Variable stiffness hybrid rock anchor

Abstract

The present invention addresses the shortcomings of conventional rock bolts during seismic events by providing a chemically end-anchored rock anchor system. The anchor comprises a bolt partially or wholly surrounded by a sleeve containing one or more adhesive cartridges. A divider within the sleeve mechanically interconnects the bolt and sleeve and controls axial movement during installation. Upon rotation of the bolt, the distal end advances into and ruptures the adhesive cartridge, ensuring thorough mixing and rapid formation of a cured resin plug that bonds the bolt to the surrounding rock. The design minimizes resin loss, ensures predictable adhesive mixing, and accommodates variations in operator technique, thereby enhancing dynamic response and reliability under rapid loading. This system effectively overcomes installation difficulties and provides consistent, long-term rock reinforcement, particularly in seismic environments where energy absorption and rapid anchorage are critical.

Description

VARIABLE STIFFNESS HYBRID ROCK ANCHORBACKGROUND OF THE INVENTION

[0001] The failure of rock bolts during seismic events presents a significant problem, even though these bolts are designed to absorb substantial amounts of energy based on laboratory tests.

[0002] While the bolts are tested to handle energy absorption up to a specific threshold (e.g., 50kJ), they often fail during seismic events because the dynamic loading occurs much more quickly than in controlled lab conditions. Laboratory tests typically simulate loading over 60-100 milliseconds, while seismic events last only 30-50 milliseconds. This discrepancy means that the bolts' energy absorption capacity is constrained by the shorter time frame of the seismic event, preventing them from dissipating energy quickly enough during the initial moments of dynamic loading.

[0003] Different bolts' varying initial stiffness and dynamic responses further contribute to the problem. Some bolts may perform better under dynamic loads by shedding load and avoiding failure, even if their energy absorption capacity is lower. However, the issue remains that many bolts cannot respond quickly enough to the rapid, high-frequency vibrations induced by seismic forces. These high-frequency oscillations lead to fatigue or resonance, causing the bolts to fail through stress concentration, material fatigue, or dynamic resonance, even though they are strong enough to withstand static loads.

[0004] To address these challenges, improving the speed at which energy absorption is available and adjusting the bolts' initial stiffness is crucial.Enhancing the dynamic response of the bolts during seismic events would help prevent failure and ensure that they perform effectively under rapid loading conditions.

[0005] Hybrid rock bolts are widely utilised in the mining industry for excavation support. These bolts generally consist of a resiliently deformable sleeve that provides initial stabilisation when inserted into a borehole. A steel bolt runs through the sleeve, transmitting forces from rock movement around the borehole. As the surrounding rock deforms, a plug at the distal end of the bolt moves through the sleeve, allowing for a gradual energy dissipation as the rock and bolt interact. While this configuration enables some energy dissipation, it lacks built-in stiffness variability, which can be problematic in dynamic conditions such as seismic events or rapid ground movements, where more adaptive responses may be required.

[0006] Prior art mechanical rock anchors typically rely on wedge inserts or supplementary anchoring components that are separate from the primary bolt structure. These arrangements add complexity and cost to both manufacturing and installation. There is accordingly a need for a rock anchor that provides reliable mechanical anchorage through an integrated wedge and sleeve interaction, without the need for separate inserts or additional anchoring components, and which can be manufactured cost-effectively using conventional materials and processes.

[0007] The present invention at least partially addresses the aforementioned problems.

[0008] In the following specification, the term “rad” refers to any elongate load¬ bearing member, which may be solid or hollow, smooth or profiled (including ribbed, threaded or deformed), and formed from any suitable material.SUMMARY OF THE INVENTION

[0009] The invention provides a variable stiffness hybrid rock anchor which includes:a radially expansible tubular sleeve that longitudinally extends between a distal end and a proximal end, with a portion being swaged to provide a confined section that terminates at the distal end,a rod extending longitudinally through the sleeve, from a leading end to a trailing end,an expansion element mounted on or integral with the rod, at or towards the leading end,a load-bearing assembly mounted on the rod that engages the proximal end of the sleeve, preventing movement of the sleeve relative to the rod when the rock anchor is inserted into a borehole,a plurality of resistant bands engaged with the confined section of the sleeve, spaced longitudinally apart from one another,wherein movement of the rod axially relative to the sleeve toward the proximal end pulls the expansion element through the confined section, causing both the confined section and the resistant bands to expand.

[0010] The sleeve is dimensioned and configured to form a friction fit within the borehole when inserted, such frictional engagement providing immediate mechanical anchorage without the need for resin, grout, or supplemental wedge inserts.

[0011] The expansion element preferably has a wedge angle of less than 20°, further preferably in the range of 10° to 11°, which provides a controlled and predictable ingress depth into the confined section. Shallower wedge angles provide reduced ingress depth.

[0012] Each resistant band may increase in stiffness, starting with a first band positioned adjacent to the distal end of the sleeve and continuing to each successive band placed between a previous band and a band that follows.

[0013] The resistant bands may comprise one or more rings or loops (collectively, “rings”) that partially or fully encircle the confined section. The resistant bands may comprise complete circular rings, C-shaped rings, pairs of opposed semi-rings, alternating ring geometries, or strap-like elements that partially or fully encircle the sleeve.

[0014] The progressive increase in stiffness of the resistant bands may be achieved through variation in one or more of: the number of rings comprising each band; the cross-sectional diameter of the rings; the material of manufacture of the rings; the shape or geometry of the rings; and the longitudinal spacing between adjacent bands along the confined section.

[0015] In one alternative, each band comprises rings that may vary in circumference but are identical in cross-sectional diameter and manufacturing material.

[0016] In this variation, each band contains an increasing number of rings, with each successive band having more rings than the one before it, increasing stiffness.

[0017] Alternatively, each band may vary in one or more of: the number of rings comprising each band; the cross-sectional diameter of the rings; the material of manufacture of the rings; the shape or geometry of the rings; and the longitudinal spacing between adjacent bands along the confined section. Any combination of these variables may be employed to achieve a desired stiffness profile along the length of the confined section.

[0018] The sleeve may be C-shaped in cross-section, defining a longitudinal slit extending between the distal and proximal ends to accommodate radial compression of the sleeve.

[0019] The load-bearing assembly may comprise a seat and a nut, the nut being configured to engage a threaded section of the rod at or towards the trailing end, and the seat being configured to engage the proximal end of the sleeve.

[0020] A faceplate may be mounted on the rod and positioned to bear against a rock face when the rock anchor is installed.

[0021] The sleeve may include a formation configured to arrest continued movement of the expansion element, thereby reinforcing the anchor whilepermitting additional rock face movement to be absorbed by elongation of the rod.

[0022] The invention extends to a method of stabilising a rock face using the variable stiffness rock anchor of the invention as described above.

[0023] The method comprises inserting the rock anchor into a predrilled borehole in the rock face, the borehole having a diameter slightly smaller than the sleeve such that insertion causes radial compression of the sleeve and establishes a friction fit; applying rotational force to the load-bearing assembly to tension the rod and draw the expansion element into the confined section, thereby creating a pre-load; and permitting subsequent static or dynamic movement of the rock face to further tension the rod, causing the expansion element to advance through the confined section and progressively engage the resistant bands, thereby increasing the stiffness of the rock anchor in response to rock movement.

[0024] BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The invention is further described by way of examples with reference to the accompanying drawings in which:Figure 1 is a schematic, partially sectioned illustration of a variable stiffness hybrid rock anchor in accordance with the invention; andFigure 2 is a schematic longitudinally sectioned illustration of a leading end of the variable stiffness hybrid rock anchor; andFigure 3 is a schematic isometric and longitudinally sectioned leading end of the hybrid rock anchor illustrated in Figure 2.DESCRIPTION OF PREFERRED EMBODIMENTSFor ease of reference and understanding, in Figures 1 to 3, like features of the invention are indicated by like numerals.

[0026] Figures 1 to 3 illustrate a variable stiffness hybrid rock anchor (10), which is in accordance with the invention.

[0027] As is typical with hybrid bolts, the variable stiffness hybrid rock anchor described here features a radially expansible tubular sleeve (12) that runs longitudinally from a distal end (14) to a proximal end (16). Inside this sleeve, an elongated rod (18) extends longitudinally between a leading end (20) and a trailing end (22).

[0028] The sleeve (12) is C-shaped in cross-section, defining a slit (24) that extends between the ends (14,16). The slit accommodates radial compression of the sleeve when the sleeve is inserted into a borehole (44) in use, as further described below.

[0029] The rod carries a conical wedge (26) that, in this embodiment, is threaded onto a distal threaded section (27) of the rod, positioned on the leading end and configured so that its trailing surface (28) is in contact with the sleeve’s distal end.

[0030] The rock anchor includes a load-bearing assembly (29) that, amongst other functions, prevents movement of the sleeve relative to the rod when the rock anchor (10) is inserted into the borehole. This assembly is mounted on the rod and engages with the proximal end of the sleeve.

[0031] The load-bearing assembly (29) consists of a seat (30) and a nut (32), which engages with the threads of a proximal threaded section (34) of the rod, located behind the seat. The seat is adapted to connect with the proximal end of the sleeve.

[0032] The sleeve (12) is swaged along a section that tapers towards its distal end (14), reducing in diameter and forming a confined section (36).

[0033] A faceplate (38) is mounted on the rod.

[0034] Along the confined section (36) of the sleeve, multiple resistant bands, respectively designated 40.1, 40.2, 40.3,…40.N are placed externally and spaced apart longitudinally. Each band increases in stiffness, starting with the first band (40.1) near the sleeve's distal end (14) and continuing with each subsequent band, which becomes stiffer as it is positioned between the previous band and the following one.

[0035] In this example, each band (40) consists of one or more rings (42.1, 42.2, 42.3, ... 42.N) that partially or fully surround the confined section. The first band (40.1) has a single ring (42.1), the second band (40.2) includes two rings (42.2, 42.3), and each following band has an additional ring compared to the preceding one.

[0036] In this embodiment, all the rings have the same cross-sectional diameter and are made from the same material, so the increase in stiffness is attributed to the number of rings in each band.

[0037] In alternative embodiments, the progressive increase in stiffness of the resistant bands may be achieved through variation in one or more of: the numberof rings comprising each band; the cross-sectional diameter of the rings; the material of manufacture of the rings; the shape or geometry of the rings; and the longitudinal spacing between adjacent bands along the confined section. Any combination of these variables may be employed to achieve a desired stiffness profile along the length of the confined section.

[0038] In use, the rock anchor (10) is inserted into a predrilled borehole (44) in a rock face (46) that needs stabilisation and support. The hole's diameter is slightly smaller than the sleeve’s (12), but slightly wider than the distal region of the sleeve. As the sleeve is driven into the hole, it compresses — helped by the slit (24) — to fit snugly. The radial forces generated during this process initially secure the rock anchor, allowing for partial load transfer from the rock strata to the sleeve.

[0039] Once the rock anchor is fully inserted, with the faceplate positioned between the seat (30) and the rock face (46), rotational motion is applied using a drill arm from a drill rig (not shown). The drill arm's chuck engages the nut (32), applying rotational force. As the chuck turns the nut, it moves along the threaded section (34) and presses against the back end of the seat (30), tightening it. This action forces the faceplate (38) into load-supporting contact with the rock face.

[0040] Any additional torquing of the nut (32) at this point will apply tension to the rod (18), which will, in turn, draw the conical wedge (26) further into the confined section of the sleeve from its distal end (14), creating partial pre-load.

[0041] The operational mechanism of the rock anchor functions as follows: when tension is applied to the rod (18), whether through torquing of the nut (32) duringinstallation or through subsequent static or dynamic movement of the rock face (46), the rod is drawn axially toward the proximal end (16) of the sleeve (12). Since the conical wedge (26) is mounted on (or integral with) the leading end (20) of the rod, the rod and wedge move together as a unitary assembly. This axial movement of the rod toward the proximal end consequently pulls the conical wedge (26) from the distal end (14) into and through the tapered confined section (36) of the sleeve. As the conical wedge advances through the confined section toward the proximal end, its increasing diameter progressively expands the sleeve radially, sequentially engaging the resistant bands (40.1, 40.2, 40.3...40. N). Since each successive resistant band is stiffer than the preceding band, the resistance to movement of the rod increases progressively as the wedge advances through the confined section. This arrangement ensures that the rock anchor initially yields under relatively low load, thereby absorbing energy, while becoming progressively stiffer to provide increasing resistance as deformation continues.

[0042] The degree of ingress of the conical wedge (26) into the confined section (36) determines the operational mode of the rock anchor. With deep ingress, the conical wedge advances through the full extent of the confined section, progressively engaging the resistant bands (40.1, 40.2, 40. 3...40.N) to provide variable stiffness characteristics as described herein. However, even with shallow ingress, where the conical wedge engages only the initial portion of the confined section adjacent the distal end (14), the rock anchor still provides utility as an end-anchored bolt through the wedge and ring interaction at the distal end. In this shallow ingress configuration, the rock anchor functions as a mechanicallyanchored bolt without necessarily exhibiting variable stiffness but nonetheless provides reliable anchorage through the interaction between the conical wedge and the resistant bands at the distal region. The shallow ingress configuration may be advantageous for applications where variable stiffness is not required, but where a cost-effective, mechanically anchored bolt with integrated wedge and ring interaction is desired.

[0043] This configuration represents a cost-effective alternative to prior art arrangements that rely on separate wedge inserts and supplementary mechanical anchoring components.

[0044] The sleeve (12) may feature a formation (not shown), positioned either internally or externally, to stop the continued movement of the conical wedge (26), thereby reinforcing the anchor while allowing any additional movement of the rock face to be absorbed by the elongation of the rod (18).

[0045] As a result, the rock anchor starts soft but becomes increasingly stiff, effectively managing dynamic loading forces and preventing anchor failure.

[0046] The invention therefore provides a variable stiffness hybrid rock anchor that addresses the shortcomings of conventional rock bolts under dynamic loading conditions. By incorporating a series of progressively stiffer resistant bands around the confined section of the sleeve, the rock anchor is able to respond adaptively to both static and dynamic rock movements. The initial low stiffness allows the anchor to yield and absorb energy during the critical early moments of a seismic event, when rapid energy dissipation is most needed. As deformation continues, the progressive engagement of increasingly stifferresistant bands ensures that the anchor's resistance builds incrementally, preventing sudden stress concentrations and reducing the risk of fatigue or resonance-induced failure. Furthermore, the friction fit engagement of the sleeve within the borehole provides immediate mechanical anchorage upon installation, eliminating the need for resin, grout, or supplemental wedge inserts. The result is a rock anchor that combines the energy-absorbing characteristics of yielding bolts with the load-bearing capacity of stiff bolts, offering enhanced performance and reliability in seismically active mining environments.

Claims

CLAIMS1. A variable stiffness hybrid rock anchor, comprising:a radially expansible tubular sleeve extending between a distal end and a proximal end, the sleeve having a confined section formed by a swaged portion that tapers towards the distal end,a rod extending longitudinally through the sleeve, from a leading end to a trailing end,an expansion element mounted on or integral with the rod, at or towards the leading end, the expansion element being configured to move axially into and through the confined section to radially expand the sleeve;a load-bearing assembly mounted on the rod and configured to engage the proximal end of the sleeve, to prevent axial movement of the sleeve relative to the rod when the rock anchor is installed within a borehole,a plurality of resistant bands positioned externally around the confined section of the sleeve and spaced apart longitudinally,wherein axial movement of the rod towards the proximal end relative to the sleeve causes the expansion element to advance into the confined section, thereby radially expanding the confined section and progressively engaging the resistant bands, whereby the stiffness of the rock anchor increases as the expansion element advances through the confined section.

2. The rock anchor as claimed in claim 1, wherein the sleeve is dimensioned and configured to form a friction fit within a borehole when inserted therein, whereinthe frictional engagement between the sleeve and the borehole wall provides immediate mechanical anchorage, without the need for resin, grout, or supplemental wedge inserts.

3. The rock anchor as claimed in claim 1 or 2, wherein the expansion element comprises a conical wedge.

4. The rock anchor as claimed in claim 3, wherein the conical wedge has a wedge angle less than 20°.

5. The rock anchor as claimed in claim 4, wherein the conical wedge has a wedge angle in the range of 10° to 11°.

6. The rock anchor as claimed in any one of claims 1 to 5, wherein stiffness, increases from a first resistant band adjacent the distal end of the sleeve towards successive bands in the proximal direction.

7. The rock anchor as claimed in claim 6, wherein each resistant band comprises one or more rings or loops (collectively referred to as rings) that partially or fully encircle the confined section.

8. The rock anchor as claimed in claim 7, wherein the rings comprise any one or more of: complete circular rings, C-shaped rings, pairs of opposed semi-rings, alternating ring geometries, or strap-like elements.

9. The rock anchor as claimed in claim 7 or 8, wherein the progressive increase in stiffness of the resistant bands is achieved through variation in one or more of: the number of rings comprising each band; the cross-sectional diameter of the rings; the material of manufacture of the rings; the shape or geometry of therings; and the longitudinal spacing between adjacent bands along the confined section.

10. The rock anchor as claimed in any one of claims 7 to 9, wherein each band comprises rings that vary in circumference but are identical in cross-sectional diameter and manufacturing material, and wherein each successive band contains an increasing number of rings compared to the preceding band.

11. The rock anchor as claimed in any one of the preceding claims, wherein the sleeve is C-shaped in cross-section, defining a longitudinal slit extending between the distal and proximal ends to accommodate radial compression of the sleeve.

12. The rock anchor as claimed in any one of the preceding claims, wherein the load-bearing assembly comprises a seat and a nut, the nut being configured to engage a threaded section of the rod at or towards the trailing end, and the seat being configured to engage the proximal end of the sleeve.

13. The rock anchor as claimed in any one of the preceding claims, further comprising a faceplate mounted on the rod and positioned to bear against a rock face when the rock anchor is installed.

14. The rock anchor as claimed in any one of the preceding claims, wherein the sleeve includes a formation configured to arrest continued movement of the expansion element, thereby reinforcing the anchor while permitting additional rock face movement to be absorbed by elongation of the rod.

15. The rock anchor as claimed in any one of claims 1 to 14, wherein shallow advancement of the expansion element into the confined section provides end-anchorage through interaction between the expansion element and the distal resistant bands, optionally without exhibiting the variable stiffness profile.

16. A method of stabilising a rock face using a variable stiffness hybrid rock anchor as claimed in any one of claims 1 to 14, comprising:inserting the rock anchor into a predrilled borehole in the rock face, the borehole having a diameter slightly smaller than the sleeve such that insertion causes radial compression of the sleeve and establishes a friction fit;applying rotational force to the load-bearing assembly to tension the rod and draw the expansion element into the confined section, thereby creating a pre- load; andpermitting subsequent static or dynamic movement of the rock face to further tension the rod, causing the expansion element to advance through the confined section and progressively engage the resistant bands, thereby increasing the stiffness of the rock anchor in response to rock movement.

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