AU2021106794A4

AU2021106794A4 - Rock bolt anchor

Info

Publication number: AU2021106794A4
Application number: AU2021106794A
Authority: AU
Inventors: Lee Mills
Original assignee: FCI Holdings Delaware Inc
Current assignee: FCI Holdings Delaware Inc
Priority date: 2021-08-24
Filing date: 2021-08-24
Publication date: 2021-11-18
Anticipated expiration: 2029-08-24

Abstract

A friction anchor (100) for a self-drilling rock bolt (10) is disclosed. Thefriction anchor (100) is mounted around the elongate rod (150) of the rock bolt adjacent the drill bit (152). The friction anchor (100) comprises generally circular tube defining a longitudinal split, the tube having a central body portion (116) having a generally constant diameter which is larger than the diameter of the elongate rod and is slightly larger than the diameter of a borehole drilled in rock by the drill bit, wherein the distal end of the friction anchor defines a tapered portion (118) and the other proximal end of the friction anchor defines a proximal end section (112) where the friction anchor has a smaller generally constant diameter having a smaller diameter than the central body portion and a typically tapered section (114) connecting the proximal end section and the central body portion in which the diameter of the friction anchor gradually increases. A driver (160) is mounted around the elongate rod (150) adjacent the proximal end of the friction anchor. A standard installation process for a self-drilling rock bolt can be used. During installation of the rock bolt in rock using rotation and percussion, the borehole (50) formed by rotation of the drill bit, is typically slightly larger than the central body portion of the friction anchor and the driver (160) transfers percussive force applied to the elongate rod to the friction anchor to force the friction anchor into the borehole. When drilling in complete the installation head can be remove and replaced with an injection head for injection of grout into the borehole via the rod (150), with the anchor retaining the rock bolt (10) in the borehole. 1/2 100 112 114 116 118 104 1Fig. 1 11214116 102 11806 112 /14 118 124 122 123 Fig. 2 10 160 100 152 150 Fig.3 Fig. 4

Description

1/2

100 112 114 116 118

104 1Fig. 1 11214116 102 11806 112 /14 118

124 122 123 Fig. 2

160 100 152

150 Fig.3

Fig. 4

"Rock bolt anchor"

Technical Field

[0001] This invention relates to a rock bolt anchor.

Background

[0002] There are various reinforcement techniques and elements currently in use for strata control in mining and also in civil applications. Self-drilling hollow rock bolts are now commonly used particularly in underground mining. These rock bolts are also known as SDA bolts.

[0003] The SDA bolt is installed using a sacrificial drill bit attached to the hollow rock bolt which drills its own borehole. The annular space between the bolt and the borehole is filled with mortar made out of cementitious grout or resin which, upon setting, ensures sufficient adherence to solidify the rod into the ground/strata. The bolt is then tensioned using a nut threaded onto the end of the bolt.

[0004] SDAs can be used in poor ground conditions where the self-drilling rock bolt acts as the drill steel and remains in the borehole after drilling. However, using this method, there are often difficulties in uncoupling the installation dolly used to install the rock bolt from the self-drilling rock bolt, if the rock bolt is not properly engaged with or locked into the borehole, which is a necessary step in order to pump the grout into the borehole. This involves a further step of connecting adaptors and setting up additional pumping equipment to inject the self-drilling rock bolt with resin or cement to lock it into the borehole after disconnection from the installation dolly. A secondary related issue arises where the boreholes are vertical or greater than horizontal, where the rock bolts have a tendency to drop out of the borehole when they are disconnected from the installation dolly, if they are not properly engaged with or locked into the borehole.

[0005] There are a number of, sometimes fairly complex expansion mechanisms which have been developed to address this issue, they typically involve several moving parts and are relatively complex and add to the manufacturing expense of the rock bolt.

[0006] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each of the appended claims.

Summary

[0007] In a first aspect of the invention there is provided a friction anchor for use with a self drilling rock bolt, the rock bolt comprising an elongate rod having a central bore and being externally threaded, the rod having a first or distal end and a second or proximal end and a drill bit mounted on the distal end for drilling a borehole in strata such as rock or the like, wherein

the friction anchor comprises a generally circular tube defining a longitudinal split, the tube having a central body portion having a generally constant diameter which is larger than the diameter of the elongate rod and is slightly larger than the diameter of borehole drilled in rock by the drill bit, wherein one distal end of the friction anchor, which in use locates adjacent the drill bit of the rock bolt, defines a tapered portion where the diameter of the friction anchor reduces from the central body portion to the distal end and the other proximal end defines a proximal end section where the friction anchor has a smaller generally constant diameter having a smaller diameter than the central body portion and a typically tapered section connecting the proximal end section and the central body portion in which the diameter of the friction anchor gradually increases.

[0008] Typically, the proximal end section at the proximal end includes a tapered cut out portion on the opposite side of the anchor to the longitudinal split.

[0009] Preferably, the tapered portion at the distal end includes a tapered cut out portion on the opposite side of the anchor to the longitudinal split.

[0010] Ina related aspect of the invention there is provided a rock bolt comprising:

an elongate rod having a central bore and being externally threaded, the rod having a first or distal end and a second or proximal end; a drill bit mounted on the distal end for drilling a borehole in strata such as rock or the like; a friction anchor mounted around the elongate rod typically adjacent the drill bit, the friction anchor comprising a generally circular tube defining a longitudinal split, the tube having a central body portion having a generally constant diameter which is larger than the diameter of the elongate rod and is slightly larger than the diameter of borehole drilled in rock by the drill bit, wherein one distal end of the friction anchor which in use locates adjacent the drill bit of the rock bolt, defines a tapered portion where the diameter of the friction anchor reduces from the central body portion to the distal end, and the other proximal end of the friction anchor defines a proximal end section where the friction anchor has a smaller generally constant diameter having a smaller diameter than the central body portion and a typically tapered section connecting the proximal end section and the central body portion in which the diameter of the friction anchor gradually increases; and a driver mounted around the elongate rod adjacent the proximal end of the friction anchor, the arrangement being such that during installation of the rock bolt in rock using rotation and percussion, the borehole formed by rotation of the drill bit, is typically slightly smaller than the central body portion of the friction anchor with the driver transferring percussive force applied to the elongate rod to the friction anchor to force the friction anchor into the borehole.

[0011] In one embodiment, the driver defines a central internally threaded bore and is threaded onto the rock bolt. Other fixing means are possible including crimping.

[0012] In an alternative embodiment, the driver is crimped onto the rock bolt.

[0013] Advantageously the method of installation is the same as that of a standard friction bolt and no additional steps are required during installation. After the borehole has been drilled to the desired depth, drilling is stopped. Radial pressure caused by the insertion of the friction anchor into the borehole, which is smaller than the outside diameter of the main section of the friction anchor compresses that section and causes radial pressure on the walls of the borehole creating frictional resistance to removal of the friction and rock bolt and retaining the rock bolt temporarily in the borehole pending grouting.

[0014] Hence a related aspect of the present invention provides a method of installing a rock bolt in rock strata or the like comprising the steps of

providing a rock bolt comprising:

an elongate rod having a central bore and being externally threaded, the rod having a first or distal end and a second or proximal end;

a drill bit mounted on the distal end for drilling a borehole in strata such as rock or the like;

a friction anchor mounted around the elongate rod typically adjacent the drill bit, the friction anchor comprising a generally circular tube defining a longitudinal split, the tube having a central body portion having a generally constant diameter which is larger than the diameter of the elongate rod and is slightly larger than the diameter of borehole drilled in rock by the drill bit, wherein one distal end of the friction anchor which in use locates adjacent the drill bit of the rock bolt, defines a tapered portion where the diameter of the friction anchor reduces from the central body portion to the distal end, and the other proximal end of the friction anchor defines a proximal end section where the friction anchor has a smaller generally constant diameter having a smaller diameter than the central body portion and a typically tapered section connecting the proximal end section and the central body portion in which the diameter of the friction anchor gradually increases; and

a driver mounted around the elongate rod adjacent the proximal end of the friction anchor, wherein the method comprises:

applying rotation and percussion to the rod and drill bit using an installation dolly to drill a borehole into the rock strata, the borehole formed by rotation of the drill bit, being smaller than the central body portion of the friction anchor with the driver transferring percussive force applied to the elongate rod to the friction anchor to force the friction anchor into the borehole compressing the central body portion;

when the borehole is drilled to a desired depth, disconnecting the installation dolly; and subsequently encasing the rock bolt in resin, cement or the like by pumping liquid resin/cement through the central bore of the rock bolt.

[0015] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Brief Description of Drawings

[0016] A specific embodiment of the present invention will now be described byway of example only and with reference to the accompanying drawings in which:

Figure 1 is a front view of an embodiment of a friction anchor for a rock bolt;

Figure 2 is a rear view of the friction anchor of Figure 1;

Figure 3 is a side view of a self-drilling rock bolt incorporating the friction anchor of Figure 1;

Figure 4 is a cross-sectional side view of the self-drilling rock bolt of Figure 3 installed in rock;

Figure 5 is a schematic diagram illustrating the path that percussive forces take through the friction anchor; and

Figure 6 shows a cross-section illustrating the radial forces that retain the friction anchor in the borehole prior to resin or grout encapsulation.

Description of Embodiments

[0017] Referring to the drawings Figures 1 and 2 show a front view and a rear view of a friction anchor 100 for installation on a self-drilling rock bolt (SDA). The anchor 100 is generally tubular and includes a longitudinally extending split 102. It is typically made from steel tube.

[0018] The anchor 100 has a proximal end 104 which, in use, is nearest to the proximal end or head of the rock bolt and a distal end 106 which, in use, is located adjacent the distal end of the rock bolt adjacent the drill bit.

[0019] Starting at the first or proximal end, the friction anchor has a first end section 112 which is part-circular in cross-section having a constant diameter then has a tapered or frusto-conical section 114 which tapers outwards to the main central body portion or section 116 which is part circular in cross-section having a constant diameter which is slightly wider than the borehole drilled by the drill bit of the rock bolt in use, and a tapering frusto-conical distal end section 118 which narrows the body of the friction anchor so that it is about the same diameter as the borehole drilled by the drill bit, which helps to guide the friction anchor in the borehole and centre it.

[0020] As is best seen in Figure 1, the longitudinal split 102 in the friction anchor is narrowest in the first section 112 which has the smallest diameter, gradually increases in size in the frusto conical section 114 is widest in the main central section 116 before tapering in the frusto-conical distal or second end section 118.

[0021] As is best seen in Figure 2, there is a tapered V-shaped cut out portion 122 in the end section 112 whose tip 123 extends just into the tapered section 114. Similarly, there is a tapered V-shaped cut out portion 124 in the tapered distal end section 118 which tapers towards the centre from a wider portion to a tip. Both cut out portions are located in the opposite side of the friction anchor to the split 102. The cut outs help to maintain the circular cross section of the taper section during the manufacture of the friction anchor while maintaining the split.

[0022] Figure 3 shows the friction anchor installed on a rock bolt 10. The rock bolt includes an externally threaded hollow steel bolt 150 with a central bore 151 and a sacrificial drill bit 152 typically threaded onto the distal end of the hollow bolt. Other options for attaching the drill bit include crimping. The friction anchor 100 is mounted around the distal end of the rock bolt. A driver 160 for the friction anchor is threaded onto the bolt 150. The driver 160 comprises a hollow steel cylinder which defines a central bore which is threaded onto the steel bolt 150 adjacent the proximal end of the friction anchor. In an alternative embodiment, not shown, the driver may be simply crimped or swaged onto the bolt 150.

[0023] Advantageously, with reference to Figures 3 to 5, the installation procedure is the same as for a standard self-drilling rock bolt. In use, the rock bolt 10 is attached to an installation dolly and drilling of a borehole in the strata is commenced at the rock face 80 by the standard combination of rotation of the rock bolt and percussion using the dolly. The rotation of the rock bolt rotates the drill bit 152 and that, in conjunction with the percussion applied to the drill bit, forms a borehole 50 in the rock strata 60. Water is flushed through the central bore 151 of the rock bolt and this washes the rock fines generated by the drilling out from the borehole along the outside of the rock bolt 10. Slots/channels in the drill bit and the slot in the friction anchor 100 assist in the clearing of the fines from the distal (drilling) end of the rock bolt.

[0024] With reference Figure 5 in particular, the percussion causes the friction anchor driver 160 to impact on the friction anchor 100 to drive it into the borehole, during which time the friction anchor is compressed as the diameter of the borehole is slightly smaller than the external diameter of the widest section of the friction anchor. This compression partially closes the split 102. The driver 160 impacts the first constant diameter section 112 which in turn transmits the percussive force to the tapered section 114 which in turn transmits the percussive forces to the widest tubular section which is pushed and dragged along the borehole. The provision of the first constant diameter section 112 transmitting to the tapered section 114 helps prevent distortion such as bending and flaring which could occur if the driver 160 impacted the flared section directly. The tapering section 118 at the front of the friction anchor acts a lead-in for the wider section and assists in maintaining the friction anchor centrally within the bore hole 50.

[0025] The rock bolt is then drilled in to the required depth. Once the hole has been drilled to the required depth, drilling ceases. The installation dolly can be disconnected and the friction anchor 100 being compressed inside the bore hole 50 retains the rock bolt in the bore hole by friction. Figure 6 is a sectional view illustrating the principals of operation of the friction anchor in which radial pressure caused by the insertion of the friction anchor 100 of the rock bolt 10 into the bore hole 50 which is smaller than the outside diameter of the main section of the friction anchor compresses that section and causes radial pressure on the walls of the borehole indicated by the arrows 200 creating frictional resistance to removal of the friction and rock bolt 10.

[0026] In civil applications, where it is required that the rock bolt is pre-tensioned prior to grouting a pre-tension may be applied to the rod using the hex nut and a washer or the like. Pre tensioning the bolt prior to grouting provides a much improved result over grouting an un tensioned bolt with the pre-tensioned bolt able to safely handle much greater loads.

[0027] In mining applications, the rock bolt will typically not be pre-tensioned to any great degree and the friction anchor simply assists in retaining the rock bolt 10 in the borehole, which may be a vertical borehole in a roof or at an angle, such as 450 to the vertical, prior to the later grouting step.

[0028] Once the installation dolly is disconnected the head on the rig is changed to an injection head which is re-connected to the rock bolt and grout is then pumped through the central bore 151 of the rock bolt into the borehole through the distal end of the rock bolt and allowed to set/cure, prior to tensioning of the rock bolt.

[0029] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. A friction anchor for use with a self-drilling rock bolt, the rock bolt comprising an elongate rod having a central bore and being externally threaded, the rod having a first or distal end and a second or proximal end and a drill bit mounted on the distal end for drilling a borehole in strata such as rock or the like, wherein

2. A friction anchor as claimed in claim 1 wherein the proximal end section at the proximal end includes a tapered cut out portion on the opposite side of the anchor to the longitudinal split

3. A friction anchor as claimed in claim 1 or claim 2 wherein the tapered portion at the distal end includes a tapered cut out portion on the opposite side of the anchor to the longitudinal split

4. A rock bolt comprising:

a drill bit mounted on the distal end for drilling a borehole in strata such as rock or the like; a friction anchor mounted around the elongate rod typically adjacent the drill bit, the friction anchor comprising a generally circular tube defining a longitudinal split, the tube having a central body portion having a generally constant diameter which is larger than the diameter of the elongate rod and is slightly larger than the diameter of borehole drilled in rock by the drill bit, wherein one distal end of the friction anchor which in use locates adjacent the drill bit of the rock bolt, defines a tapered portion where the diameter of the friction anchor reduces from the central body portion to the distal end, and the other proximal end of the friction anchor defines a proximal end section where the friction anchor has a smaller generally constant diameter having a smaller diameter than the central body portion and a typically tapered section connecting the proximal end section and the central body portion in which the diameter of the friction anchor gradually increases; and a driver mounted around the elongate rod adjacent the proximal end of the friction anchor, the arrangement being such that during installation of the rock bolt in rock using rotation and percussion, the borehole formed by rotation of the drill bit, is typically slightly smaller than the central body portion of the friction anchor with the driver transferring percussive force applied to the elongate rod to the friction anchor to force the friction anchor into the borehole.

5. A rock bolt as claimed in claim 4 wherein the driver defines a central internally threaded bore and is threaded onto the rock bolt.

6. A rock bolt as claimed in claim 4 wherein the driver is fixed onto the rock bolt by swaging or crimping.

7. A method of installing a rock bolt in rock strata or the like comprising the steps of

providing a rock bolt comprising:

a drill bit mounted on the distal end for drilling a borehole in strata such as rock or the like; a friction anchor mounted around the elongate rod typically adjacent the drill bit, the friction anchor comprising a generally circular tube defining a longitudinal split, the tube having a central body portion having a generally constant diameter which is larger than the diameter of the elongate rod and is slightly larger than the diameter of the borehole drilled in rock by the drill bit, wherein one distal end of the friction anchor which in use locates adjacent the drill bit of the rock bolt, defines a tapered portion where the diameter of the friction anchor reduces from the central body portion to the distal end, and the other proximal end of the friction anchor defines a proximal end section where the friction anchor has a smaller generally constant diameter having a smaller diameter than the central body portion and a typically tapered section connecting the proximal end section and the central body portion in which the diameter of the friction anchor gradually increases; and a driver mounted around the elongate rod adjacent the proximal end of the friction anchor, wherein the method comprises: applying rotation and percussion to the rod and drill bit using an installation dolly to drill a borehole into the rock strata, the borehole formed by rotation of the drill bit, being smaller than the central body portion of the friction anchor with the driver transferring percussive force applied to the elongate rod to the friction anchor to force the friction anchor into the borehole compressing the central body portion; when the borehole is drilled to a desired depth, disconnecting the installation dolly; and subsequently encasing the rock bolt in resin, cement or the like by pumping liquid resin/cement through the central bore of the rock bolt.