Rock bolting system and method for reinforcing rock surfaces

Abstract

A rock bolting system (1) for reinforcing rock surfaces (2) is disclosed. The rock bolting system may comprise a cable bolt (3) for being inserted into a drill hole (4), the cable bolt comprising a helical cable (5), the helical cable comprising a plurality of outer wires (9) wound around a central core wire (10), wherein each of the outer wires has an outer surface (11) having a continuous helical groove (12); and a bonding agent (6) for being injected as grouting material into the drill hole, wherein the bonding agent is an injectable resin.

Description

This application claims priority from EP 24157578.6 filed on 14 February 2024, the contents of which are incorporated herein by this reference. TECHNICAL FIELD The present disclosure relates to a rock bolting system and a method for reinforcing rock surfaces. BACKGROUND In mines, construction sites and at other work areas, there may be a need to reinforce rock surfaces and to thereby ensure their safety and suitability for their intended purposes. A common method for rock reinforcement is rock bolting. In such reinforcement systems, several rock bolts are fastened in drilled holes by a bonding agent, i.e. grouting material. In this way, rock layers are bonded together so that the risk for collapse is reduced. There are several different rock bolt types and methods for installing rock bolts. One known and widely used rock bolt is a cable bolt. Using cable bolts with cement as a bonding agent is a common practice. However, cement typically requires a significant time to cure. There are also restrictions in how the cement is mixed and placed in the drill hole, rendering the whole process difficult to automate. There is a need for replacing cements with injectable resins as bonding agents to speed up the curing and application process. Injectable resins, however, yield characteristics different from cement grouts. This unfortunately tends to increase the length of cable required to achieve a minimum embedment length for cable bolting. SUMMARY This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. A rock bolting system for reinforcing rock surfaces is disclosed. The rock bolting system may comprise a cable bolt for being inserted into a drill hole, the cable bolt comprising a helical cable, the helical cable comprising a plurality of outer wires wound around a central core wire, wherein each of the outer wires has an outer surface having a continuous helical groove; and a bonding agent for being injected as grouting material into the drill hole, wherein the bonding agent is an injectable resin. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to provide a further understanding of the described embodiments and constitute a part of this specification, illustrate various advantageous features and examples of their combinations. In the drawings: Figures 1A, IB and IC illustrate a rock bolting system for reinforcing rock surfaces, when in use, and a method for reinforcing rock surfaces using the rock bolting system; Figure 2A illustrates a part of an embodiment of the helical cable as a side view; Figure 2B shows an outer wire as a side view; Figure 3A illustrates schematically a crosssectional view of the helical cable; Figure 3B illustrates schematically a crosssectional view of an outer wire of the helical cable; and Figure 4 illustrates an embodiment of a cable bolt. DETAILED DESCRIPTION A rock bolting system for reinforcing rock surfaces is disclosed. The rock bolting system may comprise a cable bolt for being inserted into a drill hole, the cable bolt comprising a helical cable, the helical cable comprising a plurality of outer wires wound around a central core wire, wherein each of the outer wires has an outer surface having a continuous helical groove; and a bonding agent for being injected as grouting material into the drill hole, wherein the bonding agent is an injectable resin. In each of the outer wires, the helical groove may extend continuously or substantially continuously along the entire length of the outer wire. Injectable resins are typically very userfriendly and therefore desirable. Resins as bonding agents yield characteristics different from e.g. cement grouts, however. For example, resins tend to have more plastic characteristics than cement grouts. In other words, resins tend to be more elastic than cement grouts. Resins also leverage bond strength rather than system stiffness, unlike cement as a bonding agent. When the cable bolt is loaded, i.e. tensioned, the wires of the cable bolt may twist, creating a torsional force, which may compress the bonding agent surrounding the wires. Generally, when friction between the cable bolt and the bonding agent is lost around the cable bolt, the cable bolt may debond and be pulled through the bonding agent. However, with the rock bolting system according to one or more embodiments described in this specification, the bonding between the bonding agent and the cable bolt may be improved. While not to be bound by theory, it may be that the bonding agent migrates along each outer wire and locks into the outer wires and the continuous helical groove in each of the outer wires, thereby locking the cable bolt in place when the bonding agent is set and the cable bolt is loaded. The bonding agent migrates along each of the outer wires and fills the helical groove. When in use, the helical groove may engage the set bonding agent surrounding the cable bolt. When the cable bolt is loaded and the wires may twist, creating a torsional force, which may compress the bonding agent surrounding the wires, a sufficient bonding may still be achieved even with a relatively short embedment length. In other words, a shorter critical embedment length may be achieved. The critical embedment length, often also referred to as critical bond length, may be understood as the longest encapsulated length of the cable bolt at which the cable bolt is pulled out of the drill hole at a given load. Or, conversely, it may be considered to be the shortest encapsulated length of the cable bolt at which the cable bolt remains in the drill hole at a given load (i.e. the minimum embedment length). With the rock bolting system according to one or more embodiments described in this specification, a minimum embedment length (or critical embedment length) of up to 1200 mm, or up to 1100 mm, or up to 1000 mm may be used, or of 900 - 1200 mm. The continuous helical groove has been shown to provide an unexpectedly improved bonding as compared to cable bolts without any grooves or indentations or e.g. to cable bolts containing chevron indentations. The bonding agent may comprise or be a fillerless injectable resin. In other words, such a fillerless injectable resin does not contain a filler or an aggregate, such as sand or ground rock. Such a fillerless resin may be considered to be a non-frictional resin. Because the fillerless injectable resin does not contain a filler or an aggregate, such as sand, ground rock or cement, the bonding surface of the set resin is smooth and relies only on bond strength. However, at least some polyester resins may comprise a filler. The fillerless bonding agent may be a urea-silicate resin or a polyurethane resin. The properties of the bonding agent may be such that it is sufficiently fluid to fill gaps between the outer wires and the central core wire. It may permeate bulbs, when bulbs are present. The bonding agent may also be sufficient in compressive strength when set. The bonding agent may be a two-component resin, i.e. an injectable two-component resin. Such a two-component resin may be e.g. at least one of a urea-silicate resin, a polyester resin, or a polyurethane resin. The bonding agent may be a thixotropic injectable resin. Such resins exhibit reduced viscosity when stress is applied. They will also exhibit reduced tendency to flow out of the drill hole after being injected and allow for a sufficient time for inserting the cable bolt into the drill hole. For example, the bonding agent may be a two-component, thixotropic urea-silicate resin. Such injectable resins are well suited for the cable bolt system described in this specification. Further, for example, the bonding agent may be an injectable two-component, thixotropic, fillerless urea-silicate resin. The bonding agent may adapted to have a compressive strength of e.g. about 40 - 60 MPa, preferably 40 - 50 MPa, when set. The dimensions of the helical cable and / or of the helical groove are not particularly limited. However, certain dimensions may provide certain benefits. The helical groove creates a continual mechanical lock between the set bonding agent and the helical cable. The dimensions of the helical groove and the helix angle may increase or decrease the performance required. For instance, if the depth of the helical groove is smaller, the frictional / mechanical lock will be reduced and may allow the cable to yield on a lower load. A smaller helix angle of the helical groove may reduce frictional / mechanical load, and a larger helix angle may increase performance. The helical groove may have a pitch and / or a thread pitch. The pitch may, at least in some embodiments, be understood as the distance from a point in the continuous helical groove to the next corresponding point in the continuous helical groove along the longitudinal axis of the outer wire, measured parallel to the longitudinal axis. Each of the outer wires may have an outer surface having a plurality of continuous helical grooves. For example, each of the outer wires may have an outer surface having two, three, four, or more continuous helical grooves. Thus any references to a / the helical groove in this specification may be understood as referring to each of the plurality of the helical grooves. Each of the helical grooves may have a pitch. In such embodiments, each of the plurality of the helical grooves may have the same pitch. The helical groove (s) of each of the outer wires may have a helix angle in the range of about 20° - 70°, preferably about 30° - 60°, more preferably about 40° - 50°, most preferably about 45°. The thread pitch of the helical groove(s) of each of the outer wires may be in the range of about 3 - 10 mm, or preferably about 4-7 mm, more preferably about 6 mm. The thread pitch may be understood as the shortest distance from a point in a continuous helical groove to the next corresponding point in the helical 7 groove, or to the next corresponding point in the (most) adjacent continuous helical groove. The helical groove(s) may have a width in the range of about 0.5 - 3 mm, or preferably about 1 - 2.5 mm, more preferably about 2 mm. The width of the helical groove may be understood as the distance between the two edges of the helical groove measured along a straight line (i.e. as the shortest distance between the two edges). The helical groove may have a depth in the range of about 0.3 - 0.55 mm, or 0.3 - 0.55 mm. However, the depth may to some extent depend e.g. on the general dimensions of the outer wires, and / or other considerations . The helical groove may have a depth in the range of about 0.3 - 0.4 mm; or preferably about 0.32 -0.38 mm; or more preferably about 0.33 - 0.37 mm; or even more preferably about 0.34 - 0.36 mm; or most preferably about 0.35 mm. The helical groove may have a depth in the range of about 0.45 - 0.55 mm; or preferably about 0.47 - 0.53 mm; or more preferably about 0.48 -0.52 mm; or even more preferably about 0.49 - 0.52 mm; or most preferably about 0.5 mm. With such helical groove depths, it is possible to significantly improve bonding between the resin and the helical cable. Further, elongation performance of the helical cable may improve. However, with large helical groove depths, the mechanical properties, such as yield strength, may decrease. For example, increasing the depth of the helical groove from 0.15 to 0.35 mm may significantly increase the bonding performance of the helical cable. In some embodiments, the helical groove(s) may have a depth in the range of about 0.05 - 0.3 mm, or preferably about 0.1 - 0.2 mm. A very deep helical groove may reduce the tensile strength of the helical cable. The helical 8 groove(s) may in some embodiments have a depth that is at most 25 % of the width of the outer wire. The helical cable may have a width in the range of about 15 - 22 mm. Each of the outer wires may have a width in the range of about 4-7 mm, preferably in the range of about 4.5 - 6.5 mm, or 4.5 - 6.5 mm. The width of each of the outer wires may be understood as the maximum diameter of each of the outer wires, or the diameter of a circle that is defined by the outer (or outermost) surfaces of the outer wire, and / or to which the outer surfaces of the outer wire are tangent. In embodiments in which each of the outer wires has a width in the range of about 4.5 - 5.5 mm, the helical groove may have a depth (d) in the range of about 0.3 - 0.4 mm; or preferably about 0.32 - 0.38 mm; or more preferably about 0.33 - 0.37 mm; or even more preferably about 0.34 - 0.36 mm; or most preferably about 0.35 mm. In embodiments in which each of the outer wires has a width in the range of about 5.5 - 6.5 mm, the helical groove may have a depth (d) in the range of about 0.45 - 0.55 mm; or preferably about 0.47 - 0.53 mm; or more preferably about 0.48 - 0.52 mm; or even more preferably about 0.49 - 0.52 mm; or most preferably about 0.5 mm. The helical groove (s) may have a bottom wall and two side walls extending from the bottom wall towards the outer surface of the outer wire, wherein the side walls lie at an angle of about 60° - 120°, preferably at an angle of about 80° - 100°, with respect to the outer surface of the outer wire. The side walls may be perpendicular to the outer surface of the outer wire. The helical cable may be formed e.g. of steel. Other materials may also be contemplated. For example, the helical cable may be formed from a synthetic material . The helical cable may have bulbs, also commonly referred to as birdcages. The bulbs may be understood, as commonly in this field, as bulbous portions formed in the helical cable, wherein the outer wires of the helical cable are radially displaced, i.e. spaced apart from each other. The outer wires in the bulbous portions may thus be considered to spaced apart from each other further than in the portions of the helical cable outside of the bulbous portions. Thus a cavity may be formed within the bulbous portion. The dimensions of the bulbs are not particularly limited. Each bulb may have e.g. a maximum width of about 25 - 35 mm, preferably about 25 - 28 mm. The frequency of the bulbs may be selected e.g. according to geotechnical assessment at the site where the rock bolting system is to be used. The helical cable may have a plurality of bulbs arranged along the helical cable at a distance of each other. The bulbs may be arranged at a distance of e.g. about 300 - 1500 mm, preferably about 300 - 1000 mm, from each other. However, the distances are also not particularly limited. A method for reinforcing rock surfaces using the rock bolting system according to one or more embodiments disclosed in this specification is also disclosed. The method may comprise injecting the bonding agent as grouting material into a drill hole; inserting the cable bolt into the drill hole; and allowing the bonding agent to set. Any embodiments of the cable bolt and of the bonding agent described in this specification in the context of the rock bolting system may be understood as also relating to the method. The composition of the bonding agent will typically decide the time required for its setting (curing) . A skilled person is capable of selecting a 10 suitable time period and other conditions of the setting of the bonding agent. The method may further comprise cutting the cable bolt outside the drill hole and forming a trailing end for the cable bolt; mounting at least one accessory element to the trailing end of the cable bolt; and tensioning the cable bolt. EXAMPLES Reference will now be made in detail to various embodiments, an example of which is illustrated in the accompanying drawings . The description below discloses some embodiments in such a detail that a person skilled in the art is able to utilize the embodiments based on the disclosure. Not all steps or features of the embodiments are discussed in detail, as many of the steps or features will be obvious for the person skilled in the art based on this specification. Figures 1A, IB and IC illustrate a rock bolting system 1 for reinforcing rock surfaces 2, when in use, and a method for reinforcing rock surfaces 2 using the rock bolting system 1. The rock bolting system 1 comprises a cable bolt 3 for being inserted into a drill hole 4. The cable bolt 3 comprises a helical cable 5. However, the helical cable 5 is illustrated only schematically in these Figs. As shown in Figure 1A, a bonding agent 6 is injected as grouting material into the drill hole 4. The bonding agent 6 may be any bonding agent described in this specification, for example a two-component or any other suitable injectable resin. Subsequently, as shown in Figure IB, the cable bolt 3 is inserted into the drill hole 4, and the bonding agent 6 is allowed to set. The bonding agent 6 then embeds the cable bolt 3 and the helical cable 5 at least partially. In Figure IC, the cable bolt 3 is cut outside the drill hole 4 and a trailing end 7 for the cable bolt is formed. At least one accessory element 8 is mounted to the trailing end 7 of the cable bolt 3. The cable bolt 3 may then be tensioned. Figure 2A illustrates a part of an embodiment of the helical cable 5 as a side view. The helical cable 5 comprises a plurality of outer wires 9 wound around a central core wire 10. The central core wire 10 is not visible in this side view. While the number of the plurality of the outer wires 9 may in general vary, in this exemplary embodiment, there are seven outer wires 9 wound around the central core wire 10. Each outer wire 9 has an outer surface 11 having a plurality of helical grooves 12. While the number of the plurality of the helical grooves 12 may again vary, in this exemplary embodiment, each of the outer wires 9 has four helical grooves 12. The helical grooves 12 are continuous. In each of the outer wires 9, the helical groove 12, or each of the helical grooves 12, may extend continuously along the entire length, or substantially the entire length, of the outer wire 9. Figure 2B shows an outer wire 9 as a side view. The outer wire 9 has an outer surface 11 having a plurality of helical grooves 12. As mentioned above, in this exemplary embodiment, the outer wire 9 has four helical grooves 12. Each of the helical grooves 12 may extend continuously along the entire length of the outer wire 9. The outer wire 9 has a longitudinal axis 13. The helical groove 12, or each of the helical grooves 12, may have a helix angle a, for example a helix angle a in the range of 20° - 70°. The helix angle a may be about 30° - 60°, or about 40° - 50°. An optimal helix angle a may be e.g. about 45°. The helix angle a may be considered to be the angle between the longitudinal axis 13 of the outer wire 9 and the helical groove 12 extending along the cylindrical outer surface 11 of the outer wire 9. The helical groove 12, or each of the helical grooves, may have a bottom wall 15. The helical groove 12, or each of the helical grooves, may have a helix 14. The helix 14 of the helical groove 12 may be understood as referring to a continuous line extending along the bottom wall 15 of the helical groove 12. The helix 14 may be considered to follow the surface of the bottom wall 15 of the helical groove 12 and wind around the longitudinal axis 13 of the outer wire 9. The helix 14 is illustrated as a linear projection extending along a tangent of the bottom wall 15 of the helical groove 12. The helix angle a may thus be considered to be the angle between the helix 14 of the helical groove 12 and of the longitudinal axis 13 of the outer wire 9. The helical grooves 12 (or the outer wire 9) may have a thread pitch tp. The thread pitch tp may be understood as the shortest distance from a point in a continuous helical groove 12 to the next corresponding point in the (most) adjacent continuous helical groove 12. For example, as shown in Fig. 2B, the thread pitch tp may be understood as the shortest distance from an edge 17 a continuous helical groove 12 to the next corresponding edge 17 in the (most) adjacent continuous helical groove 12. In some embodiments, the thread pitch tp may be understood as the shortest distance from an edge 17 a continuous helical groove 12 to the next corresponding edge 17 in the (most) adjacent continuous helical groove 12 in the direction of (and / or along) the outer surface 12 of the outer wire 9. The thread pitch may be in the range of 3 - 10 mm, preferably about 4 -7 mm. In embodiments in which each outer wire 9 contains only a single helical groove 12, the thread pitch tp may be the shortest distance from a point in the continuous helical groove 12 to the next corresponding point in the helical groove 12. The helical groove 12, or each of the helical grooves 12, has a width Wg. The width Wg of the helical groove(s) may be measured as the shortest distance between two edges 17 of the helical groove 12 opposing each other. The width Wg of the helical groove (s) may be measured as the distance between the two edges 17 of the helical groove 12 measured along a straight line (i.e. as the shortest distance between the two edges 17) . Figure 3A illustrates schematically a crosssectional view of the helical cable 5. The helical cable 5 is, in this exemplary embodiment, similar to the one shown in Fig. 2A. The central core wire 10 is surrounded by a plurality of the outer wires 9. In this exemplary embodiment, there are seven outer wires 9 wound around the central core wire 10. The helical cable 5 has a width Wc. The width Wc may be selected based on e.g. by the requirements of the mine in which the rock bolting system is to be used; by the ease of reeling; and / or by cost considerations. The width Wc may be e.g. in the range of about 15 - 22 mm. Figure 3B illustrates schematically a crosssectional view of one of the outer wires 9 of the helical cable 5 shown in Fig. 3A. The outer wire 9 shown in this exemplary embodiment is similar to the one illustrated in Fig. 2B. The outer wire 9 has a width Ww. Thus each of the outer wires 9 of the helical cable 5 may have a width Ww. The outer wire 9, or each of the outer wires 9, may have, for example, a width Ww in the range of about 4-7 mm, or about 4.5 - 6.5 mm. The width Ww may be understood as the maximum diameter of the wire, or the diameter of a circle (illustrated in Fig. 3B with the dashed line) that is defined by the outer (or outermost) surfaces 11 of the outer wire 9, and / or to which the outer (or outermost) surfaces 11 of the outer wire 9 are tangent. The helical groove 12, or each of the helical grooves 12, has a depth d. The depth (d) may be in the range of 0.3 - 0.55 mm, or any other depth described in this specification. With such depths, the bonding between the resin and the helical cable may be significantly improved. Further, elongation performance of the helical cable may be improved. The depth d may, in some embodiments, be e.g. in the range of about 0.05 - 0.30 mm, or e.g. about 0.1 - 0.2 mm. The depth d may, in some embodiments, be e.g. at most 25 % of the width Ww of the outer wire 9, or e.g. in the range of about 5 - 25 % of the width Ww of the outer wire 9. With such dimensions, the mechanical strength of the outer wire(s) 9 may be improved. The helical groove 12, or each of the helical grooves 12, may have a bottom wall 15 and two side walls 16 extending from the bottom wall 15 towards the outer surface 11 of the outer wire 9. The helical groove 12 has two edges 17 formed between (i.e. at the junction of) the outer surface 11 of the outer wire 9 and the side walls 16. The two edges 17 may oppose each other. As mentioned in the context of Fig. 2B, the helical groove 12, or each of the helical grooves 12, has a width Wg. The width Wg of the helical groove(s) may be measured as the shortest distance between the two edges 17 of the helical groove 12 opposing each other. The width Wg of the helical groove (s) may be measured as the distance between the two edges 17 of the helical groove 12 measured along a straight line (i.e. as the shortest distance between the two edges 17), as shown in Fig. 2B. In this Fig., Wg is not indicated, as the distance between the two edges 17 of the helical groove 12 shown in Fig. 3B in the direction of the crosssection of the outer wire 9 is typically not the shortest 15 distance between the two edges 17 of the helical groove 12 and thus not the same as Wg. The helical groove 12 may have a width Wg e.g. in the range of about 0.5-3 mm. The helical groove (s) 12 may form e.g. about 10 - 80 % of the circumference of the cross-section of the outer wire 9 (or of each of the outer wires 9). The circumference of the cross-section of the outer wire 9 (or of each of the outer wires 9) may be calculated as nWw. The side walls 16 may be, for example, perpendicular to the outer surface 11 of the outer wire 9. In other embodiments, the side walls 16 may lie at an angle p of about 60° - 120°, preferably at an angle p of about 80° - 100°, with respect to the outer surface 11 of the outer wire. However, various other shapes of the helical groove 12, when viewed in cross-section of the outer wire 9, may be contemplated. Figure 4 illustrates an embodiment of a cable bolt 3. Again, the cable bolt 3 comprises a helical cable 5. The helical cable 5 has a bulb 18. Although only a single bulb 18 is depicted in the figure, the helical cable 5 may have a plurality of bulbs 18. The bulbs 18 may be arranged at a distance of each other. The bulb 18 is a bulbous portion formed in the helical cable 5, wherein the outer wires 9 of the helical cable 5 are radially displaced, i.e. spaced apart from each other. EXAMPLE 1 Pull out tests using 15.2 mm and 17.8 mm bulbed cables were conducted using either helically indented helical cables (7 outer wires, each with helical grooves) or corresponding helical cables without helical grooves . An injectable resin (Mineral Bolt SLOW, a thixotropic two-component silicate resin) was pumped into test holes (54mm x 1000mm) resulting in 1000mm of embedment of Mineral Bolt. The cables were inserted and the resin was allowed to set. The cables were pulled out after 1 hour or 18 hours. The following tests were made: - Tests no: 1-3 - 15.2mm strand smooth with no bulbs or indentations. - Tests no: 4-6 - 15.2mm helically indented with no bulbs . - Tests no: 7-9 - 15.2mm helically indented with production bulbs 33-35mm. - Tests no: 10-12 - 15.2mm helically indented with bulbs processed through the cable feeder range 29mm-25mm. - Tests 13-15 - 17.8mm helically Indented cable with no bulbs - Tests 16 - 18 - 17.8mm helically indented cable with production bulbs 34mm-31mm - Tests 19 - 21 - 17.8mm helically indented cable with bulbs processed through the cable feeder range 27mm-21mm. As a reference point, the values obtained for smooth 15.2mm cable with no bulbs were used. The results are shown in Table 1. Table 1. Comparison of the load transfer of the helically indented (cables with helical grooves) and smooth 15.2 mm and 17.8mm cables. No Sample ID Mineral Bolt Curing time Maximum force metric tons Cable 1 15.2 smOl 1 hour 7.4T 15.2mm     smooth strand 2 15.2 smOl 18 hours 8.5T 15.2mm     smooth strand 3 15.2 smOl 18 hours 11.6T 15.2mm     smooth strand 4 15.2 sindnbOl 1 hour 14. 8T 15.2mm helically indented strand with no bulbs strand 5 15.2 sindnb02 18 hours 17T 15.2mm helically indented strand with no bulbs strand 6 15.2 sindnb03 18 hours 16T 15.2mm helically indented strand with no bulbs strand 7 15.2 sindpbOOl 1 hour 19. IT 15.2mm helically indented strand with 1 x 33mm production bulbs 500mm from the tip of the strand 8 15.2 sindpb002 18 hours 21.2T 15.2mm helically indented strand with 1 x 33mm production bulb 500mm fromthe tip of the strand 9 15.2 sindpb003 18 hours 19. IT 15.2mm helically indented strand with 1 x 35mm production bulb 500mm from the tip strand 10 15.2 si- ins-qblbOOl 1 hour 21.2T 15.2mm helically indented strand with 1 x 29mm production bulb ran through the cable feeder 500mm from the tip of the strand 11 15.2 sins-qblb002 18 hours 21.2T 15.2mm helically indented strand with 1 x 27mm production bulb ran through the cable feeder 500mm from the tip of the strand 12 15.2 sins-qblb003 18 hours 20.IT 15.2mm helically indented strand with 1 x 25mm production bulb ran through the cable feeder 500mm from the tip of the strand 13 17.8 SinnbOOl 1 hour 22. 3T 17.8mm helically indented strand with no bulb 14 17.8 sinnb002 18 hours 26.5T 17.8mm helically indented strand with no bulb 15 17.8 sinnb003 18 hours 26.5T 17.8mm helically indented strand with no bulb 16 17.8 sinpbOOl 1 hour 22. 3T 17.8mm helically indented strand with 1 x 32mm production bulb 500mm from the tip of the strand 17 17.8 sinpb002 18 hours 29.7T 17.8mm helically indented strand with 1 x 34mm production bulb 500mm from the tip of the strand 18 17.8 sinpb003 18 hours 26.5T 17.8mm helically indented strand with 1 x 31mm production bulb 500mm from the tip of the strand 19 17.8 sins-qblbOOl 1 hour 19. IT 17.8mm helically indented strand with 1 x 25mm production bulb ran through the cable feeder 500mm from the tip strand 20 17.8 sins-qblbOOl 18 hours 27.6T 17.8mm helically indented strand with 1 x 27mm production bulb ran through the cable feeder 500mm from the tip strand 21 17.8 sins-qblbOOl 18 hours 25T 17.8mm helically indented strand with 1 x 21mm production bulb ran through the cable feeder 500mm from the tip strand Means average of each type of strand with 1000mm of embedment in mineral bond slow • 15.2mm smooth strand with no bulbs Ih = 7.4T, 18h =10.5T • 15.2mm helically indented strand with no bulbs strand Ih = 14.8T, 18h = 16.5T • 15.2mm helically indented strand with 1 x production bulb 500mm from the tip Ih = 19.IT, 18h = 20.15T • 15.2mm helically indented strand with 1 x production bulb ran through the cable feeder 500mm from the tip strand Ih = 21.2T, 18h = 20.65T • 17.8mm helically indented strand with no bulb Ih = 22.3T, 18h = 26.5T • 17.8mm helically indented strand with 1 x 32mm production bulb 500mm from the tip of the strand Ih = 22.3T, 18h = 28.IT • 17.8mm helically indented strand with 1 x 21mm production bulb ran through the cable feeder 500mm from the tip strand Ih = 19.IT, 18h = 26.3T The results show that the load transfer is significantly improved using the helically indented strand (with helical grooves) . This was assumed due to the increased friction and mechanical lock along the bonded length; also it is assumed that the resin is of a suitable strength as not to crush the bulb when under critical load. Other conditions had a minor or negligible effect on the critical bond length (critical embedment length). The same cable with cement as the bonding agent was found not to work in the same manner. EXAMPLE 2 Similar tests with chevron indented cables were conducted. The chevron indented cables contained columns of stacked chevron-shaped indentations on their surface. The results are shown in Table 2. Table 2. Comparison of the load transfer of the 5 chevron indented 15.2 mm and 17.8mm cables. No Sample ID Mineral Bolt Curing time (hours) Maximum force metric tons 1 15.2 smOl 1 13.8t 2 15.2 sm02 1 12.7t 3 15.2 sm03 20 16. Ot 4 15.2 sindnbOl 1 10.6t 5 15.2 sindnb03 1 14.9t 6 15.2 sindnb03 20 16. Ot 7 15.2 sindpbOOl 1 11.6t 8 15.2 sindpb002 20 13.8t 9 15.2 sindpb003 1 13.8t 10 15.2 siinsqblbOOl 1 9.6t 11 15.2 siinsqblb002 20 15.9t 12 15.2 siinsqblb003 20 15. Ot 13 17.8 SinnbOOl 1 8.5t 14 17.8 Sinnb002 20 13.8t 15 17.8 Sinnb003 20 13.8t 16 17.8 sinpbOOl 1 12.7t 17 17.8 sinpb002 20 17 . Ot 18 17.8 sinpb003 20 17 . Ot 19 17.8 sinsqblbOOl 20 15.9t 20 17.8 sinsqblb002 1 12.7t 21 17.8 sinsqblb003 20 14.8t The chevron indented cables did not provide a comparable critical bond length. 10 EXAMPLE 3 Two different helically indented helical cables (7 outer wires, each with helical grooves) were produced: cable 1 with a strand diameter of 15.3 mm, outer wire width (diameter) of 5.05-5.15 mm, and a groove depth of 0.15 mm, and cable 2 with a strand diameter of 14.6 mm, outer wire width (diameter) of 5.05-5.15 mm, and a groove depth of 0.35 mm. The yield strength and elongation of the cables were measured and are shown in Table 3. Table 3. Mechanical properties of the two ca bles tested. Steel area (mm2) Yield strength (kN) Elongation (Lok500 mm, Cable 1 139 249 5.9 Cable 2 133.3 237 6.6-7.4 It was found that the deeper groove depth weakened certain mechanical properties, such as yield strength and steel area, to some extent; however, they were still within the scope of the specification. The deeper groove depth also improved the elongation performance of the cable. It is obvious to a person skilled in the art that with the advancement of technology, the basic idea may be implemented in various ways. The embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims. The embodiments described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment. A method, a product, an arrangement, or a use, disclosed herein, may comprise at least one of the embodiments described hereinbefore. It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to 5 several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to 'an' item refers to one or more of those 10 items. The term "comprising" or "including" is used in this specification to mean including the feature(s) or act(s) followed thereafter, without excluding the presence of one or more additional features or acts.

Claims

1. A rock bolting system (1) for reinforcing rock surfaces (2), wherein the rock bolting system comprisesa cable bolt (3) for being inserted into a drill hole (4), the cable bolt comprising a helical cable (5), the helical cable comprising a plurality of outer wires (9) wound around a central core wire (10), wherein each of the outer wires has an outer surface (11) having a continuous helical groove (12); anda bonding agent (6) for being injected as grouting material into the drill hole, wherein the bonding agent is an injectable resin;wherein each of the outer wires has a width (Ww) in the range of 4.5 - 6.5 mm, andwherein the helical groove has a depth (d) in the range of 0.3 - 0.55 mm.

2. The rock bolting system according to claim 1, wherein the bonding agent comprises or is a fillerless injectable resin.

3. The rock bolting system according to claim 1 or 2, wherein the bonding agent is a two-component resin.

4. The rock bolting system according to any one of claims 1-3, wherein the bonding agent is a two-component, thixotropic urea-silicate resin.

5. The rock bolting system according to any one of claims 1-4, wherein the bonding agent is adapted to have a compressive strength of about 40 - 60 MPa, when set.

6. The rock bolting system according to any one of claims 1-5, wherein the helical groove of each of the outer wires has a helix angle (a) in the range of about 20° - 70°.

7. The rock bolting system according to any one of claims 1-6, wherein the helical groove has a width (Wg) in the range of about 0.5-3 mm.

8. The rock bolting system according to any one of claims 1-7, wherein each of the outer wires has a width (Ww) in the range of 4.5 - 5.5 mm, and wherein the helical groove has a depth (d) in the range of 0.3 - 0.4 mm.

9. The rock bolting system according to any one of claims 1-7, wherein each of the outer wires has a width (Ww) in the range of 5.5 - 6.5 mm, and wherein the helical groove has a depth (d) in the range of 0.45 -0.55 mm.

10. The rock bolting system according to any one of claims 1-9, wherein the helical cable has a width (Wc) in the range of about 15 - 22 mm.

11. The rock bolting system according to any one of claims 1 - 10, wherein the helical groove has a bottom wall (15) and two side walls (16) extending from the bottom wall towards the outer surface of the outer wire, wherein the side walls lie at an angle of about 60° - 120° with respect to the outer surface of the outer wire, or wherein the side walls are perpendicular to the outer surface of the outer wire.

12. The rock bolting system according to any one of claims 1 - 11, wherein the helical cable is formed of steel.

13. The rock bolting system according to any one of claims 1 - 12, wherein the helical cable has bulbs (18).

14. A method for reinforcing rock surfaces (2) using the rock bolting system (1) according to any one of claims 1 - 13, wherein the method comprisesinjecting the bonding agent (6) as grouting material into the drill hole (4);inserting the cable bolt (3) into the drill hole; andallowing the bonding agent to set.

15. The method according to claim 14, wherein the method further comprises cutting the cable bolt26outside the drill hole and forming a trailing end (7)for the cable bolt; mounting at least one accessory element (8) to the trailing end of the cable bolt; and tensioning the cable bolt.

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