Field of the Invention

The present invention relates to rock bolts for ground support systems typically used in mining excavation applications and relates particularly to an improved passive friction stabiliser rock bolt. A method for the manufacture of the improved rock bolt is also described.

Background to the Invention

Rock masses are strong in compression but weak in tension, similar to concrete. Concrete requires internal rebar-type steel reinforcing to give it tensile strength. The reinforcing is in two types, passive or active (prestressed).

Similarly rock masses in mining excavations require the insertion of rock bolts or cable bolts to provide tensile strength. Again they can be passive or active. In mining there is a large amount of relaxation (movement) of a rock mass after an excavation is made. Active rock bolts may have some advantages, but for most circumstances passive rock bolts are entirely adequate and generally more cost-effective.

In the 1970's Ingersoll Rand introduced to the mining industry the Split Set friction stabiliser (see US3922867 and further refinement in US4126004). This is described as a passive rock bolt, or dowel, meaning that there is no active restraining force applied to the rock surrounding the excavation. It is essentially a tube with a longitudinal split down the side which allows it to be compressed when pushed into a hole of a smaller diameter than its natural state, and load transfer is achieved through the friction created from the interference-fit in the hole. The rock bolt works by restricting movement of the rock through relaxation of the rock mass. Critical to the installation is careful control over the diameter of the hole being drilled for installation. The Ingersoll Rand Split Set rock bolt has become the industry standard in Australia due to its simplicity and ease of use, and the "single pass" installation (i.e. only having a single step installation process). However, it does have a number of drawbacks. The first is susceptibility to corrosion, which has only in part been addressed by galvanising. Many underground mines operate in a groundwater regime where the naturally occurring groundwater surrounding the mine is many times saltier than the ocean. Corrosion is both rapid and aggressive in these conditions. Typically these rock bolts suffer from corrosion approximately 100 - 200mm inside the neck, where air and moisture can readily access it. Additionally any moisture in the surrounding rock will tend to enter the bolt through the split and run down through its central opening. The Western Australian Department of Mines and Petroleum only consider these rock bolts as temporary support, rather than permanent support unless they are fully grouted due to this tendency. Fully grouting them prevents air and moisture accessing the inside of the rock bolt, and protects the rock bolt from corrosion. However, to fully grout installed rock bolts requires what is referred to as a "second pass", being a second installation process which nullifies the advantage of "single pass" installation.

A second drawback is the limited load-carrying capacity. A large number of articles have been written on the capacity of the friction stabiliser, and it is commonly accepted that a typical 2.4m long 47mm diameter bolt will carry a maximum load of between 8 and 12 tonnes capacity (i.e. 3 to 5 tonnes per metre of embedment). These performance statistics can be relatively easily - measured by pull-testing installed bolts to the point where they start to slide out. However, performance does vary due to differing rock types and hole diameter, albeit that hole diameter is carefully controlled during installation.

There are a number of patented improvements to this original design (see CA 2,690,301 , US 5,335,736, US 6,033,153) although none of these have achieved any significant level of commercial success. s noted above, the background to this invention relates to the original Ingersoll Rand Split Set friction stabiliser (see US 3,922,867 and further refinement in US 4,126,004). While there are many subsequent variations and improvements to this configuration, a further modification by Ingersoll Rand, described in US 4,490,074, seeks to modify the surface of the friction stabiliser.

US 4,490,074 describes a friction stabiliser which is inserted into a sheath of low friction polyethylene, deemed necessary for ease of installation. The sheath is preinstalled into the borehole in preparation for receiving the friction stabiliser. The sheath is primarily to reduce friction during insertion, but also is to provide corrosion resistance. US 4,490,074 describes a number of ways of bonding the friction stabiliser to the inside of the sheath - two component glue, anaerobic glue, or a plastic powder coating described as "frit". During insertion, the heat generated by the friction stabiliser melts the sheath, and on dissipation of the heat build-up the sheath bonds itself and the friction stabiliser to the surrounding rock.

US 4,490,074 also describes using a knurled surface instead of frit, or a saw tooth arrangement, to increase the mechanical interlock between the friction stabiliser and the sheath. A major, practical disadvantage of the arrangement described in US 4,490,074 is the need to preinstall a sheath into each borehole. During underground installation of these rock bolts, the area the rock bolts are being installed in is not a "safe" area, in that it is unsupported ground (in fact it is being made safe by the installation of rock bolts, but at this stage while the rock bolts are not yet installed, the area is not yet safe). This then requires some additional form of mechanical or remote installation of the sheath. Note that it cannot simply be put over the rock bolt prior to installation because the rock bolt is an interference fit in the borehole and will tear the sheath away during installation. Even if these practicalities were ignored, the installation of the rock bolt in US 4,490,074 becomes a "two stage" or "two pass" installation, with the first pass being the installation of the sheath. This will not be favoured by operators.

The present invention was developed with a view to providing an improved passive friction rock bolt that is capable of providing a reliable ground support system and that is less susceptible to the above-noted disadvantages of the prior art.

References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere.

Summary of the Invention

According to one aspect of the present invention there is provided a rock bolt for a ground support system, the rock bolt comprising: an elongate body made of resilient material with high tensile strength, the body having a head at one end and a tip at the other; a plurality of protrusions provided on an external surface of the body; and, a coating of friction-promoting material applied to the body so as to cover at least part of the external surface whereby, in use, when the body is driven into the borehole during installation at least some of the coating material is retained between the protrusions to increase the coefficient of friction between the external surface and the surrounding wall of the borehole.

Preferably the plurality of protrusions formed on the external surface are uniformly distributed over substantially the whole of the external surface of the wall of the tubular body. In one embodiment the protrusions are in the form of knurls. In the present invention the knurls typically take the form of a series of straight ridges or a series of helixes of ridges forming a cross- hatched pattern. In another embodiment the protrusions are in the form of dimples. Typically the elongate body is a tubular body with a generally annular wall having a gap extending longitudinally along at least a portion of the length of the wall, so that when the rock bolt is driven into a borehole of smaller diameter than an outer diameter of the tubular body the gap permits the wall of the tubular body to flex inwards and to frictionally engage a surrounding wall of the borehole after installation.

Advantageously the friction-promoting coating is applied to substantially the whole of the inner surface and the outer surface of the wall of the tubular body. Preferably the coating material is also corrosion resistant. Typically the friction-promoting coating material is a polymeric material that can be applied to the wall of the tubular body in liquid form. Preferably the polymeric material is an elastomer material such as natural rubber or synthetic rubber.

Preferably the gap extends substantially the full length of the tubular body from the tip to the head. Preferably the tip of the tubular body is tapered to facilitate insertion of the rock bolt into the borehole. The tubular body is preferably of substantially uniform cross-section throughout its length. Typically a retainer ring is provided at the head of the tubular body.

According to another aspect of the present invention there is provided a method of manufacturing a rock bolt for a ground support system, the method comprising the steps of: forming an elongate body of resilient material with high tensile strength, with a head at one end and a tip at the other; providing a plurality of protrusions on an external surface of the body; and, applying a coating of friction-promoting material to the body so as to cover at least part of the external surface whereby, in use, when the body is driven into the borehole during installation at least some of the coating material is retained between the protrusions to increase the coefficient of friction between the external surface and the surrounding wall of the borehole. In one embodiment of the method of the invention the step of providing a plurality of protrusions involves knurling the external surface of the body. Knurling typically involves cutting or rolling a diamond-shaped (criss-cross) pattern into a surface of the material. The step of knurling may be done before or after the step of forming the material into an elongate body.

Typically the elongate body is formed into a tubular body with a generally annular wall having a gap extending longitudinally along at least a portion of the length of the wall, so that when the rock bolt is driven into a borehole of smaller diameter than an outer diameter of the tubular body the gap permits the wall of the tubular body to flex inwards and to frictionally engage a surrounding wall of the borehole after installation.

Preferably the step of applying a coating of the friction-promoting material involves applying the material to substantially the whole of the inner surface and the outer surface of the wall of the tubular body. Typically the step of applying the friction-promoting coating involves dipping the tubular body into a bath of the coating material in liquid form.

Typically the elongate tubular body is formed of sheet metal and the method of manufacture further includes the step of galvanising the metal for corrosion protection. Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Likewise the word "preferably" or variations such as "preferred", will be understood to imply that a stated integer or group of integers is desirable but not essential to the working of the invention. Brief Description of the Drawings

The nature of the invention will be better understood from the following detailed description of a specific embodiment of a rock bolt and a method of manufacturing the same, given by way of example only, with reference to the accompanying drawings, in which:

Figure 1 illustrates a typical prior art friction stabiliser rock bolt;

Figure 2 illustrates part of the tubular body of a first embodiment of the improved rock bolt according to the present invention; and,

Figure 3 illustrates the tubular body of Figure 2 with a coating applied to the wall of the body.

Detailed Description of Preferred Embodiments

A preferred embodiment of rock bolt 10 in accordance with the invention, as illustrated in Figures 2 and 3, comprises an elongate tubular body 12 made of a resilient material with high tensile strength. In this embodiment the tubular body 12 is made of steel, however it will be appreciated that other suitable materials may also be used, for example, a carbon-fibre reinforced material. The tubular body 12 has a generally annular wall with a head 14 at one end and a tip 16 at the other. A gap 18 extends longitudinally along at least a portion of the length of the wall of the tubular body 2. When the rock bolt 10 is driven into a borehole of smaller diameter than an outer diameter of the tubular body 12 the gap 18 permits the wall of the tubular body to flex inwards and to frictionally engage a surrounding wall of the borehole after installation.

A plurality of protrusions 20 is provided on an external surface of the wall of the tubular body 12, Preferably the plurality of protrusions 20 are uniformly distributed over substantially the whole of the external surface of the wall of the tubular body 12. Typically the protrusions are in the form of knurls 20. In the illustrated embodiment the knurls 20 are in the form of a series of helixes of "straight" ridges forming a cross-hatched pattern. However it will be understood that the protrusions may be of any suitable form, for example in the form of dimples.

A coating 22 of friction-promoting material is applied to the tubular body 12 so as to cover at least part of the external surface. In use, when the rock bolt is driven into the borehole during installation, at least some of the friction coating material 22 is retained between the protrusions on the external surface of the tubular body 12 to increase the coefficient of friction between the external surface and the surrounding wall of the borehole.

It is the combined action of the protrusions 20 and coating material 22 on the external surface that increases the coefficient of friction, and which results in an improvement of the frictional interaction with the surrounding rock and load transfer to the rock bolt 10. This then translates into improved holding capacity of the rock bolt 10.

However, if a friction-promoting coating is applied on its own, it will be ineffective in increasing the load transfer. Because the rock bolt 10 has an interference fit in the borehole, any soft coating, as friction-promoting materials tend to be, will be scraped off at the collar of the borehole on insertion of the rock bolt into the borehole.

Knurling alone applied to an external surface of the rock bolt will also be ineffective, although for different reasons. Knurling is a way of improving friction between a hard and a soft surface. For instance, a weightlifters bar is knurled where gloved or bare hands are placed. Gloves or skin are relatively soft compared to the steel bar, and conform to the knurling to take advantage of the grip provided.

Contrast this with locomotive wheels, which are made of steel and run on a steel rail. These wheels are not knurled, and for good reason. Knurling in this instance would not provide any better grip - and would more likely result in faster wear of the wheel as the individual "knurls" are point loaded against a similarly hard surface and therefore wear down faster. Without digging-in to the hard rail the knurling will not provide any additional grip - in fact it may result in reduced grip due to the lower contact area.

In the case of the rock bolt 10, simply knurling the external surface is also unlikely to increase load transfer in all but the softest of rock because knurling, as noted above, is ineffective when two hard surfaces are in contact. It is the combination of both knurling and applying a friction- promoting coating to the rock bolt that increases the coefficient of friction. Applying either in isolation is ineffective. Applying both is very effective. The combination of the protrusions 20 (in this case, formed by knurling) and the friction-promoting coating 22 has the following effects:

The knurling protects the majority of the friction-promoting coating 22 during installation. As the rock bolt 10 is driven into the borehole only the peaks of the knurling 20 contact the wail of the borehole; the friction-promoting coating 22 is retained in the spaces between the knurling. The coating material extending above the ridges of the knurling is scraped off, but the coating material applied between the ridges of the knurling remains substantially in place. The ridges of the knurling 20 also act as a wedge against the friction coating 22. Any relative movement between the external surface of the rock bolt 10 and wall of the borehole will force the friction coating material into the spaces between the knurling and the borehole, ensuring contact and significant factional resistance.

In this connection it is worth noting the difference in frictional coefficients during installation and at the completion of installation. It is well understood that for two bodies being pressed together the static coefficient of friction is greater than the dynamic coefficient of friction. This is the basis of Anti-lock Braking Systems (ABS) in cars. A locked wheel will skid further than a controlled, braking wheel on a road surface. It is also well understood that in both circumstances, in wet conditions, both coefficients of friction will be lower, but even when wet the static coefficient of friction will be greater than the dynamic coefficient of friction. On installation of the friction stabiliser rock bolt 10, it is best that the process occurs in wet conditions to reduce the coefficient of friction as low as possible for ease of installation. For the purpose of analysing the invention it is also probable that some proportion of the installation process is overcoming dynamic friction rather than static friction, i.e. the rock bolt 10 is moving while it is being hammered into the borehole. Once the rock bolt 10 is installed however, and water dries or is squeezed out through the very small spaces between the friction coating 22 and the knurling 20, the applicable frictional resistance to movement will be at a maximum - that is both static and dry. Advantageously the coating 22 is applied to substantially the whole of the inner surface and the outer surface of the wall of the tubular body 12 to also provide protection from corrosion. Typically the friction-promoting coating material is a polymeric material that can be applied to the wall of the tubular body in liquid form. Preferably the coating material is also corrosion resistant. Preferably the polymeric material is an elastomer material such as natural rubber, latex or synthetic rubber.

Rubber is well known for having high friction properties, and as a sealing compound such as in O-rings. It is used for tyres, which rely on friction on the road surface to work effectively. However, rubber is not generally used for corrosion protection. Nevertheless it has quite good corrosion resistance properties, and it can seal the metal surface of the tubular body from exposure to water and air. In the present instance, by applying the rubber coating 22 to the whole of the external and internal surface of the tubular body it provides the additional benefit of corrosion protection. Additional friction requires additional strength. Therefore the rock bolt 10 can be improved in holding capacity by thickening the wall of the tubular body 12 to provide a higher load transfer from the surrounding rock to the rock bolt.

The tubular body 12 of the rock bolt 10 is similar in shape to that of a prior art rock bolt, as shown in Figure 1. Preferably the gap extends substantially the full length of the tubular body 12 from the head 14 to the tip 16. Preferably the tip 16 of the tubular body 12 is tapered to facilitate insertion of the rock bolt into the borehole. The tubular body 12 is preferably of substantially uniform cross-section throughout its length. Typically a retainer ring 24 is provided at the head 14 of the tubular body.

A preferred method of manufacturing the rock bolt 10 will now be briefly described. The method typically comprises the step of forming the elongate tubular body 12 of resilient material from sheet metal, for example, steel plate. The steel plate is rolled into a tubular shape but with the opposite parallel edges of the plate not touching so as to leave a gap 18 extending longitudinally of the tubular body 12. One end of the rolled tube may be tapered to form the tip 16, and a retainer ring 24 welded to the other end to form the head 14 of the friction stabiliser rock bolt 10. Up to this point the rock bolt 10 is of conventional construction.

Next, to provide the plurality of protrusions 20- on an external surface of the wall of the tubular body 12, the external surface of the steel plate is preferably knurled. Preferably a thicker steel plate is used to handle the higher load transfer from the surrounding rock to the rock bolt 10. Knurling could occur either immediately before or immediately after rolling of the steel plate into a tubular shape. If before, the knurled or crosshatched sheet of steel may then be rolled, resulting in a rock bolt with the knurling already present on the outer surface.

The next step in the manufacturing process would typically be galvanising for corrosion protection, as currently occurs with the prior art. This is carried out by hot-dipping the tubular body 12 in a zinc bath.

The final step in the manufacturing process is applying a coating of friction- promoting material 22 to the tubular body 12 so as to cover at least part of the external surface. This is preferably also done by dipping in a hot bath, similar to the galvanising step above, except in this instance the rock bolt 10 would preferably be dipped into a latex- or rubber-based liquid formulation to provide both increased friction and increased corrosion protection. Ideally recycled rubber may be employed in the friction-promoting formulation, but rubber has a higher melting point than the steel so the rubber cannot simply be melted to liquid form. There may be a way to. get a lower melting point recycled rubber, and certainly new rubber can be applied in liquid form at lower temperatures. The friction-promoting coating 22 will bond to the surface of the tubular body 2 like paint. Although it will require a clean surface, the newly manufactured rock bolt 10 will be sufficiently clean so that no additional cleaning process is required to clean the rock bolt. There are some bonding techniques that may be considered depending on the coating material chosen. For example, vulcanising may be employed which is a bonding and curing process for rubber. However the bonding process may not be critical to the performance of the rock bolt 10. It is the combined action of the protrusions 22 retaining the material of the friction- promoting coating 20 there-between that is critical. It is this feature that ensures that when the rock bolt 10 is driven into the borehole during installation at least some of the coating material is retained between the protrusions 20 to increase the coefficient of friction between the external surface and the surrounding wall of the borehole.

Now that preferred embodiments of the rock bolt and method of manufacture have been described in detail, it will be apparent that the described embodiments provide a number of advantages over the prior art, including the following: (i) The improved coefficient of friction provides demonstrable additional holding capacity and therefore results in a more efficient product in terms of better ground support for the same installation effort.

(ii) The additional corrosion protection provided over the prior art increases the longevity of the invention post-installation.

(iii) For an installed rock bolt in an underground mine, the vast majority of the cost is in the installation process rather than the rock bolt itself. Therefore significantly improved ground support can be afforded by a small increase in overall cost.

It will be readily apparent to persons skilled in the relevant arts that various modifications and improvements may be made to the foregoing embodiments, in addition to those already described, without departing from the basic inventive concepts of the present invention. For example, although the preferred form of the rock bolt is a split set friction stabiliser, the invention may also be embodied in other types of rock bolt. Therefore, it will be appreciated that the scope of the invention is not limited to the specific embodiments described.