Patent Application
20080260472
The present invention relates to rock bolts and methods for installing rock bolts and in particular to rock bolts which are used in combination with a cementing material. The invention has been developed primarily for rock bolts used in mining applications and will be described hereafter with reference to this application. However, it will be appreciated that the invention is not limited solely to mining applications.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
There are three main types of rock bolt known for securing supporting structures in mine cavities. The first is a resin anchored rock bolt, which is installed in a borehole by inserting the bolt into the borehole, which contains a quick-setting resin compound. Once the resin is cured the bolt can be tensioned thereby to support the structure.
The second type is a mechanically anchored rock bolt, which uses an expansion assembly located at the distal end of the rock bolt. The expansion assembly expands upon a forward rotation to anchor the rock bolt in a borehole.
The third type is a combined mechanical and resin anchored rock bolt. This type uses a resin compound in combination with a mechanical expansion assembly to form an anchor with the advantages of both types mentioned above.
Typically this type of rock bolt is installed by first inserting a frangible capsule containing a resin compound into a borehole. The rock bolt is then inserted into the borehole to rupture the resin capsule. Once the resin is released, the bolt is further inserted into the borehole until the desired position is reached. A forward rotation is then applied to activate the expansion assembly thereby locking the rock bolt in the borehole whilst the resin cures.
A disadvantage of this type of rock bolt is that during the insertion process, a user often has to axially reciprocate the bolt to sufficiently mix and distribute the resin compound such that the rock bolt forms a strong bond once the resin cures. As such, the time required to perform this reciprocation can add significantly to the installation cost.
Additionally, the axial insertion force required has been found to be excessive due to the fine clearance between the bore and the expansion assembly providing only a very small path for the resin to flow.
It should be understood that during this insertion process, and before the rock bolt reaches its desired position, only minimal rotation may be applied. That is, a forward rotation causes the expansion assembly to immediately and irreversibly lock the rock bolt before it reaches its final position, and a reverse rotation will cause the expansion assembly to potentially disengage from the rock bolt shank.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.
According to a first aspect of the invention there is provided a rock bolt having:
Preferably, the expansion assembly includes an expansion shell having two or more longitudinally extending elongate expansion leaves for locking of the rock bolt in the borehole.
Preferably, the expansion shell is slidably engaged with the shank and is supported on the shank by an abutment member. More preferably, the abutment member is a nut threadedly engaged with the shank.
Preferably, the diameter of the expansion shell is substantially equal to the diameter of the rock bolt shank such that the cementing material flows more freely to the region of the rock bolt shank below the expansion shell.
Preferably, the expansion assembly includes a chuck in threaded engagement with the shank such that rotation of the rock bolt with respect to the chuck causes axial movement of the chuck, the chuck having surfaces in sliding keying engagement with complementary surfaces on the expansion leaves to cause outward divergent deformation of the leaves upon the relative forward rotation of the rock bolt. The chuck and expansion shell are preferably adapted for conjoined rotation.
Preferably, the stop means is a stop formation disposed at the distal end of the rock bolt shank. More preferably, the stop formation is a flange member fixedly connected to the rock bolt shank. Alternatively, the stop formation may be formed from a portion of the rock bolt shank and the distal end of the rock bolt may be pressed flat to define the stop formation.
Preferably, each leaf includes a plurality of gripping formations disposed on its outer surface for gripping engagement with the borehole.
Preferably, the chuck surfaces are tapered.
Preferably, the chuck includes one or more fluid flow passageways.
According to a second aspect of the invention there is provided a method for installing a rock bolt of the first aspect in a borehole of a rock formation, including the steps of:
Preferably, a substantial portion of the rock bolt shank is encapsulated by the cementing material after step (ii).
Preferably, the reverse rotation mixes the cementing material.
Preferably, the frangible capsule in substantially shredded by the reverse rotation.
Preferably, the rock bolt head is substantially 50 mm away from the surface of the rock formation after step (ii).
Preferably, the cementing material is a two-part epoxy resin compound.
Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:
Referring to the drawings, the rock bolt includes an elongate shank 1 and an expansion assembly 2 located at the distal end of the shank. The arrangement is such that the expansion assembly expands to lock the rock bolt in a borehole 3 in a rock formation upon relative forward rotation of the rock bolt. As is common to the art, the illustrated rock bolt is used in combination with a two-part epoxy resin compound to secure the rock bolt in the borehole.
A stop formation in the form of a pressed flat 4 is located at the distal end of the rock bolt shank 1 and is used to lock the expansion assembly with respect to the rock bolt for conjoined rotation upon a relative reverse rotation of the rock bolt, as best shown in
The expansion assembly 2 includes an expansion shell 5 having three longitudinally extending leaves 6 for locking the rock bolt in the borehole. Each leaf includes a plurality of gripping serrations 7 peripherally disposed about its outer surface, for gripping engagement with the borehole.
The expansion shell 5 includes a clearance bore 8 for sliding engagement with a threaded portion 9 of the rock bolt shank 1. A threaded nut 10 supports the expansion shell on the shank so that the expansion assembly 2 is retained at the distal end of the shank.
The expansion assembly further includes a chuck 11 having a threaded bore 12 for threaded engagement with the shank 1 so that rotation of the rock bolt with respect to the chuck causes axial movement of the chuck. The chuck includes tapered surfaces 13 in sliding keying engagement with complimentary surfaces on the leaves 6 such that the axial movement of the chuck results in outward divergent deformation of the leaves upon relative forward rotation of the rock bolt. Moreover, engagement between the leaves and the tapered surfaces allows for conjoined rotation between the expansion shell and the chuck. The chuck 11 further includes fluid flow passageways 14 to allow the resin to flow through the chuck and onto the shank 1.
In the embodiment shown in
Ideally, the borehole diameter is approximately equal to, or slightly less than, the diameter of the expansion shell so that rotational resistance is created between the periphery of the expansion shell and the borehole.
As best shown in
The rock bolt is then further inserted into the borehole by continued reverse rotation and upward axial force. The resin will flow from the area above expansion assembly 2 on to the shank 1 through the passageways 14 and around the periphery of the expansion shell. It will be appreciated that due to the expansion assembly being locked with respect to the rock bolt there is no risk of the expansion assembly disengaging the rock bolt during the reverse rotation.
The reverse rotation insertion process continues to perform three main functions; firstly, to force the resin down the rock bolt thereby encapsulating a substantial portion of the shank 1. Secondly, the locked expansion assembly efficiently mixes the two-part epoxy resin compound. Finally, the rotation of the expansion assembly 2 substantially shreds the frangible resin capsule.
Once the rock bolt head 16 is approximately 50 mm away from the surface of the rock formation, it is rotated in a forward direction in the conventional manner to move the chuck axially away from the pressed flat and thereby expand the expansion assembly 2 to lock the rock bolt in the borehole.
It will be appreciated that the illustrated rock bolt reduces installation times because the reverse rotation makes it easier to overcome the resistance of inserting the expansion assembly through the resin. Further, the resin compound is efficiently mixed and the resin capsule is substantially shredded during the insertion process, which also saves significantly on installation time.
Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. For example, it should be understood that many other suitable stopping formations may be used to perform the same function without departing from the scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005200804 | Feb 2005 | AU | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US06/06393 | 2/23/2006 | WO | 00 | 2/27/2008 |
