CA3090707A1

CA3090707A1 - Rock bolt

Info

Publication number: CA3090707A1
Application number: CA3090707A
Authority: CA
Inventors: David William Evans
Original assignee: Dywidag Systems International Pty Ltd
Current assignee: DSI Underground Australia Pty Ltd
Priority date: 2018-02-08
Filing date: 2019-02-08
Publication date: 2019-08-15
Also published as: AU2019216876A1; WO2019153045A1

Abstract

Described herein is a a rock bolt (100) comprising: a bar (110) longitudinally extending between a bar leading end (111) and a bar trailing end (112), said bar comprising: a bar leading portion (113) longitudinally extending from said bar leading end (111); a first anchor portion (150) trailing said bar leading portion (113), said first anchor portion (150) comprising at least one deformation (151,152) integrally formed in said bar (110); and a first debonding portion (170) trailing said first anchor portion (150), said first debonding portion (170) being configured to be debonded from resin encapsulating said rock bolt (100), in use, at least upon application of a predetermined service load to said bar (110); said rock bolt (100) further comprising a wire (130) secured to and helically extending along said bar leading (113) portion.

Description

ROCK BOLT
Field of the Invention [0001] The present invention relates to the field of strata control in civil and mining operations and in particular relates to a rock bolt for securing the roof or wall of a mine, tunnel or other ground excavation.
Background of the Invention

[0002] One known method of stabilizing the roof or wall of mines, tunnels or other ground excavations is to secure a rock bolt into a bore hole drilled into the face of the rock to be stabilized. Rock bolts are typically formed of a rigid steel bar with a thread formed in either the trailing portion of the bar or along the full length of the bar.

[0003] A known method of installing a rock bolt involves first drilling a bore hole into the rock face. A sausage-like two-component resin filled cartridge is then inserted into the bore hole, followed by the rock bolt which pushes the resin filled cartridge towards the top of the bore hole. The rock bolt is then rotated by the installation rig which also thrusts the rock bolt upwardly whilst it is rotating to mix the resin components and shred the frangible cartridge casing, pushing it towards the blind end of the bore hole. Various means have previously been proposed to be formed with, or mounted on, the leading end portion of the rigid bar to assist in mixing of the resin components and/or assisting in anchoring the rock bolt within the resin.
Rotation of the rock bolt is then stopped for a few seconds to allow the resin to cure.

[0004] Incomplete resin mixing and the presence of air voids and pockets of unmixed or "wet"
resin can reduce the load transfer performance of an installed rock bolt. This concern is further exacerbated in dynamic ground conditions.

[0005] In one form of installation, the resin only encapsulates the leading end portion of the rigid bar, thereby forming a point anchor. In such an installation the rock bolt may be tensioned by way of a drive nut mounted on a threaded trailing end portion of the bar and which bears against a plate washer that engages the rock face adjacent the bore hole opening. For additional load transfer and corrosion protection, it is known to post-grout the annular cavity about the rigid bar extending between the resin encapsulated leading end portion and the bore hole opening.

[0006] According to another previously proposed installation configuration, the entire length of the rigid bar within the bore hole may be resin encapsulated.
Summary of Invention

[0007] In a first aspect, the present invention provides a rock bolt comprising:
a bar longitudinally extending between a bar leading end and a bar trailing end, said bar comprising:
a bar leading portion longitudinally extending from said bar leading end;
a first anchor portion trailing said bar leading portion, said first anchor portion comprising at least one deformation integrally formed in said bar; and a first debonding portion trailing said first anchor portion, said first debonding portion being configured to be debonded from resin encapsulating said rock bolt, in use, at least upon application of a predetermined service load to said bar;
said rock bolt further comprising a wire secured to and helically extending along said bar leading portion.

[0008] In one or more embodiments, said bar further comprises:
a second anchor portion trailing said first debonding portion, said second anchor portion comprising at least one deformation integrally formed in said bar; and a second debonding portion trailing said second anchor portion, said second debonding portion being configured to be debonded from resin encapsulating said rock bolt, in use, at least upon application of the predetermined service load to said bar.

[0009] In at least one embodiment, said bar further comprises:
a third anchor portion trailing said second debonding portion, said third anchor portion comprising at least one deformation integrally formed in said bar.

[0010] Typically, said bar further comprises a threaded bar trailing portion longitudinally extending from said bar trailing end.

[0011] In a preferred embodiment, each said debonding portion has a smooth outer surface configured to debond from resin encapsulating said rock bolt, in use, upon application of the predetermined service load to said bar.

[0012] In one or more preferred embodiments, each said anchor portion comprises a flattened paddle-like deformation formed in said bar.

[0013] In a preferred form, each said anchor portion comprises a pair of said paddle-like deformations, said paddle-like deformations of each said pair being arranged mutually adjacent and oriented at least substantially mutually perpendicular.

[0014] In at least one embodiment, said rock bolt further comprises a cap mounted on said leading bar portion and extending longitudinally beyond said bar leading end, said cap having a trailing sleeve portion extending partially over said leading bar portion and a leading nose portion tapering from said sleeve portion to a leading end of said cap.

[0015] Typically, said wire is located adjacent said cap.

[0016] In a preferred embodiment, the length of the bar leading portion is less than 500mm. For example, the length of the bar leading portion is between about 300mm to about 200mm, or between about 300mm to about 150mm.

[0017] Typically, the first anchor portion is substantially adjacent to a trailing end of the wire.

[0018] In a second aspect the present invention provides a method of installing the rock bolt define above, said method comprising:
drilling a bore hole with a blind end into a rock face to be stabilized;
inserting a two-component resin filled cartridge having a frangible casing into said bore hole;
inserting said rock bolt into said bore hole with said bar leading end leading;
thrusting said rock bolt towards said blind end whilst rotating said rock bolt;
puncturing said frangible casing;
mixing said resin with said wire and allowing mixed resin to flow along said bar toward said bar trailing end; and stopping thrusting and rotation of said rock bolt, allowing said resin to cure.

[0019] Typically, said method further comprises pre-tensioning said bar.
Brief Description of Drawings

[0020] Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings wherein:

[0021] Figure 1 is a fragmentary, isometric view of a rock bolt according to a first embodiment;

[0022] Figure 2 is a fragmentary, front elevation view of the rock bolt of Figure 1 with end fittings;

[0023] Figure 3 is a partially cross-sectioned, front elevation view of a partially completed rock bolt installation utilizing the rock bolt of Figure 1;

[0024] Figure 4 is a partially cross-sectioned, front elevation view of a completed rock bolt installation utilizing the rock bolt of Figure 1;

[0025] Figure 5 is a fragmentary, isometric view of a rock bolt according to a second embodiment;

[0026] Figure 6 is a fragmentary, front elevation view of the rock bolt of Figure 5 with end fittings.
Description of Embodiments

[0027] Referring to Figures 1 and 2 of the accompanying drawings, a rock bolt 100 according to a first embodiment comprises a longitudinally extending bar 110 and a cap 120 and a wire 130 each mounted on the bar 110. The bar 110 is here of conventional form and is typically formed of structural steel. The bar 110 longitudinally extends between a bar leading end 111 and a bar trailing end 112. The bar 100 has a bar leading portion 113 terminating in the bar leading end 111 and a bar trailing portion 114 terminating in the bar trailing end 112.

[0028] In typical embodiments, the bar trailing portion 114 is threaded for receipt of a drive nut 140 (shown in Figure 2) provided with a corresponding thread for tensioning of the rock bolt 100, as will be discussed below. The bar 110 may have any of various diameters selected to suit the particular bore hole in which it is to be installed, and may typically have a diameter of between 12 mm and 30 mm, with common diameters being 16, 20, 22 and 24 mm. The bar 110 will typically have a length of 1.5 to 3 metres, but again the bar 110 may have any length to suit the particular bore hole.

[0029] The bar 110 has first and second anchor portions 150, 160 formed along its length. The first anchor portion 150 trails the bar leading portion 113 and, accordingly, trails the wire 130, although some overlap between the trailing end of the wire and the first anchor portion is permissible. The first anchor portion 150 is typically arranged adjacent the trailing end of the wire 130. The second anchor portion 160 trails the first anchor portion 150 and is longitudinally spaced therefrom. Each of the anchor portions 150, 160 comprises at least one deformation integrally formed in the bar 110. In the arrangement depicted, each of the anchor portions 150, 160 comprises a pair of flattened paddle-like deformations 151, 152. Each of the paddle-like deformations 151, 152 is formed by locally forging the bar 110 to form a flattened 'paddle'.
Preferably, the paddle-like deformations are cold forged to facilitate work hardening. Each flattened paddle-like deformation 151, 152 has a reduced thickness as compared to the undeformed diameter of the bar 110 and an increased width as compared to the undeformed diameter of the bar 110. The first and second paddle-like deformations 151, 152 constituting each pair of paddle-like deformation are arranged mutually adjacent and oriented at least substantially mutually perpendicular. That is, the second paddle-like deformation 152 is flattened in a direction substantially perpendicular to the direction of flattening of the first paddle-like deformation. The broadened edges of the first paddle-like deformation 151 thus project laterally in one plane and the broadened edges of the second paddle-like deformation 152 project laterally in a substantially perpendicular plane. For a 22mm diameter bar, each of the paddle-like deformations typically has a length of the order of 60 mm, a width of the order of 30 mm and a thickness (at the location of minimum thickness) of the order of 12 mm.

[0030] The bar has first and second debonding portions 170, 180 formed along its length. The first debonding portion 170 trails the first anchor portion 150 and extends between the first anchor portion 150 and the second anchor portion 160. The second debonding portion 180 trails the second anchor portion 160 and extends between the second anchor portion 160 and the threaded bar trailing portion 114. Each of the debonding portions 170, 180 is configured to be debonded from resin encapsulating the rock bolt 100, in use, as will be further described below.

Each of the debonding portions 170, 180 may have a length of the order of 1000 mm, and will typically have lengths of between 700 mm and 1300 mm, depending upon the length of the rock bolt 100, although other arrangements are possible. Each of the debonding portions 170, 180 has a smooth outer surface that results in debonding of the debonding portion 170, 180 from the resin encapsulating the rock bolt 100 in use, upon application of a predetermined service load to the bar 110 which exceeds the shear bond strength between the resin and the smooth outer surface of the debonding portion 170, 180. Accordingly, whilst when initially installed the debonding portions 170, 180 may be relatively lightly bonded to the encapsulating resin, the resin readily debonds from the debonding portions 170, 180 upon the application of a predetermined service load. Such a load may result from movement or fracture of the rock that is resisted by yielding deformation of the debonding portions 170, 180. It is also envisaged that a surface coating, such as a low-friction paint surface coating, may be applied to the debonding portions 170, 180 to further promote debonding. Suitable surface coatings include low-friction, lubricating paints such as polyolefin based paints. It is also envisaged that the debonding portions 170, 180 may be provided by locating debonding sleeve on portions of the bar 110, the debonding sleeves each debonding the underlying portion of the bar from the encapsulating resin.

[0031] The wire 130, typically formed of steel, is secured to and helically extends along the bar leading portion 113, from adjacent the cap 120 towards the bar trailing end 112. The wire 130 here extends from a wire leading end 131, offset from the bar leading end 111 adjacent to and trailing the trailing end of the cap 120, to adjacent the first anchor portion 150. It is envisaged, however, that the wire leading end 131 may be located under the cap 120, between the trailing end of the cap 120 and the bar leading end 111. The wire 130 preferably helically extends in a direction opposing the intended direction of rotation of the bolt 100 during installation.
Accordingly, for installation of the rock bolt 100 with a standard left-handed installation rig, the wire 130 will typically helically extend in a right-handed direction as depicted. The wire 130 will typically extend over a length of the bar of the order of about 200 mm to about 300 mm.

[0032] The wire 130 may be secured to the bar 110 by tack welding, typically at the wire leading end 131 and wire trailing end 132. In some embodiments a further weld may be placed part way along the length of the wire 130. It is also envisaged, however, that the wire 130 may be secured to the bar 130 by use of one or more ferrules crimped onto the bar 110, with the wire 130 welded to the ferrules. This arrangement will avoid potential weakening of bar 110 formed of high tensile steel by annealing the steel as a result of heat generated through the welding process. If the wire 130 is secured to the bar 110 by welding, it is preferable that the welds are applied only to the bar leading portion 113. This is because the leading portion 113 of the bar does not typically have much geotechnical load carrying benefit such that the effective length essentially starts from the first anchoring portion 150 continuing toward the trailing end 112.
Generally, the welds are not applied to the first anchoring portion 150, or any other part of the bolt 100 intended to carry a substantial geotechnical load, so that the load bearing capability of the first anchoring portion 150, which may be formed of high tensile steel is not impacted by the welds. However, for ease of manufacture, it is permissible to place a tack weld on the trailing end of the wire 130 at an uppermost (leading) broadened edge of the first paddle-like deformation 151 of the first anchor portion 150.

[0033] The cap 120 is mounted on the bar leading portion 113 and extends beyond the bar leading end 111. The cap 120 has a trailing sleeve portion 121 that extends partially over the bar leading portion 113 and a leading nose portion 122 that tapers from the sleeve portion 121 to a leading end of the cap 120. The sleeve portion 121 will typically extend over a length of the bar leading portion 113 slightly less than the offset distance of the wire leading end 131 from the bar leading end 111, with the wire leading end 131 located adjacent to the sleeve portion 121, although it is envisaged that the wire leading end 131 may be located within the sleeve portion 121. In one example, the sleeve portion 121 extends over a length of about 50 mm of the bar leading portion 113. The sleeve portion 121 is typically of cylindrical form whilst the nose portion 122 is typically of a generally conical form defining a rounded cap leading end.
Alternatively, the nose portion 122 could be generally hemispherical or of an alternate tapered form.

[0034] The sleeve portion 121 may have a diameter equal to or greater than a maximum transverse dimension defined by the wire 130 helically extending along the bar 110. This maximum transverse dimension is generally defined by the diameter of the bar 110 plus twice the diameter of the wire 130. The thickness of the wall of the sleeve portion 121 will thus typically be equal to or greater than the diameter of the wire 130. The wire diameter may vary, and will typically be of the order of 3 to 6 mm. With typical wire diameters being 3.15 mm, 4.5 mm and 6.0 mm. Whilst the sleeve portion 121 may have a diameter equal to or exceeding the maximum transverse dimension defined by the wire 130, it is also envisaged that the diameter of the sleeve portion 121 may be less than the maximum transverse dimension defined by the wire 130, such that the thickness of the wall of the sleeve portion 121 is less than the diameter of the wire 130.

[0035] The sleeve portion 121 may have an interference fit with the bar leading portion 113 such that it may be mounted on the bar leading portion 113 by simply pushing the cap 120 onto the bar leading portion 113. Alternatively, the sleeve portion 121 may have a clearance fit on the bar 110 and be secured thereto by adhesive or other means. An adhesive may also be utilised in conjunction with other types of fit.

[0036] The cap 120 will typically be formed of plastics material, with polypropylene and nylon being particularly suitable materials. The cap 120 may also be formed with a high visibility colour, such as bright yellow or orange, for enhanced visibility of the leading end of the rock bolt 100 in underground applications. Embodiments are also envisaged in which the cap 120 is omitted.

[0037] As shown in Figure 2, a drive nut 140 is threaded onto the threaded bar trailing portion 114. The drive nut 140 is typically provided with a mechanism which fixes the drive nut 140 onto the threaded bar trailing portion 114 up to a predetermined torque at which the mechanism fails, enabling the drive nut 140 to be threadingly advanced along the threaded bar trailing portion 114. Such mechanism may take the form of a disc shaped insert (not depicted) located in the trailing end of the aperture extending through the drive nut 140. The insert engages the bar trailing end 112 as the drive nut 140 attempts to advance, locking the drive nut 140 onto the bar 110 until a predetermined torque is exceeded, at which the insert is ejected from the drive nut 140, allowing the drive nut 140 to advance along the threaded bar trailing portion 114. Such a drive nut 140 housing an insert is described in Australian Patent Publication No. AU
2007201848A1, the entire contents of which are incorporated herein by cross-reference. Other known mechanisms for selectively fixing the drive nut 140 to the bar 110 may, however, be utilised as desired, for example a shear pin. An anti-friction washer 141 and dome washer 142 are also mounted on the bar 110 in the usual manner.

[0038] Installation of the rock bolt 100 of the first embodiment will now be described with reference to Figures 3 and 4 of the accompanying drawings. The anti-friction washer 141, dome washer 142 and drive nut 140 are mounted on the threaded bar trailing portion 114, along with a standard plate washer 143. A bore hole 201 is drilled into the rock face 200 to be stabilised, optionally with one or more sheets of reinforcing mesh held against the rock face 201. A two-component resin filled cartridge 210 is then inserted into the bore hole 201.
The rock bolt 100, with the assembled anti-friction washer 141, dome washer 142, drive nut 140 and plate washer 143 is mounted on an installation rig and maneuvered toward the bore hole 201.
As the rock bolt 100 is guided toward the bore hole 201, the nose portion 121 of the cap 120 assists in ensuring the blunt bar leading end 111 and protruding wire leading end 131 do not catch on the reinforcing mesh or edge of the bore hole 201.

[0039] Once the cap 120 is guided into the bore hole 200, the rock bolt 100 is thrust deeper into the bore hole 201 using the installation rig, pushing the resin filled cartridge 210 toward the blind end of the bore hole 201. As the rock bolt 100 is thrust toward the blind end of the bore hole, the installation rig also rotates the drive head 140 of the rock bolt 100, here in an anti-clockwise direction. The insert (or other mechanism for selectively fixing the drive nut 140 to the bar 110) retains the drive nut 140 in a fixed relationship to the bar 110, such that rotation of the drive head 140 results in rotation of the bar 110 in unison with the drive nut 140. When the cartridge 210 advances to the blind end of the bore hole 201, the cap 120 ruptures the frangible casing of the cartridge 210, resulting in the resin flowing over the bar leading portion 113 in the narrow annulus defined between the bar 110 and the wall of the bore hole 201, where the two components of the resin are thoroughly mixed by the wire 130. The wire 130 also assists in shredding the casing of the cartridge 210. With the wire 130 helically extending in an opposing direction to the direction of rotation of the rock bolt 100, the resin is actively encouraged via an Archimedean pumping action towards the blind end of the bore hole 201, although sufficient resin will typically be utilized to substantially fill the annulus, thereby fully resin encapsulating the rock bolt 110. As the resin flows past the anchor portions 150, 160, the paddle-like deformations 151, 152 assists in further mixing of the resin, although they are typically less effective than the wire 130 in mixing the resin. The resin is then allowed to cure for a few seconds.

[0040] After the resin has cured, the drive nut 140 is driven by the installation rig at an increased torque that results in failure of the insert fixing the drive nut 140 onto the bar 110.
Further torque applied to the drive head 140 thus threads the drive nut 140 along the threaded bar trailing portion 114. Continued driving of the drive nut 140 bears the drive nut 140 against the anti-friction washer 141 which in turn bears the dome washer 142 and plate washer 143 against the rock face 200, thereby resulting in tensioning of the bar 110. In a particularly preferred installation method, the two-component resin filled cartridge 210 contains two separate resins, with a relatively fast setting resin at the top of the cartridge 210 used to point anchor the bar leading portion 113, and a slower setting resin at the bottom of the cartridge used to encapsulate the anchor portions 150, 160 and debonding portions 170, 180.
In this configuration, the rock bolt 100 will be pretensioned once the first, faster setting resin has cured, anchoring the bar leading portion 113, the first anchoring portion 150 and at least a portion of the first debonding portion 170, but prior to the second, slower setting resin curing. This enables elongation of the anchor portions 150, 160 and debonding portions 170, 180 within the second, slower setting resin during the pretensioning process. The separate fast and slow setting resins could alternatively be housed in separate cartridges.

[0041] In the completed installation, depicted in Figure 4, the first and second anchor portions 150, 160 are firmly anchored within the resin, whilst the first and second debonding portions 170, 180 are debonded from, or only lightly bonded to, the resin by virtue of the smooth outer surface of the debonding portions 170, 180. As a result, in the event of any movement or fracture of unstable rock the first and second debonding portions 170, 180 are able to yield over their full length, effectively "slipping" within the encapsulating resin, allowing the bar 110 to absorb movement of the rock over the debonding portion 170, 180. This can be contrasted against an installation where the bar remains bonded to the resin along its entire length such that the bar must endeavor to absorb any movement locally at the failure plane of the rock over a very short distance, often resulting in catastrophic failure of the bar. In the event of a significant shock load or slow yielding ground movement through the strata, the first and second anchor portions 150, 160 are able to be drawn through the encapsulating resin, absorbing further energy without catastrophic failure of the bar 110.

[0042] A rock bolt 300 according to a second embodiment is depicted in Figures 5 and 6 of the accompanying drawings. The rock bolt 300 is of an identical configuration to the rock bolt 100 of the first embodiment, apart from the provision of an additional anchoring portion 350 on the bar 310, trailing the second debonding portion 180 and extending between the second debonding portion 180 and the threaded bar trailing portion 114. The remaining features of the rock bolt 300 that are identical to the rock bolt 100 will thus not be further described and are provided with identical reference numerals in Figures 5 and 6. The rock bolt 300 is installed in the same manner as the rock bolt 100 of the first embodiment as described above, and behaves under load in substantially the same manner as the rock bolt 100 of the first embodiment, except that the additional anchor portion 350 provides additional anchoring of the rock bolt 300 within the encapsulating resin. It is also envisaged that further debonding portions and/or further anchor portions may be formed in conjunction with a longer length of the bar 110.
Embodiments are also envisaged incorporating a single anchor portion only trailed by a single debonding portion.

[0043] As discussed above, the paddle-like deformations 151, 152 provide a secondary mixing action to that of the wire 130. In isolation, the secondary mixing action of the paddle-like deformations 151,152 typically provides for a weak or less effective mixing action than the wire 130. However, it has been surprisingly found that the combination of the wire 130 and the paddle-like deformations 151,152 provides for good mixing results, so much so that the helical extension of the wire 130 can be reduced while still providing for sufficient mixing of the resin.
Being able to reduce the helical extension of the wire 130 along the bar leading portion 113 allows the length of the bar leading portion 133 to in turn be reduced.

[0044] For example, for a typical 35mm borehole, it would generally be thought that a bar leading portion 113 carrying the wire 130 extending substantially along this length would need to be about 500mm long to provide for sufficient mixing of the resin. However, with the wire 130 combining with the paddle like deformations 151,152, it has surprisingly been found that the bar leading portion 113 carrying the wire 130 can be substantially less than 500mm, with good mixing results observed for a bar leading portion 113 with 300mm length and 200mm length. It is envisaged that that a bar leading portion 113 of 150mm carrying the wire 130 that substantially extends this length may also provide for sufficient mixing results. It is further envisaged that similar results can be achieved for a larger borehole by increasing the gauge of the wire 130 beyond the typical 4.5mm size and/or increasing the diameter of the bar 110.

[0045] The above result appears contrary to the conventional approach that a relatively long leading portion 113 with the wire 130 extending therealong is required to achieve sufficient resin mixing, and that the paddle-like deformations 151,152 in isolation only provide for a weak or limited mixing effect. For example, a leading portion 113 length of at least about 500mm has typically been thought necessary to achieve thorough mixing for rock bolts 100 without any paddle-like deformations 151,152. It is thought that the combination of the paddle-like deformations 151,152 and the wire 130 may enhance the mixing contribution of the paddle-like deformations 151,152 so as to provide a synergistic mixing effect beyond what may be expected from the individual mixing effects of the wire and the paddles.

[0046] Without wishing to be bound by theory, it is thought that the combined interaction of the wire 130 and the paddle like deformations 151,152 may actually improve the mixing effect of the paddles by limiting the amount of shredded cartridge casing in the vicinity of the paddle-like deformations 151,152. As the bolt 100 is inserted into the borehole, the wire 130 ruptures and gathers the cartridge casing toward the blind end of the borehole. As the catalyst and resin mastic within the cartridge is pumped toward the blind end and then down the borehole annulus toward the trailing end 112 of the bolt 100, the catalyst and resin mastic are mixed as a predominantly fluid resin that passes the paddle-like deformations 151, 152 of the first anchor portion 150 with limited cartridge casing. Limiting the cartridge casing in the vicinity of the paddle like deformations 151, 152 may increase their mixing effectiveness by avoiding the casing wrapping around the paddles which may otherwise reduce mixing effectiveness.
Furthermore, increasing the mixing effectiveness of the paddle-like deformations 151,152 through combination with the wire 130 is thought to be additionally beneficial in that once the rock bolt is installed within the borehole and continues rotating for a short period, the enhanced operation of the paddle-like deformation 151,152 is thought to provide an enhanced radial mixing action in the vicinity of the anchoring portions. In this manner, mixing of resin in the vicinity of the anchoring portions may be improved, increasing the effectiveness of the rockbolt.

[0047] Furthermore, it was surprisingly found that a wire 130 helically extending about 200mm along a shortened bar leading portion 113 may actually provide for an enhanced mixing effect compared to a longer bar leading potion 113 with a wire 130 helically extending about 300mm along the length thereof. This result was further unexpected since the reduced length leading portion 113/wire 130 facilitated an increased insertion speed of the rock bolt 100 into the borehole, which would typically be expected to reduce mixing effectiveness.
The increased insertion speed is thought to come about due to a lower back pressure within the borehole being generated by the bolt 100 with the 200mm length leading portion/wire as opposed to a 300mm length leading portion/wire. The suprising and counter intuitive result the shorter leading portion/wire producing in a better mixing of the resin with a shorter installation time provides substantial benefits in operational efficiency.

[0048] The bar leading portion 113 carrying the wire 130 does not contribute substantially to the geotechnical load carrying benefit of the rock bolt. Instead, the effective geotechnical length of the rock bolt starts at the first anchor portion 150. Regardless of the exact mechanism resulting in the combined mixing effectiveness of the wire 130 and the paddle-like deformations 151,152, reducing the extension of the wire 130 due to the combined effect with the paddle-like deformations 151,152 allows the bar leading portion 113 length to be reduced with sufficient resin mixing being achieved, which can result in shorter rock bolts 100 and boreholes to produce the same or similar geotechnical load carrying benefit.

[0049] In an alternative embodiment to that described above, each or any of the anchor portions may have more than a pair of flattened paddle-like deformations. For example, embodiments with four or five paddle-like deformations comprising a first anchoring portion may serve to increase resin mixing and subsequent anchoring proximal to the leading end of the bolt.
Furthermore, although it is preferred for a pair of paddle-like deformations to be orientated mutually perpendicular, they are not limited to this arrangement. When an anchoring portion comprises more than a pair of paddle-like deformations, they may be orientated at various angles to one another.

[0050] A person skilled in the art will appreciate various other modifications and alterations of the rock bolt may be made to suit specific applications.

Claims

14

1. A rock bolt comprising:
a bar longitudinally extending between a bar leading end and a bar trailing end, said bar comprising:
a bar leading portion longitudinally extending from said bar leading end;
a first anchor portion trailing said bar leading portion, said first anchor portion comprising at least one deformation integrally formed in said bar; and a first debonding portion trailing said first anchor portion, said first debonding portion being configured to be debonded from resin encapsulating said rock bolt, in use, at least upon application of a predetermined service load to said bar;
said rock bolt further comprising a wire secured to and helically extending along said bar leading portion.

2. The rock bolt according to claim 1, wherein said bar further comprises:
a second anchor portion trailing said first debonding portion, said second anchor portion comprising at least one deformation integrally formed in said bar; and a second debonding portion trailing said second anchor portion, said second debonding portion being configured to be debonded from resin encapsulating said rock bolt, in use, at least upon application of the predetermined service load to said bar.

3. The rock bolt according to claim 2, wherein said bar further comprises:
a third anchor portion trailing said second debonding portion, said third anchor portion comprising at least one deformation integrally formed in said bar.

4. The rock bolt according to any one of the preceding claims, wherein said bar further comprises a threaded bar trailing portion longitudinally extending from said bar trailing end.

5. The rock bolt according to any one of the preceding claims, wherein each said debonding portion has a smooth outer surface configured to debond from resin encapsulating said rock bolt, in use, upon application of the predetermined service load to said bar.

6. The rock bolt according to any one of the preceding claims, wherein each said anchor portion comprises a flattened paddle-like deformation formed in said bar.

7. The rock bolt according to any one of the preceding claims, wherein each said anchor portion comprises a pair of said paddle-like deformations, said paddle-like deformations of each said pair being arranged mutually adjacent and oriented at least substantially mutually perpendicular.

8. The rock bolt according to any one of the preceding claims, wherein said rock bolt further comprises a cap mounted on said leading bar portion and extending longitudinally beyond said bar leading end, said cap having a trailing sleeve portion extending partially over said leading bar portion and a leading nose portion tapering from said sleeve portion to a leading end of said cap.

9. The rock bolt according to claim 8, wherein the wire is located adjacent to said cap.

10. The rock bolt according to any one of the preceding claims, wherein the length of the bar leading portion is less than 500mm.

11. The rock bolt according to claim 10, wherein the length of the bar leading portion is between about 300mm to about 200mm.

12. The rock bolt according to claim 11, wherein the length of the bar leading portion is between about 200mm to about 150mm.

13. The rock bolt according to any one of the preceding claims, wherein the first anchor portion is substantially adjacent to a trailing end of the wire.

14. A method of installing the rock bolt of any one of the preceding claims, said method comprising:
drilling a bore hole with a blind end into a rock face to be stabilized;
inserting a two-component resin filled cartridge having a frangible casing into said bore hole;
inserting said rock bolt into said bore hole with said bar leading end leading;
thrusting said rock bolt towards said blind end whilst rotating said rock bolt;
puncturing said frangible casing;
mixing said resin with said wire and allowing mixed resin to flow along said bar toward said bar trailing end; and stopping thrusting and rotation of said rock bolt, allowing said resin to cure.

15. The method according to claim 14, wherein the method further comprises pre-tensioning said bar.