bolt.

Use of passive sensors (and/or RFID tags) is also advantageous as it allows the rock bolts to be monitored even when the bolt heads (and hence the deformable element) cannot be easily seen (for example, if the bolt heads are positioned high up such that the deformable element is not clearly visible from ground level).

One example of a deformation indicator is a displacement sensor. The displacement sensor may comprise a switch and a switch monitor, and may be operable to output information regarding the status of the switch (open/closed).

The switch may change from an open state to a closed state (or from a closed state to an open state) at a predetermined displacement of the deformable element, i.e. when the deformable element is deformed to a predetermined amount. The displacement sensor may be configured to output information about the status of the switch (i.e. open or closed). The displacement sensor may be a passive sensor, as discussed above. The displacement sensor may be configured to output a unique identification number.

The deformation indicator may comprise a strain sensor. The strain sensor may be a passive sensor, as discussed above. The strain sensor may be configured to output a unique identification number.

The strain sensor may comprise an insulating flexible backing which supports a metallic foil pattern. The strain sensor may be attached to the deformable element. Deformation of the deformable element causes the metallic foil pattern to be deformed, causing its electrical resistance to change. This resistance change is related to the amount of deformation.

Alternatively, the strain sensor may comprise a piezoresistor.

Other forms of strain sensor may of course be used, as appropriate.

In some embodiments, both a sensor (for example, a displacement sensor or strain sensor) and a visual indicator may be provided (for example, a length gauge comprising a scale) on the deformable element.

Where a deformation indicator is provided, if the degree of deformation (e.g. displacement or strain) is known and the deformation properties (e.g. displacement or strain as a function of load) of the deformable element are known, the load on the rock bolt can be calculated accordingly. For example, a calibration curve may be prepared which correlates the amount of deformation (displacement or strain) with the load applied.

The invention also extends to a rock bolt comprising: a rod; a faceplate positioned on the rod; a nut (or other fastener) positioned on the rod; and a deformable element according to the first aspect (optionally including any of the optional features described above), positioned on the rod.

Without a deformable element, under loading, and particularly as a result of deformation in the rock mass, the rod of the rock bolt generally deforms (in particular, it elongates). Under extreme loading, the rod may deform plastically, and then may fracture. The failure of the rock bolt can result in the failure of neighbouring rock bolts, and in a worst case, in destabilisation of the excavation.

The deformable element may be configured to deform under loading more readily than the rod of the rock bolt. Thus, when the rock bolt experiences loading, deformation preferentially takes place in the deformable element, rather than in the rod of the rock bolt (at least until the load exceeds the load capacity of the deformable element). The deformable element is configured to take up the load as it deforms, to maintain the loading properties of the rock bolt. The deformable element therefore increases the ability of the rock bolt to absorb deformation (compared to the case where the rock bolt is not provided with a deformable element).

That is, at least embodiments of the invention allow the full loading capacity of the rod to be maintained, even with large deformation of the rock.

When a deformable element is provided, when there is deformation in the rock, the deformable element deforms by a length up to the deformation length of the deformable element. The rod is then not forced to elongate, as long as the deformable element does not reach the end of its working range. Absent the deformable element, deformation in the rock causes the rod to elongate and may break. Thus, in at least some embodiments, the deformable element is for increasing the capacity of the bolt.

Thus, the deformable element of the present invention (and a rock bolt incorporating such) is particularly suitable for use where large deformations of the rock mass are expected to occur, and/or where it is important to survey the degree of deformation in the rock mass.

The deformable element can therefore contribute to a safer environment within an excavation, for example, by providing an indication of when rock bolts have reached their full capacity and may be near failure.

The deformable element can be sized to fit any type of rock bolt, and can be designed to handle the particular degree of loading expected in a given installation.

The rock bolt may comprise a plurality of deformable elements installed along the rod of the rock bolt.

The rock bolt may be provided in combination with surface support on a rock face. The surface support may for example comprise a reinforcement mesh and/or sprayed concrete. When the rock moves (as a result of a seismic event, for example) the rock will push on the surface support, and the surface support will transfer load to the rock bolt.

A second aspect of the invention provides a method of method of stabilising a rock mass using a rock bolt, the method comprising: installing a rock bolt as described above. The method may include installing the rock bolt in combination with surface support at a rock face of the rock mass. The surface support optionally comprises a reinforcement mesh and/or sprayed concrete.

The method may comprise installing a plurality of rock bolts.

The method may further include monitoring the deformable element(s) to determine their degree of deformation (displacement). The degree of deformation may be monitored visually. The deformable element may comprise sensors, optionally passive sensors, (for example, strain sensors, or displacement sensors), and the degree of deformation may be monitored using the sensors.

The method may comprise installing additional rock bolts in the vicinity of one or more rock bolts exhibiting a degree of deformation that is above a predetermined threshold.

The method may comprise replacing a deformable element on a rock bolt if the existing rock bolt is close to failure.

The method may comprise installing a plurality of deformable elements (for example, two deformable elements) on a rock bolt. That is, a plurality of deformable elements may be installed in series (i.e. one after another) along the rod of the rock bolt.

The method may comprise determining that the rock mass is stable if the deformation in a plurality (optionally all) of the rock bolts has stopped, or the rate of deformation has reduced below a predetermined threshold.

The method may comprise spraying the rock bolt with concrete to encapsulate the deformable element, once the rock mass has been determined as stable.

According to a third aspect of the present invention, there is provided a deformable element for a rock bolt, wherein the deformable element is configured to deform under load such a first end of the deformable element is displaced towards a second end, wherein the magnitude of the displacement is an increasing function of the load.

By“an increasing function of the load”, it is meant that the displacement- load curve is a smooth curve without dips. The magnitude of the displacement may be an increasing function of the load up to the end of the working range of the deformable element.

According to a fourth aspect of the present invention, there is provided a rock bolt comprising: a rod; a faceplate positioned on the rod; a nut positioned on the rod; and a deformable element positioned on the rod between the faceplate and the nut, wherein the deformable element is configured to deform under load such a first end of the deformable element is displaced towards a second end, wherein the magnitude of the displacement is an increasing function of the load.

Advantages, and optional or preferred features, of the deformable element of the third aspect of the invention, and of the rock bolt of the fourth aspect of the invention are discussed below.

By the feature that“a first end of the deformable element is displaced towards a second end”, it is meant that either the deformable element as a whole is shortened, i.e. reduced in length, for example along its axial extent, or that one part of the deformable element is displaced towards another (for example, in an axial direction).

Under loading, and particularly as a result of deformation in the rock mass, the rod of the rock bolt generally deforms (in particular, it elongates). Under extreme loading, the rod may deform plastically, and then may fracture. The failure of the rock bolt can result in the failure of neighbouring rock bolts, and in a worst case, in destabilisation of the excavation.

The deformable element may comprise a structure that deforms more than the rod of the rock bolt. Thus, when the rock bolt experiences loading, deformation preferentially takes place in the deformable element, rather than in the rod of the rock bolt (at least until the load exceeds the load capacity of the deformable element). The deformable element is configured to take up the load as it deforms, to maintain the loading properties of the rock bolt. The deformable element therefore increases the ability of the rock bolt to absorb deformation (compared to the case where the rock bolt is not provided with a deformable element).

That is, at least preferred embodiments of the invention allow the full loading capacity of the rod to be maintained, even with large deformation of the rock. When a deformable element is provided, when there is deformation in the rock, the deformable element deforms by a length up to the deformation length of the deformable element. The rod is then not forced to elongate, as long as the deformable element does not reach the end of its working range. Absent the deformable element, deformation in the rock causes the rod to elongate and may break. Thus, in at least preferred embodiments, the deformable element is for increasing the capacity of the bolt and gives an indication (for example, a visual indication) of the loading of the bolt as the rock deforms. Thus, the deformable element of the present invention (and a rock bolt incorporating such) is particularly suitable for use where large deformations of the rock mass are expected to occur, and/or where it is important to survey the degree of deformation in the rock mass.

The deformable element may be configured to have a deformation length of between 50mm and 300mm, for example 50 to 100mm, and optionally has a deformation length of approximately 80mm. Here, the deformation is the maximum displacement of the deformable element.

The deformable element of the present invention may be placed on the bolt head (between the faceplate and the nut), i.e. outside the rock mass.

Due to the location of the deformable element on the bolt head (i.e. between the faceplate and the nut), deformation of the deformable element happens outside the rock mass, and therefore is visible. When a deformation indicator is provided, the degree of deformation can be measured quantitatively and/or monitored.

Additionally, the remaining capacity of the deformable element (and hence, of the rock bolt itself) can be assessed. It is advantageous to be able to survey the degree of deformation in existing rock bolts to determine if the installation of additional bolts is required, to provide increased support. In particular, this can be assessed before the maximum strength limit of the rock bolt is reached. Moreover, it is possible to determine when deformation within the rock mass has stopped, at which point it can be concluded that the tunnel/cavern etc. is safe. At this stage, the bolt head can be covered with sprayed concrete if necessary, for example, if that is needed to obtain a sufficient lifespan for permanent support.

The deformable element may comprise a passive RFID tag. The RFID tag may be integrated with a sensor. In other embodiments, RFID tags may be provided even where the deformable element does not comprise a sensor. The use of such RFID tags allows a large number of bolts to be monitored easily and quickly. By comparing the deformation/loading at each bolt, and optionally with knowledge of the relative positions of each bolt, the stability of the rock mass as a whole can be monitored, rather than merely the deformation at each bolt. Use of passive sensors (and/or RFID tags) is also advantageous as it allows the rock bolts to be monitored even when the bolt heads (and hence the deformable element) cannot be easily seen (for example, if the bolt heads are positioned high up such that the deformable element is not clearly visible from ground level).

Alternatively, the strain sensor may comprise a piezoresistor.

Other forms of strain sensor may of course be used, as appropriate.

Optionally, the deformable element has a length (i.e. in the axial dimension) of less than 300mm, less than 200mm, less than 150mm, or less than 100mm. The longer the deformable element is, the greater the distance by which is projects into the tunnel/cavern, and the greater the reduction in the headspace. Optionally, the deformable element has a length (i.e. in the axial dimension) of greater than 50mm.

A first type of deformable element is now described. The deformable element may comprise a right circular hollow cylinder formed by a cylindrical wall, comprising a first hole in the cylindrical wall and a second hole diametrically opposite the first hole.

The holes in the cylindrical wall may be sized to accommodate the rock bolt rod. i.e. the first and second holes are sized to allow the rod of the rock bolt to pass through the first and second holes. The holes in the cylindrical wall may be oversized compared to the diameter of the rod. This, the diameter of the holes may be approximately 1.2 times the diameter of the rod. In one example the diameter of the rod is 20 mm. In that case, the diameter of the holes in the cylindrical wall may be 24 mm. The over-sizing of the holes (relative to the diameter of the rod) allows to reduce contact between the periphery of the holes and the rod. This contact (causing friction between the cylinder and the rod) can lead to spikes in the displacement-load curve. It is desirable to avoid such oscillations.

In use, the cylinder is installed between the nut and the faceplate of the rock bolt. Under increasing loading, the cylinder deforms to form an oval shape, shortening (and flattening) along the direction of the axis of the rock bolt, and elongating (spreading out) in the transverse direction.

The maximum displacement of the deformable element broadly corresponds to the inner diameter of the right circular hollow cylinder.

The cylinder wall may comprise a material with a minimum yield strength of greater than 250 N/mm², optionally greater than 300 N/mm², for example, 355 N/mm².

The cylinder wall may comprise a material with a minimum yield strength of less than 650 N/mm².

The cylinder wall may comprise a material with a minimum yield strength of between 250 and 650 N/mm².

The cylinder wall may comprise a material with a Young’s modulus (E) of approximately 210,000 N/mm².

The cylinder wall may comprise a material with a Poisson’s ratio of 0.26 to 0.31 , for example approximately 0.29. The hollow cylinder may comprise a length of metal tubing, for example steel tubing. The steel may be carbon steel, for example S355 steel, or stainless steel.

The cylinder wall may have a thickness of between 5mm and 30 mm, and optionally has a thickness of 8 to 14 mm, for example 11 mm.

The cylinder may have an outer diameter of between 40 mm and 150 mm, and optionally has an outer diameter of about 100 mm.

The cylinder may have an inner diameter of between 30 mm and 140 mm, and optionally has an inner diameter of about 80 mm (for example, 78 mm).

The ratio of the wall thickness to the outer diameter may be around 1 :4.

The cylinder may have a length of 50 mm to 100 mm, and optionally has a length of around 65mm.

The holes for receiving the rod in the cylinder may have a diameter that is optionally 10% to 30% larger than the diameter of the rod. Advantageously, this may reduce undesirable deviations in the deformation-load curve caused by contact between the edge of the holes and the rod.

The cylinder may be thought of as the combination of two pairs of deformable portions: a top-bottom pair and a side-side pair. Each of the top, bottom and two sides may form a quarter of the cylinder. The top and bottom portions include the holes for the rod to pass through, and the side portions are the remaining portions. Each quarter may be a portion of the cylinder subtended by 90°, wherein the middle of the top portion for example is aligned with the middle of the top hole for the rod to pass through.

The way that the cylinder deforms (and hence the shape of the deformation- load curve) may be influenced by the relative amounts of material in the top-bottom pair and the side-side pair.

The relative amounts of material in the top-bottom pair and the side-side pair may be varied by providing extra material in the top-bottom pair or the side-side pair, for example by forming the cylinder not to have a flat front and back face, but rather forming the cylinder such that each of the front and back ends has two faces, meeting at two outwardly-projecting points (either at the top and bottom of the front and back faces, in which case the material in the top-bottom pair is increased relative to the material in the side-side pair, or at each side of the front and back faces, in which case the material in the side-side pair is increased relative to the material in the top-bottom pair. In one example, the angle of each face to the line perpendicular to the longitudinal axis of the cylinder is between 3° and 15°, for example between 5° and 10°. Such a cylinder may be formed from a length of tubing from which several cylinders can be cut. Here, the tubing is formed with a plurality of holes for the rod to pass through. Two diagonal cuts are made at each end of each cut cylinder, to form the angled front and back ends.

As an alternative, it is possible to reduce the amount of material in the top- bottom pair relative to the amount of material in the side-side pair by forming each end of the cylinder to have two faces, meeting at an inwardly-projecting point at the top of the cylinder, and an inwardly projecting point at the bottom of the cylinder. Correspondingly, of course the amount of material in the side-side pair could be reduced in the same way.

The relative amounts of material in the top-bottom pair and the side-side pair may be varied by reducing the material in the top-bottom pair or the side-side pair, for example by providing opposed pairs of cut-outs in the front and back faces of the cylinder. Specifically, opposed pairs of cut-outs in the front and back faces of the cylinder may be formed in either the top-bottom pair or the side-side pair. The cut-outs may be semi-circular in shape, but cut-outs of another shape could be used.

Two cut-outs may be formed on each of the front and back faces, the two cut-outs on the top front and back faces being aligned with the top hole for the rod to pass through, and the two cut-outs on the bottom front and back faces being aligned with the bottom hole for the rod to pass through.

Such a cylinder may be formed from a length of tubing from which several cylinders can be cut. Here, the tubing is formed with a plurality of holes for the rod to pass through, alternating with a plurality of circular holes to form semi-circular cut outs. The tubing is cut into lengths perpendicular to the longitudinal axis of the tubing, bisecting each of the cut-out circles to form semi-circular cut-outs in each of the front and back faces of the cylinder.

In one example, the diameter of each cut-out circle is between 20 mm and 50mm, optionally between 25mm and 40mm, for example 35mm.

Whilst cut-outs have been described in the top-bottom pair, it will be appreciated that they could instead be formed in the side-side pair.

Whilst a right circular hollow cylinder has been described above, the deformable element may instead have a cross-section other than a circle. The cross-section may be an oval, for example, in which case the deformable element comprises a right oval hollow cylinder. The cross-section may be an ellipse (in which case the deformable element comprises a right elliptic hollow cylinder) or a stadium-shape. The major axis of each may be in the range of: 70mm to 150mm. The minor axis of each may be in the range of: 60mm to 140mm. The ratio of the minor axis to the major axis may be approximately 0.9 to 0.5.

The deformable element may comprise a first annular disk and a second annular disk adjoining the cylindrical wall around the first and second hole, respectively. These help to spread evenly the load from the faceplate and nut to the cylinder. The annular disks may each comprise a hole sized to allow the rod of the rock bolt to pass through each annular disk. The annular disk provided abutting the faceplate may have a broadly hemispherical shape for providing a good interface with the faceplate.

The disks may be separate components from the cylinder, i.e. the disks are not formed integrally with the cylinder, and are not attached to the cylinder.

Alternatively, the disks might be attached to the cylinder, either by integrally forming the disks and cylinder, or by forming the cylinder and disks separately, and then attaching them together e.g. by welding.

The disks may have an outer diameter of between 30 mm and 60 mm, and optionally have an outer diameter of 45 mm to 50 mm. The disks may have a thickness of 5 mm to 30 mm, and optionally have a thickness of approximately 10 mm.

The disks optionally comprise the same material as the cylinder.

Where the cylinder is provided with a deformation indicator, the deformation indicator may be used to read off the degree of shortening along the direction of the axis of the rock bolt, or the degree of elongation in the transverse direction.

A length gauge may be provided which measures the degree of elongation in the transverse direction. For example, the length gauge may comprise a scale (for example, marked on a rigid rod) housed within a housing, wherein the housing is attached to one side of the cylinder (on the side which will elongate in the transverse direction), and the scale is attached diametrically opposite. Optionally, the scale is entirely hidden within the housing when the cylinder in not deformed.

As load is applied, the opposite sides of the cylinder move outwardly, causing the scale and housing to move relative to one another so that the scale moves out of the housing. The degree of deformation can then be read from the exposed scale. The scale may be marked with units of measurement. Alternatively or additionally, the scale may be marked with colour-coding.

A second type of deformable element is now described. The deformable element may comprise a sphere, spherical cap or spherical segment and a sleeve for receiving the sphere, spherical cap or spherical segment inside (at least partially).

Here, a spherical cap is a portion of a sphere cut off by a plane. The spherical cap may be a hemisphere, such that the height of the cap is equal to the radius of the sphere. The spherical cap may be larger than a hemisphere, such that (for example) the height of the cap is equal to 2/3 of the diameter of the sphere, or 3/4 of the diameter of the sphere.

The sleeve has a first end that optionally has an inner diameter similar to or slightly larger than the diameter of the sphere, spherical cap or spherical segment and a second end that has a smaller inner diameter, smaller than the diameter of the sphere, spherical cap or spherical segment. The inner diameter of the sleeve may decrease smoothly from the first end to the second end. Optionally, the sleeve comprises an inner surface forming the shape of a truncated cone, with the wide end of the truncated cone at the first end and the narrow end of the truncated cone at the second end. Optionally, the sleeve comprises an outer surface forming a cylinder, such that the walls of the sleeve increase in thickness from the first end to the second end.

The sleeve may be an open cylinder at its first end. The sleeve may be a closed cylinder at its second end, except that a through-hole may pass through the otherwise closed end, to enable the sleeve to be slid onto a rod of a rock bolt.

The sphere, spherical cap or spherical segment may comprise a through- hole sized so that the rod of the rock bolt can pass through the through-hole, i.e. so that the sphere, spherical cap or spherical segment can be installed onto the rod of the rock bolt. Where a spherical cap or spherical segment is used, the through-hole is optionally perpendicular to the plane of the spherical cap or the planes of the spherical segment.

In use, the sleeve is slid onto the rod, with the second end of the sleeve (i.e. the end of the sleeve which has a smaller inner diameter, and which is closed except for the through-hole) closest to the faceplate. Then, the sphere, spherical cap or spherical segment is slid onto the rod, to be received (at least partially) within the sleeve at its first end (i.e. the end of the sleeve which has a larger inner diameter, and which is open). The rod may be threaded at the location where the sphere, spherical cap or spherical segment is provided, and a nut may be screwed onto the rod to secure the sphere, spherical cap or spherical segment (and hence also the sleeve) in place.

An annular disk having a broadly hemispherical shape may be provided between the faceplate and the sleeve (i.e. it may be slid onto the rod before the sleeve is slid on), with the curved surface of the hemisphere providing a good interface with the faceplate, and the flat surface of the hemisphere providing a good interface with the sleeve.

When installed, loading on the deformable element causes relative movement of the sphere, spherical cap or spherical segment and the sleeve, such that the sphere, spherical cap or spherical segment moves along the sleeve from the first end to the second end. Due to the decreasing internal diameter of the sleeve, the sphere, spherical cap or spherical segment experiences an increasing resistance as it moves through the sleeve.

The maximum displacement of the deformable element broadly corresponds to the length of the sleeve.

The deformable element may comprise a deformation indicator, wherein optionally the deformation indicator is a length gauge attached to the sphere, spherical cap or spherical segment.

For example, the length gauge may comprise a scale marked on a rigid rod, projecting from the top surface of the sphere, spherical cap or spherical segment.

As the sphere, spherical cap or spherical segment moves inside the sleeve, so too does the length gauge. The displacement can then be read off from the part of the length gauge still projecting outside of the sleeve.

Alternatively, the length gauge may comprise a scale marked on the exterior of the sleeve, and the position of the sphere, spherical cap or spherical segment within the sleeve may be read off from the sleeve based on the deformation of the sleeve around the position of the sphere, spherical cap or spherical segment.

The sleeve may comprise a material with a minimum yield strength of greater than 250 N/mm², optionally greater than 300 N/mm², for example 355 N/mm². The sleeve may comprise a material with a minimum yield strength of less than 650 N/mm².

The sleeve and sphere, spherical cap or spherical segment may comprise metal, for example steel, and optionally comprise carbon steel, for example S355 steel or stainless steel.

The sleeve may have a length of between 100 mm and 300 mm, and optionally has a length between 150 mm and 250 mm, and for example has a length of approximately 200 mm (for example, 205 mm).

The sleeve wall may have a minimum thickness (at the first end) of 3 mm to 7 mm, for example approximately 5 mm. The sleeve wall may have a maximum thickness (at the second end) of 8 mm to 12 mm, for example 10mm.

The sphere, spherical cap or spherical segment may have a diameter of between 50 mm and 100 mm, and optionally has a diameter of between 60 mm and 80 mm, and optionally has a diameter of approximately 70 mm, for example 68 mm.

The inner diameter of the sleeve at the first end is the same as or slightly larger than the diameter of the sphere, spherical cap or spherical segment. The inner diameter of the sleeve at the second end is 80% to 95% (optionally approximately 85%) of the inner diameter at the first end.

The sleeve may have an outer diameter (which is constant along its length) of 50 mm to 100 mm, for example 70 mm to 90 mm, and optionally approximately 80 mm, for example 78 mm.

In one embodiment, the sleeve has a length of 205 mm, and an outer diameter of 78 mm. In this embodiment, the sleeve wall is 5 mm thick at its first end and 10 mm thick at its second end. In this embodiment, the sphere, spherical cap or spherical segment has a diameter of 68 mm.

Another type of deformable element is now described. The deformable element comprises deformable or breakable material sandwiched between two (optionally parallel) flanges. A first flange is towards a first end of the deformable element, and a second flange is towards a second end of the deformable element. The first and second flanges may form end caps.

The first flange is optionally provided by an inner cylinder comprising an outwardly-extending boss at its first end, wherein the boss provides the flange. The second flange is optionally provided by a cap which is received over the second end of the inner cylinder. A plurality of washers may be provided between the two flanges. The function of the washers is to hold in place the deformable or breakable material in a plurality of rings and to prevent the material buckling outwards.

The maximum displacement of the deformable element broadly corresponds to the distance between the two flanges.

The deformable or breakable material may comprise plastic, vulcanized rubber, concrete, or graphite, for example.

The material properties of the rings (for example, the radial thickness, the axial length, and the material) may be chosen to provide a desired response under loading. The properties may be chosen according to the expected load.

The deformable element may comprise two rings separated by a washer. The deformable element may comprise three rings, wherein two washers are provided to separate neighbouring rings. The deformable element may comprise four rings, wherein three washers are provided to separate neighbouring rings. The deformable element may comprise five rings, wherein four washers are provided to separate neighbouring rings. The deformable element may comprise more than five rings, and an appropriate number of washers to separate neighbouring rings.

Each ring may comprise an annular deformable or breakable disk.

Alternatively, each ring may comprise a plurality of such annular deformable or breakable disks. A plurality of materials may be used in each ring - for example, each ring may comprise alternating disks made of different materials. Alternatively, each ring may comprise only one material (in a single disk, or a plurality of disks).

Other configurations of the deformable or breakable material are possible. For example, the deformable or breakable elements may be wedge-shaped.

The deformable or breakable elements may comprise cylindrical grids of material which form each ring. Here, a grid is a 3-dimensional lattice of material comprising lines of material crossing each other to form a repeating pattern of spaces. Each cylinder may have different material properties. For example, each may be made from a different material, or may have different dimensions, such as different axial length, or may have a different grid pattern, or may have the same grid pattern with the lines of material being of differing thickness.

Each cylindrical grid may have the same or different radial thickness. Here, radial thickness is the difference between the outer diameter of the cylindrical grid, and the inner diameter of the cylindrical grid. The radial thickness may be between 10 and 20 mm, for example 13 to 17 mm, and optionally is approximately 14 mm.

The axial length of each cylindrical grid may be between 10 mm and 20 mm, and optionally is between 12 and 18 mm.

Each ring may comprise a single cylinder, or a plurality of concentric cylinders.

The cylindrical grids may be made of steel, aluminium or plastic, for example.

Where the deformable or breakable elements are wedge-shaped or comprise cylindrical grids, in some embodiments, washers are not provided to separate the respective rings.

As will be appreciated from the foregoing, in at least preferred

embodiments, the deformable element is for increasing the capacity of the bolt and gives an indication (for example, a visual indication) of the loading of the bolt as the rock deforms.

Further optional/preferred features of the rod of the rock bolt (which may be combined in a rock bolt with any of the herein described deformable elements) are discussed below.

The rod may be a solid rod, or may be a hollow rod, i.e. a tube/pipe. The rod may be a multi-strand rod (in the case of a rock anchor, for example). The rod may be end-anchored or fully grouted.

The rod of the rock bolt may be a standard rebar (reinforcement bar). Such a rebar may have protrusions, ribs (ridges) and/or depressions on its surface to increase the bond strength with grout (for example, concrete or resin) to reduce slippage between the rod and the rock mass. Alternatively, the rod may be smooth along its length.

The rod may be a D-bolt (for example, as disclosed in WO/2008/079021 ).

The rod of the rock bolt may comprise an anti-corrosion treatment, for example a corrosion-resistant coating. Examples of such corrosion-resistant coatings are those produced by hot-dip galvanizing or both hot-dip galvanizing and powder coating. As a result of the reduced deformation in the rod due to the use of the deformable element, the anti-corrosion treatment may remain intact, where otherwise it would have cracked.

The rock bolt may be any rock bolt known in the art. The rock bolt may be mechanically or chemically anchored at the end furthest into the rock mass (and may then be grouted, though this is often not necessary), or may be fully grouted. The rock bolt may be a CT-bolt type, a tube bolt, or may comprise a rebar, a smooth rod, a friction bolt or a D-bolt. The rock bolt may comprise a multi-strand anchor (as in the case of a rock anchor, for example).

The rock bolt may comprise a plurality of deformable elements (for example, two deformable elements). That is, a plurality of deformable elements may be installed in series (i.e. one after another) along the rod of the rock bolt. Any of the deformable elements described herein may be combined with any type of rock bolt.

According to a final aspect, the invention provides a method of stabilising a rock mass using a rock bolt, the method comprising: installing a rock bolt in the rock mass, wherein the rock bolt comprises a deformable element which deforms under load, such that a first end of the deformable element is displaced towards a second end, wherein the magnitude of the displacement is an increasing function of the load.

The method may comprise use of the deformable element as set out in the third aspect of the invention, and including any of the preferred/optional features thereof.

The method may comprise use of the rock bolt as set out in the fourth aspect of the invention, and including any of the preferred/optional features thereof.

The method may comprise installing a plurality of rock bolts.

The method may comprise replacing a deformable element on a rock bolt if the existing rock bolt is close to failure. The method may comprise installing a plurality of deformable elements (for example, two deformable elements) on a rock bolt. That is, a plurality of deformable elements may be installed in series (i.e. one after another) along the rod of the rock bolt.

The method may comprise determining that the rock mass is stable if the deformation in a plurality (optionally all) of the rock bolts has stopped, or the rate of deformation has reduced below a predetermined threshold. The method may comprise spraying the rock bolt with concrete to encapsulate the deformable element, once the rock mass has been determined as stable.

Certain embodiments will now be described by way of example only, and with reference to the accompanying drawings in which:

Figure 1 shows a perspective view of a rock bolt comprising a first deformable element;

Figure 2 shows the rock bolt of Figure 1 installed in a rock mass (Figure 2a) and deformed under increased loading following a rock burst in the rock mass (Figure 2b);

Figure 3a shows a plan elevation of the rock bolt of Figure 1 , showing section lines A-A and B-B; Figure 3b shows a cross-section through the rock bolt of Figure 1 along section line A-A, and Figure 3c shows a cross-section through the rock bolt of Figure 1 along section line B-B;

Figure 4a shows a deformation indicator installed on the rock bolt of Figure 1 in an unloaded condition, and Figure 4b shows the rock bolt under load such that the deformable element has deformed, and the resultant change in the deformation indicator;

Figure 5 shows a simulation of the stresses applied to the deformable element of the rock bolt of Figure 1 at a stage close to the maximum deformation of the deformable element;

Figure 6 shows the ideal deformation-load curve for the each of the deformable elements described herein;

Figures 7a and 7b show a first modification of the first deformable element;

Figures 8a and 8b show a second modification of the first deformable element, and Figure 8c shows a length of tubing from which the deformable element can be cut;

Figure 9a shows a second deformable element, and Figure 9b shows the same deformable element in cross-section; Figure 10a shows a cross-sectional view of a third deformable element as it would appear in an installed position, prior to a substantial load being applied; Figure 10b shows the same deformable element once a high load has been applied, and Figure 10c shows the geometry of the deformable element

exaggerated for clarity;

Figures 1 1a to 11 c show an alternative deformation indicator;

Figure 12 show another alternative deformation indicator;

Figures 13a and 13b show alternative shapes for the plug of the third deformable element;

Figure 14a shows a cross-sectional view of a fourth deformable element, Figure 14b shows a perspective view of the fourth deformable element installed on a rock bolt, Figure 14c shows the geometry of the fourth deformable element exaggerated for clarity and Figures 14d to 14j show a deformation indicator suitable for use with the fourth deformable element,

Figure 15 shows a cross-sectional view of a fifth deformable element;

Figure 16 shows an expanded view of the cross-section shown in Figure 15;

Figure 17 shows a perspective view of a rock bolt incorporating the deformable element of Figure 15, including a deformation indicator;

Figure 18 shows the rock bolt of Figure 17 installed in a rock mass (Figure 18a) and deformed under increased loading following a rock burst in the rock mass (Figure 18b);

Figures 19a and 19b show a first modification of the deformable element of Figure 15;

Figures 20a and 20b show a second modification of the deformable element of Figure 15; and

Figures 21a and 21 b show a third modification of the deformable element of Figure 15.

In the following, like features have been labelled with the same reference numeral. For brevity, each feature is described just once, and the description of a given feature in reference to an earlier figure should be taken also to apply to the same feature in a later figure labelled with the same reference numeral.

Figure 1 shows a perspective view of a rock bolt 100 according to a first embodiment. The rock bolt 100 comprises a rod 6, a faceplate 4, and a nut 2 located on a screw-threaded end 6a of the rod. Between the nut 2 and faceplate 4 is provided a first deformable element 101 , comprising a hollow cylinder 105 and two annular disks 110a and 110b. The cylinder 105 comprises a first hole 105a in the cylinder and a second hole diametrically opposite the first hole, the holes being sized so as to allow the rod 6 to pass therethrough, so that the cylinder 105 can be mounted onto the rod 6. The first disk 110a and second disk 110b also each comprise a hole that is sized so as to allow the rod 6 to pass therethrough. The disks 110a, 110b are not attached to the cylinder 105, but serve to wedge the cylinder 105 between the nut 2 and faceplate 4. Under load, the first disk 110a and annular disk 110b adjoin the cylinder 105 and help to spread evenly the load from the faceplate 4 and nut 2 to the cylinder 105.

The cylinder 105 and annular disks 110a, 110b are made of S355 steel, which has a minimum yield strength of 355N/mm², a Young’s modulus (E) of 210,000 N/mm² and a Poisson’s ratio of approximately 0.29.

The cylinder wall has a thickness of 11 mm, and has an outer diameter of 100mm. The cylinder has a length of 70mm. The disks have an outer diameter of between 48 and 50 mm, and a thickness of 10 mm. The holes 105a in the cylinder 105 and disks 110a, 110b have a diameter of 24 mm (to receive a rod having a diameter of 20 mm).

Figure 2 shows the rock bolt 100 of Figure 1 installed in a rock mass 1 (Figure 2a) and deformed under increased loading following a rock burst in the rock mass 1 (Figure 2b). A comparison of Figures 2a and 2b shows that, under increasing loading, the cylinder 105 deforms to form an oval shape, shortening (and flattening) along the direction of the axis X of the rock bolt 100 and elongating in the perpendicular direction Y.

Figure 3a shows a plan elevation of the rock bolt of Figure 1 , showing section lines A-A and B-B; Figure 3b shows a cross-section through the rock bolt 100 of Figure 1 along section line A-A, and Figure 3c shows a cross-section through the rock bolt 100 of Figure 1 along section line B-B.

From the cross-sectional views, the shape of the first and second annular disks 110a, 110b can be appreciated. The first annular disk 110a broadly has the shape of a truncated cone, broadening at the base of the truncated cone which contacts the cylinder 105. The top of the truncated cone has a diameter broadly similar to that of the nut 2. The second annular disk 110b has a broadly hemispherical shape, with the curved surface of the hemisphere providing a good interface with the faceplate 4, and the flat surface of the hemisphere providing a good interface with the cylinder 105. Figure 4a shows a deformation indicator 8 installed on the rock bolt 100 of Figure 1 in an unloaded condition, and Figure 4b shows the rock bolt 100 under load such that the cylinder 105 of the deformable element 101 has deformed, resulting in a change in indication shown by the deformation indicator 8.

The deformation indicator 8 shown here measures the degree of elongation (displacement) in the direction transverse to the axis of the rock bolt 100, i.e. the direction labelled B in Figures 2a and 2b. The deformation indicator 8 comprises a length gauge comprising a scale marked on a rigid rod 9. The rigid rod 9 is housed completely, or almost completely, within a housing 10. The rigid rod 9 and housing 10 are attached to opposite sides of the cylinder 105 (i.e. at the sides which elongate in the transverse direction under loading). As load is applied, the opposite sides of the cylinder 105 move outwardly, causing the rigid rod 9 and housing 10 to move relative to one another so that the rigid rod 9 moves out of the housing 10. The degree of deformation can then be read from the exposed scale. In this case, the scale is marked with colour coding. In this example, green signifies that the cylinder 105 is not deformed or only deformed by a small amount, yellow indicates some (more) deformation, orange indicates even more deformation, etc. If the loading properties of the deformable element 101 are known in advance (i.e. the displacement as a function of load), the degree of deformation can be correlated to the load, for example using a calibration curve.

An deformation indicator (not shown) is a glue-on wire strain sensor. The strain sensor may comprise an insulating flexible backing which supports a metallic foil pattern. Alternatively, the strain sensor may comprise a piezoresistor.

The strain sensor may be combined with an RFID tag. Battery-free and wire-free surveillance of the degree of deformation is then possible using an antenna to read the degree of deformation. Real-time monitoring may be used to trigger an audible alarm and/or text-message alerts (or similar) if the degree of deformation exceeds pre-set thresholds in one or a plurality of rock bolts. Figure 5 shows a simulation of the stresses applied to the deformable element of the rock bolt of Figure 1 at a stage close to the maximum deformation of the deformable element 101. When the cylinder 105 is buckled inwards and experiences self- contact, as shown in Figure 5, the ability of the cylinder 105 to deform further is much reduced.

Figure 6 shows the ideal deformation-load curve for the deformable element of Figure 1. Here, the degree of deformation corresponds to the shortening in the diameter of the cylinder 105 in the axial direction, or the broadening of the diameter of the cylinder 105 in the transverse direction. The curve is a smooth curve, with no steps or dips, such that the deformation is an increasing function of the load, over the working range of the deformable element 101 (i.e. up until the cylinder 105 experiences self-contact).

The ideal deformation-load curve for the deformable element in general is one in which load initially increases rapidly with low deformation (i.e. an initial steep gradient portion) with a slower development under increasing load (i.e. a shallower gradient portion) as the bolt yield strength is approached. This allows pre- tensioning of the rock bolt, without a great deal of deformation, and then as the load increases, the deformation gradually increases until the bolt starts yielding.

Testing has shown that the size of the holes 105a in the cylinder 105 may have an effect on the deformation-load curve. If the diameter of the holes 105a is only slightly larger than the diameter of the rod 6, then contact between the edge of the hole 105a and the rod 6 may cause deviations from the expected smooth curve. The diameter of the holes 105a is optionally 10 to 30% larger than the diameter of the rod 6.

The cylinder 105 may be thought of as the combination of two pairs of deformable portions: a top-bottom pair and a side-side pair. Each of the top, bottom and two sides forms a quarter of the cylinder. The top and bottom portions include the holes 105a for the rod 6 to pass through, and the side portions are the remaining portions. Each quarter is a portion of the cylinder subtended by 90°, wherein the middle of the top portion for example is aligned with the middle of the top hole 105a.

The way that the cylinder 105 deforms (and hence the shape of the deformation-load curve) may be influenced by the relative amounts of material in the top-bottom pair and the side-side pair. Figures 7a, 7b, and 8a to 8c

demonstrate ways that the relative amounts of material in the top-bottom pair and side-side pair can be varied.

Figures 7a and 7b demonstrate that the amount of material in the top- bottom pair can be increased relative to the amount of material in the side-side pair by forming the cylinder 105 such that the front and back faces of the cylinder are not flat, as in the previous embodiment. Rather, each end of the cylinder is formed to have two faces, meeting at an outwardly-projecting point at the top of the cylinder, and an outwardly projecting point at the bottom of the cylinder. Looking down on the cylinder 105 from above along the axis which passes through the middle of each hole 105a, the cylinder then has a hexagonal shape in plan view. This is shown in Figure 7b. The points of the opposite ends of the cylinder may be collinear with the middle of the hole 105a.

In one example, the angle a (see Figure 7b) of each face to the line perpendicular to the longitudinal axis Z of the cylinder is between 3° and 15°, for example between 5 and 10°.

Such a cylinder may be formed from a length of tubing from which several cylinders can be cut. Here, the tubing is formed with a plurality of holes 105a (for the rod to pass through). Two diagonal cuts are made at each end of each cut cylinder, to form the angled front and back ends.

As an alternative, it is possible to reduce the amount of material in the top- bottom pair relative to the amount of material in the side-side pair by forming each end of the cylinder 105 to have two faces, meeting at an inwardly- projecting point at the top of the cylinder, and an inwardly projecting point at the bottom of the cylinder.

Figures 8a to 8c demonstrate a further way of changing the amount of material in the top-bottom pair compared to the amount of material in the side-side pair. Specifically, opposed pairs of cut-outs in the front and back faces of the cylinder can be formed in either the top-bottom pair or the side-side pair.

In Figures 8a and 8b, cut-outs 105b are formed in the top-bottom pair. The cut-outs shown are semi-circular in shape, but cut-outs of another shape could be used. The two cut-outs 105b on the top front and back faces are aligned with the top hole 105a for the rod 6 to pass through. The two cut-outs 105b on the bottom front and back faces are aligned with the bottom hole 105a for the rod 6 to pass through. Figure 8c shows a length of tubing 105c from which several such cylinders can be cut. Here, cut-out circles 105b alternate with the holes 105a for the rod to pass through. The tubing 105c is cut into lengths perpendicular to the longitudinal axis Z of the tubing, bisecting each of the cut-out circles to form semi- circular cut-outs 105b in each of the front and back faces of the cylinder 105.

Whilst cut-outs are shown in the top-bottom pair in Figures 8a to 8c, it will be appreciated that they could instead be formed in the side-side pair.

Figures 9a and 9b show views of a second deformable element 201 according to a second embodiment. The second deformable element 201 comprises a sleeve 215 and a spherical cap 220 which is received (at least partially) within the sleeve 215. The sleeve 215 has a first end 215a which has an inner diameter similar to or slightly larger than the diameter of the spherical cap 220, and a second end 215b that has a smaller inner diameter that is smaller than the diameter of the spherical cap. The inner diameter of the sleeve decreases smoothly from the first end 215a to the second end 215b. The sleeve 215 comprises an inner surface 215c forming the shape of a truncated cone, with the wide end of the truncated cone at the first end 215a and the narrow end of the truncated cone at the second end 215b, and an outer surface 215d forming a cylinder, such that the walls of the sleeve 215 increase in thickness from the first end 215a to the second end 215b.

The spherical cap 220 comprises a through-hole sized so that the rod 6 of the rock bolt can pass through the through-hole, i.e. so that the spherical cap 220 can be installed onto the rod 6 of the rock bolt.

The deformable element 201 may comprise a deformation indicator (not shown), for example a length gauge attached to the spherical cap 220. The length gauge may comprise a scale marked on a rigid rod, projecting from the top surface of the spherical cap 220. As the spherical cap 220 moves inside the sleeve 215, so too does the length gauge. The displacement can then be read off from the part of the length gauge still projecting outside of the sleeve 215.

Alternatively, the length gauge may comprise a scale (not shown) marked on the exterior of the sleeve 215, and the position of the spherical cap 220 within the sleeve 215 may be read off from the sleeve 215 based on the deformation of the sleeve 215 around the position of the spherical cap 220.

To install the second deformable element 201 , firstly an annular disk 210 having a broadly hemispherical shape is slid onto the rod 6 to abut the faceplate 4. The sleeve 215 is slid onto the rod, with the second end of the sleeve 215b (i.e. the end of the sleeve which has a smaller inner diameter) closest to the faceplate 4. Then, the spherical cap 220 is slid onto the rod 6, to be received (at least partially) within the sleeve at its first end 215a (i.e. the end of the sleeve which has a larger inner diameter). The rod 6 is threaded at the location where the spherical cap 220 is provided, and a nut 2 is screwed onto the rod 6 to secure the spherical cap 220 in place.

When installed, loading on the deformable element causes relative movement of the spherical cap 220 and the sleeve 215, such that the spherical cap 220 moves along the sleeve 215 from the first end 215a to the second end 215b. Due to the decreasing internal diameter of the sleeve 215, the spherical cap 220 experiences an increasing resistance as it moves through the sleeve 215.

The sleeve 215 and sphere 220 are made of S355 steel, which has a minimum yield strength of 355N/mm², a Young’s modulus (E) of 210,000 N/mm² and a Poisson’s ratio of approximately 0.29.

The sleeve 215 has a length of 205 mm. The sleeve 215 wall has a minimum thickness (at the first end 215a) of 5 mm. The sleeve 215 wall has a maximum thickness (at the second end 215b) of 10 mm. The sphere 220 has a diameter of 68 mm.

The inner diameter of the sleeve 215 at the first end 215a is 68 mm. The inner diameter of the sleeve at the second end 215b is 58 mm. The sleeve has an outer diameter (which is constant along its length) of 78 mm.

The ideal deformation-load curve for this embodiment is the same as that shown in Figure 6. Here, the degree of deformation corresponds to the distance that the spherical cap 220 has moved into the sleeve 215. The curve is a smooth curve, with no steps or dips, such that the deformation is an increasing function of the load, over the working range of the deformable element (i.e. up until the spherical cap 220 reaches the second end 215b of the sleeve 215 - i.e. the end of the sleeve 215 closest to the faceplate 4).

Figure 10a shows a cross-sectional view of a third deformable element 250 as it would appear in an installed position, prior to a substantial load being applied. Figure 10b shows the same deformable element 250 once a high load has been applied.

The third deformable element 250 comprises a sleeve 255 and a plug 260 which is received (at least partially) within the sleeve 255. The sleeve 255 has a first end 256 which has an inner diameter d₁ (see Figure 10c) similar to or slightly larger than the width of the plug 260 at its widest point, and a second end 257 that has a smaller inner diameter that is smaller than the width of the plug 260 at its widest point. The inner surface of the sleeve 255 comprises the surfaces of two notional adjacent truncated conical sections 255a and 255b, each having a different base angle (q-i and q₂, respectively). The first truncated conical section 255a (closest to the first end 256 of the sleeve) has a smaller base angle (q^ than the base angle (q₂) of the second truncated conical section 255b. This means that the first truncated conical section 255a has a shallower slope than the second truncated conical section 255b. In general, the height hn of the first truncated conical section 255a is smaller than the height h₂ of the second truncated conical section 255b. Adjacent the second truncated conical section 255b, and forming the second end 257 of the sleeve 255 is a hemispherical section 255c. A through-hole passes through the second end 257 of the sleeve 255 to allow the sleeve 255 to be placed onto the rod 6 of the rock bolt. The hemispherical section 255c is provided in order to engage with the faceplate in such a way as to allow for the rock bolt to be placed at an angle other than perpendicular to the rock face.

The sleeve 255 comprises the surfaces of two notional adjacent truncated conical sections in order to control the deformation-load curve of the deformable element 250. As discussed above, a desirable deformation-load curve is one which load initially increases rapidly with low deformation. This is achieved by providing a first truncated conical section 255a with a shallower slope than the second truncated conical section 255b. This means that it is harder for the plug 260 to pass through the first truncated conical section 255a than the second truncated conical section 255b. This gives the desired deformation-load curve.

Whilst a sleeve 255 comprising the surfaces of two notional adjacent truncated conical sections has been described above, it will be appreciated that the sleeve could be formed from more than two notional adjacent truncated conical sections, for example three, or four notional adjacent truncated conical sections. The base angle of each may increase, moving from the first end 256 to the second end 257, to give the desired deformation-load curve.

The geometry of the sleeve 255 is shown in an exaggerated form in Figure 10c. Exemplary values for the parameters shown in the figure are as follows: d^ 47mm

h^ 15mm

qi : 83°

d₂: 43mm

h₂: 70mm

q₂: 88°

d₃: 38mm

The sleeve 255 has a length of 107mm. The sleeve 255 wall has a thickness of approximately 5mm to 6mm.

The sleeve 255 may be formed from a cylindrical steel pipe having an inner diameter of 47mm and wall thickness of 5mm (for example). The pipe may be formed into the sleeve 255 by applying radial pressure or by pressing the pipe into mould. The process of forming the pipe into the sleeve 255 increases the wall thickness moving from the first end 256 down the sleeve towards the second end 257, such that the wall thickness at the first end 256 is 5mm, and this increases to is just under 6mm at the second end 257.

The plug 260 in this case has a spherical cap shape and comprises a through-hole sized so that the rod 6 of the rock bolt can pass through the through- hole, i.e. so that the plug 260 can be installed onto the rod 6 of the rock bolt. In this case, the plug 260 has a diameter of 46 mm, and the through-hole has a diameter of 22mm.

To install the third deformable element 250, firstly the rod 6 is installed in a hole in the rock mass, and a faceplate is slid over the end of the rod 6 to abut the rock face. Then, the sleeve 255 is slid onto the rod 6 to abut the faceplate 4, with the second end of the sleeve 257 (i.e. the end of the sleeve which has a smaller inner diameter) closest to the faceplate 4. Then, the plug 260 is slid onto the rod 6, to be received (at least partially) within the sleeve 255 at its first end 256 (i.e. the end of the sleeve which has a larger inner diameter d^. The rod 6 is threaded at the location where the plug 260 is provided, and a nut 2 is screwed onto the rod 6 to secure the plug 260 in place.

When installed, loading on the deformable element 250 causes relative movement of the plug 260 and the sleeve 255, such that the plug 260 moves along the sleeve 255 from the first end 256 to the second end 257. Due to the decreasing internal diameter of the sleeve 255, the plug 260 experiences an increasing resistance as it moves through the sleeve 255. This seems to be due to three factors:

- As the radius reduces, a larger deformation of the sleeve is needed for further advancement of plug.

- The angle between the deformation axis and the plug contact is larger with increased deformation.

- The contact area between the plug and the sphere is larger with

increased deformation.

The sleeve 255 and plug 260 are made of S355 steel, which has a minimum yield strength of 355N/mm², a Young’s modulus (E) of 210,000 N/mm² and a Poisson’s ratio of approximately 0.29. The ideal deformation-load curve for this embodiment is the same as that shown in Figure 6. Here, the degree of deformation corresponds to the distance that the plug 260 has moved into the sleeve 255. The curve is a smooth curve, with no steps or dips, such that the deformation is an increasing function of the load, over the working range of the deformable element (i.e. up until the plug 260 reaches the second end 257 of the sleeve 255 - i.e. the end of the sleeve 255 closest to the faceplate 4).

The deformable element 250 may comprise a deformation indicator as discussed above in relation to the second deformable element 210 (for example, a length gauge comprising a scale marked on a rigid rod attached to the plug 260, or a scale (not shown) marked on the exterior of the sleeve 255).

An alternative deformation indicator 265 is shown in Figures 11 a to 11 c.

The deformation indicator 265 comprises a flexible strip 265a having approximately the same length as the deformation length of the deformable element 250. The flexible strip 265a is attached at one end to the plug 260 and is received within a cover portion 265b on the exterior of the sleeve 255. The cover portion 265b does not cover the entirety of the flexible strip 265a, leaving an exposed position where the flexible strip 265a can be seen. The flexible strip 265a strip is colour-coded such that as the plug 260 is pulled through the sleeve 255, the flexible strip 265a is pulled out of the cover portion 265b and different colours of the flexible strip 265a are visible at the exposed position. Figure 11 c shows that a portion of the flexible strip 265a visible when the deformable element 250 is initially installed is a first colour (for example, green). Under an additional load, a second colour (for example, yellow) is exposed. Under further loading, a third colour (for example, red) is exposed. Of course, any number of colours may be chosen.

A further alternative deformation indicator 267 is shown in Figure 12. This deformation indicator 267 is a glue-on wire strain sensor. The strain sensor may comprise an insulating flexible backing which supports a metallic foil pattern.

Alternatively, the strain sensor may comprise a piezoresistor.

The strain sensor may be combined with an RFID tag. Battery-free and wire-free surveillance of the degree of deformation is then possible using an antenna to read the degree of deformation. Real-time monitoring may be used to trigger an audible alarm and/or text-message alerts (or similar) if the degree of deformation exceeds pre-set thresholds in one or a plurality of rock bolts. Whilst the plug 260 shown in Figures 10 to 12 has a spherical cap shape (as shown in cross-section in Figure 13a), other shapes are possible for the plug 260. Figure 13b shows one such alternative. Here, the plug 260’ extends between a first plug end 261 having a first outer diameter D1 to a second plug end 262 having a second outer diameter D2, and tapers outwardly from each of the first and second plug ends to a maximum diameter portion 263 between the first and second plug ends having a maximum diameter D.

Figure 14a shows a cross-sectional view of a fourth deformable element 270. Figure 14b shows a perspective view of the fourth deformable element 270 installed on a rock bolt 6.

The fourth deformable element 270 comprises a sleeve 255’ and a plug 260 which is received (at least partially) within the sleeve 255’. The fourth deformable element 270 differs from the third deformable element 250 in that, rather than being placed outside of the rock face (which is where the third deformable element 250 is installed), the fourth deformable element 270 is designed so that the majority of the deformable element 270 is received within the hole in the rock mass.

In order to anchor the sleeve 255’ relative to the faceplate 4, the sleeve 255’ has a widest section 255d which abuts the faceplate 4 (as shown in Figures 14a and 14b), whilst the remainder of the sleeve 255’ is received within the hole in the rock mass. The widest section 255d forms the surface of a partial sphere, to allow for the rock bolt to be placed at an angle other than perpendicular to the rock face. The widest section may for example have a radius of curvature of 60 to 90mm, for example 75mm.

The remaining structure of the sleeve 255’ is similar to that described in respect of the third deformable element 250. The sleeve 255’ has a first portion 256’ which has an inner diameter di (see Figure 14c) similar to or slightly larger than the width of the plug 260 at its widest point, and a second end 257’ that has a smaller inner diameter that is smaller than the width of the plug 260 at its widest point. The inner surface of the sleeve 255’ comprises the surfaces of two notional adjacent truncated conical sections 255a’ and 255b’, each having a different base angle (q^ and q₂’, respectively). The first truncated conical section 255a’ (the closest truncated conical section to the first portion 256’ of the sleeve) has a smaller base angle (Q- ) than the base angle (q₂’) of the second truncated conical section 255b’. This means that the first truncated conical section 255a’ has a shallower slope than the second truncated conical section 255b’. In general, the height h- of the first truncated conical section 255a’ is smaller than the height h₂ of the second truncated conical section 255b’. Adjacent the second truncated conical section 255b’, and forming the second end 257’ of the sleeve 255’ is a hemispherical section 255c’. A through-hole passes through the second end 257’ of the sleeve 255’ to allow the sleeve 255’ to be placed onto the rod 6 of the rock bolt. The hemispherical section 255c’ need not be hemispherical, but should be closed (except for the through-hole) in order to avoid the plug 260’ being pulled out of the sleeve 255’.

The sleeve 255’ comprises the surfaces of two notional adjacent truncated conical sections in order to control the deformation-load curve of the deformable element 250. As discussed above, a desirable deformation-load curve is one which load initially increases rapidly with low deformation. This is achieved by providing a first truncated conical section 255a’ with a shallower slope than the second truncated conical section 255b’. This means that it is harder for the plug 260 to pass through the first truncated conical section 255a’ than the second truncated conical section 255b’. This gives the desired deformation-load curve.

Whilst a sleeve 255’ comprising the surfaces of two notional adjacent truncated conical sections has been described above, it will be appreciated that the sleeve could be formed from more than two notional adjacent truncated conical sections, for example three, or four notional adjacent truncated conical sections. The base angle of each may increase, moving from the first end 256’ to the second end 257’, to give the desired deformation-load curve.

The geometry of the sleeve is shown in an exaggerated form in Figure 14c. Exemplary values for the parameters shown in the figure are as follows:

d₄: 100mm (and widest section 255d has a radius of curvature of 75mm) h₃: 20mm

di: 42mm

h_t:7mm

Q’,: 83°

d₂: 38mm

h₂: 70mm

0’₂: 88°

d₃: 34mm

The sleeve 255’ has a length of 120mm. The sleeve 255’ wall has a thickness of approximately 5mm to 6mm The main part of the sleeve 255’ (the part forming a plurality of truncated conical sections, and the hemispherical end) may be formed from a cylindrical steel pipe having an inner diameter of 47mm and wall thickness of 5mm (for example). The pipe may be formed into the main body of the sleeve 255’ by applying radial pressure or by pressing the pipe into mould. The process of forming the pipe into the main body of the sleeve 255’ increases the wall thickness moving from the first end 256’ down the sleeve towards the second end 257’, such that the wall thickness at the first end 256’ is 5mm, and this increases to is just under 6mm at the second end 257’. The widest section 255d may need to be formed separately from the main body of the sleeve 255’ and then attached (for example, by welding) onto the main body of the sleeve 255’.

The plug 260 in this case has a spherical cap shape and comprises a through-hole sized so that the rod 6 of the rock bolt can pass through the through- hole, i.e. so that the plug 260 can be installed onto the rod 6 of the rock bolt. In this case, the plug 260 has a diameter of 40mm, and the through-hole has a diameter of 22mm.

To install the fourth deformable element 270, firstly the rod 6 is installed in a hole in the rock mass, and a faceplate is slid over the end of the rod 6 to abut the rock face. Then, the sleeve 255’ is slid onto the rod 6 and into the hole in the rock mass, until the movement of the sleeve 255’ into the hole in the rock mass is stopped by the widest section 255d of the sleeve 255’ abutting the faceplate.

Then, the plug 260 is slid onto the rod 6, to be received (at least partially) within the sleeve 255’ at its first end 256’. The rod 6 is threaded at the location where the plug 260 is provided, and a nut 2 is screwed onto the rod 6 to secure the plug 260 in place.

When installed, loading on the deformable element 270 causes relative movement of the plug 260 and the sleeve 255’, such that the plug 260 moves along the sleeve 255’ from the first end 256’ to the second end 257’. Due to the decreasing internal diameter of the sleeve 255', the plug 260 experiences an increasing resistance as it moves through the sleeve 255’.

The sleeve 255’ and plug 260 are made of S355 steel, which has a minimum yield strength of 355N/mm², a Young’s modulus (E) of 210,000 N/mm² and a Poisson’s ratio of approximately 0.29.

The ideal deformation-load curve for this embodiment is the same as that shown in Figure 6. Here, the degree of deformation corresponds to the distance that the plug 260 has moved into the sleeve 255’. The curve is a smooth curve, with no steps or dips, such that the deformation is an increasing function of the load, over the working range of the deformable element (i.e. up until the plug 260 reaches the second end 257’ of the sleeve 255’).

The deformable element 270 may comprise a deformation indicator (not shown) as discussed above in relation to the second deformable element 210 (for example, a length gauge comprising a scale marked on a rigid rod attached to the plug 260).

An alternative deformation indicator (not shown) is one similar to the one shown in Figures 11a to 11c, i.e. comprising a flexible colour-coded strip having approximately the same length as the deformation length of the deformable element 270, attached at one end to the plug 260 and received within a cover portion.

Whereas in Figures 11a to 11 c, the cover portion 265b is provided on the exterior of the sleeve 255, this is not possible for the fourth deformable element 270, as the sleeve 255’ is received partially within the hole in the rock mass. Instead, the cover portion can be provided on the faceplate, leaving an exposed position between the plug and the faceplate where the flexible strip can be seen.

An alternative deformation indicator is shown in Figures 14d to 14j. The deformation indicator 280 comprises three parts: a nut 281 , an inner sheath 282, and an outer cover 283. The nut 281 (shown from the top in Figure 14d and from the side in cross-section in Figure 14e) comprises an inner threaded portion 281a to engage with the threaded end 6a of the rock bolt 6 (for example, to secure the plug 260 against the sleeve 255’). The nut 281 also comprises two pins 281b protruding from opposite sides of the nut 281 , towards the top of the nut (i.e. the end of the nut furthest from the deformable element). The inner sheath 282 (shown from the top in Figure 14f and from the side in Figure 14g) is a cylinder having two closed slots 282a (on opposite sides of the sheath) extending along most of the longitudinal extent of the sheath (but not right to the top and bottom of the sheath). The inner sheath 282 also comprises two sets of diagonal stripes 282b (for example, colour coded green, yellow and red) extending around the sheath from just below each slot 282a to just above the other slot 282a. The two sets of stripes are separated by an unmarked area 282c. The outer cover 283 (shown from the top in Figure 14h and from the side in in Figure 14i) comprises a piece of material shaped as a parallelogram wrapped round to partially form a cylinder, leaving an open part through which the inner sheath 282 can be seen. The outer cover 283 comprises two slots 283a running along two of the sides of the parallelogram (the sides which run diagonally up, rather than the sides that run in parallel horizontal planes).

The deformation indicator 280 is shown assembled on an in-hole

deformable element 270 in Figure 14j. The deformation indicator 280 could equally be used in the external variant 250.

The nut 281 is screwed on to the rod 6. The inner sheath 282 covers the nut 281 and the outer cover 283 covers only partially the inner sheath 282. The outer cover 283 is sized so that it cannot be pulled through the sleeve 255’ along with the plug 260; however, the nut 281 and inner sheath 282 can be pulled through the sleeve 255’. The pins 281b are received through the slots 282a in the inner sheath 282 and in the slots 283a in the outer cover 283.

When the deformation element deforms (i.e. when the plug 260 moves through the sleeve 255’) the downward movement of the pins 281 b will turn the outer cover 283, revealing the coloured stripes 282b printed on the inner sheath 282.

Whilst the plug 260 shown in Figure 14a has a spherical cap shape (as shown in cross-section in Figure 13a), other shapes are possible for the plug 260. Figure 13b shows one such alternative, described above.

Figure 15 shows a cross-sectional view of a fifth type of deformable element 301 , which is also shown as an expanded view in Figure 16.

The deformable element 301 comprises deformable or breakable material 338a, 338b, 338c sandwiched between two flanges 332a and 334a. A first flange 332a is towards a first end 301a of the deformable element 301 , and a second flange 334a is towards a second end 301 b of the deformable element 301. The first flange 332a is provided by an inner cylinder 332 comprising an outwardly-extending boss at its first end 332b, wherein the boss provides the flange 332a. The second flange 334a is provided by a cap 334 which is received over the second end 332c of the inner cylinder. A plurality of washers 336 may be provided between the two flanges 332b, 334a. The function of the washers 336 is to hold in place the deformable or breakable material in a plurality of rings 338a, 338b, 338c and to prevent the material buckling outwards.

The deformable or breakable material 338a, 338b, 338c may comprise plastic, vulcanized rubber, concrete, or graphite, for example.

The material properties of the rings 338a, 338b, 338c (for example, the radial thickness, the axial length, the material) may be chosen to provide a desired response under loading. The properties may be chosen according to the expected load.

In this embodiment, the deformable element 301 comprises three rings 338a, 338b, 338c, wherein two washers 336 are provided to separate neighbouring rings. Each ring 338a, 338b, 338c comprises an annular deformable or breakable disk. In alternative embodiments, each ring 338a, 338b, 338c may comprise a plurality of such annular deformable or breakable disks, for example, each ring may comprise two alternating materials.

Figure 17 shows a perspective view of the fourth deformable element 301 installed on a rock bolt 300 between a nut 2 and a faceplate 4. Here, the deformable element 301 includes a deformation indicator 340. The deformation indicator comprises a length gauge 340 configured to measure the shortening of the deformable element 301 in the axial direction X under loading. In this case, the length gauge 340 is striped, with the width of the stripes being 10mm (for example). This enables the degree of deformation to be visually determined. If the loading properties of the deformable element 301 are known in advance, the degree of deformation can be correlated to the load, for example using a calibration curve.

Figure 18a and 18b show the rock bolt 300 installed in a rock mass 1 (Figure 18a) and deformed under increased loading following a rock burst in the rock mass 1 (Figure 18b). A comparison of Figures 18a and 18b shows that, under increasing loading, the deformable element 301 shortens along the direction of the axis X (shown in Figure 17) of the rock bolt 300.

Figures 19 to 21 show modifications to the deformable element 301 of Figures 15 to 18, in which the structure of the breakable or deformable material is different, and/or the structure of the deformable member which forms the flanges which sandwich the breakable or deformable material is different.

Figures 19a and 19b shows a partial perspective view and a cross-sectional view, respectively, of an embodiment of the deformable element 351 in which the deformable or breakable elements comprise cylindrical grids of material which are coaxial with the axis of the deformable element 351 , and which form rings 358a, 358b, 358c. The grids are made from steel, aluminium or plastic, for example.

Here, the three rings 358a, 358b, 358c comprise the same grid pattern, but the thickness of the material forming the crossing lines, and the spaces between the crossing lines, are different in each case. This gives the rings 358a, 358b, 358c different material properties. The radial thickness of the rings 358a, 358b, 358c is 14 mm. In this example, a first (upper) grid 358a has an axial length of 12 mm, a second (middle) grid 358b has an axial length of 13 mm, and a third (lower) grid 358c has an axial length of 14 mm.

Figures 20a and 20b show a partial perspective view and a cross-sectional view, respectively, of an embodiment of the deformable element 361 which is similar to that shown in Figure 12. In this embodiment, the rings 368a, 368b, 368c are the same as shown in Figures 12a and 12b. The difference is in the structure of the deformable element 361 which forms the flanges (i.e. to sandwich the rings 368a, 368b, 368c). In this case, there is no central cylinder portion around which the rings 368a, 368b, 368c are mounted; the rings are mounted directly around the rod of the rock bolt.

Figure 21 shows an embodiment of the deformable element 371 similar to the preceding embodiment in which the deformable or breakable elements comprise wedges 376 and rings 378. Under load, the outer wedges will be pressed outwards and deformed in tension. The inner rings will be squeezed together.

The ideal deformation-load curve for the embodiments shown in Figures 15 to 21 is the same as that shown in Figure 6. Here, the degree of deformation corresponds to the degree of shortening of the deformable element. The curve is a smooth curve, with no steps or dips, such that the deformation is an increasing function of the load, over the working range of the deformable element (i.e. up until the flanges of the deformable element contact one another).

In the foregoing, each deformation indicator described could be used with other embodiments of the deformable element, as well as with the embodiment of the deformable element which it is described in conjunction with.

The rock bolt may be any rock bolt known in the art. The rock bolt may be mechanically or chemically anchored at the end furthest into the rock mass (and then grouted), or may be fully grouted. The rock bolt may be a CT-bolt type, a tube bolt, or may comprise a rebar, a smooth rod, a friction bolt or a D-bolt. The rock bolt may comprise a multi-strand anchor (as in the case of a rock anchor, for example).

Thus, there is described herein various deformable elements for increasing the capacity of a rock bolt, which and give an indication of the loading of the bolt as the rock deforms. The following clauses set out features of the invention which may not presently be claimed in this application, but which may form the basis for future amendment or a divisional application.

1. A deformable element for a rock bolt, wherein the deformable element is configured to deform under load such a first end of the deformable element is displaced towards a second end, wherein the magnitude of the displacement is an increasing function of the load.

2. A deformable element according to clause 1 , wherein the deformable element comprises an RFID tag.

3. A deformable element according to any preceding clause, wherein the deformable element comprises a deformation indicator configured to indicate whether the deformable element has been deformed.

4. A deformable element according to clause 3, wherein the deformation indicator comprises a sensor, and wherein optionally the sensor is a passive sensor.

5. A deformable element according to clause 3 or 4, wherein the deformation indicator comprises a displacement sensor comprising a switch and a switch monitor, or wherein the deformation indicator comprises a strain sensor.

6. A deformable element according to clause 3, 4 or 5, wherein the deformation indicator comprises a length gauge, optionally marked with a scale showing units of measurement and/or marked with a colour-coded scale.

7. A deformable element according to any preceding clause, wherein the deformable element has a deformation length of between 50 mm and 300 mm.

8. A deformable element according to any preceding clause, comprising a hollow cylinder formed by a cylindrical wall, wherein first and second through-holes are provided in the cylindrical wall, diametrically opposite each other, and wherein the first and second through-holes are sized to allow a rod of the rock bolt to pass through. 9. A deformable element according to clause 8 wherein the cylinder wall comprises a material with a minimum yield strength of between 250 N/mm² and 650 N/mm², and/or comprises a material with a Young’s modulus (E) of approximately 210,000 N/mm², and/or comprises a material with a Poisson’s ratio of 0.26 to 0.31.

10. A deformable element according to clause 8 or 9, comprising a first annular disk and a second annular disk adjoining the cylindrical wall around the first and second hole, respectively, optionally wherein one of the annular disks has a broadly hemispherical shape.

11. A deformable element according to any of clauses 8 to 10, comprising a deformation indicator which allows to read off the degree of shortening of the deformable element along its axial extent, or the degree of elongation in the transverse direction.

12. A deformable element according to clause 11 , comprising a length gauge for measuring the degree of elongation in the transverse direction, wherein the length gauge optionally comprises a scale marked on a rigid rod which is housed within a housing, and optionally wherein the housing is attached to one side of the cylinder on a side which will elongate in the transverse direction, and the rigid rod is attached diametrically opposite.

13. A deformable element according to any of clauses 1 to 7, comprising a sphere, spherical cap or spherical segment and a sleeve for receiving the sphere, spherical cap or spherical segment inside, at least partially.

14. A deformable element according to clause 13, wherein the sleeve has a first end that has an inner diameter the same as or slightly larger than the diameter of the sphere, spherical cap or spherical segment and a second end that has a smaller inner diameter, smaller than the diameter of the sphere, spherical cap or spherical segment.

15. A deformable element according to clause 14, wherein the inner diameter of the sleeve decreases smoothly from the first end to the second end. 16. A deformable element according to any of clauses 13 to 15, comprising an annular disk having a broadly hemispherical shape, provided to abut the second end of the sleeve and a faceplate of the rod.

17. A deformable element according to any of clauses 13 to 16, comprising a length gauge deformation indicator, wherein the length gauge comprises: a scale marked on a rigid rod, projecting from the top surface of the sphere, spherical cap or spherical segment; or a scale marked on the exterior of the sleeve.

18. A deformable element according to any of clauses 13 to 17, wherein the sleeve comprises a material with a minimum yield strength of between 250 N/mm² and 650 N/mm², and/or comprises a material with a Young’s modulus (E) of approximately 210,000 N/mm², and/or comprises a material with a Poisson’s ratio of 0.26 to 0.31.

19. A deformable element according to any of clauses 13 to 18, wherein the inner diameter of the sleeve at the second end is 80% to 95% of the inner diameter at the first end.

20. A deformable element according to any of clauses 1 to 7, wherein the deformable element comprises deformable or breakable material sandwiched between first and second flanges.

21. A deformable element according to clause 20, wherein the first and second flanges form end caps of the deformable element.

22. A deformable element according to clause 21 , wherein the first flange is provided by an inner cylinder comprising an outwardly-extending boss at its first end, wherein the boss provides the flange, and the second flange is provided by a cap which is received over the second end of the inner cylinder.

23. A deformable element according to any of clauses 20 to 22, wherein a plurality of washers are provided between the two flanges, for holding in place the deformable or breakable material in a plurality of rings.

24. A deformable element according to any of clauses 20 to 23, wherein the deformable or breakable material comprises plastic, vulcanized rubber, concrete, or graphite, and/or wherein the deformable or breakable material is provided in disks or wedges.

25. A deformable element according to any of clauses 20 to 24, wherein the deformable or breakable material is provided in cylindrical grids of material, and optionally wherein the cylindrical grids comprise steel, aluminium or plastic.

26. A rock bolt comprising:

a rod;

a faceplate positioned on the rod;

a nut positioned on the rod; and

a deformable element according to any preceding clause, positioned on the rod between the faceplate and the nut.

27. A rock bolt according to clause 26, wherein the deformable element is configured to deform under loading more readily than the rod of the rock bolt.

28. A rock bolt according to clauses 26 or 27, comprising a plurality of deformable elements installed along the rod of the rock bolt.

29. A method of stabilising a rock mass using a rock bolt, the method comprising: installing a rock bolt in the rock mass, wherein the rock bolt comprises a deformable element which deforms under load, such that a first end of the deformable element is displaced towards a second end, wherein the magnitude of the displacement is an increasing function of the load.

30. A method according to clause 29, comprising using the deformable element according to any of clauses 1 to 25, or the rock bolt according to any of clauses 26 to 29.

31. A method according to clause 29 or 30, comprising monitoring the deformable elements to determine the degree of deformation, and optionally monitoring the degree of deformation visually, and/or using sensors. 32. A method according to any of clauses 29 to 31 , comprising installing additional rock bolts in the vicinity of a rock bolt exhibiting a degree of deformation that is above a predetermined threshold, and/or replacing a deformable element on a rock bolt if the existing rock bolt is close to failure.

33. A method according to any of clauses 29 to 32, comprising installing a plurality of deformable elements on a rock bolt.

34. A method according to any of clauses 29 to 33, comprising determining that the rock mass is stable if the deformation in a plurality of rock bolts has stopped, or the rate of deformation has reduced below a predetermined threshold.

35. A method according to any of clauses 29 to 34, comprising spraying the rock bolt with concrete to encapsulate the deformable element, once the rock mass has been determined as stable.

Claims

1. A deformable element for a rock bolt, wherein the deformable element comprises a sleeve and a plug, wherein the plug is configured to be received at least partially within the sleeve at a first end of the sleeve, and is configured to be held in frictional engagement with the sleeve, and is configured to be fixed in place relative to an end of the rock bolt by securing a fastener onto the end of the rock bolt against the plug to secure the plug between the fastener and the sleeve,

wherein the sleeve has an inner diameter which decreases from a maximum diameter at the first end of the sleeve to a minimum diameter at an opposed second end of the sleeve, the maximum diameter of the sleeve being the same as or larger than a maximum diameter of the plug, and the minimum diameter being smaller than the maximum diameter of the plug,

wherein the deformable element is configured such that, in use, when a load is applied to the deformable element, the plug is drawn through the sleeve towards the second end of the sleeve.

2. The deformable element according to claim 1 wherein the inner surface of the sleeve comprises the surfaces of a plurality of notional adjacent truncated conical sections, each having a different base angle, wherein a first truncated conical section that is closer to the first end of the sleeve than an adjoining truncated conical section has a smaller base angle than the adjoining truncated conical section.

3. The deformable element according to claim 2 wherein the inner surface comprises the surface of two stacked truncated conical sections, wherein the first truncated conical section has a base angle of between 80 and 85 degrees, optionally 83 degrees, and the adjoining truncated conical section has a base angle of between 86 and 89 degrees, optionally approximately 88 degrees.

4. The deformable element according to claim 2 or 3 wherein the inner surface comprises the surface of two stacked truncated conical sections, wherein the first conical section has a height of 10 to 30mm, optionally approximately 15mm, and the adjoining conical section has a height of 50 to 90mm, optionally approximately 70mm.

5. The deformable element according to any preceding claim, wherein the sleeve is shaped and sized to engage with a faceplate of the rock bolt towards the second end of the sleeve, optionally wherein the second end of the sleeve comprises a hemispherical portion for engaging with the faceplate of the rock bolt .

6. The deformable element according to any of claims 1 to 4 wherein the sleeve is shaped and sized to engage with a faceplate of the rock bolt towards the first end of the sleeve, optionally wherein the sleeve comprises a widest portion having a surface shaped as a partial sphere, for engaging with the faceplate of the rock bolt.

7. The deformable element according to any of claims 1 to 4 or 6, wherein an outer diameter of the first end is larger than a diameter of a hole in a faceplate for the rock bolt, and the outer diameter of a majority of the length of the sleeve is smaller than the diameter of the hole in the faceplate, such that the sleeve can pass partially through the hole in the faceplate.

8. The deformable element according to any preceding claim, wherein the second end of the sleeve is formed into a broadly hemispherical shape, the hemispherical-shaped second end having a through-hole therethrough sized to allow a rock bolt to pass through the hole.

9. The deformable element according to claim 1 wherein an inner diameter of the sleeve decreases linearly from the first end to the second end, such that the inside surface of the sleeve is a truncated conical surface.

10. A deformable element according to claim 9, wherein the inner diameter of the sleeve at the second end is 80% to 95% of the inner diameter at the first end.

11. A deformable element according to claims 1 , 9 or 10, comprising an annular disk having a broadly hemispherical shape, provided to abut the second end of the sleeve and a faceplate of the rod.

12. The deformable element according to any preceding claim, wherein the plug is a sphere, or is a spherical cap, or is a spherical segment.

13. The deformable element according to any of claims 1 to 11 , wherein the plug extends between a first plug end having a first outer diameter to a second plug end having a second outer diameter, and tapers outwardly from each of the first and second plug ends to a maximum diameter portion between the first and second plug ends.

14. A deformable element according to any preceding claim, wherein the deformable element is configured to deform such that the magnitude of the displacement is an increasing function of the load.

15. A deformable element according to any preceding claim, wherein the sleeve comprises a material with a minimum yield strength of between 250 N/mm² and 650 N/mm², and/or comprises a material with a Young’s modulus (E) of approximately 210,000 N/mm², and/or comprises a material with a Poisson’s ratio of 0.26 to 0.31.

16. A deformable element according to any preceding claim, wherein the deformable element has a deformation length of between 50 mm and 300 mm.

17. A deformable element according to any preceding claim, wherein the deformable element comprises an RFID tag.

18. A deformable element according to any preceding claim, wherein the deformable element comprises a deformation indicator configured to indicate whether the deformable element has been deformed.

19. A deformable element according to claim 18, wherein the deformation indicator comprises a flexible strip having approximately the same length as the deformation length of the deformable element, wherein the flexible strip is attached at one end to the plug and is received within a cover portion, and the cover portion does not cover the entirety of the strip, leaving an exposed position where the strip can be seen, and wherein the strip is colour-coded such that as the plug is pulled through the sleeve, the strip is pulled out of the cover portion and different colours of the strip are visible at the exposed position.

20. A deformable element according to claim 18, comprising a length gauge deformation indicator, wherein the length gauge comprises: a scale marked on a rigid rod projecting from the top surface of the plug, or a scale marked on the exterior of the sleeve, optionally wherein the scale is colour coded.

21. A deformable element according to claim 18, wherein the deformation indicator comprises a sensor, and wherein optionally the sensor is a passive sensor.

22. A deformable element according to claim 18, wherein the deformation indicator comprises a displacement sensor comprising a switch and a switch monitor, or wherein the deformation indicator comprises a strain sensor.

23. A rock bolt comprising:

a rod;

a faceplate positioned on the rod;

a nut positioned on the rod; and

a deformable element according to any preceding claim, positioned on the rod.

24. A rock bolt according to claim 23, wherein the deformable element is configured to deform under loading more readily than the rod of the rock bolt.

25. A rock bolt according to claim 23 or 24 in combination with surface support on a rock face, optionally wherein the surface support comprises a reinforcement mesh and/or sprayed concrete.

26. A method of stabilising a rock mass using a rock bolt, the method comprising: installing a rock bolt according to any of claims 23 to 25 in the rock mass.

27. A method according to claim 26, including installing the rock bolt in combination with surface support at a rock face of the rock mass, optionally wherein the surface support comprises a reinforcement mesh and/or sprayed concrete.

28. A method according to claim 26 or 27, comprising monitoring the

deformable elements to determine the degree of deformation, and optionally monitoring the degree of deformation visually, and/or using sensors.

29. A method according to claim 26, 27 or 28, comprising installing additional rock bolts in the vicinity of a rock bolt exhibiting a degree of deformation that is above a predetermined threshold, and/or replacing a deformable element on a rock bolt if the existing rock bolt is close to failure.

30. A method according to any of claims 26 to 29, comprising determining that the rock mass is stable if the deformation in a plurality of rock bolts has stopped, or the rate of deformation has reduced below a predetermined threshold.

31. A method according to any of claims 26 to 30, comprising spraying the rock bolt with concrete to encapsulate the deformable element, once the rock mass has been determined as stable.