Patent Application
20230009400
BACKGROUND OF INVENTION
The invention relates to a resin bolt adapted for use in supporting a rock wall by being grouted in a hole drilled into the rock wall.
Current hollow bar resin bolt design relies on at least part of the external profile of the bolt providing anchorage with the encapsulating resin/grout. To ensure that there is sufficient anchorage between the bolt and the encapsulating resin to enable the bolt to be loaded to its tensile capacity, and with conventional profile, the bolt has to anchor along a relatively long length to achieve adequate anchorage.
A problem occurs when such a bolt is required to yield. Anchorage along a significant length of the bolt will impact on the ability of the bolt to elongate which is required for a bolt to absorb energy when subjected to dynamic loading once installed. To overcome the problem you need to minimise the anchor length preferably at each end of the bolt so that the unbonded length over which the bolt can elongate is maximised.
One of the solutions to the problem is to weld a smooth portion of hollow bar between two end portions of threaded bar. In this configuration, each end portion serves to anchor the bolt whilst the smooth middle portion yields. A downside to this solution is that welding affects the material properties of the steel, reducing the ultimate tensile capacity of the bolt. Another solution is to roll a thread portion onto each end of a smooth bar.
Hereinafter the terms “resin” and “grout” are interchangeably to refer to an adhesive material which has the effect of adhering a rock bolt into a rock hole.
The invention at least partially solves the aforementioned problems.
The invention provides a rock bolt for use in supporting a rock wall by being grouted in a rock hole drilled into the rock wall which includes an elongate body of a suitable metal material which extends between a distal end and a proximal end and which body includes a threaded portion at the proximal end, a resin bore which extends through the elongate body, opening at each of the ends, a first integral anchor at or towards the distal end that comprises a plurality of integrally formed paddle formations, a second integral anchor between the first integral anchor and the threaded portion that comprises a plurality of integrally formed paddle formations, and a stem portion between the first integral anchor and the second integral anchor which has a smooth cylindrical exterior surface.
The rock bolt may have an inner diameter (ID), being the diameter of the resin bore, to an outer diameter (OD), being the outer diameter of the elongate body, ratio in the range 1:1.8 to 1:3.0. Preferably, the ID to OD ratio is in the range 1:1.85 to 1:2.8; 1:1.9 to 1:2.8; 1:1:95 to 1:2.8 or 1:2.0 to 1:2.8.
In another aspect, the invention provides a self-drilling resin anchored rock bolt which includes an elongate body of a suitable metal material which extends between a distal end and a proximal end; and a drill bit engaged to the distal end; which body includes a threaded portion at the proximal end, a resin bore which extends through the elongate body, opening at each of the ends, a first integral anchor on the body, comprised of a plurality of integrally formed paddle formations; wherein the drill bit engaged with the body has a resin conduit which communicates an exterior of the drill bit with the resin bore; wherein the threaded portion has a first section, with a first thread density, extending from the proximal end, and a second section, with a second thread density, starting at the end of the first section and extending towards the distal end; and wherein the first thread density is lower than the second thread density.
The first thread density may be lower or coarser than the second thread density.
The first portion may have an R or T series thread and the second portion may have a metric or RD type thread.
The resin bolt may have an ID to OD ratio in the range 1:1.8 to 1:3.0. Preferably, the ID to OD ratio is in the range 1:1.85 to 1:2.8; 1:1.9 to 1:2.8; 1:1:95 to 1:1.28 or 1:2.0 to 1:2.8.
The invention extends to a method of manufacturing a hollow resin anchored rock bolt which includes the steps of:
The method may include the additional steps of turning the blank about its elongate axis and repeating step (d) at a second location on the blank so as to provide a second paddle formation which is radially offset from the first paddle formation.
The invention is described with reference to the following drawings in which:
Referring to the
The bolt 10 has an elongate body 12, made of a suitable metal material, which extends between a first or leading end 14 and a second or trailing end 16. The body has a threaded end section 18 which terminates at the second end. The threaded section is formed with threads which are adapted to receive a nut 20 in threaded engagement thereon.
The leading end 14 can be shaped to a point 22, with the end surface being diagonally orientated relatively to the axis of the bolt body. Providing this point aids in pushing the bolt into a pre-drilled rock hole. A leading edge 24 can be bevelled to further reduce resistance to the bolt's passage into the rock hole.
The rock bolt body 12 is hollow, formed with a resin bore 26 which extends between, and opens at, the ends (14, 16). It is through this bore that a resin is pumped from the distal end 16, in use of the bolt.
The rock bolt 10 is adapted with a first and a second integral anchor, respectively designated 28 and 30, to anchor the bolt within the rock hole, after a resin is introduced into the rock hole through the bore 26 and hardens. Each of these integral anchors is designed to resist passage through a column of hardened resin which fills the annular space between the bolt and rock hole wall.
In describing the integral anchors (28, 30), for ease of description, reference is made to the first integral anchor. Although, the description that follows equally applies to the second integral anchor.
The first integral anchor 28 is formed from the same smooth surface hollow blank as the body and will be described below in greater detail. The anchor comprises, for example, a set of four or five end-to-end paddle formations, respectively designated 32A, 32B . . . 32N. Each paddle formation lies in a plane which, for a five set paddle formation anchor for example, is radially offset from adjacent paddle formations by 45°. It is anticipated within the scope of the invention, that the integral anchor is comprised of any number of paddle formations. If, for example, there are four paddle formations making up the anchor (28 or 30), then each formation will be radially offset from adjacent formations by 60°.
Each paddle formation 32 is formed by pressing the body 12 of the bolt 10, between a pair of dies, respectively designated 34A and 34B (see
Once the integral anchors (28, 30) are formed, a first stem portion 36 and a second stem portion 40 are defined between the first integral anchor 28 and the second integral anchor 30, and the second integral anchor and the second end 16 respectively. The stem portions retain the smooth cylindrical exterior of the original blank. Having a non-resistive exterior, the stem portions will move axially relatively to the rock hole in use of the bolt 10 to accommodate the body 12 stretching—under passive load exerted on a faceplate (not shown) mounted on the rock bolt body 14 ahead of the nut 20—whilst the integral anchors locally anchor the bolt, substantially preventing any relative axial movement there-along.
The problem with forming the paddle formations 32 as described is that in compressing the bolt body 14, the resin bore 26 will partially change shape from circular to ovoid. In this shape-changing process, the bore may become partially occluded due to a reduction in the cross-sectional area of the bore, reducing the flow rate of the resin through the bore. With a quick setting resin, this reduction in the flow-rate is particularly problematic as the resin could set, or partially set, within the bore.
The solution to the problem lies in the configuration of the blank that is used and the dies that are used to compress the blank to form the paddles formations.
Regarding the configuration of the blank, a hollow bolt body 14 needs to be chosen that has sufficient material in a wall of the body to form the paddles whilst providing the bolt with as large a diameter bore as possible without compromising on the bolt's load bearing capacity. To achieve a compromise between bore diameter and strength, the applicant has found that ratio of the ID to OD must be in the range 1:1.8 to 1:4.5. Preferably, the ID to OD ratio is in the range 1:1.8 to 1:3.0, more preferably 1:1.87 to 1:2.8. In one example of the rock bolt, the OD is 28 mm and the ID is 10 mm. In another, the OD is 28 mm and the ID is 15 mm. In a third example, the OD is 32 mm and the ID is 16 mm.
In the forming method, to provide the paddle formations 32, which method is illustrated in
In order to limit the deformation of the bore 26, each die 40 is adapted with a channel 46 which runs across the pressing surface 44 in a direction that is axial to the orientation of the bolt body 14 blank when it is inserted between the dies. The channel width is significantly (+/−50%) less than the width of the blank. With the dies attached to the press, the channels 46 vertically align.
During the forming process, the bolt body 14 blank is inserted between the dies, in axial alignment with the channels 46. When the dies are brought together in the pressing action, the parts of the blank which extend beyond the width of the channel (hereinafter, the side parts 48) are squeezed laterally into forming a respective projection of a paddle formation 32. The centre part 50 of the blank, lying substantially within the width of the channels 45, is not exposed to full deformation between the closing dies, due to the space provided by the channels. And, although the centre part is deformed from a circular to an avoid shape, and so too the resin bore (see
The extent to which the dies close is limited by an internal mechanical stopper mechanism inherent in the process tooling or through use of electro-mechanical instrumentation and control. By adjusting the internal stopper mechanism, the pressing step can be controlled to stop with the dies set apart by a certain distance so as not to produce close the resin bore completely.
A resin anchored rock bolt 10A in accordance with the other aspect of the invention is illustrated in
The essential differences between this bolt 10A and the earlier described bolt 10 is that bolt 10A is self-drilling, provided with a drill bit 52 which is engaged to the leading end 14 of the bolt body 12, and has a single integral anchor 28 which extends along a leading end portion of the body, terminating at the leading end.
In one variant of the bolt 10A, the threaded section 18 has a thread of a single thread density. In another variant, the threaded end the threaded end section 18 is divided into a first portion 18A, of a first thread density, and a second portion 18B, of a second thread density. The first portion extends from the trailing end 16, to a mid-point 54, and the second portion extends towards the leading end, from the mid-point.
The first thread density is significantly lower or coarser than the second thread density. For example, the first portion may have an R or T series thread such as a 28, 32, 38 or 45 thread, adapted for optimal transfer of energy during a percussive drilling step. The second portion may have a metric or RD type thread, for better axial load hold.
The nut 20 is threadedly engaged with the threads on the second portion 18B.
In use, an adapter 56 (such as that described in 2020/00977 which specification is herein incorporated by reference) is engaged, at one end 58, to a shaft of a rock drill (not shown) and, at the other end 60, to the threads of the first portion 18A of the threaded end section 18 of the rock bolt body.
Actuating the drill will cause the rock bolt 10A to spin and, with the drill bit 52 forced against a rock wall (not shown), to drill a hole in the rock.
Once the rock hole is created with the self-drilling action of the rock bolt 10A, the adapter can be relatively easily disengaged from the rock bolt, by reversing the rotational direction of the rock drill, as the threads of the first portion 18A do not have a high torque to axial load ratio. Because of this low tensile strength, these threads would not be suitable for holding the load applied axially to the bolt by rock movement once the bolt is installed and grouted in the rock hole
With the adapter disengaged, a tensioning tool 62 is indexed into place. This tool, as illustrated in
Because of the finer thread density of the threads of the second portion 18B, there is a higher torque to axial load ratio than the counterpart threads of the first portion 18A. However, this does not cause a problem with disengagement of the tensioning tool because this tool is not threadedly engaged. Having a higher thread density however does provides greater axial strength which is necessary as the reactive axial load, after tensioning, is placed on the nut.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2019/08131 | Dec 2019 | ZA | national |
| 2020/02462 | May 2020 | ZA | national |
This application is the U.S. national phase of International Application No. PCT/ZA2020/050069 filed Dec. 9, 2020 which designated the U.S. and claims priority to ZA Patent Application No. 2019/08131 filed Dec. 9, 2019, and ZA Patent Application No. 2020/02462 filed May 6, 2020, the entire contents of each of which are hereby incorporated by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/ZA2020/050069 | 12/9/2020 | WO |
