AU2016102140A4 - Rock bolt - Google Patents

Abstract

A rock bolt (100) for fully resin encapsulated installation in a bore hole (181) has an elongate bar (101) longitudinally extending between a bar leading end (102) and bar trailing end (103). One or more surface protrusions (105b) helically extend in a first direction about a first bar portion (109) located toward the bar leading end (102). One or more surface protrusions (105b) helically extend in the first direction about a second bar portion (119) toward the bar trailing end (103). The rock bolt (100) is configured to provide a debonded bar portion (129) between the first bar portion (109) and the second bar portion (119). The debonded bar portion (129) is debonded from resin (141) encapsulating the rock bolt (100) in use, at least upon application of a predetermined service load to the bar (101).

Description

ι 2016102140 20 Dec 2016

ROCK BOLT

Related Applications [0001] The present application is a divisional application of Australian Patent Application No. 2014203249, the entire contents of which are incorporated herein by cross reference.

Field [0002] The present invention relates to strata control in civil engineering 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 [0003] One known method of stabilising the wall or roof of an underground mine is to secure a rock bolt into a bore hole drilled in to the face of the rock to be stabilised. Rock bolts are formed with an elongate load bearing element, commonly formed of a rigid steel bar.

[0004] A known method of installing such 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 blind end 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.

[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. (11928523_l):dah 2 2016102140 20 Dec 2016 [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 for fully resin encapsulated installation in a bore hole, said rock bolt comprising: an elongate bar longitudinally extending between a bar leading end and a bar trailing end; one more surface protrusions helically extending in a first direction about a first portion of said bar located towards said bar leading end; and one or more surface protrusions helically extending in said first direction about a second portion of said bar located towards said bar trailing end; wherein said rock bolt is configured to provide a debonded length of said bar between said first end portion of said bar and said second end portion of said bar, said debonded length being debonded from resin encapsulating said rock bolt, in use, at least upon application of a predetermined service load to said bar.

[0008] In an embodiment, said debonded bar portion is provided with 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.

[0009] In an embodiment, said bar has a threaded trailing end bar portion located between said second bar portion and said bar trailing end said threaded trailing end portion having a thread extending in a second direction opposing said first direction, said rock bolt further comprising a threaded drive nut mounted on said threaded trailing end bar portion.

[0010] In a second aspect the present invention provides a rock bolt for fully resin encapsulated installation in a bore hole, said rock bolt comprising: an elongate bar longitudinally extending between a bar leading end and a bar trailing end; a first thread helically extending in a first direction about a first portion of said bar located towards said bar leading end; and a second thread helically extending in said first direction about a second portion of said bar located towards said bar trailing end; wherein said rock bolt is configured to provide a debonded length of said bar between said first end portion of said bar and said second end portion of said bar, said debonded length being debonded from resin encapsulating said rock bolt, in use, at least upon application of a predetermined service load to said bar. (11928523_l):dah 3 2016102140 20 Dec 2016 [0011] In an embodiment said bar has a threaded trailing end bar portion located between said second bar portion and said bar trailing end said threaded trailing end portion having a thread extending in a second direction opposing said first direction, said rock bolt further comprising a threaded drive nut mounted on said threaded trailing end bar portion.

Brief Description of Drawings [0012] Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings wherein: [0013] Figure 1 is a fragmentary front elevation view of a rock bolt according to a first embodiment; [0014] Figure 2 is a partially cross-sectioned front elevation view of a partially completed rock bolt installation utilizing the rock bolt of Figure 1; [0015] Figure 3 is a partially cross-sectioned front elevation view of a completed rock bolt installation utilizing the rock bolt of Figure 1; [0016] Figure 4 is a front elevation view of a rock bolt according to a second embodiment; [0017] Figure 5 is a fragmentary front elevation view of a rock bolt according to a third embodiment; [0018] Figure 6 is a front elevation view of a rock bolt according to a fourth embodiment; and [0019] Figure 7 is a side elevation view of the rock bolt of Figure 6.

Description of Embodiments [0020] Referring to Figure 1 of the accompanying drawings, a rock bolt 100 has an elongate load bearing element in the form of a rigid bar 101 which longitudinally extends between a bar leading end 102 and a bar trailing end 103. The bar 101 will typically be formed of steel.

[0021] The bar 101 typically has a nominal diameter of between 12 and 30 mm, with common nominal diameters being 16, 20, 24 and 27 mm to suit a 35 mm diameter borehole. The bar 101 (11928523_l):dah 4 2016102140 20 Dec 2016 will typically have a length of the order of 1.8 to 2.4 m, but this may vary to suit the particular application.

[0022] The bar 101 has a first bar portion 109 located toward the bar leading end 102, a second bar portion 119 located toward, and spaced from, the bar trailing end 103 and a threaded trailing end portion 104 extending from the bar trailing end 103 to the second bar portion 119. A debonded bar portion 129 is located between the first bar portion 109 and the second bar portion 119. The first and second bar portions 109, 119 are formed with surface protrusions 105. In the configuration depicted, the surface protrusions 105 comprise a longitudinally extending rib 105a extending longitudinally along each of the first and second bar portions 109, 119 and a plurality of helically extending ribs 105b each extending helically about one of the first and second bar portions 109, 119. Each of the helically extending ribs 105b extends in a direction opposing the intended direction rotation of the rock bolt 100 during installation, being the direction of the thread of the threaded trailing end portion 104. In the configuration depicted, the thread of the threaded trailing end portion 104 is a left-handed thread rolled into the bar 201 to suit a standard left-hand installation ring and each of the helically extending ribs 105b helically extends in a right-handed direction. It is equally envisaged, however, that the thread of the threaded trailing end portion 104 could be a right-handed thread and the helically extending ribs 105b could extend in a left-handed direction. The surface protrusions 105 formed in the first and second bar portions 109, 119 are formed by a cold working process, particularly cold rolling. The cold working process hardens and increases the yield strength of the first and second bar portions 109, 119, whilst leaving the debonded bar portion 129 in its original softer and ductile form. As a result, upon loading of the rock bolt 100 beyond the yield strength of the debonded bar portion 129, the debonded bar portion 129 will yield without yielding of the first and second bar portions 109, 119 allowing the first and second bar portions 109, 119. The surface protrusions 205 also assist load transfer with the encapsulating resin and further assist in mixing of the resin during installation, as will be further discussed below.

[0023] In the arrangement depicted, an end fitting in the form of a hexagonal threaded drive nut 106 and dome washer 107 is mounted on the threaded trailing end portion 104, with the drive nut 106 threadingly engaging the thread of the threaded trailing end portion 104. A standard plate washer 108 is mounted on the rock bolt 100, engaging the dome washer 107 in a known manner. (11928523_l):dah 5 2016102140 20 Dec 2016 [0024] The drive nut 106 will typically be fixed in relation to the bar 101 by way of a locking means that is configured to fail upon application of a predetermined torque during the installation process. This allows the drive nut 106 to advance along the threaded trailing end portion 104 of the bar 101 upon application of torque exceeding the predetermined torque during post-tensioning of the rock bolt 100, as will be discussed further below. In the configuration depicted, the locking means is in the form of a shear pin 106a that locks the drive nut 106 to the bar 101. The locking means could alternatively be in the form of a slightly deformed thread of the threaded trailing end portion 104 of the bar 101. In another alternate form, the locking means could be in the form of a disc mounted in the trailing end of the drive nut 106 and configured to prevent the bar 101 from passing through the trailing end of the drive nut 106 until application of a torque exceeding the predetermined torque, at which the disc ruptures. The locking means may also take any other suitable form.

[0025] In the first embodiment, a first wire 110 helically extends about the first bar portion 109 of the bar 101. In the arrangement depicted, the first wire 110 extends to the bar leading end 102. The wire 110 extends from a first wire leading end 111 to a first wire trailing end 112. The first wire 110 is typically formed of steel and, in the arrangement depicted, has a diameter of about 3.15 mm although wire diameters of up to about 6.3 mm or greater may also be suitable for some applications. The first wire 110 helically extends in the same direction as the helically extending ribs 105b, being the direction opposing the intended direction of rotation of the rock bolt 100 during installation. Accordingly, for a standard left-handed installation rig, the first wire 110 will helically extend in a right-handed direction as depicted in the accompanying drawings, and in an opposing direction to the left-handed thread of the threaded trailing end portion 104 of the bar 101. In one example, the first wire 110 extends over a length of about 500 mm. The first wire 110 will typically extend over a length of between 300 mm and 1000 mm and between 3 and 20 revolutions.

[0026] In the arrangement depicted, the first wire 110 is fixed to the first bar portion 109 by welding, here being welded towards the first wire leading and trailing ends 111, 112 and partway along the length of the first wire 110. Alternatively, the first wire leading end 111 may be secured in a slot formed in the bar leading end 102. In a still further alternative, particularly suitable for applications with high tensile steel bars which might otherwise be weakened through inadvertent heat treatment of the bar during a welding process, the first wire trailing end 112 may (11928523_l):dah 6 2016102140 20 Dec 2016 be fixed to the bar 101 by way of a ferrule that is crimped onto the bar 101. A person skilled in the art will appreciate other suitable means for securing the first wire 110 to the bar 101.

[0027] With the rock bolt 100 being intended for full resin encapsulation along its length, the first wire 110 and helically extending ribs 105b serve three basic purposes, being to mix the two-component resin during the installation process, to pump the resin towards the blind end of the bore hole during installation and to assist in anchoring the first bar portion 109 within the resin upon the application of tensile load to the rock bolt 100 in service.

[0028] In one example, the first wire 110 has a helical pitch of about 75mm. The helical pitch of the first wire 110 will typically be between 50 mm and 100 mm.

[0029] In the first embodiment, the second wire 120 helically extends about the second bar portion 119 of the bar 101. The second wire 120 extends from a second wire leading end 121 to a second wire trailing end 122. The second wire 120 is typically formed of steel and, in the arrangement depicted, has a diameter of about 3.15 mm although wire diameters of up to about 6.3 mm or greater may also be suitable for some applications. The second wire 120 will typically have the same diameter as the first wire 110. The second wire 120 typically helically extends in the same direction as that of the first wire 110 and the helically extending ribs 105b, being in the direction opposing the intended direction of rotation of the rock bolt 100 during installation. Accordingly, for a standard left-handed installation rig, the second wire 120 will helically extend in a right-handed direction as depicted in the accompanying drawings. The second wire 120 will typically extend over a length of between 100 mm and 500 mm, although it is envisaged that, when the second wire is employed merely to act as a resin dam, that it may extend over a shorter length, such as about 50 mm. For long rock bolts, where it is desired to lock the second bar portion 119 over an extended length, that the second wire 120 may have lengths of up to about 800 mm. The second wire 120 will typically extend over at least 3 revolutions.

[0030] In the arrangement depicted, the second wire 120 is fixed to the second bar portion 119 by welding, here being welded towards the second wire leading and trailing ends 121, 122 and partway along the length of the second wire 120. In an alternative, particularly suitable for applications with high tensile steel bars which might otherwise be weakened through inadvertent heat treatment of the bar during a welding process, the second wire leading and trailing end 121, (11928523_l):dah 7 2016102140 20 Dec 2016 122 may be fixed to the bar 101 by way of ferrules crimped onto the bar 101. A person skilled in the art will appreciate other suitable means for securing the second wire 120 to the bar 101.

[0031] With the rock bolt 100 being intended for full resin encapsulation along its length, the second wire 120 serves the same three basic purposes as the first wire 110 and helically extending ribs 105b, being to mix the two-component resin during the installation process, to pump the resin towards the blind end of the bore hole during installation and to assist in anchoring the second bar portion 119 within the resin upon the application of tensile load to the rock bolt 100 in service.

[0032] The second wire 120 may have a helical pitch approximately the same as that of the first wire 110 or alternatively, and more preferably, have a helical pitch less than that of the first wire 110. The helical pitch of the second wire 120 will typically be between 10 mm and 75 mm, and more typically between 25 mm and 50 mm.

[0033] As will be further described below, the debonding bar portion 129 is debonded from resin encapsulating the rock bolt 100 in use. The debonded bar portion 129 will typically 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. The debonded bar portion 129 has a smooth outer surface that results in debonding of the debonded bar portion 129 from the resin encapsulating the rock bolt 100 in use, upon application of a predetermined service load to the bar 101 which exceeds the shear bond strength between the resin and the smooth outer surface of the debonded bar portion 129. Accordingly, whilst when initially installed, the debonded bar portion 129 may be relatively lightly bonded to the encapsulating resin, the resin readily debonds from the debonded bar portion 129 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 debonded bar portion 129. It is also envisaged that a surface coating, such as a low-friction paint surface coating, may be applied to the debonded bar portion 129 to further promote debonding of the debonded bar portion 129. Suitable surface coatings include low-friction, lubricating paints such as polyolefin based paints. It is also envisaged that the debonded bar portion may be provided by locating a debonding sleeve on a central portion of a bar with surface portions extending along its length, the debonding sleeve debonding the underlying portion of the bar from the encapsulating resin. (11928523_l):dah 8 2016102140 20 Dec 2016 [0034] Installation of the rock bolt 100 will now be described with reference to Figures 2 and 3. A bore hole 181 is first drilled into the rock face 180. A two-component resin filled cartridge 140 is then inserted into the bore hole 181. The rock bolt 100, with the plate washer 108 mounted on the bar 101 in front of the dome washer 107, is then inserted into the bore hole 181 with the bar leading end 102 leading. The rock bolt 100 is thrust towards the blind end 182 of the bore hole 181, utilizing a standard rock bolt installation rig, thereby pushing the resin filled cartridge 140 against the bore hole blind end 182 and rupturing the frangible casing of the cartridge 140.

[0035] Whilst the installation rig thrusts the rock bolt 100 towards the bore hole blind end 182, it also rotates the drive nut 106, here in an anticlockwise direction, thereby rotating the entire rock bolt 100 in a direction opposing the direction of helical winding of the first and second wires 110, 120 and helically extending ribs 105b. As the rock bolt 100 is rotated and advanced the first bar portion 109, and particularly the first wire 110 and the helically extending ribs 105b of the first bar portion 109, actively mix the two components of the resin. As the resin 141 flows down over the debonded bar portion 129 and about the second bar portion 119, the second wire 120 and helically extending ribs 105b of the second par portion 119 will further mix the two components of the resin 141. With the first and second wires 110, 120 and helically extending ribs 105b helically extending in an opposing direction to the direction of rotation of the rock bolt 100, the resin 141 is actively pumped towards the bore hole blind end 182. In arrangements where the second wire 120 has a shorter helical pitch than the first wire 110, the shorter pitch will provide an enhanced pumping effect to assist in better consolidating the resin. Thrusting and rotation of the rock bolt 100 is then ceased, allowing the resin 141 to cure for a few seconds.

[0036] After the resin 141 has cured, the drive nut 106 is driven by the installation rig at an increased torque that is equal to or in excess of the predetermined torque at which the shear pin 106a fails. The torque applied to the drive nut 106 then threads the drive nut 106 along the threaded trailing end portion 104. Continued driving of the drive head 106 bears the drive nut 106 against the dome washer 107, which in turn bears against the plate washer 108 and in turn the rock face 181, thereby resulting in tensioning of the bar 101. In a particularly preferred installation method, the two-component resin filled cartridge 140 contains two separate resins, with a relatively fast setting resin at the top of the cartridge 140 used to point anchor the first bar portion 109, and a slower setting resin at the bottom of the cartridge used to encapsulate the second bar portion 119. In this configuration, the rock bolt 100 will be pretensioned once the (11928523_l):dah 9 2016102140 20 Dec 2016 first, faster setting resin has cured, anchoring the first bar portion 109, but prior to the second, slower setting resin curing. This enables elongation of the second bar portion 119 and debonded bar portion 129 within the second, slower setting resin during the pretensioning process. The separate fast and slow setting resins could alternatively be housed in separate cartridges.

[0037] In the completed installation, depicted in Figure 3, the first and second bar portions 109, 119 are firmly anchored within the resin 141, whilst the debonded bar portion 129 is debonded from, or only lightly bonded to, the resin 141 by virtue of the smooth outer surface of the debonded portion 129. As a result, in the event of any movement or fracture of unstable rock the debonded bar portion 129 is able to yield over its full length, effectively “slipping” within the encapsulating resin 141, allowing the bar 101 to absorb movement of the rock over the debonded bar portion 129. 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. Further, with the first and second bar portions 109, 119 being hardened through cold working formation of the ribbed protrusions 205, the debonded bar portion 129 is able to yield without yielding of the first and second bar portions 109, 119, allowing the first and second bar portions 109, 119 to remain firmly anchored within the resin.

[0038] A rock bolt 200 according to a second embodiment is depicted in Figure 4. The rock bolt 200 is of an identical configuration to the rock bolt 100 of the first embodiment, except that it omits the first and second wires 110, 120. In this configuration, the helically extending ribs 105b perform the function of mixing of the resin 141 and enhancing load transfer between the first and second bar portions 109, 119 of the bar 101 and the resin 141 without assistance from the first and second wires. Omitting the wires enables the provision of a smaller resin annulus between the bar 101 and borehole wall, allowing for increased bar diameters for a given diameter borehole. For example, for a 35 mm borehole, rigid bar diameters of 27 mm or even 30 mm would be suitable. The mechanical properties of such a larger rigid bar 101 will increase if desired, depending upon the selected steel grade. Load transfer between the rigid bar 101 and borehole wall will also typically improve with a smaller resin filled annulus. Such applications may be particularly suitable for hard rock applications. The rock bolt 200 is in the same manner as the rock bolt 160 of the first embodiment described above. (11928523_l):dah 10 2016102140 20 Dec 2016 [0039] A rock bolt 300 according to a third embodiment is depicted in Figure 5. The rock bolt 300 is identical to the rock bolt 200 of the second embodiment, except that the surface protrusions 105 of the rock bolt 200 of the second embodiment are replaced with helically extending first and second threads 305a, 305b rolled into the first and second bar portions 309, 319 of the bar 301 respectively. The first and second threads 305a, 305b extend in an opposing direction to the threads of the threaded trailing portion 304 of the bar 301. Accordingly, for a standard left-handed installation rig, with a left-handed thread on the threaded trailing end portion 304, the first and second threads 305a, 305b on the first and second bar portions 309, 319 are right-handed threads. The rock bolt 300 is installed in the same manner as the rock bolt 100 of the first embodiment described above and the threads 305 act to mix the resin and enhance load transfer between the rigid bar 301 and borehole wall in the same general manner as the helically extending ribs 105b of the rock bolts 100, 200 of the first and second embodiments.

[0040] A rock bolt 400 according to a fourth embodiment is depicted in Figures 6 and 7. The rock bolt 400 is identical to the rock bolt 300 of the third embodiment, apart from the specific configuration of the first and second bar portions 409, 419 of the bar 401. The first bar portion 409 has a leading section with a flattened region 430 formed in the bar 401. The flattened region 430 may be formed by locally forging the bar 401 to form a flattened “paddle” which assists in mixing of the resin and in anchoring the first bar portion 409 within the resin once secured. A trailing section of the first bar portion 409 is provided with a first thread 405a of the same form as the first thread 305a of the rock bolt 300 of the third embodiment discussed above. The second bar portion 419 has a leading section provided with a second thread 405b of the same form as the second thread 305b of the rock bolt 300 of the third embodiment. The first and second threads 405a, 405b are right-handed threads, configured for use with a standard left-handed installation rig with a left-handed thread on the threaded trailing portion 404, as with the third embodiment. A trailing section of the second bar portion 419, located between the second thread 405b and the threaded trailing portion 404, is provided with a flattened region 431, identical to the flattened region 430, again typically formed by locally forging the bar to form a flattened “paddle”.

[0041] The rock bolts 200, 300, 400 of the second to fourth embodiments described above each have a debonded bar portion 129 identical to that of the rock bolt 100 of the first embodiment.

As with the first embodiment, the debonded bar portion 129 may thus be in the form of a smooth (11928523_l):dah 11 2016102140 20 Dec 2016 outer surface. Alternatively, the debonded bar portion may be provided by locating a debonding sleeve on a central portion of the bar.

[0042] A person skilled in the art will appreciate other possible modifications and variations of the rock bolts described. (11928523_l):dah

Claims (5)

1. A rock bolt for fully resin encapsulated installation in a bore hole, said rock bolt comprising: an elongate bar longitudinally extending between a bar leading end and a bar trailing end one more surface protrusions helically extending in a first direction about a first portion of said bar located towards said bar leading end; and one or more surface protrusions helically extending in said first direction about a second portion of said bar located towards said bar trailing end; wherein said rock bolt is configured to provide a debonded length of said bar between said first end portion of said bar and said second end portion of said bar, said debonded length being debonded from resin encapsulating said rock bolt, in use, at least upon application of a predetermined service load to said bar.

2. The rock bolt of claim 1, wherein said debonded bar portion is provided with 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.

3. The rock bolt of either one of claims 1 and 2, wherein said bar has a threaded trailing end bar portion located between said second bar portion and said bar trailing end said threaded trailing end portion having a thread extending in a second direction opposing said first direction, said rock bolt further comprising a threaded drive nut mounted on said threaded trailing end bar portion.

4. A rock bolt for fully resin encapsulated installation in a bore hole, said rock bolt comprising: an elongate bar longitudinally extending between a bar leading end and a bar trailing end a first thread helically extending in a first direction about a first portion of said bar located towards said bar leading end; and a second thread helically extending in said first direction about a second portion of said bar located towards said bar trailing end; wherein said rock bolt is configured to provide a debonded length of said bar between said first end portion of said bar and said second end portion of said bar, said debonded length being debonded from resin encapsulating said rock bolt, in use, at least upon application of a predetermined service load to said bar.

5. The rock bolt of claim 4, wherein said bar has a threaded trailing end bar portion located between said second bar portion and said bar trailing end said threaded trailing end portion having a thread extending in a second direction opposing said first direction, said rock bolt further comprising a threaded drive nut mounted on said threaded trailing end bar portion.