AU2803501A - Improvements in relation to strata control - Google Patents

Description

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION STANDARD PATENT "Improvements in Relation to Strata Control" The following statement is a full description of this invention, including the best method of performing it known to me: IMPROVEMENTS IN RELATION TO STRATA CONTROL Background and Summary of the Invention This invention concerns the use of rock bolts to stabilise the ground in civil engineering and mining operations. It has particular applicability to roof stabilisation in underground mines.

Supporting ground and strata in civil engineering and mining has traditionally been done with soil nails, rock bolts, and cables. Situations and conditions can arise which require support bolts or cables of greater length than the standard support bolts. This means, in the case of coal mines, supports having a length greater than the seam height or mining height of the opening.

Such long support has in the past been fulfilled by either cable support, or by means of joining or coupling rock bolts together. The use of cable bolt support requires special handling, straightening, specialist fittings, etc. Coupled bolts, are by comparison easier to manufacture, easier to handle in the field than cable bolts and do not require too much in the way of specialist equipment to install them.

Coupled bolts are traditionally joined together by means of a separate sleeve (coupler) which is threaded internally and allows the threaded ends of two rock bolts to be screwed into the sleeve. The outside diameter of the sleeve is, of necessity, substantially larger than the male threads on the bolt ends, and therefore significantly larger than the bolt diameter in order for the threaded portion of the coupled bolts to possess adequate physical strength.

The large outside diameter of the sleeve means that, in order to install such coupled bolts into a drilled hole in rock strata, the portion of the hole closest the drilling face, at least up to the coupling, needs to be drilled to a greater diameter than the sleeve. In practice this usually entails reaming out the ground with a reaming drill bit, which can be a slow and awkward process.

The larger hole diameter also means that coupled bolts tend to have very poor load transfer to the rock behind the coupled section due to the large annulus caused by reaming. This usually means that coupled bolts are point anchored (rather than having the more effective encapsulation of the bolt over its full length) and the system accordingly needs to take into account the consequent poor load transfer characteristics.

Coupling of bolts by way of a separate threaded sleeve can involve a lack of certainty as to the position of the bolt ends within the coupling. Whilst it is hoped that the end faces of the bolts are centrally located within the coupling, there is usually no practical way to ensure this is the case.

An object of the present invention is to alleviate the disadvantages of the prior art.

Accordingly, in one aspect the invention provides a coupled rock bolt comprising two lengths of steel bar, each said bar length comprising: a body portion having a generally cylindrical form, said generally cylindrical form defining a diameter, a coupling end portion integrally formed with said body portion; a first of said bar lengths having a male thread formed on its respective coupling end portion, the second of said bar lengths having a socket formed on its respective coupling end portion, said socket having formed therein a female thread adapted to mate with said male thread, said socket having an outside diameter no greater than 15% larger than the larger of said diameters of the two body portions, and said male thread matingly engaged with said female thread to couple said two bar lengths together coaxially aligned.

Raised deformations may be formed along the length of at least one of said body portions and in this case said diameter of said one body portion is taken to be the outside diameter (or clearance diameter) of said one body portion.

The outside diameter of the socket is preferably no more than 10% larger, and more preferably no more than 5% larger, than the larger of said diameters of said two body portions.

A hollow bore may be formed in the bar lengths so that when the lengths are coupled together the bore runs for its full length of the coupled bolt including through said coupling end portions.

The raised deformations may form a helical pattern along the bolt such that the handedness of the helix is opposite to the handedness of said coupling thread. In this situation, if the raised deformations form a left-handed helical pattern along the bolt then the coupling thread would be right handed. Conversely, if the raised deformations form a right-handed helical pattern along the bolt then the coupling thread would be left handed.

Preferably the male and female threads on the coupling end portions have a rounded threadform in which the ratio of thread depth to pitch is less than 0.30, preferably less than 0.25, and more preferably less than 0.20.

In another aspect the invention provides a method of stabilising ground comprising: drilling a hole into the ground to be stabilised, assembling into the hole a rock bolt according to any one of the previous claims, said assembly comprising: inserting into the hole one of said two bar lengths, leaving its respective coupling end portion accessible, (ii) screwing onto said accessible coupling end portion the coupling end portion of the other of said two bar lengths such that said male thread is matingly engaged with said female thread to couple said two bar lengths together, pushing the coupled bolt into the hole, adhesively bonding the bolt into the hole, and tensioning the coupled bolt to induce a compressive load in the ground surrounding the hole.

The bolts may be made from smooth round steel bar, but are preferably made from a steel bar known as "deformed bar". This is a hot rolled product having a pattern of ribs rising from the generally cylindrical surface of the bar. The bar therefore has a small or inside diameter, corresponding to the cylindrical surface of the bar, and a large or outside diameter corresponding to a measurement across the tops of the ribs.

This outside diameter may also be termed the clearance diameter of the deformed bar as it is the diameter of a hole through which the bar would just pass through.

The bolts are joined together by means of a coupling which has an outside diameter which is no more than 15% larger, preferably no more than 10% larger, and more preferably no more than 5% larger, than the outside diameter of the parent bolt. The coupling is most preferably substantially the same diameter as the outside diameter of the parent bolt.

The joining mechanism does not require a separate component or sleeve to join the bolts together but is formed from the material of the parent bar lengths. The joining mechanism is a high-speed connection, which reduces handling and installation times and ensures that the engagement of mating threads is fully in place. The high-speed connection is a mating male and female thread of right or left-hand thread depending on the rotation direction of the drilling machine. The bolt can be installed into a hole of one continuous diameter, without the need for separate reaming. The bolt can be fully encapsulated in a bonding agent such as the two-part epoxy bonding agents well known to the rock bolting practitioner. The load transfer of the bolt is improved, with continuous load transfer over the full length of the bolt.

The coupling end portions of the bolt may be heat treated to give them different metallurgical properties to the body portions and so improve the strength of the joint.

The bolt may have the surfaces of the coupling end portions treated differently to the body portion to resist corrosion.

The bolt may be made from solid bar or it may have a hollow bore running for its full length including through its end portions. Coupled bolts would thus be able to carry fluid flow during installation. This fluid could be a drilling fluid, an air flow to clean the hole, or the bonding agent which could be injected into the hole via the bore.

Brief Description of the Drawings In order that the invention may be more fully understood there will now be described, by way of example only, preferred embodiments and other elements of the invention with reference to the accompanying drawings where: Figure 1 is a side view of one end of one length of steel bar forming one part of one embodiment of a rock bolt according to the present invention, Figure 2 is an end view of the bar length shown in Figure 1, Figure 3 is a side view of the other length of bar forming another part of the rock bolt, and Figure 4 is an end view of the bar length shown in Figure 3.

Detailed Description of Examples of the Invention Referring to the Figures, the male bar length 2 comprises a body portion 4 and a male end portion 20. The female bar length 10 comprises a body portion 12 and a socket end portion 21. The body portions 4 and 12 have a generally cylindrical main surface 14 carrying raised deformations 16. The deformations comprise some 17 which create a helical pattern and a pair of longitudinal ribs 18. The helical deformations 17 form a twin-start helix sharing its axis 19 with the longitudinal axis of the bar.

The male end portion 20 carries a single start male thread 22 showing ridges 24 and valleys 26 on Figure 1. The male thread 22 is unusual in that it is relatively shallow and has a long pitch relative to conventional bolt threads of that diameter. In this embodiment the ratio of thread depth (dimension marked t) to thread pitch (dimension marked p) is approximately 0.15, which is substantially less than the ratio of around 0.7 to 1.0 used for most conventional bolt threads. For the present invention the ratio is preferably less than 0.30, more preferably less than 0.25 and even more preferably less than 0.20.

The socket end portion has a smooth external cylindrical surface 30 and carries a cylindrical recess 31 to which is applied a female thread 32 which is sized and shaped to provide a matched engagement with a male end portion The male end portion 20 carries an annular neck 28 where the male end portion joins the body portion 12. This has the advantage that in dirty situations dirt and dust may be pressed into this space without hindering the full tightening of a male end portion into a socket end portion of a co-operating bolt.

The helical deformations 17 are left-handed in their orientation whereas the threads 22 and 32 are right-handed. This means that, firstly, when two bar lengths are engaged via their co-operating threads, and are rotated in the hole in the rock in a clockwise direction to mix the bonding agent, they do not unscrew from each other and, secondly, the left-handed helical deformations create a screw action upon the bonding agent in the hole which tends to "pump" the bonding agent into the hole. If the helical deformations 17 were of the same handedness as the threads 22 and 32, then it would not be possible to at the same time have both a self-tightening action on the thread and the required adhesive-retaining action provided by the deformations.

The male end portion 20 and socket end portion 21 are formed from the parent deformed bar by a multi-step forging operation and each end is then heat treated in order to provide increased strength with minimum wall thickness on the socket and minimum diameter of the male end portion 20 which together allows for the minimum possible outer diameter of the socket end portion 21.

For the embodiment shown in the Figures: outside diameter of the socket end portion 21 25.0 mm core diameter of body portion 12 (ie diameter of main surface 14) 21.7 mm.

height of deformations 1.3 mm maximum diameter of body portion 12 (ie to tops of deformations) 24.3 mm.

Thus the ratio of the outside diameter of the socket 21 to the outside diameter of the body portion 12 is: 25.0 24.3 1.029.

In other words, the socket has an outside diameter 2.9% larger than the outside diameter of the body portion of the bolt.

The present invention requires that the ratio calculated above is no more than 1.15 or, in other words, that the socket has an outside diameter no more than 15% larger, and preferably no more than 10% larger, than the outside diameter of the body portions 4 and 12.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within its spirit and scope.

It will be also understood that where the word "comprise", and variations such as "comprises" and "comprising", are used in this specification, unless the context requires otherwise such use is intended to imply the inclusion of a stated feature or features but is not to be taken as excluding the presence of other feature or features.

9 The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge in Australia.

Claims (13)

1. A coupled rock bolt comprising two lengths of steel bar, each said bar length comprising: a body portion having a generally cylindrical form, said generally cylindrical form defining a diameter, a coupling end portion integrally formed with said body portion; a first of said bar lengths having a male thread formed on its respective coupling end portion, the second of said bar lengths having a socket formed on its respective coupling end portion, said socket having formed therein a female thread adapted to mate with said male thread, said socket having an outside diameter no greater than 15% larger than the larger of said diameters of the two body portions, and said male thread matingly engaged with said female thread to couple said two bar lengths together coaxially aligned.

2. A rock bolt according to claim 1 wherein raised deformations are formed along the length of at least one of said body portions and said diameter of said one body portion is the outside diameter of said one body portion.

3. A rock bolt according to any one of the previous claims wherein the outside diameter of the socket is no more than 10% larger than the larger of said diameters of said two body portions.

4. A rock bolt according to any one of the previous claims wherein the outside diameter of the socket is no more than 5% larger than the larger of said diameters of said two body portions. A rock bolt according to any one of the previous claims further characterised by having a hollow bore running for its full length including through said coupling end portions.

6. A rock bolt according to any one of claims 2 to 5 wherein said raised deformations form a helical pattern along the bolt and the handedness of the helix is opposite to the handedness of said coupling thread.

7. A rock bolt according to claim 6 wherein the raised deformations form a left- handed helical pattern along the bolt and the coupling thread is right handed.

8. A rock bolt according to claim 6 wherein the raised deformations form a right- handed helical pattern along the bolt and the coupling thread is left handed.

9. A rock bolt according to any one of the previous claims further characterised by said male and female threads having a rounded threadform exhibiting a ratio of thread depth to pitch of less than 0.30. A rock bolt according to claim 9 wherein said ratio of thread depth to pitch is less than 0.25.

11. A rock bolt according to claim 10 wherein said ratio of thread depth to pitch is less than 0.20.

12. A rock bolt according to any one of the previous claims further characterised by said coupling end portions having been heat treated to give them different metallurgical properties to the body portion.

13. A rock bolt according to any one of the previous claims further characterised by the surfaces of said coupling end portions having been treated differently to the body portion to resist corrosion.

14. A method of stabilising ground comprising: drilling a hole into the ground to be stabilised, assembling into the hole a rock bolt according to any one of the previous claims, said assembly comprising: inserting into the hole one of said two bar lengths, leaving its respective coupling end portion accessible, (ii) screwing onto said accessible coupling end portion the coupling end portion of the other of said two bar lengths such that said male thread is matingly engaged with said female thread to couple said two bar lengths together, pushing the coupled bolt into the hole, adhesively bonding the bolt into the hole, and tensioning the coupled bolt to induce a compressive load in the ground surrounding the hole. A rock bolt as described in this specification with reference to the Figures.

16. A method of manufacturing a coupled rock bolt as described in this specification with reference to the Figures. Dated this 15th day of March 2001 Celtite Pty Ltd by their patent attorneys Morcom Pernat